Biomechanical Basis of Human Movement, 3rd Edition

Section I - Foundations of Human Movement

Chapter 2

Skeletal Considerations for Movement

Objectives

After reading this chapter, the student will be able to:

  1. Define how the mechanical properties of a structure can be expressed in terms of its stress-strain relationship.
  2. Define stress, strain, elastic region, plastic region, yield point, failure point, and elastic modulus.
  3. Identify the elastic region, yield point, plastic region, and failure point on a stress-strain curve.
  4. Describe the difference between elastic and a viscoelastic material.
  5. Differentiate between brittle, stiff, and compliant materials.
  6. List the functions of bone tissue that makes up the skeletal system.
  7. Describe the composition of bone tissue and the characteristics of cortical and cancellous bone.
  8. Identify the types of bones found in the skeletal system and describe the role each type of bone plays in human movement or support.
  9. Describe how bone tissue forms and the differences between modeling and remodeling.
  10. Discuss the impact of activity and inactivity on bone formation.
  11. Define osteoporosis and discuss the development of osteoporosis.
  12. Discuss the strength and stiffness of bone as well as bone's anisotropic and viscoelastic properties.
  13. Define the following types of loads that bone must absorb and provide an example to illustrate each load on the skeletal system: compression, tension, shear, bending, and torsion.
  14. Describe stress fractures and other common injuries to the skeletal system and explain the load causing the injury.
  15. Describe the types of cartilage and their function in the skeletal system.
  16. Describe the function of ligaments in the skeletal system.
  17. Describe all of the components of the diarthrodial joint, factors that contribute to joint stability, and examples of injury to the diarthrodial joint.
  18. List the seven different types of diarthrodial joints and provide examples of each one.
  19. Describe the characteristics of the synarthrodial and amphiarthrodial joint and provide an example of each.
  20. Define osteoarthritisand discuss the development of osteoarthritis.

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Measuring the Mechanical Properties of Body Tissues

Bone, tendon, ligament, and muscle are some of the basic structures that make up the human body. Of great interest to biomechanists are the mechanical properties of these tissues. Generally, when analyzing the mechanical properties of such structures, we discern the external forces that are applied to the structure and relate them to the resulting deformation of the structure. The ability of a structure to resist deformation depends on its material organization and overall shape. Therefore, this type of analysis is important because it provides information on the mechanical properties of the structure that may ultimately influence its function.

Basic Structural Analysis

Stress and Strain

The force applied to deform a structure and the resulting deformation is referred to as stress and strain, respectively. To enable comparison of structures of different sizes, stress and strain are scaled quantities of the force applied and the deformation of the structure, respectively. The values of stress and strain are measured on a machine that can place either tension (pulling stress) or compression (pushing stress) on the structure. In Figure 2-1, the load cell measures the tension, or pulling force, applied to the tendon, and the extensiometer measures the length to which the tendon is stretched. The actuator is a motor that initiates the pull on the tendon. Figure 2-2 shows a similar setup to determine the compression stress on an amputated foot. The graph relating stress to strain is the stress-strain curve of a structure. A stress-strain analysis can be used to discern how a material changes with age, how materials react to different force applications, and how a material reacts to lack of everyday stress. Figure 2-3 illustrates the stress-strain relationships of bone vertebrae of normal rhesus monkeys versus those that have been immobilized. A stress-strain analysis can be performed with pulling force (tension), pushing force (compression), or shear force (a push or pull along the surface of the material). This book deals only with tension and compression stress-strain relationships.

In this type of test, stress is defined as the force per unit area and is designated with the Greek letter sigma (<r). Stress is calculated thus:

where Fis the applied force and A is the unit area over which the force is applied. The force is applied perpendicular to the

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surface of the structure over a predetermined area. The unit in which a force is measured is the newton (N). The unit of area is the square meter (m2). Thus, the unit of stress is new-tons per square meter (N/m2), or the pascal (Pa).

 

FIGURE 2-1 A testing machine that determines the stress-strain properties of a tendon. The actuator stretches the tendon. (Reprinted with permission from 

Alexander, R. M. (1992). The Human Machine. New York: Columbia University Press.

)

 

FIGURE 2-2 A testing machine that determines the stress-strain properties of an amputated foot. (Reprinted with permission from 

Alexander, R. M. (1992). The Human Machine.New York: Columbia University Press.

)

Deformation or strain is also scaled to the initial length of the structure being tested. That is, the deformation caused by the applied stress is compared with the initial, or resting, length of the material, when no force is applied. Strain, designated by the Greek letter epsilon (e), is therefore defined as the ratio of the change in length to the resting length. Thus:

ε=ΔL/L

where δL is the change in length of the structure and L is the initial length. Because we are dividing a length by a length, there are no units, so strain is a dimensionless number.

 

FIGURE 2-3 Stress-strain curves for bone vertebral segments from a normal and immobilized rhesus monkey (Adapted from 

Kazarian, L. E., Von Gierke, H. E. [1969]. Bone loss as a result of immobilization and chelation. Preliminary results in Macaca mulatta. Clinical Orthopaedics, 65:67-75.

)

A stress-strain curve is presented in Figure 2-4. A number of key points on this curve are important to the ultimate function of the structure. In this curve, the slope of the linear portion of the curve is the elastic modulus, or stiffness of the material. Stiffness is thus calculated as:

k = Stress/Strain = σ/ε

As greater force is applied to the structure, the slope of the curve eventually decreases. At this point, the structure is said to yield or reach its yield point. Up to the yield point, the structure is said to be in the elastic region. If the stress is removed while the material is in this region, the material will return to its original length with no structural damage. After the yield point, the molecular components of the material are permanently displaced with respect to each other, and if the applied force is removed, the material will not return to its original length (Fig. 2-5). The difference between the original length of the material and the (resting) length resulting from stress into the plastic region is the residual strain.

The region after the yield point is the plastic region. For rigid materials, such as bone, the yield or plastic region is relatively small, but for other materials, it can be relatively large. If the applied force continues beyond the plastic region, the structure will eventually reach failure, at which point the stress quickly falls to zero. The maximal stress reached when failure occurs determines the failure strength and failure strain of the material.

 

FIGURE 2-4 An idealized stress-strain curve showing the elastic and plastic regions and the elastic modulus.

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FIGURE 2-5 A stress-strain curve of a material that has been elongated into the plastic region. A, The period of the applied load. B, The period when the applied load is removed. The residual strain results because of the reorganization of the material at the molecular level.

In normal functional activities, the stress applied will not cause a strain that reaches the yield point. When structures are designed by an engineer, the engineer considers a safety factor when determining the stress-strain relationship of the structure. This safety factor is generally in the range of 5 to 10 times the stress that would normally be placed on the structure. That is, the applied force to reach the yield point is significantly greater than the force generally applied in everyday activities. It is obvious and has been suggested that biological materials and biological structures must have a significantly large safety factor. Needless to say, the stresses placed on a biological structure in everyday activities are much less than the structure can handle. Figure 2-6 illustrates a stress-strain curve for a human adult tibia and the actual stress-strain relationship during jogging.

 

FIGURE 2-6 The shaded area represents the tension stress-strain values of an adult human tibia during jogging, and the solid line represents bone samples tested to failure (Adapted with permission from 

Nordin, M., Frankel, V. H. [1989]. Basic Biomechanics of the Musculoskeletal System. Philadelphia: Lea & Febiger.

)

When a structure is deformed by an applied force, the strain developed in the material relates to the mechanical energy absorbed by the material. The amount of mechanical energy stored is proportional to the area under the stress-strain curve (Fig. 2-7). That is, the stored mechanical energy is:

ME = 1/2σε

When the applied force is removed, the stored energy is released. For example, a rubber band can be stretched by pulling on both ends. When one end is released, the rubber band rebounds back to its original length but, in doing so, releases the energy stored during stretching. For practical purposes, this is the same concept as a trampoline. The weight of the person bouncing on it deforms the bed and stores energy. The trampoline rebounds and releases the stored energy to the person.

Types of Materials

Elastic

The idealized material described in Figure 2-4 is an elastic material. In this type of material, a linear relationship exists between the stress and strain. That is, when the material is deformed by the applied force, the amount of deformation is the same for a given amount of stress. When the applied load is removed, the material returns to its resting length as long as the material did not reach its yield point. In an elastic material, the mechanical energy that was stored is fully recovered.

Viscoelastic

As opposed to elastic structures, certain materials show stress-strain characteristics that are not strictly linear; these are viscoelastic materials. These structures have nonlinear or viscous properties in combination with linear elastic properties. The combination of these properties results in the magnitude of the stress being dependent on the rate of loading, or how fast the load is applied. Nearly all biological materials, such as tendon and ligament, show some level of viscoelasticity.

 

FIGURE 2-7 The stored mechanical energy (shaded area) is equal to the area under the stress-strain curve.

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FIGURE 2-8 A stress-strain curve of a typical viscoelastic material. The elastic modulus (slope of the curve) varies according to the portion of the curve on which it is calculated.

Figure 2-8 illustrates a viscoelastic material. On a stress-strain curve of a viscoelastic material, the terms stiffness, yield point, and failure point also apply. The elastic and plastic regions are defined similarly as in an elastic material. In contrast to an elastic structure, however, stiffness has several values that can be determined by where it was calculated on the curve. In Figure 2-8, the stiffness designated by E1 is less than that of E2. E3, however, is certainly less than E2. In addition, in a viscoelastic material, the stored mechanical energy is not completely returned when the applied load is removed. Thus, the energy returned is not equal to the energy stored. The energy that is lost is hysteresis (Fig. 2-9).

Materials, whether they are elastic or viscoelastic, are often referred to as stiff, compliant, or brittle, depending on the elastic modulus. Stress-strain curves of these materials are presented in Figure 2-10. A compliant material has an elastic modulus less than that of a stiff material. The compliant material stores considerably more energy than a stiff material. On the other hand, a brittle material has a greater elastic modulus and stores less energy than a stiff material. Nonetheless, all of these terms are relative. Depending on the materials being tested, a brittle material may be considered stiff relative to one material and compliant relative to another. For example, bone is brittle relative to tendon but compliant relative to glass.

 

FIGURE 2-9 A stress-strain curve of a typical viscoelastic material showing the energy recovered when the material is allowed to return to its resting length. The hysteresis or energy lost is equal to the energy stored when the material is deformed minus the energy recovered.

 

FIGURE 2-10 Stress-strain curves of compliant, stiff, and brittle materials. The elastic modulus is significantly different in the three materials.

Biomechanical Characteristics of Bone

Bone Tissue Function

The skeleton is built of bone tissue. Joints, or articulations, are the intersections between bones. Ligaments connect bones at the articulations, thus reinforcing the joints. The skeleton consists of approximately 20% of total body weight. The skeletal system is generally broken down into axial and appendicular skeletons. The major bones of the body are presented in Figure 2-11. Bone tissue performs many functions, including support, attachment sites, leverage, protection, storage, and blood cell formation.

Support

The skeleton provides significant structural support and can maintain a posture while accommodating large external forces, such as those involved in jumping. The bones increase in size from top to bottom in proportion to the amount of body weight they bear; thus, the bones of the lower extremities and the lower vertebrae and pelvic bones are larger than their upper extremity and upper torso counterparts. A visual comparison of the humerus and femur or the cervical vertebrae and lumbar vertebrae demonstrates these size relationships. Internally, bones also protect the internal organs.

Attachment Sites

Bones provide sites of attachment for tendons, muscles, and ligaments, allowing for the generation of movement through force applications to the bones through these sites. Knowledge of attachment sites on each bone provides good information about the movement potential of

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specific muscles, support offered by ligaments, and potential sites of injury.

 

FIGURE 2-11 Anterior view (left) and posterior view (right) of the bones of the human body. (Reprinted with permission from 

Willis, M. C. [1996]. Medical Terminology: The Language of Health Care. Baltimore: Williams & Wilkins.

)

Leverage

The skeletal system provides the levers and axes of rotation about which the muscular system generates the movements. A lever is a simple machine that magnifies the force, speed, or both of movement and consists of a rigid rod that is rotated about a fixed point or axis called the fulcrum. The rigid rod in a skeletal lever system is primarily the longer bones of the body, and the fixed point of rotation or axis is the joints where the bones meet. The skeletal lever system transmits movement generated by muscles or external forces. Chapter 10 provides an in-depth discussion of levers.

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Other Functions

Three additional bone functions are not specifically related to movement: protection, storage, and blood cell formation. The bones protect the brain and internal organs. Bone also stores fat and minerals and is the main store of calcium and phosphate. Finally, blood cell formation, called hematopoiesis, takes place within the cavities of bone.

Composition of Bone Tissue

Bone, or osseous tissue, is a remarkable material with properties that make it ideal for its support and movement functions. It is light but it has high tensile and compressive strength and a significant amount of elasticity. Bone is also a very dynamic material with minerals moving in and out it constantly. As much as a half a gram of calcium may enter or leave the adult skeleton every day, and humans recycle 5% to 7% of their bone mass every week. Bone can also be made to grow in different ways, and it is tissue that is continually being modified, reshaped, remodeled, and overhauled.

Osseous tissue is strong and is one of the body's hardest structures because of its combination of inorganic and organic elements. Bone is composed of a matrix of inorganic salts and collagen, an organic material found in all connective tissue. The inorganic minerals, calcium and phosphate, along with the organic collagen fibers, make up approximately 60% to 70% of bone tissue. Water constitutes approximately 25% to 30% of the weight of bone tissue (43). Collagen provides bone with tensile strength and flexibility, and the bone minerals provide compressive strength and rigidity (38).

Bone cells are referred to as osteocytes. The two types of these cells are referred to as osteoblasts and osteoclasts. These cells are responsible for remodeling bone. Osteoclasts are the cells that break down bone and convert calcium salts into a soluble form that passes easily into the blood. Osteoblasts produce the organic fibers on which the calcium salts are deposited. A balance in the activities of these two cells maintains a constant bone mass.

Macroscopic Structure of Bone

Bone is composed of two types of tissue: cortical bone and cancellous bone. The hard outer layer is cortical bone; internal to this is cancellous bone. A section of the femoral head presented in Figure 2-12 illustrates the architecture of the long bone. The architectural arrangement of bony tissue is remarkably well suited for the mechanical demands imposed on the skeletal system during physical activity.

Cortical Bone

Cortical bone is often referred to as compact bone and constitutes about 80% of the skeleton. Cortical bone looks solid, but closer examination reveals many passageways for blood vessels and nerves. The exterior layer of bone is very dense and has a porosity less than 15% (48). Porosity is the ratio of pore space to the total volume; when porosity increases, bone mechanical strength deteriorates. Small changes in porosity can lead to significant changes in the stiffness and strength of bone.

 

FIGURE 2-12 Midsection of the proximal end of the femur showing both cortical and cancellous bone. The dense cortical bone lines the outside of the bone, continuing down to form the shaft of the bone. Cancellous bone is found in the ends and is distinguishable by its lattice-like appearance. Note the curvature in the trabeculae, which forms to withstand the stresses.

Cortical bone consists of a system of hollow tubes called lamellae that are placed inside one another. Lamellae are composed of collagen fibers, all running in a single direction. The collagen fibers of adjacent lamellae always run in different directions. A series of lamellae form an osteon or Haversian system. Osteons are pillar-like structures that are oriented parallel to the stresses that are placed on the bone. The arrangement of these weight-bearing pillars and the density of the cortical bone provide strength and stiffness to the skeletal system. Cortical bone can withstand high levels of weight bearing and muscle tension in the longitudinal direction before it fails and fractures (46).

Cortical bone is especially capable of absorbing tensile loads if the collagen fibers are parallel to the load. Typically, the collagen is arranged in layers running in longitudinal, circumferential, and oblique configurations. This offers resistance to tensile forces in different directions because the more layers there are, the greater the strength and stiffness the bone has. Also, where muscles, ligaments, and tendons attach to the skeleton, collagen fibers are arranged parallel to the insertion of the soft tissue, thereby offering greater tensile strength for these attachments.

A thick layer of cortical bone is found in the shafts of long bones, where strength is necessary to respond to the high loads imposed down the length of the bone during weight bearing or in response to muscular tension. The thickness is greater in the middle part of the long bones because of the increased bending and torsion forces (9). Thin layers of cortical bone are found on the ends of the long bones, the epiphyses, and covering the short and irregular bones.

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Cancellous Bone

The bone tissue interior to cortical bone is referred to as cancellous or spongy bone. Cancellous bone is found in the ends of the long bones, in the body of the vertebrae, and in the scapulae and pelvis. This type of bone has a lattice-like structure with a porosity greater than 70% (48). Cancellous bone structure, although quite rigid, is weaker and less stiff than cortical bone. Cancellous bone is not as dense as cortical bone because it is filled with spaces. The small, flat pieces of bone that serve as small beams between the spaces are called trabeculae (Fig. 2-12). The trabeculae adapt to the direction of the imposed stress on the bone, providing strength without adding much weight (11). Collagen runs along the axis of the trabeculae and provides cancellous bone with both tensile and compressive resistance.

The high porosity gives cancellous bone high-energy storage capacity, so that this type of bone becomes a crucial element in energy absorption and stress distribution when loads are applied to the skeletal structure (43). This type of bone is more metabolically active and responsive to stimuli than cortical bone (30). It has a much higher turnover rate than cortical bone, ending up with more remodeling along lines of stress (38). Cancellous bone is not as strong as cortical bone, and there is a high incidence of fracture in the cancellous bone of elderly individuals. This is believed to be caused by loss of compressive strength because of mineral loss (osteoporosis).

 

FIGURE 2-13 Various types of bones serve specific functions. A. Long bones serve as levers. B. Short bones offer support and shock absorption. C. Flat bones protect and offer large muscular attachment sites. D. Irregular bones have specialized functions. E. Sesamoid bones alter the angle of muscular insertion.

Anatomical Classification of Bones

The skeletal system has two main parts: the axial (skull, spine, ribs, and sternum) and the appendicular (shoulder and pelvic girdles and arms and legs) skeleton. Making up each section of the skeleton are four types of bones (Fig. 2-13). These include bones designated as long, short, flat, and irregular. Each type of bone performs specific functions.

Long Bones

The long bones are longer than they are wide. The long bones in the body are the clavicle, humerus, radius, ulna, femur, tibia, fibula, metatarsals, metacarpals, and phalanges. The long bone has a shaft, the diaphysis, a thick layer of cortical bone surrounding the bone marrow cavity (Fig. 2-14). The shaft widens toward the end into the section called the metaphysis. In the immature

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skeleton, the end of the long bone, the epiphysis, is separated from the diaphysis by a cartilaginous disc. The epiphyses consist of a thin outer layer of cortical bone covering cancellous inner bone. A thin white membrane, the periosteum, covers the outside of the bone with the exception of the parts covered by cartilage.

 

FIGURE 2-14 The long bone has a shaft, or diaphysis (A), which broadens out into the metaphysis (B) and the epiphysis (C). Layers of cortical bone make up the diaphysis. The metaphysis and epiphysis are made up of cancellous bone inside a thin layer of cortical bone.

Long bones offer the body support and provide the interconnected set of levers and linkages that allow us to move. A long bone can act as a column by supporting loads along its long axis. Long bones typically are not straight; rather, they are beam shaped, which creates a stronger structure so bones can handle and minimize bending loads imposed on them. A long bone is strongest when it is stressed by forces acting along the long axis of the bone. Muscle attachment sites and protuberances are formed by tensile forces of muscles pulling on the bones.

Short Bones

The short bones, such as the carpals of the hand and tarsals of the foot, consist primarily of cancellous bone covered with a thin layer of cortical bone. These bones play an important role in shock absorption and the transmission of forces. A special type of short bone, the sesamoid bone, is embedded in a tendon or joint capsule. The patella is a sesamoid bone at the knee joint that is embedded in the tendon of the quadriceps. Other sesamoid bones can be found at the base of the first metatarsal in the foot, where the bones are embedded in the distal tendon of the flexor hallucis brevis muscle, and at the thumb, where the bones are embedded in the tendon of the flexor pollicis brevis muscle. The role of the sesamoid bone is to alter the angle of insertion of the muscle and to diminish friction created by the muscle.

Flat Bones

A third type of bone, the flat bone, is represented by the ribs, ilium, sternum, and scapula. These bones consist of two layers of cortical bone with cancellous bone and marrow in between. Flat bones protect internal structures and offer broad surfaces for muscular attachment.

Irregular Bones

Irregular bones, such as those found in the skull, pelvis, and vertebrae, consist of cancellous bone with a thin exterior of cortical bone. These bones are termed irregular because of their specialized shapes and functions. The irregular bones perform a variety of functions, including supporting weight, dissipating loads, protecting the spinal cord, contributing to movement, and providing sites for muscular attachment.

Bone Formation

Bone is a highly adaptive material that is very sensitive to disuse, immobilization, or vigorous activity and high levels of loading. Across the lifespan, bone is continually optimized for its load-bearing role through functionally adaptive remodeling. Changes occur in whole bone architecture and bone mass as a functional adaptation occurs where the bone mass and architecture is matched to functional demand (40). In the appendicular skeleton, this is particularly important because of the load bearing. Adaptive changes are maximum in growing bone and decrease with aging but still occur to some level as the skeleton adapts to changes in mechanical use. Bone tissue is self-repairing and can alter its properties and configuration in response to mechanical demand. This was first determined by the German anatomist Julius Wolff, who provided the theory of bone development, termed Wolff's law. This law states: “Every change in the form and function of a bone or of their function alone is followed by certain definitive changes in their internal architecture and equally definite secondary alteration in their external conformation, in accordance with mathematical laws” (32).

Ossification, Modeling, and Remodeling

The formation of bone is a complex process that cannot be fully explored in detail here. Bone is always formed by the replacement of some preexisting tissue. In fetuses, much of the preexisting tissue is hyaline cartilage. Ossification is the formation of bone by the activity of the osteoblasts and osteoclasts. In fetuses, the cartilage is slowly replaced through this process so that at the time of birth, many of the bones have been at least partially ossified. In sites such as the flat bones of the skull, bone replaces a soft fibrous tissue instead of cartilage.

Long bones grow from birth through adolescence through activity at cartilage plates located between the shaft and the heads of the bones. These epiphyseal plates expand as new

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cells are formed and the bone is lengthened until the thicknesses of the plates diminish to reach what is called full ossification. This occurs in different bones at different ages but is usually complete by age 25 years.

Modeling occurs during growth to create new bone as bone resorption and bone formation (ossification) occur at different locations and rates to change the bone shape and size. In growing bone, bone properties are related to the growth-related demands on size and to changes in tensile and compressive forces acting on the body. Bone is deposited by osteoblasts while it is also being resorbed by osteoclasts. In the process of resorption, old bone tissue is broken down and digested by the body. This process is not the same in all bones or even in a single bone. For example, whereas the bone in the distal part of the femur is replaced every 5 to 6 months, the bone in the shaft is replaced much more slowly.

Living bone is always undergoing remodeling in which the bone matrix is constantly being removed and replaced. The thickness and strength of bone must be continually maintained by the body, and this is done by an ongoing cycle of replacing old bone with new bone. A dynamic steady state is maintained by replacing a small amount of bone at the same site while leaving the size and shape of the remodeled bone basically the same. At least some new bone is being formed continually, and bone remodeling is the process through which bone mass adapts to the demands placed on it. After an individual is past the growing stage, the rate of bone deposit and resorption are equal to each other, keeping the total bone mass fairly constant. Through exercise, however, bone mass can be increased, even up through young adulthood. Bone deposits exceed bone resorption when greater strength is required or when an injury has occurred. Thus, weight lifters develop thickenings at the insertion of very active muscles, and bones are densest where stresses are greatest. The dominant arms of professional tennis players have cortical thicknesses that are 35% greater than the contralateral arm (32). The shape of bone also changes during fracture healing.

This ongoing rebuilding process continues up until age 40 years, when the osteoblastic activity slows and bones become more brittle. This remodeling process has two major benefits: The skeleton is reshaped to respond to gravitational, muscular, and external contact forces, and blood calcium levels are maintained for important physiological functions.

Bone Tissue Changes Across the Lifespan

In immature bone, the fibers are randomly distributed, providing strength in multiple directions but lower overall strength. In mature bone, mineralization takes place, haversian canals are created and lined with bone, and fibers are oriented in the primary load-bearing directions. Bone continues to reorganize throughout life to mend damage and to repair wear on the bone. In older bone, bone restoration still occurs, but the Haversian system is smaller, and the canals are larger because of slower bone deposits. There is some indication that this structural adjustment may be a result of decreased muscle strength, leading to partial disuse and subsequent bone remodeling that reduces strength (20).

Physical Activity and Inactivity and Bone Formation

Physical Activity

Bones require mechanical stress to grow and strengthen. Bones slowly add or lose mass and alter form in response to alterations in mechanical loading. Thus, physical activity is an important component of the development and maintenance of skeletal integrity and strength. Bone tissue must have a daily stimulus to maintain health. Muscle contraction in active movement coupled with external forces exerts the biggest pressure on bones. Not all exercises are equally effective. Overloading forces must be applied to the bone to stimulate and adaptive force, and continued adaptation requires a progressive overload (33).

Generally, dynamic loading is better for bone formation than static loading, loading at higher frequencies is more effective, and prolonged exercise has diminishing returns (52). Repetitive, coordinated bone loading associated with habitual activity may have little role in preserving bone mass and may even reduce osteogenic potential because bone tissue becomes desensitized (40). Shorter periods of vigorous activity are more efficient in promoting an increase in bone mass (40). To stimulate an osteogenic effect in adult bone, four cycles a day of loading has been shown to be sufficient to stop bone loss (40). The daily applied loading history, comprising the number of loading cycles and the stress magnitude, influences the density of the bone. Again, it is recommended that one long session be broken into smaller sessions such as four sessions per day or three to five daily sessions per week (47,51).

The effect of physical activity on increasing bone mass varies across the lifespan. In the growing skeleton, loads applied to the skeleton provide a much greater stimulus than to the mature skeleton (52). In older adults with low bone mass, exercise is only moderately effective in bone building. The goal is to maximize the gain in bone mineral density in the first three decades of life and then minimize the decline after age 40 years (33). Bone mass reaches maximum levels between the ages of 18 and 35 years (9) and then decrease by about 0.5% per year after age 40 years (33). In adulthood, bone mass is the maximum bone mass minus the quantity lost, so exercise may be effective in just attenuating the rate of loss rather than increasing bone (33).

Inactivity

Bone loss after a decrease in the activity level may be significant (56). When under loaded in conditions such as fixation or bed rest, bone mass is resorbed, resulting in reduced bone mass and thinning. The skeletal system senses changes in load patterns and adapts to carry the load most efficiently using the least amount of bone mass. In microgravity conditions, astronauts, subjected to reduced activity and the loss of body weight influences, lose significant bone mass in relatively short periods. Some

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of the changes that occur to bone as a result of space travel include loss of rigidity, increased bending displacement, a decrease in bone length and cortical cross-section, and slowing of bone formation (57).

What exercise prescription will facilitate bone growth in children and adolescents?

Exercise Mode

Exercise Intensity

Exercise Frequency

Exercise Duration

Impact activities (gymnastics, plyometrics, jumping); moderate-intensity resistance training; sports involving running and jumping

High; weight training <60% of 1RM for safety purposes

At least 3 days per week

10-20 minutes

Source: Kohrt et al (33).

What exercise prescription will help preserve bone health in adults?

Exercise Mode

Exercise Intensity

Exercise Frequency

Exercise Duration

Weight-bearing endurance activities (tennis, stair climbing, jogging); activities involving jumping (basketball, volleyball); resistance exercise

Moderate to high

Weight-bearing endurance activities 3 to 5 days per week; resistance training 2 days per week

30–60 minutes

Source: Kohrt et al (33).

Osteoporosis

In osteoporosis, bone resorption exceeds bone deposits. Osteoporosis is a disease of increasing bone fragility that is initially subtle, affecting only the trabeculae in cancellous bone, but leads to more severe examples in which one might experience an osteoporotic vertebral fracture just opening a window or rising from a chair (40,51). Bone fragility depends on the ultimate strength of the bone, the level of brittleness in the bone, and the amount of energy bone can absorb (51). These factors are influenced by bone size, bone shape, bone architecture, and bone tissue quality. The symptoms of osteoporosis often begin to appear in elderly individuals, especially postmenopausal women. Osteoporosis may begin earlier in life, however, when bone mineral density decreases. When bone deposition cannot keep up with bone resorption, bone mineral mass decreases, resulting in reduced bone density accompanied by loss of trabecular integrity. The loss of bone mineral density means loss of the stiffness in bone, and the loss of trabecular integrity weakens the structure. Both of these losses create the potential for a much greater incidence of fracture (12), ranging from 2.0% to 3.7% in nonosteoporotic individuals and almost doubling to 5% to 7% in osteoporotic individuals (28). A reason for higher fracture rates may be the higher strain in the osteoporotic bone under similar loading patterns. For example, the osteoporotic femoral head was shown to handle only 59% of the original external load in walking with strains 70% higher than normal and less uniformly distributed (53).

The exact causes of osteoporosis are not fully understood, but the condition has been shown to be related to genetics, hormonal factors, nutritional imbalances, and lack of exercise. Normal bone volume is 1.5 to 2 L, and the cortical diameter of bone is at its maximum between ages 30 and 40 years for both men and women (19,44). After age 30 years, a 0.2% to 0.5% yearly loss in the mineral weight of bone occurs (56), accelerating after menopause in women to bone loss that is 50% greater than in men of similar age (44). It is speculated that a substantial proportion of this bone loss may be related to the accompanying reduction in activity level (56).

Lifestyle and activity habits seem to play an important role in the maintenance of bone health (13). In one study, the incidence of osteoporosis was 47% in a sedentary population compared with only 23% in a population whose occupations included hard physical labor (8). It is clear that elderly individuals may benefit from some form of weight-bearing exercise that is progressive and of at least moderate resistance.

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Estrogen levels in anorexic women and amenorrheic female athletes have also been related to the presence of osteoporosis in this population. There is speculation that stress fractures in the femoral neck of female runners may be related to a noted loss of bone mineral density caused by osteoporosis (12). Elite female athletes in a variety of sports have had bone loss, usually associated with bouts of heavy training and associated menstrual irregularity. Some of these athletes have lost so much bone mass that their skeletal characteristics resemble those of elderly women.

Mechanical Properties of Bone

The mechanical properties of bone are as complex and varied as its composition. The measurement of bone strength, stiffness, and energy depends on both the material composition and the structural properties of bone. In addition, the mechanical properties also vary with age and gender and with the location of the bone, such as the humerus versus the tibia. Additional variation may result from other factors such as orientation of load applied to the bone, rate of load application, and type of load.

Bone must be capable of withstanding a variety of imposed forces simultaneously. In a static position, bone resists the force of gravity, supports the weight of the body, and absorbs muscular activity produced to maintain the static posture. In a dynamic mode such as running, the forces are magnified many times and become multidirectional.

Strength and Stiffness of Bone

The behavior of any material under loading conditions is determined by its strength and stiffness. When an external force is applied to a bone or any other material, an internal reaction occurs. The strength can be evaluated by examining the relationship between the load imposed (external force) and the amount of deformation (internal reaction) occurring in the material.

As noted earlier, bone must be stiff yet flexible and strong yet light. Strength is necessary for load bearing, and lightness is necessary to allow movement. The strength in weight-bearing bones lies in their ability to resist bending by being stiff. Flexibility is needed to absorb high-impact forces, and the elastic properties of bone allow it to absorb energy by changing shape without failing and then return to its normal length. If the imparted energy exceeds the zone of elastic deformation, plastic deformation occurs at the price of microdamage to the bone. If both the elastic and plastic zones are exceeded, the imparted energy is released in the form of a fracture.

Strength

The strength of bone or any other material is defined by the failure point or the load sustained before failure. The overall ability of bone to bear a load depends on having sufficient bone mass with adequate material properties as well as fiber arrangement that resist loading possibilities in different directions (40). The failure of bone depends on the type of load imposed (Fig. 2-15); there is actually no standardized strength value for bone because the measurement is so dependent on the type of bone and testing site. Failure of bone involves either a single traumatic event or the accumulation of microfractures. Thus, both fracture and fatigue behaviors of bone are important. Strength of bone is provided by the mineralization of its tissue: the greater the tissue mineral content, the stiffer and stronger the material. If bone becomes too mineralized, however, it becomes brittle and does not give during impact loading. Strength is assessed in terms of energy storage or the area under a stress-strain curve.

The compressive strength of cortical bone is greater than that of concrete, wood, or glass (Fig 2-16). The strength of cortical bone in the middle of long bones is demonstrated in the ability to tolerate large impact loads and resist bending. Cancellous bone strength is less than that of cortical bone, but cancellous bone can undergo more deformation before failure.

What makes a skeleton stronger and less brittle?

  1. An increase in bone mass
  2. Effective distribution of bone mass so more bone is present where mechanical demand is greater
  3. Improved bone material properties
 

FIGURE 2-15 Ultimate stress for human adult cortical bone specimens (Adapted with permission from 

Nordin, M. & Frankel, V.H. [Eds.] [1989] Basic Biomechanics of the Musculoskeletal System. Philadelphia: Lea & Febiger.

)

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FIGURE 2-16 The strength and stiffness of a variety of materials are plotted in four quadrants representing material that is flexible and weak (A), stiff and weak (B), stiff and strong (C), and flexible and strong (D). Bone is categorized as being flexible and weak, along with other materials, such as spider web and oak wood. (Adapted with permission from 

Shipman, P., Walker, A., and Bichell, D. [1985]. The Human Skeleton. Cambridge, MA: Harvard University Press.

)

Stiffness

Stiffness, or the modulus of elasticity, is determined by the slope of the load deformation curve in the elastic response range and is representative of the material's resistance to load as the structure deforms. The stress-strain curve for ductile, brittle, and bone material is shown in Figure 2-17, and Figure 2-16 plots a variety of materials according to strength and stiffness (49). Metal is a type of ductile material that has high stiffness, and at stresses beyond its yield point, exhibits ductile behavior in which it undergoes large plastic deformation before failure. Glass is a brittle material that is stiff but fails early, having no plastic region. Bone is not as stiff as glass or metal, and unlike these materials, it does not respond in a linear fashion because it yields and deforms nonuniformly during the loading phase (43). Bone has a much lower level of stiffness than metal or glass and fractures after very little plastic deformation.

At the onset of loading, bone exhibits a linearly elastic response. When a load is first applied, a bone deforms through a change in length or angular shape. Bone deforms no more than approximately 3% (50). This is considered in the elastic region of the stress-strain curve because when the load is removed, the bone recovers and returns to its original shape or length. With continued loading, the bone tissue reaches its yield point, after which its outer fibers begin to yield, with microtears and debonding of the material in the bone. This is termed the plastic region of the stress-strain curve. The bone tissue begins to deform permanently and eventually fractures if loading continues in the plastic region. Thus, when the load is removed, the bone tissue does not return to its original length but stays permanently elongated. Although bone can exhibit a plastic response, normal loading remains well within the elastic region. Bone behaves largely like a brittle material, exhibiting very little permanent plastic deformation to failure.

 

FIGURE 2-17 Stress-strain curves illustrating the differences in the behavior between ductile material (A), brittle material (B), and bone (C), which has both brittle and ductile properties. When a load is applied, a brittle material responds linearly and fails or fractures before undergoing any permanent deformation. The ductile material enters the plastic region and deforms considerably before failure or fracture. Bone deforms slightly before failure.

Anisotropic Characteristics

Bone tissue is an anisotropic material, which means that the behavior of bone varies with the direction of the load application (Fig. 2-18). The differences between the properties

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of the cancellous and cortical bone contribute to the anisotropy of bone. The contribution of cancellous and cortical components of whole bone to overall strength varies with anatomical location because variable amounts of cortical and cancellous are present at every site. Cancellous bone provides bending strength, and cortical bone provides significant compressive strength. Within each bone, considerable variation exists, as seen in the femur, where the bone is weaker and less stiff in the anterior and posterior aspects than in the medial and lateral aspects. Even though the properties of bone are direction dependent, in general, tissue of long bones can handle the greatest loads in the longitudinal direction and the least amount of load across the surface of the bone (46). Long bones are stronger withstanding longitudinal loads because they are habitually loaded in that direction.

 

FIGURE 2-18 Bone is considered anisotropic because it responds differently if forces are applied in different directions. A. Bone can handle large forces applied in the longitudinal direction. B. Bone is not as strong in handling forces applied transversely across its surface.

Viscoelastic Characteristics

Bone is viscoelastic, meaning that its response depends on the rate and duration of the load. At a higher speed of loading, bone is stiffer and tougher because it can absorb more energy to failure the more rapidly it is loaded. These strain rates are seen in high impact situations involving falls or vehicular accidents. As shown in Figure 2-19, a bone loaded slowly fractures at a load that is approximately half of the load handled by the bone at a fast rate of loading.

Bone tissue is a viscoelastic material whose mechanical properties are affected by its deformation rate. The ductile properties of bone are provided by its collagenous material. The collagen content gives bone the ability to withstand tensile loads. Bone is also brittle, and its strength depends on the loading mechanism. The brittleness of bone is provided by the mineral constituents that provide bone with the ability to withstand compressive loads.

Loads Applied to Bone

The skeletal system is subject to a variety of applied forces as bone is loaded in various directions. Loads are produced by weight bearing, gravity, muscular forces, and external forces. Internally, loads can be applied to bones through the joints by means of ligaments or at tendinous insertions, and these loads are usually below any fracture level. Externally, bone accommodates multiple forces from the environment that have no limit on magnitude or direction.

 

FIGURE 2-19 Bone is considered viscoelastic because it responds differently when loaded at different rates. A. When loaded quickly, bone responds with more stiffness and can handle a greater load before fracturing. B. When loaded slowly, bone is not as stiff or strong, fracturing under lower loads.

Muscular activity can also influence the loads that bone can manage. Muscles alter the forces applied to the bone by creating compressive and tensile forces. These muscular forces may reduce tensile forces or redistribute the forces on the bone. Because most bones can handle greater compressive forces, the total amount of load can increase with the muscular contribution. If muscles fatigue during an exercise bout, their ability to alleviate the load on the bone diminishes. The altered stress distribution or increase in tensile forces leaves the athlete or performer susceptible to injury.

The stress and strain produced by forces applied to bones are responsible for facilitating the deposit of osseous material. Stress can be perpendicular to the plane of a cross-section of the loaded object. This is termed normal stress. If stress is applied parallel to the plane of the cross-section, it is termed shear stress. Each type of stress produces a strain. For example, whereas normal strain involves a change in the length of an object, shear strain is characterized by a change in the original angle of the object. An example of both normal strain and shear strain is the response of the femur to weight bearing. The femur shortens in response to normal strain and bends anteriorly in response to shear strain imposed by the body weight (46). Normal stress and shear stress, developed in response to tension applied to the tibia, are presented in Figure 2-20. Normal and shear strain, developed in response to compression of the femur, are also illustrated.

Whether or not a bone incurs an injury as a result of an applied force is determined by the critical strength limits of the material and the loading history of the bone. External factors related to fracture include the magnitude, direction, and duration of the force coupled with the rate at which the bone is loaded. The ability of a bone to resist fracture is related to its energy-absorbing capacity. The ability of a bone to resist deformation varies through its length because of the different makeup of cortical and cancellous bone (38). Cancellous bone, depending on its architecture, can deform more and can absorb considerably more energy than cortical bone (38). These limits are primarily influenced by the loading on the bone. The loading of the bone can be increased or decreased by physical activity and conditioning, immobilization, and skeletal maturity of the individual. The rate of loading is also important because the response and tolerance of bone is rate sensitive. At high rates of loading, when bone tissue cannot deform fast enough, an injury can occur.

The five types of forces applying loads to bone are compression, tension, shear, bending, and torsion. These forces are summarized in Table 2-1 and illustrated in Figure 2-21.

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FIGURE 2-20 Stress, or force per unit area, can be perpendicular to the plane (normal stress) (A) or parallel to the plane (shear stress) (B). Strain, or deformation of the material, is normal (C), in which the length varies, or shear (D), in which the angle changes.

 

FIGURE 2-21 The skeletal system is subjected to a variety of loads that alter the stresses in the bone. The square in the femur indicates the original state of the bone tissue. The colored area illustrates the effect of the force applied to the bone. A. Compressive force causes shortening and widening. B. Tensile force causes narrowing and lengthening. C and D. Shear force and torsion create angular distortion. E. Bending force includes all of the changes seen in compression, tension, and shear.

TABLE 2-1 Different Types of Loads Acting on Bone

Load

Type of Force

Source

Stress/Strain

Compression

Presses ends of bones together to cause widening and shortening

Muscles, weight bearing, gravity or external forces

Maximal stress on the plane perpendicular to the applied load

Tension

Pulls or stretches bone to cause lengthening and narrowing

Usually pull of contracting muscle tendon

Maximal stress on the plane perpendicular to the applied load

Shear

Force applied parallel to surface, causing internal deformation in an angular direction

Compressive or tension force application or external force

Maximum stress at on the plane parallel to the applied load

Bending

Force applied to bone having no direct support from the structure

Weight bearing or multiple forces applied at different points on the bone

Maximum tensile forces on the convex surface of the bent member and maximum compression forces on the concave side

Torsion

Twisting force

Force applied with one end of the bone fixed

Maximum shear stress on both the perpendicular and parallel to axes of bone with tension and compression forces also present at an angle across the surface

Compression Forces

Compressive forces are necessary for development and growth in the bone. Specific bones need to be more suited to handle compressive forces. For example, the femur carries a large portion of the body's weight and needs to be stiff to avoid compression when loaded. The loads acting on the femur have been measured in the range of 1.8 to 2.7 body weight during one-leg standing and as high as 1.5 body weight in a leg lift in bed (2).

If a large compressive force is applied and if the load surpasses the stress limits of the structure, a compression fracture will occur. Numerous sites in the body are susceptible

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to compressive fractures. Compressive forces are responsible for patellar pain and softening and destruction of the cartilage underneath the patella. As the knee joint moves through a range of motion, the patella moves up and down in the femoral groove. The load between the patella and the femur increases and decreases to a point at which the compressive patellofemoral force is greatest at approximately 50° of flexion and least at full extension or hyper-extension of the knee joint. The high-compressive force in flexion, primarily on the lateral patellofemoral surface, is the source of the destructive process that breaks down the cartilage and underlying surface of the patella (17).

Compression is also the source of fractures to the vertebrae (18). Fractures to the cervical area have been reported in activities such as water sports, gymnastics, wrestling, rugby, ice hockey, and football. Normally the cervical spine is slightly extended with a curve anteriorly convex. If the head is lowered, the cervical spine will flatten out to approximately 30° of flexion. If a force is applied against the top of the head when it is in this position, the cervical vertebrae are loaded along the length of the cervical vertebrae by a compressive force, creating a dislocation or fracture-dislocation of the facets of the vertebrae. When spearing or butting during tackling with the head in flexion was outlawed in football, the number of cervical spine injuries was dramatically reduced (18).

Compression fractures in the lumbar vertebrae of weight lifters, football linemen, and gymnasts who load the vertebrae while the spine is held in hyperlordotic or sway-back position have also been reported (23). Figure 2-22 is a radiograph of a fracture to the lumbar vertebrae, demonstrating the shortening and widening effect of the compressive force. Finally, compression fractures are common in individuals with osteoporosis.

Specific lifts in weight training result in spondylolysis, a stress fracture of the pars interarticularis section of the vertebra. Lifts that have a high incidence of this fracture are the clean and jerk and the snatch from the Olympic lifts and the squat and dead lift from power lifting (22,23). In gymnasts, it is associated with extreme extension positions in the lumbar vertebrae. This injury will be discussed in greater detail in Chapter 7, when the trunk is reviewed.

A compressive force at the hip joint can increase or decrease the injury potential of the femoral neck. The hip joint must absorb compressive forces of approximately three to seven times body weight during walking (43,46). Compressive forces are up to 15 to 20 times body weight in jumping (46). In a normal standing posture, the hip joint assumes approximately one third of the body weight if both limbs are on the ground (43). This creates large compressive forces on the inferior portion of the femoral neck and a large pulling, or tensile, force on the superior portion of the neck. Figure 2-23 shows how this happens as the body pushes down on the femoral head, pushing the bottom of the femoral neck together and pulling the top of the femoral neck apart as it creates bending.

The hip abductors, specifically the gluteus medius, contract to counteract the body weight during stance. As shown in Figure 2-23, they also produce a compressive load on the superior aspect of the femoral neck that reduces the tensile forces and injury potential in the femoral neck because bone usually fractures sooner with a tensile force (43). It is proposed that runners develop femoral neck fractures because the gluteus medius fatigues and cannot maintain its reduction of the high tensile force, producing the fracture (29,46). A femoral neck fracture can also be produced by a strong co-contraction of the hip muscles, specifically the abductors and adductors, creating excessive compressive forces on the superior neck.

 

FIGURE 2-22 The lumbar vertebrae can incur compressive fracture (arrow), in which the body of the vertebra is shortened and widened This type of fracture has been associated with loading of the vertebrae while maintaining a hyperlordotic position. (Reprinted with permission from 

Nordin, M., and Frankel, V.H. [1989]. Basic Biomechanics of the Musculoskeletal System. [2nd Ed.]. Philadelphia: Lea & Febiger.

)

Tension Forces

When muscle applies a tensile force to the system through the tendon, the collagen in the bone tissue arranges itself in line with the tensile force of the tendon. Figure 2-24 shows an example of collagen alignment at the tibial tuberosity. This figure also illustrates the influence of tensile forces on the development of apophyses. An apophysis is bony outgrowth, such as a process, tubercle, or tuberosity. Figure 2-24 illustrates how an apophysis, the tibial tuberosity, is formed by tensile forces.

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FIGURE 2-23 A. During standing or in the stance phase of walking and running, bending force applied to the femoral neck creates a large compressive force on the inferior neck and tensile force on the superior neck. B. If the gluteus medius contracts, the compressive force increases, and the tensile force decreases. This reduces the injury potential because injury is more likely to occur in tension.

Failure of the bone usually occurs at the site of muscle insertion. Tensile forces can also create ligament avulsions. A ligament avulsion, or an avulsion fracture, occurs when a portion of the bone at the insertion of the ligament is torn away. This occurs more frequently in children than in adults. Avulsion fractures occur when the tensile strength of the bone is not sufficient to prevent the fracture. This is typical of some of the injuries occurring in the high-velocity throwing motion of a Little Leaguer's pitching arm. The avulsion fracture in this case is commonly on the medial epi-condyle as a result of tension generated in the wrist flexors.

Two other common tension-produced fractures are at the fifth metatarsal, caused by the tensile forces generated by the peroneal muscle group, and at the calcaneus, where the forces are generated by the triceps surae muscle group. The tensile force on the calcaneus can also be produced in the stance phase of gait as the arch is depressed and the plantar fascia covering the plantar surface of the foot tightens, exerting tensile force on the calcaneus. Some sites of avulsion fractures for the pelvic region, presented in Figure 2-25, include the anterior superior and inferior spines, lesser trochanter, ischial tuberosity, and pubic bone.

Tension forces are generally responsible for sprains and strains. For example, the typical ankle inversion sprain occurs when the foot is oversupinated. That is, the foot rolls over its lateral border, stretching the ligaments on the lateral side of the ankle. Tensile forces are also identified with shin splints. This injury occurs when the tibialis anterior pulls on its attachment site on the tibia and on the interosseous membrane between the tibia and the fibula.

 

FIGURE 2-24 A. When tensile forces are applied to the skeletal system, the bone strengthens in the direction of the pull as collagen fibers align with the pull of the tendon or ligament. B. Tensile forces are also responsible for the development of apophyses, bony outgrowths such as processes, tubercles, and tuberosities.

Another site exposed to high-tensile forces is the tibial tuberosity, which transmits very high tensile forces when the quadriceps femoris muscle group is active. This tensile force, if sufficient in magnitude and duration, may cause

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tendinitis or inflammation of the tendon in older participants. In younger participants, however, the damage usually occurs at the site of tendon-bone attachment and can result in inflammation, bony deposits, or an avulsion fracture of the tibial tuberosity. Osgood-Schlatter is characterized by inflammation and formation of bony deposits at the tendon-bone junction.

 

FIGURE 2-25 Avulsion fractures can result from tension applied by a tendon or a ligament. Sites of avulsion fractures in the pelvic region include the anterior superior spine (A), anterior inferior spine (B), ischial tuberosity (C), pubic bone (D), and lesser trochanter (E).

Bone responds to the demands placed upon it, as described by Wolff's law (32). Therefore, different bones and different sections in a bone respond to tension and compressive forces differently. For example, the tibia and femur participate in weight bearing in the lower extremity and are strongest when loaded with a compressive force. The fibula, which does not participate significantly in weight bearing but is a site for muscle attachment, is strongest when tensile forces are applied (43). An evaluation of the differences found in the femur has uncovered greater tensile strength capabilities in the middle third of the shaft, which is loaded through a bending force in weight bearing. In the femoral neck, the bone can withstand large compressive forces, and the attachment sites of the muscles have great tensile strength (43).

Shear Forces

Shear forces are responsible for some vertebral disc problems, such as spondylolisthesis, in which the vertebrae slip anteriorly over one another. In the lumbar vertebrae, shear force increases with increased swayback, or hyper-lordosis (22). The pull of the psoas muscle on the lumbar vertebrae also increases shear force on the vertebrae. This injury is discussed in greater detail in Chapter 7.

Examples of fractures caused by shear forces are commonly found in the femoral condyles and the tibial plateau. The mechanism of injury for both is usually hyperextension in the knee through some fixation of the foot and valgus or medial force to the thigh or shank. In adults, this shear force can fracture a bone as well as injure the collateral or cruciate ligaments (37). In developing children, this shear force can create epiphyseal fractures, such as in the distal femoral epiphysis. The mechanism of injury and the resulting epiphyseal damage are presented in Figure 2-26. The effects of such a fracture in developing children can be significant because this epiphysis accounts for approximately 37% of the bone growth in length (15).

Compressive, tensile, and shear forces applied simultaneously to the bone are important in the development of bone strength. Figure 2-27 illustrates both compressive and tensile stress lines in the tibia and femur during running. Bone strength develops along these lines of stress.

Bending Forces

Bone is regularly subjected to large bending forces. For example, during gait, the lower extremity bones are subjected to bending forces caused by alternating tension and compression forces. During normal stance, both the femur and the tibia bend. The femur bends both anteriorly and laterally because of its shape and the manner of the force transmission caused by weight bearing. Also, weight bearing produces an anterior bend in the tibia. Although these bending forces are not injury producing, the bone is strongest in the regions where the bending force is greatest (46).

 

FIGURE 2-26 Fracture of the distal femoral epiphysis is usually created by shear force. This is commonly produced by a valgus force applied to the thigh or shank with the foot fixed and the knee hyperextended.

Typically, a bone fails and fractures on the convex side in response to high tensile forces because bone can withstand greater compressive forces than tensile forces (43). The magnitude of the compressive and tensile forces produced by bending increases with distance from the axis of the bone. Thus, the force magnitudes are greater on the outer portions of the bone.

 

FIGURE 2-27 The lines of compressive stress (bold black lines) and tension stress (lighter black and red lines) for the distal femur and proximal tibia during the stance phase of running.

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FIGURE 2-28 The ski boot fracture, created by a three-point bending load, occurs when the ski stops abruptly. Compressive force is created on the anterior tibia and tensile force on the posterior tibia. The tibia usually fractures on the posterior side.

Injury-producing bending loads are caused by multiple forces applied at different points on the bone. Generally, these situations are called three- or four-point force applications. A force is usually applied perpendicular to the bone at both ends of the bone and a force applied in the opposite direction at some point between the other two forces. The bone will break at the point of the middle force application, as is the case in a ski boot fracture shown in Figure 2-28. This fracture is produced as the skier falls over the top of the boot, with the ski and boot pushing in the other direction. The bone usually fractures on the posterior side because that is where the convexity and the tensile forces are applied. Ski boot fractures have been significantly reduced because of improvements in bindings, skis that turn more easily, well-groomed slopes, and a change in skiing technique that puts the weight forward on the skis. The reduction of tibial fractures through the improvement of equipment and technique, however, has led to an increase in the number of knee injuries, for the same reasons (14).

 

FIGURE 2-29 Three-point bending loads are used in many braces. A. The Milwaukee brace, used for correction of lateral curvature of the spine, applies three-point bending force to the spine. B. The Jewett brace applies three-point bending force to the thoracic spine to create spinal extension in that region.

The three-point bending force is also responsible for injuries to a finger that is jammed and forced into hyper-extension and to the knee or lower extremity when the foot is fixed in the ground and the lower body bends. Simply eliminating the long cleats in the shoes of football players and playing on good resurfaced fields reduce this type of injury by half (22). Three-point bending force applications are also used in bracing. Figure 2-29 presents two brace applications using the three-point force application to correct a postural deviation or stabilize a region.

A four-point bending load is two equal and opposite pairs of forces at each end of the bone. In the case of four-point bending, the bone breaks at its weakest point. This is illustrated in Figure 2-30 with the application of a four-point bending force to the femur.

 

FIGURE 2-30 Hypothetical example of four-point bending load applied to the femur, creating a fracture or failure at the weakest point.

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FIGURE 2-31 Example of torsion applied to the humerus, creating shear stress across the surface.

Torsional Forces

Fractures resulting from torsional force can occur in the humerus when poor throwing technique creates a twist on the arm (46) and in the lower extremity when the foot is planted and the body changes direction. A spiral fracture is a result of torsional force. An example of the mechanism of spiral fracture to the humerus in a pitcher is shown in Figure 2-31. Spiral fractures usually begin on the outside of the bone parallel to the middle of the bone. Torsional loading of the lower extremity is also responsible for knee cartilage and ligament injuries (22) and can occur when the foot is caught while the body is spinning.

Combined Loading

Tension, compression, shear, bending, and torsion represent simple and pure modes of loading. It is more common to incur various combinations of loads acting simultaneously on the body. For example, the lower

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extremity bones are loaded in multiple directions during exercise. The mechanical loading provides the stimulus for bone adaptation and selection of exercises for this purpose becomes an important consideration. Because bone responds more stiffly at higher rates of loading, the strain rate also becomes important. In Figure 2-32, the bone strain in the tibia during the performance of the leg press, bicycle, stepmaster, and running is compared with baseline walking values (39). Whereas compression and shear values produced during the leg press, stepmaster, and running are higher than walking, tension strain values vary between exercise modes. Bicycling results in lower tension, compression, and shear values than walking. When the rate of loading is evaluated (Fig. 2-33), however, only running produces higher strain rates than walking.

 

FIGURE 2-32 Comparison of in vivo tibial strain during four exercises compared with walking. (Adapted from 

Milgrom, C, et al. (2000). Journal of Bone and Joint Surgery, 82-B:591-594.

)

 

FIGURE 2-33 Comparison of in vivo tibial strain rates during four exercises compared with walking. (Adapted from 

Milgrom, C, et al. (2000). Journal of Bone and Joint Surgery, 82-B:591-594.

)

Stress Fractures

Injury to the skeletal system can be produced by a single high-magnitude application of one of these types of loads or by repeated application of a low-magnitude load over time. The former injury is referred to as a traumatic fracture. The latter type is a stress fracture, fatigue fracture, or bone strain. Figure 2-34 shows a radiograph of a stress fracture to the metatarsal. These fractures occur as a consequence of cumulative microtrauma imposed upon the skeletal system when loading of the system is so frequent that bone repair cannot keep up with the breakdown of bone tissue.

A stress fracture occurs when bone resorption weakens the bone too much and the bone deposit does not occur rapidly enough to strengthen the area. Stress fractures in the lower extremity can be attributed to muscle fatigue that reduces shock absorption and causes redistribution of forces to specific focal points in the bone. In the upper extremity, stress fractures result from repetitive muscular forces pulling on the bone. Stress fractures account for 10% of injuries to athletes (36).

 

FIGURE 2-34 Stress fractures occur in response to overloading of the skeletal system so that cumulative microtraumas occur in the bone. A stress fracture to the second metatarsal, as shown in this radiograph (arrow), is caused by running on hard surfaces or in stiff shoes. It is also associated with persons with high arches and can be created by fatigue of the surrounding muscles. (Reprinted with permission from 

Fu, H. F., and Stone, D. A. [1994]. Sports Injuries. Baltimore: Williams & Wilkins.

)

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TABLE 2-2 Injuries to the Skeletal System

Type of Injury

Activity Examples

Load Causing Injury

Mechanism of Injury

Tibial stress

Dancing, running, basketball

Compression

Poor conditioning, stiff footwear, unyielding surfaces, hypermobile foot (overpronation)

Medial epicondyle fracture

Gymnastics

Tension, compression

Too much work on floor exercise and tumbling

Stress fracture of the big toe

Sprinting, fencing, rugby

Tension

Toe extensors create bowstring effect on the big toe when up on the toes; primarily in individuals with hallux valgus

Stress fracture of the femoral neck

Running, gymnastics

Compression

Muscle fatigue, high-arched foot

Stress fracture in the calcaneus

Running, basketball, volleybal

Compression

Hard surface, stiff footwear

Stress fracture in the umbar vertebrae

Weight lifting, gymnastics, football

Compression, tension

High loads with hyperlordotic low-back posture

Tibial plateau fractures

Skiing

Compression

Hyperextension and valgus of the knee, as in turning, with the force on the inside edge of the downhill ski, abruptly halted with heavy snow

Stress fracture to the medial malleolus

Running

Compression

Ankle sprain to the outside, causing compression between the talus and medial malleolus or excessive pronation because the medial malleolus rotates in with tibial rotation and pronation

Hamate fracture of the hand

Baseball, golf, tennis

Compression

Relaxed grip in the swing that stops suddenly at the end of the swing as the club hits the ground, the bat is forcefully checked, or the racquet sports racket is out of contro

Fracture of the tibia

Skiing

Bending, compression, tension

Three-point bending fall in which the body weight, boot, and ground bend the tibia posteriorly

Fracture of the femora condyles

Skiing, football

Shear

Hyperextension of knee with valgus force

Stress fracture in the fibula

Running, aerobics, jumping

Tension

Jumping or deep-knee bends with a walk; pull by soleus, tibialis posterior, peroneals, and toe flexors, pulling the tibia and fibula together

Meniscus tear of the knee

Basketball, football, jumping, volleyball, soccer

Compression, Torsion

Turning on a weight-bearing limb or valgus force to the knee

Stress fracture in the metatarsal

Running

Compression

Hard surfaces, stiff footwear, high-arched foot, fatigue

Stress fracture in the femoral shaft

Running, triathlon

Tension

Excessive training and mileage; created by pull of the vastus medialis or adductor brevis

The typical stress fracture injury occurs during a load application that produces shear or tensile strain and results in laceration, fracture, rupture, or avulsion. Bone tissue can also develop a stress fracture in response to compressive or tensile loading that overloads the system, either through excessive force applied one or a few times or through too-frequent application of a low or moderate level of force (29,34,36). Fatigue microdamage occurs under cyclic loading that needs to be repaired before the bone progresses to failure, resulting in a stress fracture. The relationship between the magnitude and frequency of applications of load on bone is presented in Figure 2-35. The tolerance of bone to injury is a function of the load and the cycles of loading.

Examples of injuries to the skeletal system are presented in Table 2-2. The activity associated with the injury, the type of load causing the injury, and the mechanism of injury are summarized. It is still not clear why some athletes participating in the same activity acquire a stress fracture injury and others do not. It has been suggested that other factors such as limb alignment and soft tissue dampening of imposed loads may play a role in influencing the risk of fracture (5).

Cartilage

Cartilage is a firm, flexible tissue made up of cells called chondrocytes surrounded by an extracellular matrix. The two main types of cartilage that will be discussed in this chapter are articular or hyaline cartilage and fibro-cartilage.

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FIGURE 2-35 Injury can occur when a high load is applied a small number of times or when low loads are applied numerous times. It is important to remain within the injury tolerance range.

Articular Cartilage

Articulating joints connect the different bones of the skeleton. In freely moving joints, the articulating ends of the bones are covered with a connective tissue referred to as articular cartilage.

Articular or hyaline cartilage is an avascular substance consisting of 60% to 80% water and a solid matrix composed of collagen and proteoglycan. Collagen is a protein with the important mechanical properties of stiffness and strength. Proteoglycan is a highly hydrated gel. It is unclear how collagen and the proteoglycan gel interact during stress to the cartilage. However, the interaction between the two materials determines cartilage's mechanical properties. Cartilage has no blood supply and no nerves and is nourished by the fluid within the joint (41).

Articular cartilage is anisotropic, meaning it has different material properties for different orientations relative to the joint surface. The properties of cartilage make it well suited to resisting shear forces because it responds to load in a viscoelastic manner. It deforms instantaneously to a low or moderate load, and if rapidly loaded, it becomes stiffer and deforms over a longer period. The force distribution across the area in the joint determines the stress in the cartilage, and the distribution of the force depends on the cartilage's thickness.

What is the role of articular cartilage?

  1. transmit compressive forces across the joint
  2. allows motion in the joint with minimal friction and wear
  3. redistributes contact stresses over a larger area
  4. protects underlying bone

Cartilage is important to the stability and function of a joint because it distributes loads over the surface and reduces the contact stresses by half (50). Collagen fibers are arranged to withstand load bearing. For example, in the knee, the medial meniscus transmits 50% of the compression load. Removal of just a small part of the cartilage has been shown to increase the contact stress by as much as 350% (25). Several years ago, a cartilage tear would have meant removal of the whole cartilage, but today orthopedists trim the cartilage and remove only minimal amounts to maintain as much shock absorption and stability in the joint as possible.

Cartilage is 1 to 7 mm thick, depending on the stress and the incongruity of the joint surfaces (26). For example, in the ankle and the elbow joints, the cartilage is very thin, but at the hip and knee joints, it is thick. The cartilage is thin in the ankle because of the ankle's architecture. A substantial area of force distribution imposes less stress on the cartilage. Conversely, the knee joint is exposed to lower forces, but the area of force distribution is smaller, imposing more stress on the cartilage. Some of the thickest cartilage in the body, approximately 5 mm, lies on the underside of the patella (54).

Articular cartilage allows movement between two bones with minimal friction and wear. The joint surfaces have remarkably low coefficients of friction. Articular cartilage contributes significantly to this. The coefficient of friction in some joints has been reported to range from 0.01 to 0.04; the coefficient of friction of ice at 0°C is about 0.1. These almost frictionless surfaces allow the surfaces to glide over each other smoothly.

Cartilage growth across the lifespan is dynamic. At maturity, stabilization of articular cartilage thickness occurs, but ossification does not entirely cease (4). The interface between the cartilage and the underlying subchondral bone remains active and is responsible for the gradual change in joint shape during aging. The amount of cartilage growth is regulated by compressive stress, and the higher the joint contact pressures, the thicker the cartilage. In activities of daily living across the lifespan, the changes in joint use cause a change in cartilage, resulting in thinning or thickening.

Fibrocartilage

Another type of cartilage is fibrocartilage, which is often found where articular cartilage meets a tendon or a ligament. Fibrocartilage acts as an intermediary between hyaline cartilage and the other connective tissues. Fibrocartilage is found where both tensile strength and the ability to withstand high pressures are necessary, such as in the intervertebral disks, the jaw, and the knee joint. A fibrocartilage structure is referred to as an articular disc, or meniscus. The menisci also improve the fit between articulating bones that have slightly different shapes. Meniscus tears usually occur during a sudden change of direction with the weight all on one limb. The resultant compression and

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tension on the meniscus tear the fibrocartilage. No pain is associated with the actual tear; rather, the peripheral attachment sites are the site of the irritation and resulting sensitivity.

Ligaments

A ligament is a short band of tough fibrous connective tissue that binds bone to bone and consists of collagen, elastin, and reticulin fibers (55). The ligament usually provides support in one direction and often blends with the capsule of the joint. Ligaments can be capsular, extracapsular, or intra-articular. Capsular ligaments are simply thickenings in the wall of the capsule, much like the glenohumeral ligaments in the front of the shoulder capsule. Extracapsular ligaments lie outside the joint itself. The collateral ligaments found in numerous joints are extracapsular (i.e., fibular collateral ligament of the knee). Finally, intra-articular ligaments, such as the cruciate ligaments of the knee and the capitate ligaments in the hip, are located inside a joint.

The maximum stress that a ligament can endure is related to its cross-sectional area. Ligaments exhibit vis-coelastic behavior, which helps to control the dissipation of energy and controls the potential for injury (7). Ligaments respond to loads by becoming stronger and stiffer over time, demonstrating both a time-dependent and a nonlinear stress-strain response. The collagen fibers in a ligament are arranged so the ligament can handle both tensile loads and shear loads; however, ligaments are best suited for tensile loading. An example of viscoelastic behavior is presented in Figure 2-36. The collagen fibers in ligament have a nearly parallel configuration. When unloaded, they have a wavy or crimped configuration. At low stresses, the crimp in the collagen fibers of the ligament disappears. At this point, the ligament behaves almost linearly, with strains that are relatively small and within the physiological limit. At greater stresses, the ligament tears, either partially or completely. Generally, when a tensile load is applied to a joint very quickly, the ligament can dissipate energy quickly and the chance of failure is more likely to be at the bone rather than in the ligament.

 

FIGURE 2-36 A stress-strain curve for a ligament. In the toe region, the collagen fibers of the ligament are wavy. The fibers straighten out in the inear region. In the plastic region, some of the collagen fibers tear (Adapted with permission from 

Butler, D. L, Grood, E. S., Noyes, F. R., and Zernike, R. F. [1978]. Biomechanics of ligaments and tendons Exercise and Sports Science Reviews, 6:125-181.

)

The strength of a ligament also diminishes rapidly with immobilization. A tensile injury to a ligament is termed a sprain. Sprains are rated 1, 2, or 3 in severity, depending on whether there is a partial tear of the fibers (rated 1), a tear with some loss of stability (rated 2), or a complete tear with loss of joint stability (rated 3) (26).

At the end of the range of motion for every joint, a ligament usually tightens up to terminate the motion. Ligaments provide passive restraint and transfers loads to the bone. A ligament can be subjected to extreme stress and damaged while overloaded while performing the role of restricting abnormal motion. Because the ligaments stabilize, control, and limit joint motion, any injury to a ligament influences joint motion. Ligament damage can result joint instability, which in turn, can cause altered joint kinematics, resulting in altered load distribution and vulnerability to injury.

What is the function of a ligament?

  1. To guide normal joint function
  2. To restrict abnormal joint movement

Bony Articulations

The Diarthrodial or Synovial Joint

Movement potential of a segment is determined by the structure and function of the diarthrodial or synovial joint. The diarthrodial joint provides low-friction articulation capable of withstanding significant wear and tear. The characteristics of all diarthrodial joints are similar. For example, the knee has similar structures to the finger joints. Because of this similarity, it is worthwhile to look at the various components of the diarthrodial joint to gain general knowledge about joint function, support, and nourishment. Figure 2-37 shows the characteristics of the diarthrodial joint.

Characteristics of the Diarthrodial Joint

Covering the ends of the bones is the articular end plate, a thin layer of cortical bone over cancellous bone. On top

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of the end plate is articular cartilage. This cartilage in the joint offers additional load transmission, stability, improved fit of the surfaces, protection of the joint edges, and lubrication.

 

FIGURE 2-37 The diarthrodial joints have similar characteristics. If you study the knee, interphalanges, elbow, or any other diarthrodial joint, you will find the same structures. These include (A) articular or hyaline cartilage, (B) capsule, (C) synovial membrane, and (D) ligaments.

Another important characteristic of the diarthrodial joint is the capsule, a fibrous white connective tissue made primarily of collagen. It protects the joint. Thickenings in the capsule, known as ligaments, are common where additional support is needed. The capsule basically defines the joint, creating the interarticular portion, or inside, of the joint, which has a joint cavity and a reduced atmospheric pressure (50). Although soft tissue loads are difficult to compute, the capsule sustains some of the load imposed on the joint (27).

Any immobilization of the capsule alters the mechanical properties of the capsular tissue and may result in joint stiffness. Likewise, injury to the capsule usually results in the development of a thick or fibrous section that can be externally palpable (16).

On the inner surface of the joint capsule is the synovial membrane, a loose, vascularized connective tissue that secretes synovial fluid into the joint to lubricate and provide nutrition to the joint. The fluid, which has the consistency of an egg white, decreases in viscosity as shear rates increase. The consistency is similar to catsup, hard to start but easy to move after it is going. When the joint moves slowly, the fluid is highly viscous, and the support is high. Conversely, when the joint moves rapidly, the fluid is elastic in its response, decreasing the friction in the joint (50).

A healthy joint provides effortless motion along preferred anatomical directions with an accompanying restriction of abnormal joint motion. Freedom of mobility is also provided by the lubricating action of articular cartilage. The healthy joint is also stable as a result of the interaction between bony connections, ligaments, and other soft tissue. Finally, the ligaments operate to guide and restrict motion, which defines the normal envelope of passive motion of the joint.

Any injury to the joint is noticeable in both a thickening in the membrane and a change in the consistency of the fluid. The fluid fills the capsular compartment and creates pain in the joint. Physicians drain the joint to relieve the pressure, often finding that the fluid is bloodstained.

Stability of the Diarthrodial Joint

Stability in a diarthrodial joint is provided by the structure— the ligaments surrounding the joints, the capsule, and the tendons spanning the joint—gravity, and the vacuum in the joint produced by negative atmospheric pressure. The hip is one of the most stable joints in the body because it has good muscular, capsular, and ligamentous support. The hip joint has congruency between the surfaces, with a high degree of bone-to-bone contact. Most of the stability in the hip, however, is derived from the effects of gravity and the vacuum in the joint (26). The negative pressure in the joint is sufficient to hold the femur in the joint if all other structures, such as supporting ligaments and muscles, are removed.

In contrast, the stability of the shoulder is supplied only by the capsule and the muscles surrounding the joint. Also, the congruency of the shoulder joint is limited, with only a small proportion of the head of the humerus making contact with the glenoid cavity.

Close-Packed versus Loose-Packed Positions

As movement through a range of motion occurs, the actual contact area varies between the articulating surfaces. When the joint position is such that the two adjacent bones fit together best and maximum contact exists between the two surfaces, the joint is considered to be in a close-packed position. This is the position of maximum compression of the joint, in which the ligaments and the capsule are tense and the forces travel through the joint as if it did not exist. Examples of close-packed positions are full extension for the knee, extension of the wrist, extension of the interphalangeal joints, and maximum dorsiflexion of the foot (50). All other joint positions are termed loose-packed positions because there is less contact area between the two surfaces and the contact areas are frequently changing. There is more sliding and rolling of the bones over one another in a loose-packed position. This position allows for continuous movement, reducing the friction in the joint. Although the loose-packed joint position is less stable than the close-packed position, it is not as susceptible to injury because of its mobility. The close- and loose-packed positions of the knee joint are

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presented in Figure 2-38. Note the greater contact area in the close-packed position.

 

FIGURE 2-38 In the close-packed position, contact between the two joint surfaces is maximal and mobility is minimal. In the loose-packed joint position, there is less contact between the surfaces in the joint and more mobility and movement between the two surfaces.

While in the close-packed position, the joint is very stable but vulnerable to injury because the structures are taut and the joint surfaces are pressed together. The joint is especially susceptible to injury if hit by an external force, such as hitting the knee when it is fully extended.

Types of Diarthrodial Joints

A classification system categorizes seven types of diarthrodial joints according to the differences in articulating surfaces, the directions of motion allowed by the joint, and the type of movement occurring between the segments. Figure 2-39 offers a graphic representation of these seven joints.

Plane or Gliding Joint

The first type of joint is the plane or gliding joint, found in the foot among the tarsals and in the hand among the carpals. Movement at this type of joint is termed nonaxial because it consists of two flat surfaces that slide over each other rather than around an axis.

In the hand, for example, the carpals slide over each other as the hand moves to positions of flexion, extension, radial deviation, and ulnar deviation. Likewise, in the foot, the tarsals shift during pronation and supination, sliding over each other in the process.

Hinge Joint

The hinge joint allows movement in one plane (flexion, extension); it is uniaxial. Examples of the hinge joint in the body are the interphalangeal joints in the foot and hand and the ulnohumeral articulation at the elbow.

Pivot Joint

The pivot joint also allows movement in one plane (rotation; pronation, supination) and is uniaxial. Pivot joints are found at the superior and inferior radioulnar joint and the atlantoaxial articulation at the base of the skull.

Condylar Joint

The condylar joint allows a primary movement in one plane (flexion and extension) with small amounts of movement in another plane (rotation). Examples are the metacarpals, interphalangeal, metacarpals, and the temporomandibular joint. The knee joint is also referred to as a condylar joint because of the articulation between the two condyles of the femur and the tibial plateau. However, because of the mechanical linkages created by the ligaments, the knee joint functions as a hinge and is referred to as a modified hinge joint in the literature for this reason.

Ellipsoid Joint

The ellipsoid joint allows movement in two planes (flexion and extension; abduction and adduction) and is biaxial. Examples of this joint are the radiocarpal articulation at the wrist and the metacarpophalangeal articulation in the phalanges.

Saddle Joint

The saddle joint, found only at the carpometacarpal articulation of the thumb, allows two planes of motion (flexion and extension; abduction and adduction) plus a small amount of rotation. It is similar to the ellipsoid joint in function.

Ball-and-Socket

Joint The last type of diarthrodial joint, the ball-and-socket joint, allows movement in three planes (flexion and extension; abduction and adduction; rotation) and is the most mobile of the diarthrodial joints. The hip and shoulder are examples of ball-and-socket joints. A summary of the major joints in the body is presented in Table 2-3.

Other Types of Joints

Synarthrodial or Fibrous Joints

Other articulations are limited in movement characteristics but nonetheless play important roles in stabilization of the skeletal system. Some bones are held together by fibrous articulations such as those found in the sutures of the skull. These articulations, referred to as synarthrodial joints, allow little or no movement between the bones and hold the bones firmly together (Fig. 2-40).

Amphiarthrodial or Cartilaginous Joints

Cartilaginous or amphiarthrodial joints hold bones together with either hyaline cartilage, such as is found at the epiphyseal plates, or fibrocartilage, as in the pubic symphysis and the intervertebral articulations. The movement at these articulations is also limited, although not to the degree of the synarthrodial joints.

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FIGURE 2-39 The seven types of diarthrodial joints. The nonaxial joint is the plane or gliding joint. Uniaxial joints include the hinge and pivot joints; biaxial joints are the condylar, ellipsoid, and saddle joint. The ball-and-socket joint is the only triaxial diarthrodial joint.

Osteoarthritis

Injury to the structures of the diarthrodial joint can occur during high load or through repetitive loading over an extended period. The articular cartilage in the joint is especially subject to wear during one's lifetime. Osteoarthritis is a disease characterized by degeneration of the articular cartilage, which leads to fissures, fibrillation, and finally disappearance of the full thickness of the articular cartilage. Osteoarthritis is the leading chronic

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medical condition and is the leading cause of disability for persons age 65 years and older (4). Osteoarthritis starts as a result of trauma to or repeated wear on the joint that causes a change in the articular substance to the point of removal of actual material by mechanical action. This results in diminished contact areas and erosion of the cartilage through development of rough spots in the cartilage. The rough spots develop into fissures and eventually go deep enough that only subchondral bone is exposed. Osteophytes or cysts form in and around the joint, and this is the beginning of degenerative joint disease, or osteoarthritis. The radiographs in Figure 2-41 show the areas of joint degeneration associated with osteoarthritis in the hip and vertebrae.

TABLE 2-3 Major Joints of the Body

Joint

Type

Degrees of Freedom

Vertebra

Amphiarthrodial

3

Hip

Ball-and-socket

3

Shoulder

Ball-and-socket

3

Knee

Condyloid

3

Wrist

Ellipsoid

3

Metacarpophalangeal (fingers)

Ellipsoid

3

Carpometacarpal (thumb)

Saddle

2

Elbow

Hinge

1

Radioulnar

Pivot

1

Atlantoaxial

Pivot

1

Ankle

Hinge

1

Interphalangeal

Hinge

1

It is theorized that osteoarthritis develops first in the subchondral, or cancellous, bone underlying the joint (45). The cartilage overlying the bone in the joint is thin; consequently, the underlying subchondral bone absorbs the shock of loading. Repetitive loading or unequal loading in the joint causes microfractures in the subchondral bone. When the microfractures heal, the subchondral bone is stiffer and less able to absorb shock, passing this role on to the cartilage. The cartilage deteriorates as a consequence of this overloading, and the body lays down bone in the form of osteophytes to increase the contact area.

Osteoarthritis has been shown to have no relationship to hyperlaxity in the joint (6), levels of osteoporosis (24), or general physical activity (35). An injured joint deteriorates at a faster rate, however, making it more susceptible to the development of osteoarthritis. Additionally, the risk of osteoarthritis is increased by factors such as occupation, level of sports participation, and exercise intensity levels (21). Heavy loading and twisting are seen as contributing factors, but elevated physical activities do not appear to be a risk factor.

 

FIGURE 2-40 A. An example of the synarthrodial joint is the fibrous articulation at the distal tibiofibular joint. B. The amphiarthrodial, or cartilaginous, joint can be found between the vertebrae or in the epiphyseal plate of a growing bone.

Osteoarthritis can also be created by joint immobilization because the joint and the cartilage require loading and compression to exchange nutrients and wastes (42). After only 30 days of immobilization, the fluid in the cartilage is increased, and an early form of osteoarthritis develops. Fortunately, this process can be reversed with a return to activity.

Injury to other structures in the diarthrodial joint can also be serious. An injury to the joint capsule results in formation of more fibrous tissue and possibly stretching of the capsule (16). Injury to the meniscus can create instability, loss of range of motion, and an increase in synovial effusion into the joint (swelling). Injury to the synovial membrane causes an increase in vascularity and produces gradual fibrosis of the tissue, eventually leading to chronic synovitis or inflammation of the membrane. Amazingly, many of these injury responses can also be reproduced through immobilization of the joint, which can produce adhesions, loss of range of motion, fibrosis, and synovitis.

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FIGURE 2-41 Osteoarthritis is characterized by physical changes in the joint consisting of cartilage erosion and formation of cysts and osteophytes. This radiograph shows osteoarthritis in the hip and vertebrae.

Summary

Individual structures of the human body may be analyzed mechanically using a stress-strain curve to help determine its basic properties. Stress-strain curves illustrate the elastic and plastic regions and the elastic modulus of the structure. Structures and materials can be differentiated as elastic or viscoelastic based on their stress-strain curves. These basic mechanical properties may give insight into how a movement may take place.

The skeleton is composed of bones, joints, cartilage, and ligaments. It provides a system of levers that allows a variety of movements at the joints, provides a support structure, serves as a site for muscular attachment, protects the internal structures, stores fats and minerals, and participates in blood cell formation. Bone is an organ with blood vessels and nerves running through it.

The types of bones that compose the skeletal system (long, short, flat, irregular) are shaped differently, perform different functions, and are made up of different proportions of cancellous and cortical bone tissue.

Bone tissue is one of the body's hardest structures because of its organic and inorganic components. Bone tissue continuously remodels through deposition and resorption of tissue. Modeling of bone is responsible for shaping both the shape and size of bone, and remodeling maintains bone mass by resorption and deposit at the same site. Bone is sensitive to disuse and loading. Bone tissue is deposited in response to stress on the bone and removed through resorption when not stressed. One of the ways of increasing the strength and density of bone is through a program of physical activity. Osteoporosis occurs when bone resorption exceeds bone deposit and the bone becomes weak.

The study of the architecture of bone tissue has identified two types of bone, cortical and cancellous. Cortical bone, found on the exterior of bone and in the shaft of the long bones, is suited to handling high levels of compression and high tensile loads produced by the muscles. Cancellous bone is suited for high-energy storage and facilitates stress distribution within the bone.

Bone is both anisotropic and viscoelastic in its response to loads and responds differently to variety in the direction of the load and to the rate at which the load is applied. When first loaded, bone responds by deforming through a change in length or shape, known as the elastic response. With continued loading, microtears occur in the bone as it yields during the plastic phase. Bone is considered to be a flexible and weak material compared with other materials such as glass and steel.

The skeletal system is subject to a variety of loads and can handle larger compressive loads than tensile or shear loads. Commonly, bone is loaded in more than one direction, as with bending, in which both compression and

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tension are applied, and in torsion loads, in which shear, compression, and tensile loads are all produced. Injury to bone occurs when the applied load exceeds the strength of the material.

Two types of cartilage are found in the skeletal system. Articular or hyaline cartilage covers the ends of the bones at synovial joints. This cartilage is composed of water and a solid matrix of collagen and proteoglycan. Articular cartilage functions to attenuate shock in the joint, improve the fit of the joint, and provide minimal friction in the joint. Cartilage has viscoelastic properties in its response to loads. A second type of cartilage, fibrocartilage, offers additional load transmission and stability in a joint. Fibrocartilage is often referred to as an articular disc or meniscus.

Ligaments connect bone to bone and are categorized as capsular, intracapsular, or extracapsular, depending on their location relative to the joint capsule. Ligaments exhibit viscoelastic behavior. They respond to loads by becoming stiffer as the load increases.

The movements of the long bones occur at a synovial joint, a joint with common characteristics such as articular cartilage, a capsule, a synovial membrane, and ligaments. The synovial joint can be injured through a sprain, in which the ligaments are injured. Joints are also susceptible to degeneration characterized by breakdown in the cartilage and bone. This degeneration is known as osteoarthritis.

The amount of motion between two segments is largely influenced by the type of synovial joint. For example, the planar joint allows simple translation between the joint surfaces; the hinge joint allows flexion and extension; the pivot joint allows rotation; the condylar joint allows flexion and extension with some rotation; the ellipsoid and the saddle joints allow flexion, extension, abduction, and adduction; and the ball-and-socket joint allows flexion, extension, abduction, adduction, and rotation. Other types of joints—synarthrodial and amphiarthrodial—allow little or no movement.

Review Questions

True or False

  1. ___Hysteresis is the energy stored in a stress-strain test.
  2. ___A viscoelastic material has no elastic properties at all
  3. ___The stiffness of a material can be determined by calculating the slope of the linear portion of the stress-strain curve
  4. ___A lever alters only the speed of a movement.
  5. ___Bones decrease in size from top to bottom
  6. ___Cancellous bone is not very porous.
  7. ___Cancellous bone transmits energy.
  8. ___The phalanges are short bones
  9. ____The shaft of a long bone is called the epiphysis
  10. ____The diaphysis is composed primarily of cortical bone.
  11. ____The role of sesamoid bone is to protect internal structures.
  12. ____Bone is approximately 10% to 15% inorganic.
  13. ___Bone tissue is a viscoelastic material whose mechanica properties are affected by its structure.
  14. ___The mineral constituents of bone allow it to withstand compressive loads
  15. ___Bone adapts to both internal and external forces
  16. ___Bones do not require mechanical stress to grow and function
  17. ___Immobilization has little effect on bone.
  18. ___A condition in which bone resorption exceeds bone deposits is called osteopetrosis
  19. ___The term that best describes the fact that the behavior of bone depends on the direction of loading is viscoelastic.
  20. ___Cartilage is approximately 70% water and has little blood supply.
  21. ___Some of the thickest cartilage in the body is found at the ankle joint.
  22. ___A ligament connects muscle to bone
  23. ___The shoulder has substantial ligamentous support.
  24. ___It is theorized that osteoarthritis develops first in the cartilage
  25. ___The knee is a complex joint.

Multiple Choice

  1. The force applied to a structure will cause a___.
  2. strain
  3. stress
  4. Both A and B
  5. Neither A nor B
  6. To compare the stress-strain properties of two different materials, the applied force must be applied to___.
  7. both materials at the same time
  8. the same side of each materia
  9. the same volume
  10. the same area
  11. When stressed in its elastic region, a material will exhibit___.
  12. structural reorganization
  13. failure
  14. no effect
  15. residual strain
  16. A viscoelastic material___.
  17. has elastic and viscous properties
  18. exhibits nonlinear behavior
  19. has multiple stiffnesses
  20. All of the above
  21. A lever alters the____of a movement.
  22. speed
  23. force
  24. change in direction
  25. Both A and B
  26. Hematopoiesis is___.
  27. internal bleeding
  28. inflammation of bone marrow
  29. blood cell formation
  30. None of the above

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  1. The shaft of a long bone is called the___.
  2. epiphysis
  3. diaphysis
  4. metaphysis
  5. duraphysis
  6. Bone tissue is a viscoelastic material whose mechanica properties are affected by its___.
  7. structure
  8. deformation rate
  9. percentage of cortical bone
  10. Both A and B
  11. The bone in the distal part of the femur is replaced every___.
  12. 5-6 months
  13. 10-12 months
  14. 2 years
  15. 4 years
  16. After age 30 years, there is a____yearly loss in the minera weight of bone
  17. 0.1-0.3%
  18. 0.2-0.5%
  19. 0.3-0.6%
  20. 1-2%
  21. Cortical bone is less than____% porous.
  22. 2
  23. 5
  24. 10
  25. 15
  26. At muscle insertion sites, collagen fibers are arranged____, maximizing strength
  27. obliquely
  28. circumferentially
  29. longitudinally
  30. in series
  31. The small, flat pieces of bone making up the cancellous bone are called___.
  32. tolmetin
  33. trabeculae
  34. treponema
  35. lacunae
  36. The term that best describes the fact that the response of bone depends on the rate of loading is___.
  37. isotropic
  38. anisotropic
  39. anisotonic
  40. viscoelastic
  41. A stress fracture can be the result of___.
  42. a single high-magnitude force
  43. a repeated low-magnitude force
  44. a repeated high-magnitude force
  45. None of the above
  46. A standing person has____forces on the inferior portion and____forces on the superior portion of the femoral neck.
  47. torsion, tensile
  48. tensile, compressive
  49. compressive, tensile
  50. tensile, torsion
  51. When a bone is subjected to an excessive bending force, it will fail on the____side because it is weaker under____forces
  52. concave, tensile
  53. convex, tensile
  54. convex, compressive
  55. None of the above
  56. Cartilage reduces contact forces by___.
  57. 50%
  58. 60%
  59. 70%
  60. None of the above
  61. Cartilage___.
  62. improves the fit of the ends of the bones in a joint
  63. decreases the friction in a joint
  64. helps to attenuate shock
  65. All of the above
  66. Cartilage exhibits____characteristics
  67. isotropic
  68. anisotropic
  69. Both A and B
  70. Neither A nor B
  71. With no tension on it, the collagen fibers in a ligament are___.
  72. perpendicular to the length
  73. crimped
  74. stretched
  75. no specific pattern
  76. The strength of a ligament____with immobilization
  77. increases
  78. is not affected
  79. decreases
  80. None of the above
  81. The knee is an example of a(n)____joint.
  82. ellipsoid
  83. hinge
  84. condylar
  85. simple
  86. Synarthrodial joints allow for____ movements
  87. smal
  88. large
  89. no movement
  90. none of the above
  91. Amphiarthrodial joints allow____ movement compared with synarthrodial joints
  92. more
  93. less
  94. the same
  95. All of the above
  96. Alexander, R. M. (1992). The Human Machine.New York: Columbia University Press.
  97. An, K.N., et al. (1991). Pressure distribution on articular surfaces: Application to joint stability evaluation. Journal of Biomechanics, 23:1013.
  98. Antao, N. A. (1988). Myositis of the hip in a professional soccer player. American Journal of Sports Medicine, 16:82.
  99. Beaupre, G. S., et al. (2000). Mechanobiology in the development, maintenance and degeneration of articular cartilage. Journal of Rehabilitation Research and Development, 37:145-151.

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  12. Egan, J. M. (1987). A constitutive model for the mechanical behavior of soft connective tissues. Journal of Biomechanics, 20:681-692,
  13. Eisele, S. A. (1991). A precise approach to anterior knee pain. Physician and Sports Medicine, 19(6):126-130, 137-139.
  14. Fine, K. M., et al. (1991). Prevention of cervical spine injuries in football. Physician and Sports Medicine, 19(10):54-64.
  15. Frost, H. M. (1985). The pathomechanics of osteoporoses. Clinical Orthopaedics and Related Research, 200:198.
  16. Frost, H. M. (1997). On our age-related bone loss: Insights from a new paradigm. Journal of Bone and Mineral Research, 12:1539-1546.
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Glossary

Glossary

Amphiarthrodial Joint

A type of joint whose bones are connected by cartilage; some movement may be allowed at these joints. Also called cartilaginous joint.

Anisotropic

Having different properties in different directions.

Apophysis

A bony outgrowth such as a process, tubercle, or tuberosity.

Articular Cartilage

Hyaline cartilage consisting of tough, fibrous connective tissue.

Avulsion Fracture

The tearing away of a part of bone when a tensile force is applied.

Ball-and-Socket Joint

A type of diarthrodial joint that allows motion through three planes.

Bending Force

A force causing a change in the angle of the bone, offsetting in the horizontal plane. The material bends in the region of no direct structural support.

Bone Strain

See Stress Fracture.

Bone Mineral Density

Amount of mineral measured per unit area or volume of bone tissue.

Cancellous Bone

See Spongy Bone.

Capsular Ligament

Ligaments within the wall of the capsule; thickening in the capsule wall.

Capsule

A fibrous connective tissue that encloses the diarthrodial joint.

Cartilaginous Joint

A type of joint whose bones are connected by cartilage. Some movement may be allowed at these joints. Also called amphiarthrodial joint.

Chondrocytes

Cartilage cells.

Close-Packed Position

The joint position with maximum contact between the two joint surfaces and in which the ligaments are taut, forcing the two bones to act as a single unit.

Collagen

A connective tissue that is the main protein of skin, tendon, ligament, bone, and cartilage.

Cortical Bone

A dense, compact tissue on the exterior of bone that provides strength and stiffness to the skeletal system. Also called compact bone.

Complex Joint

A joint in which two or more bones articulate and a disc or fibrocartilage is present.

Compound Joint

A joint in which three or more bones articulate.

Compression Force

A force pressing the ends of the bone together, shortening and widening the structure.

Condylar Joint

A type of diarthrodial joint that is biaxial, with one plane of movement that dominates the movement in the joint.

Cortical Bone

Dense, compact tissue on the exterior of bone that provides strength and stiffness to the skeletal system. Also called compact bone.

Deposition

A phase of bone remodeling during which bone is formed through osteoblastic activity.

Diaphysis

The shaft of a long bone.

Diarthrodial Joint

Freely movable joint; also called synovial joint.

Elastic Material

A material that exhibits only elastic properties on a stress-strain curve.

Elastic Modulus

The linear portion of a stress-strain curve.

Elastic Region

The area of a stress-strain curve before the yield point. The area in which the material will return to its resting length when the applied force is removed.

Ellipsoid Joint

A type of diarthrodial joint with two degrees of freedom that resembles the ball-and-socket joint.

Epiphyseal Plate

The disc of cartilage between the meta-physis and the epiphysis of an immature long bone that permits growth in length.

Epiphysis

The ends of a long bone.

Extracapsular Ligament

Ligament outside of the joint capsule.

Failure Point

The point on a stress-strain curve when the applied force causes a complete rupture of the material.

Fibrocartilage

A type of cartilage that has parallel thick, collagenous bundles.

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Fibrous Joint

A type of joint whose bones are connected by fibrous material; little or no movement is allowed at these joints; also called synarthrodial joint.

Flat Bone

A thin bone consisting of thin layers of cortical and cancellous bone.

Gliding Joint

A type of diarthrodial joint with flat surfaces that allows translation between the two bones; also called plane joint.

Haversian System

See Osteon.

Hinge Joint

A type of diarthrodial joint that allows one degree of freedom.

Hyaline Cartilage

See articular cartilage.

Hysteresis

The mechanical energy lost by a material that has been deformed.

Intra-articular Ligament

Ligament inside a joint.

Irregular Bone

A bone that has a specialized shape and function.

Lamellae

The concentric tubes of collagen that encircle an osteon.

Lever

A simple machine that magnifies force or speed of movement.

Ligament

A band of fibrous, collagenous tissue connecting bone or cartilage to each other; supports the joint.

Long Bone

A bone that is longer than it is wide, having a shaft, diaphysis, and wide ends, the epiphyses.

Loose-Packed Position

The joint position with less than maximum contact between the two joint surfaces and in which contact areas frequently change.

Meniscus

Crescent-shaped disc of fibrocartilage.

Metaphysis

The wide shaft toward the end of a long bone.

Microtrauma

A disturbance or abnormal condition that is initially too small to be seen.

Modeling

Bone resorption and deposit that forms bone at different sites and rates, resulting in altered size and shape.

Normal Stress

The amount of load per cross-sectional area applied perpendicular to the plane of a cross-section of the loaded object.

Normal Strain

Deformation in a material involving a change in the length of the object.

Osseous

Having the nature or quality of bone.

Ossification

The formation of bone.

Osteoarthritis

Degenerative joint disease characterized by degeneration in the articular cartilage, osteophyte formation, and reduction in the joint space.

Osteocyte

A bone cell.

Osteoblast

A type of bone cell responsible for bone deposition.

Osteoclast

A type of bone cell responsible for bone resorption.

Osteogenesis

The formation of bone.

Osteon

Long cylindrical structure in bone that serves as a weight-bearing pillar.

Osteoporosis

A condition in which the rate of bone formation is decreased, demineralization occurs, and the bone softens.

Periosteum

A white membrane of connective tissue that covers the outer surface of a bone except over articular cartilage.

Pivot Joint

A type of diarthrodial joint that allows movement in one plane; pronation, supination, or rotation.

Plane Joint

A type of diarthrodial joint with flat surfaces that allows translation between the two bones; also called gliding joint.

Plastic Region

The region between the yield point and the failure point on a stress-strain curve; the region in which the material will not return to its initial length after it is deformed.

Porosity

The ratio of pore space to the total volume.

Remodeling

Sequential bone resorption and formation at the same site which does not change the size and shape of bone.

Residual Strain

The difference between the initial length of a material and the length when the material has gone beyond its yield point.

Resorption

A phase of bone remodeling in which bone is lost through osteoclastic activity.

Saddle Joint

A type of diarthrodial joint that has two saddle-shaped surfaces, allowing two degrees of freedom.

Safety Factor

The ratio of the stress to reach the yield point to the stress of everyday activity.

Sesamoid Bone

A type of short bone embedded in a tendon or joint capsule.

Shear Force

A force applied parallel to the surface, creating deformation internally in an angular direction.

Shear Strain

Deformation in a material involving a change in the original angle of the object.

Shear Stress

The amount of load per cross-sectional area applied parallel to the plane of a cross-section of the loaded object.

Short Bone

A bone having dimensions that are approximately equal.

Simple Joint

A joint with only two articulating surfaces.

Spongy Bone

Bone tissue that is lattice-like, with high porosity; capable of high energy storage; also called cancellous bone.

Stress

Force per unit area.

Stress Fracture

A fracture created when loading of the skeletal system is so frequent that bone repair cannot keep up with the breakdown of bone tissue; also called fatigue fracture.

Stress-Strain Curve

A plot of the stress placed on a material against the strain imposed by the stress.

Synarthrodial Joint

A type of joint whose bones are connected by fibrous material; little or no movement is allowed at these joints; also called fibrous joint.

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Synovial Fluid

Liquid secreted by the synovial membrane that reduces friction in the joint; the fluid changes viscosity in response to the speed of joint movement.

Synovial Joint

Freely movable joint; also called diarthrodial joint.

Synovial Membrane

Loose vascularized connective tissue that lines the joint capsule.

Tension Force

Force pulling a bone apart, lengthening and narrowing the bone.

Torsion Force

Twisting force that creates shear stress over the entire material.

Trabeculae

Strands within cancellous bone that adapt to the direction of stress on the bone.

Traumatic Fracture

A break in a bone as a result of a single high-magnitude force application.

Viscoelastic Material

A material that exhibits nonlinear properties on a stress-strain curve.

Yield Point

The point on a stress-strain curve at which the material reaches the plastic region.