I THINK THERE IS MORE THAN THE BODY, BUT THE BODY IS ALL YOU CAN GET YOUR HANDS ON.
—IDA ROLF
THE KNEE JOINT is formed by the femur and tibia. Both the fibula and the patella are extra-articular, which means that neither is directly involved in the knee joint proper.
The knee joint is at the apex of the longest lever in the body, the femur, and is a major weight-bearing joint (Figures 9.1 and 9.2). Because of these two facts, the knee is subject to extreme stresses and strains. These stresses occur during both flex-ion and extension of the knee joint, as well as during the rotational components the joint undergoes. Unfortunately, while the joint is also affected by a number of very powerful muscles, it is not directly supported very much by these muscles. In order to understand the dynamics of this joint, therefore, pay careful attention to its unique structure.
9.1 (FAR LEFT) ANTERIOR KNEE JOINT
9.2 (LEFT) POSTERIOR KNEE JOINT
BONES
As we have seen, at the proximal end of the femur, the neck is angled at approximately 130 degrees. Remember that this is the angle between the neck of the femur as is exits the acetabulum and as it bends at the beginning of the shaft of the femur.
If this angle is changed by misplacement of the femoral head in the socket, it can cause alignment difficulties for the knee joint. If this angle is decreased, the head of the femur articulates with the acetablulum in such a way that the femur is in a more abducted position at rest. This condition is referred to as genus varus and is commonly called bow legs. It is more common in men. If this femoral angle is increased, the head of the femur comes into the acetabulum such that the femur is in a more adducted position at rest. This condition is referred to as genus valgus, commonly called knock knees. It is more common in women.
The femur ends in two condyles that are connected to each other on the anterior side but are separate on the posterior side, where they form a hollow called the intercondylar fossa. Just proximal to the condyles are the epicondyles, both lateral and medial prominences that serve as attachment points for soft tissue.
The tibia is the next largest bone in the body after the femur; it forms the other half of the knee joint. Like the distal femur, the superior tibia has two condyles, medial and lateral. The superior surface of these condyles is the surface that is the receiving end of the condyles of the femur. This is the true knee joint. The surface of the medial condyle of the tibia is concave to receive the medial condyle of the femur, while the lateral condyle is less so (Figure 9.3)
On the anterior superior tibia is the tuberosity of the tibia, a large and easily palpable bony prominence that serves as the insertion point for the quadriceps femoris muscle. Find it on yourself now.
Sit comfortably on the floor with your right knee flexed and half way toward your chest, foot resting on the floor. Cup your patella with your right hand so that your fingers are facing in the direction of your foot. Notice the depression at the distal patella. Just below that depression you will feel the bony prominence of the tibial tuberosity sticking out at the center of the tibial shaft.
The lateral surface of the distal tibia is flattened to join with the fibula (Figure 9.4). The distal tibia is elongated to form the medial malleolus, which articulates with the ankle joint.
The fibula joins with the lateral tibia superiorly and forms the lateral part of the ankle joint distally. This part of the fibula is called the lateral malleolus. The fibula is not involved in the knee joint ; its major function is to act as a supporting strut to the lateral leg.
The patella is a sesamoid bone, a type of bone that develops inside a tendon. At the knee joint, this usually happens during the second year of life. The tendon is the quadriceps femoris as it crosses over the joint (Figure 9.5).
The functions of the patella are to cover the knee joint to protect it and to provide increased leverage for the quadriceps femoris muscle as it crosses the knee at the patella. This is how it happens: It is well known that increased leverage is created by the presence of a fulcrum. As the quadriceps femoris tendon passes over the knee joint, it is lifted by the thickness of the patella, which is acting as a fulcrum. This slight elevation of the tendon at the joint creates increased power on knee extension. In fact the patella actually increases the leverage of the quadriceps femoris at the joint from 15 to 40 percent.
9.3 MEDIAL KNEE JOINT
9.4 LATERAL KNEE JOINT
9.5 PATELLOFEMORAL JOINT
When the knee joint is in extension, the patella sits slightly medially. When the knee is in flexion, the patella sits slightly laterally. However, it is not unusual for the patella to drift either too far medially or laterally and remain there. This can sometimes be seen in athletes.
This positional fault of the patella is usually caused by an imbalance in muscular strength in the medial or lateral head of the vasti (the other three heads of the quadriceps femoris) respectively. This condition is called patello-femoral syndrome. Usually it can be improved by specific strengthening exercises for the weak part of the vasti.
JOINTS
The knee joint consists solely of the articulation of the distal femur and the proximal tibia. The center of the joint is to be found on a line drawn directly down from the center of the acetabulum. This line shows that the femur must curve inward from the hip joint to bring the knee joint directly under the acetabulum and that the femur has a medial curve. This is accomplished in part by the medial curve of the femur created by the angle of the neck. This means that the femoral head sits in the actetabulum at an angle, with the effect that the greater trochanter is actually anterior of the hip joint proper. The knee joint is completed by the presence of the patella, which creates the patellofemoral joint.
The space at the back of the knee is called the popliteal space. Its boundary on the proximal lateral side is the biceps femoris and on the distal lateral side is the lateral head of the gastrocnemius muscle. Its medial borders are the semitendinosus, the semimembranosus, and the medial head of the gastrocnemius. It contains arteries, veins, bursa, the common peroneal nerve, and the posterior tibial nerve.
Another joint around the knee is the articulation of the tibia and fibula. The lateral tibia joins the head of the fibula at a site called the tibiofibular joint. It is a gliding joint with the ability to move slightly anteriorly and posteriorly.
CONNECTIVE TISSUE
The tibia and fibula are joined by the interosseous membrane. It not only keeps the tibia and fibula connected but also divides the muscles of the leg into anterior and posterior compartments.
9.6 (RIGHT) MEDIAL COLLATERAL LIGAMENT
9.7 (FAR RIGHT) LATERAL COLLATERAL LIGAMENT
The main ligaments of the moving knee joint are called the medial and lateral collateral ligaments (Figures 9.6 and 9.7). These two collateral ligaments give stability to any rotational movements at the joint. The medial collateral ligament is stronger because of the increased need for stabilization on the medial side of the joint, at the site where the femur and tibia join. The distal femur is at an increased angle here, and more movement is allowed than at the lateral knee joint; thus more support is needed. The lateral collateral ligament runs from the lateral epicondyle of the femur to the lateral fibular head and supports the lateral joint.
The medial collateral ligament is the stronger of the two and runs from the medial epicondyle of the femur to the proximal medial shaft of the tibia. Another important distinction between the collateral ligaments at the knee is that the medial collateral ligament is also connected to the medial meniscus. This is one of the factors that allow an increased potential for injury at the medial knee joint, especially twisting injuries during weight bearing.
These types of injuries can occur when the foot is securely planted, the knee flexed, and the femur adducted and internally rotated. In this position, the knee joint is opened at the medial side and made more vulnerable, because during flexion there is less congruence between the femur and tibia and thus less stability. Any added force to the joint at this point, like a tackle in a football game, can cause injury to the medial muscle tendons as well as to the collateral tendon and its adjoining medial meniscus all at once. This injury is called “the unhappy triad of O’Donohue,” probably the most colorful name of any orthopedic injury. This connection of muscle tendons, ligament, and meniscus does not exist on the lateral side, which is one of the reasons that the lateral knee is less likely to be injured.
The internal ligaments of the knee joint are called cruciate because they are crossed. They are named for the part of the tibia to which they are connected. Because of the oblique orientation of the cruciates, flexion is allowed and at the same time they restrain tibial-femoral displacement. The anterior cruciate ligament (ACL) is attached to the anterior intercondylar area of the tibia and the lateral meniscus; it runs posteriorly and superiorly to attach to the lateral condyle of the femur (Figure 9.8). The ACL has several functions. It acts as a restraint to hyperextension of the joint. It is the main structure that restrains anterior tibial displacement, and it limits internal rotation of the tibia. The posterior cruciate ligament (PCL) originates from the posterior intercondylar area of the tibia and attaches at the anterior medial condyle of the femur (Figure 9.9). The PCL passively aids in flexion of the joint
9.8 (FAR LEFT) ANTERIOR CRUCIATE LIGAMENT WITH THE MENISCUS
9.9 (LEFT) POSTERIOR CRUCIATE LIGAMENT WITH THE MENISCUS
The knee joint has unique connective tissue structures called menisci. These structures act as pads for the knee joint as well as functioning to create a deeper cavity in the superior surface of the tibia for the distal femoral condyles. The menisci maintain an even covering of synovial fluid and thus help keep the joint lubricated for easy movement and reduced friction. Irritation and even destruction of these cartilage surfaces tend to decrease the surface available for weight bearing, thus decreasing this natural lubrication and contributing to degeneration.
Both menisci are thick on their outer rim and thinner in the middle. Each meniscus is attached to a portion of the joint capsule. They move slightly during locomotion. During extension of the knee joint, the mensci move forward slightly, and on flexion they move backward. This is to facilitate the easy movement of the femur on the tibia.
The knee joint has thirteen bursae that act as cushions. They are located anteriorly, laterally, and medially around the joint. The largest of these bursae is located between the patella and the skin. This bursa can be irritated by excessive kneeling, a condition called housemaid’s knee. All the bursae function to cushion soft tissue, like tendons and ligaments, from rubbing against bone and increasing the wear on them.
NERVES
The two main nerves around the knee joint are branches of the sciatic nerve. (For more discussion on the sciatic nerve, see chapter 6.) These divisions are called the tibial nerve and the common peroneal nerve. This division occurs just superior to the posterior knee joint.
The tibial nerve is the larger; it passes directly through the popliteal fossa at the back of the joint and continues distally. It is deep in the calf muscles as it descends on the medial side of the Achilles tendon. It finally ends in the connective tissue at the bottom of the foot, where it further divides into the medial and lateral plantar nerves.
The tibial nerve has branches to the knee joint proper and the ankle joint, and it controls the muscles of the calf and muscles of the flexor surface of the foot as well as other muscles in this area.
The common peroneal nerve descends on the lateral side of the popliteal fossa; as it continues it winds around the head of the fibula, courses deep into the peroneus longus muscle, and divides into deep and superficial branches.
9.10 SUPERFICIAL AND DEEP HAMSTRINGS
9.11 POPLITEUS MUSCLE AND POSTERIOR TIBIALIS
9.12 GASTROCSOLEUS
9.14 MUSCLES ACTING SPECIFICALLY ON THE KNEE JOINT
9.13 OTHER MUSCLES OF THE LEG
9.15 ROTATIONAL COMPONENTS DURING FLEXION AND EXTENSION
The common peroneal nerve also has branches to the knee joint, as well as to muscles of the lateral and posterior aspects of the leg. Additionally it offers innervation to the tibialis anterior muscles and some of the muscles of the foot.
MUSCLES
The muscles that surround the knee joint are some of the most powerful in the body. The quadriceps femoris, the adductors, and the hamstrings are presented in chapter 8. Here we look at the muscles that are attached to the leg and act upon the knee joint specifically, not the hip joint (Figures 9.10, 9.11, 9.12, and 9.13). The origin, insertions, and actions of these muscles are presented in Figure 9.14.
These muscles are sometimes called secondary knee flexors. When the foot is stabilized or bearing weight, they flex the knee. But when there is no weight on the foot, these muscles will plantar flex the ankle. In other words, they increase the angle between the foot and the leg.
KINESIOLOGY
One of the misconceptions about the knee joint is that it acts as a hinge. Instead, the knee moves with a rolling and gliding action during flexion and extension. During flexion the femur rolls forward on the tibial shelf, while the tibia glides backward on the femur.
The healthily functioning knee joint also has a rotational component that occurs during movement (Figure 9.15). During flexion, there is an unlocking mechanism in which the femur rotates slightly externally on the tibia. This external rotation of the femur on the tibia is the freest at 90 degrees of flexion. This is the position of the front knee joint in Utthita Parsvakonasana or Virabhadrasana I and Virabhadrasana II.
9.16 HYPEREXTENSION OF THE KNEE
During extension of the knee, the femur rolls backward on the tibial shelf, while the tibia glides forward on the femur. In addition, during extension the femur rotates slightly internally on the tibia in such a way that causes a locking mechanism. This movement is created in part by the shape of the right medial condyle of the tibia and the shape of the medial meniscus. Because of the shape of these structures, the femur moves farther into extension on the medial side of the joint. This contributes to the stability of the joint in extension by helping to create the locking mechanism.
Hyperextension of the knee joint occurs when the backward glide of the tibia on the femur is excessive, so that the joint moves past 180 degrees, or a straight line (Figure 9.16). This hyperextension of the joint contributes to instability in extension and stress on the ligaments of the knee. Hyperextension is inhibited especially by the anterior cruciate ligament but also by the oblique popliteal and collateral ligaments.
Other positional faults of the knee joint are genus valgus and genus varus (discussed on page 110). Genus valgus (knock knees) is more common in women and is caused in part by a decreased angle of the neck of the femur. This causes the knees to come together. A student suffering from genus valgus sometimes is unable to put her feet together in Tadasana because her medial knees meet first. Genus varus (bow legs) is associated with the increased angle of the neck of the femur. It is more common in men. In this case, it might be difficult for the student to put his knees close together in Tadasana.
EXPERIENTIAL ANATOMY
For Practicing
9.17 DANDASANA, WITH ONE LEG BENT
Applied Practice 1: Normal Patellar Movement
Prop: 1 nonskid mat
Take Care: Stop immediately if this practice causes any discomfort.
WHEN THE QUADRICEPS muscles are relaxed, the patella is easily moveable. To experience this, sit on your nonskid mat with one leg bent and the other leg extended (Figure 9.17). Relax the quadriceps muscle in the extended leg and grasp the sides of your patella between your thumb and index finger.
Now move your patella gently from side to side; it should move easily and with no pain about an inch in each direction. Now release the patella and contract your quadriceps. You will find that the patella is held firmly against the femur, and you will be unable to move the patella at all. As simple as this technique is, it can be a helpful way to communicate exactly what a contraction of the quadriceps feels like, especially in standing poses.
9.18 DANDASANA, TESTING FOR KNEE HYPEREXTENSION
Applied Practice 2: Testing for Hyperextended Knees
Prop: 1 nonskid mat
Take Care: Do not force your knees to the floor in this test.
AN EASY WAY to determine if your knees are hyperextended can be done while sitting. Sit in Dandasana on your nonskid mat (Figure 9.18). Make sure your knees are not rotating externally. Now contract your quadriceps very strongly, while keeping your feet completely relaxed. This part is very important; if you dorsiflex your feet, it will interfere with the test.
When you contract your quadriceps, if your heels lift off the floor, it is very likely that your knees have the ability to hyperextend. This can be problematic during the practice of asana, so care should be taken to avoid hyperextending your knees whenever possible.
For Teaching
9.19 TADASANA
Applied Teaching 1: Observing the Knee in Tadasana
Prop: 1 nonskid mat
Take Care: Stand on an even surface.
HAVE YOUR STUDENT stand on her mat in Tadasana (Figure 9.19). Make sure that the lateral borders of her feet are parallel to the walls of the room, so that she is not standing in external rotation from her hip.
Now observe her knee joints from the side. If you draw an imaginary vertical plumb line from her outer ear downward through the center of her external shoulder joint and external hip joint, it should pass through the anterior half of her knee joint. Thus in an aligned Tadasana, weight is being carried through the anterior part of the joint.
This relationship means that not only does the structure of the joint itself help to create stability during extension, but that the force of gravity is aiding in creating a position of stability as well. Encourage your student to stand in Tadasana with the knee joint in pure extension, without hyperextending, to provide a position of maximal stability and minimal effort.
Applied Teaching 2: Observing the Knee in Adho Mukha Svanasana
Prop: 1 nonskid mat
Take Care: If your student has wrist pain, practice with the hands on the wall instead.
9.20 ADHO MUKHA SVANASANA
HAVE YOUR STUDENT practice Adho Mukha Svanasana, and, when he is in the pose, stand behind him and observe the backs of his knees (Figure 9.20). Notice two things: first, that the surface at the back of the knee is not parallel to the wall behind you but rather that it is slightly internally rotated. Second, notice that the back of his medial knee is slightly lower than his lateral knee. Both of these phenomena are due to the fact that the femur internally rotates on extension to help to provide stability in the joint. This relationship is the normal shape of the back of the knee in extension.
LINKS
I recommend these two books about knees. Peter Egosue’s Pain Free (New York: Bantam, 2000) details exercises that I have found very helpful, if not practically miraculous, for students who exhibit a patella that is either internally or externally rotated too much. These exercises help to align the patella, thus increasing the efficiency of the quadriceps muscle at the knee joint and decreasing the strain on surrounding tissues. Sandy Blaine’s Yoga for Healthy Knees(Berkeley, CA: Rodmell Press, 2005) offers a comprehensive yoga program for pain prevention and rehabilitation.