Sectional anatomy for imaging professionals, 4th edition

Chapter 10. Lower Extremity

And well observe Hippocrates’ old rule, the only medicine for the foote is rest.

Thomas Nash (1567-1601), Summer’s Last Will and Testament

The complex anatomy of the lower extremity is responsible for bearing the entire upper body weight and for accommodating the demands of movement placed on this system (Fig. 10.1).

FIG. 10.1 Multiple fractures of the distal tibia and fibula

OBJECTIVES

• Identify the bony anatomy of the lower extremity.

• Identify and state the actions of the lower extremity muscles, as well as their origins and insertions.

• Describe the labrum and articular capsule of the hip.

• List and describe the ligaments, retinacula, and tendons of the lower extremity joints.

• Define and identify the meniscus and articular capsule of the knee.

• Identify the bursae of the hip and knee.

• List and identify the major arteries and veins of the lower extremity.

• Describe the nerves that innervate the lower extremity.

HIP

The hip provides strength to carry the weight of the body in an erect position. This synovial ball-and-socket joint, created by the articulation of the femoral head with the acetabulum of the pelvis, allows for a wide range of motion.

Bony Anatomy

Acetabulum. A cuplike cavity termed the acetabulum is created by the three bones of the pelvis: ilium, ischium, and pubis (Fig. 10.2; see also Chapter 8, bony anatomy of the pelvis). In axial cross-section, this area can be divided into sections known as the anterior and posterior columns. The posterior column is stronger and contributes to the weight-bearing portion of the acetabulum (Fig. 10.3). Within the acetabulum is a centrally located, nonarticulating depression called the acetabular fossa. It is formed mainly by the ischium and is filled with fat (Fig. 10.4). A continuation of the acetabular fossa is the acetabular notch, which interrupts the smooth circumference of the acetabular rim inferiorly and functions as an attachment site for the transverse acetabular ligament (Fig. 10.2).

Femur. The femur is the longest, heaviest, and strongest bone in the body. The proximal end of the femur consists of a head, a neck, and two large processes: the greater and lesser trochanters (Figs. 10.5-10.12). On the proximal portion of the femur is the smooth, rounded femoral head. The femoral head is covered entirely by articular cartilage, with the exception of a small centrally located pit termed the fovea capitis. The fovea capitis is an attachment site for the ligamentum teres (Figs. 10.3-10.6, 10.9, and 10.12). Connecting the head of the femur to the femoral shaft is the femoral neck. The neck extends obliquely from the head at an angle of approximately 120 degrees in an inferolateral direction to meet the shaft (Fig. 10.5). The result of this angle is increased freedom of movement within the hip joint. At the distal end of the neck are two large bony prominences termed trochanters (Figs. 10.5-10.11). The greater trochanter is situated at the junction of the neck with the shaft. The superior portion of the greater trochanter projects above the neck and curves slightly posteriorly and medially (Figs. 10.4, 10.7, 10.9, and 10.10). The greater trochanter provides attachment for numerous muscles of the gluteal region (see Table 10.1). The lesser trochanter is at the posteromedial portion of the proximal shaft and provides insertion for the tendon of the iliopsoas muscle (Figs. 10.6, 10.9, and 10.10). The prominent ridge extending posteriorly between the trochanters at the base of the neck is the intertrochanteric crest (Figs. 10.9 and 10.11). It provides an attachment site for the ischiofemoral ligament and part of the quad- ratus femoris tendon. Connecting the trochanters anteriorly is the less prominent ridge termed the intertrochanteric line, which provides attachment for the iliofemoral ligament and part of the vastus lateralis tendon (Figs. 10.9 and 10.10). On the posterior surface of the femoral shaft is a raised ridge termed the linea aspera. Its medial and lateral lips provide attachment sites for muscles of the posterior and medial compartments of the thigh (Fig. 10.9). The pectineal line runs from the medial lip of the linea aspera to the lesser trochanter of the femur (Figs. 10.9 and 10.11). It is the insertion site for the tendon of the pectineus muscle. The lateral lip is very rough and runs almost vertically upward to the base of the greater trochanter. The widened portion of the lateral lip, the gluteal tuberosity, is an attachment site for the gluteus maximus and adductor magnus muscles (Figs. 10.9 and 10.11). The linea aspera extends down to the popliteal surface of the femur.

Avascular necrosis (AVN) is a major concern following subcapital fractures of the femoral head. Disruption of the arterial supply to the femoral head is the most significant factor leading to AVN.

Labrum and Ligaments

The femoral head is held to the acetabulum by the acetabular labrum and several major ligaments. The acetabular labrum, transverse acetabular ligament, iliofemoral ligament, ischiofemoral ligament, pubofemoral ligament, and ligamentum teres are shown in Figs. 10.12-10.26.

Labrum. The acetabular labrum creates a fibrocartilaginous rim attached to the margin of the acetabulum.

This labrum closely surrounds the femoral head, helping to hold it in place by deepening the acetabular fossa, which adds increased stability to the joint (Figs. 10.12 and 10.21-10.24).

Ligaments. The inferior margin of the acetabulum is incomplete and is reinforced by the transverse acetabular ligament that spans the acetabular notch at the inferior edge of the acetabulum (Figs. 10.2, 10.12, 10.20, and 10.24). As the transverse acetabular ligament abridges the acetabular notch, it transforms it into the acetabular foramen, which allows nerves and blood vessels to pass to and from the hip joint.

The iliofemoral ligament is among the strongest of the body, with a major stabilizing function as it spans from the anterior inferior iliac spine and rim of the acetabulum to insert on the intertrochanteric line of the femur (Figs. 10.13 and 10.14). A primary function of this ligament is to provide a thick reinforcement to the anterior part of the hip joint. The ischiofemoral and pubofemoral ligaments, although difficult to distinguish, present a spiral configuration of femoral attachment (Figs. 10.13 and 10.14). The ischiofemoral ligament arises from the ischium and courses in a spiral above the femoral neck to insert on the posterior femoral neck, making it the only ligament located on the posterior aspect of the hip. The pubofemoral ligament arises from the superior pubic ramus to radiate and insert onto the iliofemoral ligament and intertrochanteric line.

The spiral configuration of these two ligaments is unique to humans and ensures the stability and function while controlling the overall position of the lower limb. The ligamentum teres is a somewhat flattened band that extends from the fovea capitis of the femoral head to attach to the rim of the acetabular notch, as well as blend with the transverse acetabular ligament (Figs. 10.12, 10.18, and 10.24). It is located entirely within the hip joint and contains nerves and vessels that pass to the head of the femur. Recent studies suggest the ligamentum teres may be a strong intrinsic stabilizer that can assist to prevent dislocation of the hip. Ligaments of the hip are identified in Figs. 10.12-10.26.

Joint Capsule

The joint capsule is composed of fibrous and synovial capsules.

The fibrous capsule of the hip is strong and dense, being reinforced by the iliofemoral, ischiofemoral, and pubofemoral ligaments. It forms a sleeve that encloses the hip joint and most of the neck of the femur. Proximally, it is attached to the edge of the acetabulum, just distal to the acetabular labrum, and to the transverse acetabular ligament. Distally, the fibrous capsule is attached to the femoral neck, intertrochanteric line, and the greater trochanter, anteriorly. Posteriorly, it attaches to the neck, just proximal to the intertrochanteric crest (Figs. 10.5 and 10.21). The capsule consists of deep circular fibers that form the zona orbicularis, a sling or collar around the femoral neck that constricts the capsule and helps hold the femoral head in the acetabulum (Figs. 10.20, 10.22-10.24, and 10.26).

The synovial capsule of the hip joint lines the internal surface of the fibrous capsule. The synovial capsule forms a sleeve for the ligamentum teres, lines the acetabular fossa, and covers the fat pad in the acetabular notch (Fig. 10.5). It is attached to the edges of the acetabular fossa and to the transverse acetabular ligament. The synovial capsule protrudes inferior to the fibrous capsule on the posterior aspect and forms the obturator externus bursa, which protects the tendon of the obturator exter- nus muscle (Fig. 10.14).

Bursae

There are many bursae located around the hip owing to the number of muscles associated with this joint. The hip bursae vary in number and position and act to reduce friction at locations where tendons and muscle pass over bone. Major bursae of the hip include the trochanteric, iliopsoas, and ischial. The trochanteric (subgluteus maximus) bursa is located between the insertion site of the gluteus muscles, the vastus lateralis muscle, and the greater trochanter of the femur. The iliopsoas (iliopectineal) bursa is situated between the iliopsoas tendon and the lesser trochanter of the femur. Located between the ischial tuberosity and the gluteus maximus muscle is the ischial bursa (Fig. 10.27).

Snapping hip syndrome is a condition characterized by a snapping or popping sensation when the hip is flexed and extended and may cause discomfort or pain. The most common cause of the snapping sound is due to tendons catching on bony prominences with movement of the hip. The hip bursae, greater trochanteric, and iliopsoas may become inflamed with this syndrome and result in pain caused by bursitis.

Muscles of the Hip and Thigh

A complex arrangement of muscles around the hip joint and thigh produces the movements of the hip. They are described in this section as gluteal muscles and muscles of the thigh and are illustrated in Figs. 10.15-10.20 and 10.28-10.54 and Table 10.1. The muscles of the gluteal region and thigh muscles may be separated into compartments by thickened sheets of deep fascia, thus allowing muscles of the lower extremity to be classified according to specific compartments in which they are located, such as the gluteal compartment and the anterior, medial, and posterior thigh compartments.

Gluteal Compartment—Superficial Muscles

Gluteus maximus muscle Gluteus medius muscle Gluteus minimus muscle

The muscles of the gluteal compartment are identified in Figs. 10.28-10.36, and their functions are presented in Table 10.1.

The gluteus maximus muscle is the largest muscle in the body and most superficial of the gluteal muscle group. It is situated on the posterior aspect of the hip joint and acts primarily as a powerful extensor of the hip. It also plays an important role in lifting, walking, and running. The gluteus maximus originates from the ilium, sacrum, and coccyx to insert just distal to the gluteal tuberosity of the greater trochanter (Figs. 10.28-10.36).

The gluteus medius muscle is located on the lateral and upper part of the buttock. It originates from the iliac crest, just lateral to the gluteus maximus muscle, and is partially covered by the gluteus maximus muscle along its medial third. The gluteus medius muscle is fan-shaped as it spans from the iliac crest to insert on the superolateral aspect of the greater trochanter of the femur (Figs. 10.28-10.36).

The gluteus minimus muscle is the smallest of the gluteal muscles. It is also fan-shaped and completely covered by the gluteus medius muscle. The upper attachment of the gluteus minimus muscle is from the gluteal surface of the ilium, just inferior to that of the gluteus medius muscle. Its tendon attaches to the anterosuperior aspect of the greater trochanter of the femur (Figs. 10.2910.36). The gluteus medius and minimus muscles act to abduct and medially rotate the thigh.

Gluteal Compartment—Deep Muscles

Piriformis muscle Obturator internus muscle Obturator externus muscle Gemellus muscles Quadratus femoris muscle

The muscles of the gluteal compartment are identified in Figs. 10.28-10.36, and their functions are presented in Table 10.1. The deep muscles of the gluteal compartment are the main lateral rotators of the thigh at the hip joint and also act to stabilize the hip joint.

The piriformis muscle originates from the inner surface of the sacrum between the sacral foramina. It passes laterally and anteriorly through the greater sciatic foramen to attach to the superior boundary of the greater trochanter of the femur (Figs. 10.29, 10.30, 10.32, and 10.35). The actions of the piriformis muscle include lateral rotation and abduction of the thigh.

The obturator internus muscle is a thick, fan-shaped muscle that originates from the inner border of the obturator foramen and travels through the lesser sciatic foramen. At this point, it changes shape as it becomes tendinous and courses laterally to attach to the greater trochanter of the femur (Figs. 10.15, 10.18, 10.29, 10.33, and 10.36). Its primary actions are the same as those of the piriformis muscle: lateral rotation and abduction of the thigh.

The obturator externus muscle arises from the outer border of the obturator foramen in the pelvis to essentially mirror the obturator internus muscle. It courses laterally around the posterior side of the neck of the femur to insert into the medial side of the greater trochanter and acts to laterally rotate the thigh (Figs. 10.18, 10.29, 10.30, 10.33, and 10.36).

FIG. 10.31 Axial, Tl-weighted MRI of gluteal muscles.

FIG. 10.32 Axial, T1-weighted MRI of piriformis and iliopsoas muscles.

FIG. 10.34 Axial CT with psoas major and iliacus muscles.

FIG. 10.35 Axial CT with iliopsoas muscle.

The two gemellus muscles (superior and inferior) are located along the superior and inferior boundaries of the obturator internus muscle and tendon (Fig. 10.29). The superior gemellus muscle arises from the ischial spine, whereas the inferior gemellus muscle arises from the ischial tuberosity. Both muscles join with the tendon of the obturator internus muscle to insert into the medial surface of the greater trochanter (Figs. 10.15, 10.16, 10.20, 10.29, and 10.30). The gemellus muscles also act to laterally rotate and abduct the thigh.

The rectangular quadratus femoris muscle is located inferior to the obturator internus and gemellus muscles. It arises from the lateral border of the ischial tuberosity and then courses laterally to insert on the intertrochanteric crest of the femur. The primary action of the quad- ratus femoris muscle is lateral rotation of the thigh (Figs. 10.29, 10.30, 10.33, and 10.36).

TABLE 10.1 Muscles of the Gluteal Compartment

Muscle	Proximal Insertion	Distal Insertion	Action
Superficial Muscles of the Gluteal Compartment
Gluteus maximus	Ilium, sacrum, coccyx	Gluteal tuberosity of greater	Extensor of the hip; maintains erect
		trochanter	position of the body
Gluteus medius	Iliac crest	Superolateral aspect of greater trochanter	Abducts and medially rotates the thigh
Gluteus minimus	Gluteal surface of ilium	Anterosuperior aspect of greater trochanter	Abducts and medially rotates the thigh
Deep Muscles of the Gluteal Compartment
Piriformis	Sacrum	Superior boundary of greater trochanter	Lateral rotation and abduction of the thigh
Obturator internus	Inner border of obturator foramen	Greater trochanter	Lateral rotation and abduction of the
			thigh
Obturator externus	Outer border of obturator foramen	Greater trochanter, medial side	Lateral rotation of the thigh
Superior gemellus	Ischial spine	Joins with tendon of obturator internus,	Lateral rotation and abduction of
		medial surface greater trochanter	the thigh
Inferior gemellus	Ischial tuberosity	Joins with tendon of obturator inter-	Lateral rotation and abduction of the
		nus, greater trochanter	thigh
Quadratus femoris	Lateral border of ischial tuberosity	Intertrochanteric crest of femur	Lateral rotation of the thigh

Muscles of the Anterior Thigh Compartment

Iliopsoas muscle Sartorius muscle Quadriceps femoris muscle Tensor fasciae latae muscle

The muscles of the anterior thigh compartment are shown in Figs. 10.30-10.54 and described in Table 10.2. These muscles act both to flex the hip joint and extend the knee joint.

The powerful iliopsoas muscle is composed of the psoas major and iliacus muscles (Figs. 10.30, 10.34, 10.35, and 10.37). The psoas major muscle arises from the transverse processes of the lumbar vertebrae and courses inferiorly within the pelvis. It exits the pelvis as it courses under the inguinal ligament to enter the anterior compartment of the thigh. The tendon of the psoas major joins with the tendon of the iliacus muscle to pass anterior to the hip joint capsule and attach to the lesser trochanter of the femur (Fig. 10.37). The iliacus muscle arises from the iliac fossa and courses along the lateral side of the psoas major muscle in the pelvis. These muscles act conjointly in flexing the thigh at the hip and stabilizing the hip joint (Fig. 10.37).

The sartorius muscle is known as the longest muscle in the body; it extends from the anterior superior iliac spine to the medial surface of the tibia near the tuberosity (Figs. 10.37 and 10.40-10.54). It acts to flex, abduct, and laterally rotate the thigh.

The quadriceps femoris muscle covers almost all of the anterior surface and sides of the femur. It originates as four heads (rectus femoris, vastus lateralis, vastus media- lis, and vastus intermedius) to create a powerful extensor of the knee (Figs. 10.37-10.54). The superior ends of the four heads of the quadriceps femoris muscle arise from different locations, but their inferior tendons merge to form the quadriceps femoris tendon that courses over the patella and continues as the patellar ligament in the knee to insert on the tibial tuberosity (Fig. 10.37). The rectus femoris originates from the anterior inferior iliac spine, the vastus lateralis from the greater trochanter and lateral lip of the linea aspera of the femur, the vastus medialis from the intertrochanteric line and medial lip of the linea aspera of the femur, and the vastus intermedius from the anterior and lateral surfaces of the body of the femur

(Figs. 10.44 and 10.47). All the parts of the quadriceps femoris act to extend the leg at the knee joint and, through the actions of the rectus femoris, flex the hip joint.

The tensor fasciae latae muscle is a short, thick, teardropshaped muscle located on the anterolateral aspect of the thigh, enclosed between two layers of the fascia. As its name implies, it tightens the lateral fascia, thereby enabling the thigh muscles to act with increased power. It abducts, medially rotates, and flexes the thigh and helps keep the knee extended.

It originates from the anterior superior iliac spine and anterior part of the iliac crest and ends where the muscle inserts in the iliotibial band (Figs. 10.32, 10.33, 10.35-10.37, and 10.43). The iliotibial (IT) band is a long, wide band of fascia that lies over the muscles on the outer surface of the thigh. This band is a thickening of the normal fascia that surrounds the entire leg. It arises from tendinous fibers of the tensor fasciae latae and the gluteus maximus muscles. Acting almost like a ligament, this tendon helps mainly to stabilize the knee joint but also acts in flexing and extending the knee. It extends downward to insert on the lateral condyle of the tibia at Gerdy tubercle (Figs. 10.6, 10.37, 10.43, and 10.49).

Muscles of the Medial Thigh Compartment

Gracilis muscle Pectineus muscle Adductor longus muscle Adductor brevis muscle Adductor magnus muscle

The muscles of the medial thigh compartment are shown in Figs. 10.37-10.54 and described in Table 10.2. Adduction is the primary action of the medial thigh muscles.

The long, straplike gracilis muscle lies along the medial side of the thigh and knee. The gracilis muscle extends from the inferior pubic ramus and pubic symphysis and is the only medial thigh muscle to cross the knee joint as it inserts onto the anterior surface of the tibia just inferior to the medial condyle. In addition to adducting the thigh, the gracilis muscle acts to flex the leg and helps medially rotate the thigh (Figs. 10.37 and 10.40-10.51).

Arising from the pectineal line of the pubis is the short, flat pectineus muscle. It lies medial to the psoas major muscle in the superior thigh, then narrows as it courses inferiorly to insert on the medial lip of the linea aspera, at the pectineal line, distal to the lesser trochanter of the femur. It acts to adduct and flex the thigh (Figs. 10.33, 10.36, 10.37, 10.49, and 10.50).

The adductor muscle group, as named, acts to adduct the thigh and is composed of three muscles: the adductor longus, adductor brevis, and adductor magnus (Figs. 10.39-10.50). These muscles originate on the body and inferior pubic ramus and fan out to insert along the length of the medial aspect of the femur.

TABLE 10.2 Muscles of the Thigh

Muscle	Proximal Insertion	Distal Insertion	Action
Anterior Thigh
Psoas major	Transverse processes of lumbar vertebrae	Lesser trochanter	Flexes the thigh at hip and stabilizes the hip joint
Iliacus	Iliac fossa	Lesser trochanter	Flexes the thigh at hip and stabilizes the hip joint
Sartorius	Anterior superior iliac spine and iliac crest	Medial surface of tibia near tuberosity	Flexes, abducts, and laterally rotates the thigh
Tensor fasciae Anterior superior iliac spine latae Quadriceps Femoris		Iliotibial (IT) band	Abducts, medially rotates, and flexes the thigh; helps maintain extension of the knee
Rectus femoris	Anterior inferior iliac spine	Tibial tuberosity via patellar ligament	Extends the leg at knee joint and flexes the hip joint
Vastus lateralis	Greater trochanter and lateral lip of linea aspera	Tibial tuberosity via patellar ligament	Extends leg at knee joint
Vastus medialis	Intertrochanteric line and medial lip of linea aspera	Tibial tuberosity via patellar ligament	Extends leg at knee joint
Vastus intermedius Medial Thigh	Anterior and lateral surfaces of femoral body	Tibial tuberosity via patellar ligament	Extends leg at knee joint
Gracilis	Inferior pubic ramus and pubic symphysis	Anterior surface of tibia just inferior to medial condyle	Adducts thigh, flexes leg, and medially rotates thigh
Pectineus	Pectineal line of pubis	Medial lip of linea aspera and lesser trochanter	Adducts and flexes the thigh
Adductor longus	Body and inferior pubic ramus	Middle third of linea aspera	Adducts the thigh
Adductor brevis	Body and inferior pubic ramus	Superior linea aspera and posterior surface of proximal femur	Adducts the thigh
Adductor magnus	Body and inferior pubic ramus	Posterior surface of proximal femur, linea aspera, and adductor tubercle of medial condyle of knee	Adducts the thigh
Posterior Thigh Hamstrings
Semitendinosus	Ischial tuberosity	Anterior tibia, medial side	Extend the thigh, flex and medially rotate the leg, and extend the trunk when hip and knee are flexed
Semimembranosus	Ischial tuberosity	Medial condyle of tibia, posterior aspect	Extend the thigh, flex and medially rotate the leg, and extend the trunk when hip and knee are flexed
Biceps femoris	Long head: ischial tuberosity Short head: lateral lip of linea aspera	Lateral surface of fibular head and lateral tibial condyle	Flex the leg at the knee and laterally rotate the leg when hip is flexed

The adductor longus muscle is the most anterior muscle in the adductor group and attaches to the middle third of the linea aspera of the femur.

The shorter adductor brevis muscle lies deep to the pectineus and adductor longus muscles. Its distal attachment is between the lesser trochanter and superior end of the linea aspera of the femur.

The largest and most medial of the adductor group is the adductor magnus muscle. It is situated posterior to the adductor brevis and adductor longus and anterior to the semitendinosus and semimembranosus muscles. It forms a large triangular sheet of muscle in the thigh and is composed of two parts, an adductor part and a hamstring part. The adductor portion of the adductor magnus has an extensive distal attachment on the linea aspera of the femur, whereas the hamstring part attaches to the adductor tubercle on top of the medial condyle of the femur.

Muscles of the Posterior Thigh Compartment

Semitendinosus muscle Semimembranosus muscle Biceps femoris muscle

The muscles of the posterior thigh compartment are shown in Figs. 10.28, 10.40-10.48 and 10.51-10.54 and described in Table 10.2. The semitendinosus, semimembranosus, and biceps femoris muscles are collectively known as the hamstrings. They make up the large mass of muscles that can be palpated on the posterior aspect of the thigh and are involved with extension of the hip, flexion of the knee, and rotation of the flexed knee.

The semitendinosus muscle extends from the ischial tuberosity, courses on the medial aspect of the femur, then continues inferiorly around the medial tibial condyle and attaches to the medial side of the anterior tibial surface (Figs. 10.28, 10.40-10.48, and 10.52-10.54). It acts to extend the thigh, flex and medially rotate the leg, and extend the trunk when the thigh and leg are flexed.

The semimembranosus muscle also originates from the ischial tuberosity but attaches to the posterior part of the medial condyle of the tibia (Figs. 10.28, 10.40-10.48, and 10.52-10.54). Along with the semitendinosus muscle, it acts to extend the thigh, flex and medially rotate the leg, and extend the trunk when the thigh and leg are flexed.

The biceps femoris muscle, as named, has two heads (long and short). The long head extends from the ischial tuberosity, whereas the short head extends from the lateral lip of the linea aspera of the femur. The biceps femoris muscle extends inferiorly over the lateral part of the posterior surface of the knee to insert on the lateral surface of the fibular head and lateral tibial condyle (Figs. 10.48, 10.51, and 10.54). It acts to flex the leg at the knee joint and laterally rotates the leg when the leg is flexed.

KNEE AND LOWER LEG

Bony Anatomy

The bones that contribute to the knee joint and lower leg are the femur, tibia, patella, and fibula (Figs. 10.55 and 10.56). Cartilage covers the articular surfaces of the femur, tibia, and patella and helps provide smooth movement within the knee joint.

Distal Femur. The distal portion of the femur broadens into two articular cartilage-covered projections called the medial and lateral condyles (Figs. 10.9, 10.55-10.61). The lateral femoral condyle is wider in front than in the back, whereas the medial femoral condyle remains more consistent in width. The femoral condyles are anteriorly connected by the smooth patellar surface and posteriorly separated by the intercondylar fossa (Figs. 10.9, 10.57, and 10.58). On the side of each condyle is a raised edge called the medial and lateral epicondyle for the attachment of ligaments and muscles (Figs. 10.59 and 10.60). A small projection located above the medial epicondyle is the adductor tubercle, which serves as an attachment for a portion of the adductor magnus muscle (Figs. 10.9 and 10.11). On the posterior surface of the distal femur is a triangular area called the popliteal surface. The base of the triangle is located at the intercondylar line, which marks the beginning of the intercondylar fossa. The sides of the triangle are formed by the medial and lateral supracondylar lines, which are continuations of the linea aspera (Figs. 10.9 and 10.11).

Tibia. The tibia has a widened proximal end that has two cartilage-covered projections: the medial and lateral condyles (Fig. 10.62). The superior articular surface of both condyles has flattened surfaces called tibial plateaus for articulation with the femoral condyles (Figs. 10.62 and 10.63). The tibial condyles are separated by the intercondylar eminence (tibial spine), which ends in two peaks called the medial and lateral intercondylar tubercles. The intercondylar tubercles and the roughened areas around them serve as attachment sites for the cruciate ligaments and meniscus (Figs. 10.55, 10.56, 10.59, and 10.62). The lateral tibial condyle has a small articular surface called the articular fibular surface, which articulates with the head of the fibula, creating the proximal tibiofibular joint (Figs. 10.62 and 10.64).

The shaft of the tibia is triangular with a sharp anterior edge or crest that contains a bony projection called the tibial tuberosity for the attachment of the patellar ligament (Figs. 10.65 and 10.66). Lateral and superior to the tibial tuberosity is the attachment site for the iliotibial (IT) band, a roughened area of the lateral tibial condyle commonly called Gerdy tubercle (Figs. 10.37 and 10.56). The tibiofibular joint the shaft lies directly beneath the skin and is devoid of muscle, whereas the lateral surface serves as the attachment site for the interosseous membrane. The posterior surface has an obliquely oriented bony ridge called the soleal (popliteal) line, which provides attachment to tendon fibers of the soleus muscle (Fig. 10.62).

The distal tibia has a flattened articular end with a medial extension that forms the medial malleolus, which articulates with the talus (Figs. 10.62, 10.67, and 10.68). The posterior surface of the medial malleolus has a small indentation, the malleolar groove, for the passage of the tibialis posterior and flexor digitorum longus tendons (Fig. 10.62). On the lateral side of the distal tibia is a shallow indentation called the fibular notch, which articulates with the distal fibula, forming the distal tibiofibular articulation (Fig. 10.62).

FIG. 10.66 Midsagittal, CT reformat of tibia.

FIG. 10.67 Axial, Tl-weighted MRI of right ankle with malleoli.

FIG. 10.68 Axial CT of left ankle with malleoli.

Fibula. The fibula is a long, relatively thin bone that has expanded proximal and distal ends. The majority of its length is covered by muscles. The proximal end is the head, which ends in a sharp superior apex. Medially, it has an articular surface for articulation with the lateral condyle of the tibia (Figs. 10.62 and 10.64). The distal end forms the lateral malleolus, which extends farther distally than the medial malleolus of the tibia (Figs. 10.62, 10.67, and 10.68). The medial surface of the lateral malleolus has an articular facet that articulates with the talus. Posterior to the malleolar articular surface is a small cavity called the malleolar fossa, which is where the posterior talofibular ligament is anchored (Figs. 10.62 and 10.67).

Patella and Patellofemoral Joint. The patella is the largest sesamoid bone in the body and is embedded within the quadriceps tendon. It is a flat, triangular bone with a proximal broad base and a distally pointed apex (Fig. 10.69). The patella has an anterior and posterior surface and three borders: superior, medial, and lateral. The base of the patella has roughened areas for the attachment of the rectus femoris and vastus intermedius muscles, whereas the roughened medial and lateral borders receive attachment for the vastus medialis and lateralis muscles. The posterior surface is covered with the thickest articular cartilage found in the body and is centrally divided by a broad vertical ridge into medial and lateral facets. The larger lateral facet articulates with the lateral femoral condyle, and the smaller medial facet articulates with the medial femoral condyle (Fig. 10.70). The patella protects the anterior joint surface of the knee and functions to increase the leverage of the quadriceps extensor system (Figs. 10.55-10.58, 10.65, and 10.66).

Knee Joint

The knee is the largest and one of the most complex joints of the body. The bones that contribute to the knee joint are the femur, tibia, and patella (Figs. 10.55 and 10.56). The knee has three separate articulations: two femorotibial and a patellofemoral articulation within the same synovial membrane. A supporting network of menisci, ligaments, tendons, fasciae, and muscles functions together to meet the demands made on the knee (Figs. 10.71-10.77).

Joint Capsule. The joint capsule of the knee consists of a strong, fibrous membrane that is medially, laterally, and posteriorly reinforced by extracapsular ligaments.

These include the patellar, lateral (fibular) collateral, medial (tibial) collateral, oblique popliteal, and arcuate ligaments. Anteriorly, the capsule blends with the quadriceps tendon and the medial and lateral retinacula (Figs. 10.77-10.80). The synovial membrane of the knee is the largest and most extensive synovial cavity in the body. It lines the inner surface of the fibrous membrane attaching to the articular margins of the femur, tibia, and patella. It is reflected across the anterior surface of the cruciate ligaments, so they are located intracapsular but extrasynovial (Fig. 10.71). Synovial recesses can be found close to the patella and popliteus tendon, and behind each femoral condyle. They may appear like a pouch, extending beyond the articular margin of the femoral condyles.

Menisci. Located between the femoral condyles and tibial plateaus are the paired menisci (Figs. 10.7210.79). These C-shaped menisci, composed of fibrous connective tissue, cushion the articulation between the femoral condyles and tibial plateaus and are commonly divided into anterior and posterior horns. In crosssection they appear wedge-shaped, with a thickened outer margin that flattens medially (Figs. 10.73, 10.74, and 10.76). Their outer margins fuse with the joint capsule, and their anterior and posterior horns attach to the intercondylar eminence of the tibia. The menisci differ in size and shape. The medial meniscus is crescent-shaped, with the posterior horn being wider than the anterior horn. The medial meniscus is attached to the medial collateral ligament, making it far less mobile than the lateral meniscus. The lateral meniscus almost forms a closed ring with anterior and posterior horns of approximately the same width (Fig. 10.73). Two ligaments arise from the posterior horn of the lateral meniscus. The posterior meniscofemoral ligament (ligament of Wrisberg) passes behind the posterior cruciate ligament to attach to the medial femoral condyle (Figs. 10.75 and 10.78). The anterior meniscofemoral ligament (ligament of Humphry) connects the posterior horn to the medial condyle, passing in front of the posterior cruciate ligament (Fig. 10.73). The two menisci are connected anteriorly by the transverse ligament (Figs. 10.72, 10.73, and 10.79).

Ligaments. The ligaments of the knee are divided into external (extracapsular) and internal (intracapsular) ligaments. The external ligaments are arranged around the knee and serve to strengthen and support the joint capsule. The internal ligaments are found within the joint capsule and serve to provide stability to the tibia and femur. Ligaments of the knee are identified in Figs. 10.80-10.98.

External Ligaments. The external ligaments of the knee include the collateral, patellar, patellar retinaculum, oblique popliteal, and arcuate popliteal ligaments (Figs. 10.80 and 10.81). The collateral ligaments provide support for the knee by reinforcing the joint capsule on the medial and lateral sides (Figs. 10.72-10.75, 10.78, and 10.79). The medial (tibial) collateral ligament is a flattened triangular ligament that originates from the medial femoral epicondyle and extends to the medial tibial condyle, continuing to the medial shaft of the tibia. Along its path, it fuses with the medial meniscus. The shorter, lateral (fibular) collateral ligament is more of a rounded cord arising from the lateral femoral epicondyle and attaching to the head of the fibula.

The anterior joint capsule is strengthened by the patellar ligament and patellar retinaculum. The patellar ligament is the strong, thick band representing the continuation of the quadriceps tendon and extends from the patella to the tibial tuberosity (Figs. 10.80A, 10.87, 10.88, 10.94-10.96, and 10.98). The patellar retinaculum is formed mainly by fibrous extensions and fascia of various muscles about the knee (Figs. 10.79 and 10.80A). The medial patellar retinaculum is formed mainly by fibers from the vastus medialis muscle and runs distally to attach to the tibia just anterior to the medial collateral ligament. The lateral patellar retinaculum consists of fibers from the vastus lateralis muscle, as well as the iliotibial band, and attaches distally to the lateral margin of the tibial tuberosity to increase stability of the lateral joint capsule (Figs. 10.80A and 10.93-10.95). The oblique and arcuate popliteal ligaments help reinforce the dorsal surface of the joint capsule. The oblique popliteal ligament is an expansion of the semimembranosus tendon that reinforces the central region of the posterior joint capsule. It extends laterally to attach to the intercondylar line of the femur (Figs. 10.77, 10.79, 10.81, 10.90, 10.95, and 10.98). The inferolateral portion of the posterior capsule is strengthened by the arcuate popliteal ligament as it passes superiorly from the apex of the fibular head to spread out over the posterior capsule with fibers continuing to the posterior intercondylar area and to the posterior surface of the lateral femoral condyle (Figs. 10.76, 10.81, 10.82, and 10.87).

Internal Ligaments. The cruciate (cross-shaped) ligaments are strong bands of fibers that provide anterior and posterior stability to the knee. They are located within the joint capsule but outside the synovial membrane (Figs. 10.71-10.73). The anterior cruciate ligament arises from the medial intercondylar tubercle and extends to attach to the posteromedial surface of the lateral femoral condyle (Figs. 10.83, 10.84, 10.88, 10.94, 10.95, and 10.97). It helps prevent hyperextension and anterior displacement of the tibia. The posterior cruciate ligament is the stronger of the two and extends from the posterior aspect of the intercondylar eminence to the anteromedial surface of the medial femoral condyle. It functions to prevent hyperflexion and posterior displacement of the tibia (Figs. 10.83, 10.84, 10.88, 10.89, 10.95, and 10.97).

FIG. 10.84 Coronal, T1-weighted MRI of right knee with collateral ligaments

FIG. 10.88 Sagittal, T1-weighted MRI of knee with anterior cruciate ligament.

FIG. 10.89 Sagittal, T1-weighted MRI of knee with posterior cruciate ligament.

FIG. 10.90 Sagittal, T1-weighted MRI of knee with medial meniscus.

FIG. 10.91 Sagittal, T1-weighted MRI of knee with semimembranosus tendon

FIG. 10.92 Axial, T1-weighted MRI of right knee with quadriceps tendon

FIG. 10.93 Axial, T1-weighted MRI of right knee with patellofemoral joint.

FIG. 10.94 Axial, T1-weighted MRI of right knee with patellar ligament.

FIG. 10.95 Axial, T1-weighted MRI of right knee with posterior cruciate ligament.

FIG. 10.96 Axial, Tl-weighted MRI of right knee with popliteus muscle.

FIG. 10.97 Axial CT scan of right knee with cruciate and collateral ligaments.

FIG. 10.98 Axial, CT scan of right knee

Tendons

The tendons of the sartorius, gracilis, and semitendino- sus muscles merge to form a conjoined tendon commonly referred to as the pes anserinus. It inserts onto the anteromedial surface of the proximal tibia just superficial to the medial collateral ligament (Figs. 10.64, 10.80A, and 10.91-10.96).

Bursae

There are more than 10 bursae located around the knee joint because of the number of muscles associated with the knee. The major bursae include the suprapatellar, prepatellar, infrapatellar (superficial and deep), gastrocnemius (medial and lateral), semimembranosus, and popliteal bursae (Fig. 10.99). The suprapatellar (quadriceps) bursa is a large extension of the synovial capsule located between the femur and the quadriceps tendon. The prepatellar bursa lies between the anterior surface of the patella and the skin, whereas the superficial infrapatellar bursa lies over the patellar ligament between the skin and the tibial tuberosity. The deep infrapatellar bursa is a small bursa located beneath the patellar ligament and anterior to the tibia just above the tibial tuberosity. Behind each femoral condyle is usually a bursa for the respective head of the gastrocnemius muscle.

The gastrocnemius bursae are located between each muscle head and the joint capsule. The semimembranosus bursa is located between the medial head of the gastrocnemius and semimembranosus tendon, and the small popliteal bursa lies between the lateral tibial condyle and the popliteus tendon. There exists a small bursa adjacent to the tendons of the pes anserinus on the anteromedial surface of the knee called the pes anserinus bursa. Bursae are difficult to see in cross-section images unless they are abnormal.

Muscles of the Lower Leg

With the exception of the popliteus, all muscles arising from the lower leg are attached to bones of the foot. The muscles of the lower leg can be classified according to their location. They are divided into anterior and posterior groups by the tibia, fibula, and interosseous membrane. The two main groups are divided again into subgroups or layers. These muscles are shown in Figs. 10.100-10.116 and described in Table 10.3.

Anterior Group. The anterior muscle group can be subdivided into the extensor group located anteriorly and the peroneus group located laterally.

FIG. 10.99 Knee bursae. Left, Lateral view. Right, Posterior view.

FIG. 10.100 Anterior view of muscles and retinacula of right lower leg.

FIG. 10.101 Lateral view of superficial muscles of the right lower leg.

FIG. 10.102 Axial view of knee with femoral condyles.

FIG. 10.103 Axial view proximal portion of left lower leg.

Anterior Group—Extensor Group

Tibialis anterior Extensor digitorum longus Extensor hallucis longus Peroneus tertius

The tibialis anterior muscle is a long, spindle-shaped muscle located just lateral to the anterior surface of the tibia. It arises from the upper two-thirds of the lateral surface of the tibia and adjoining interosseous membrane, becoming tendinous over the lower third. The tibialis anterior muscle runs distally and medially over the tibia to insert on the plantar surface of the medial cuneiform and first metatarsal. Its actions include dorsiflexion of the foot at the ankle joint, and together with the tibialis posterior muscle, it inverts the foot (Figs. 10.100-10.109).

FIG. 10.104 Axial view middle portion of left lower leg.

FIG. 10.105 Axial view distal portion of left lower leg.

The extensor digitorum longus muscle is located lateral to the tibialis anterior muscle in the anterior aspect of the leg. It arises from the upper two-thirds of the fibula and adjoining interosseous membrane and the lateral condyle of the tibia. The tendon of the extensor digitorum longus muscle passes over the front of the ankle joint and gives rise to four separate tendons at the level of the inferior extensor retinaculum that run to the dorsal surface of the second through fifth digits. The extensor digitorum longus muscle is an extensor of the lateral four digits at the metatarsophalangeal joints (Figs. 10.100, 10.101, 10.103-10.109, and 10.112).

The extensor hallucis longus muscle lies posterior to and between the tibialis anterior and extensor digitorum longus muscles. It arises from the middle half of the anterior fibula and interosseous membrane. The tendon of the extensor hallucis longus muscle passes through the inferior extensor retinaculum to the base of the great toe and inserts into the distal phalanx. The extensor hallucis longus muscle extends the joints of the great toe and provides dorsiflexion of the foot at the ankle joint (Figs. 10.100, 10.101, 10.104-10.109, and 10.112).

FIG. 10.106 Axial, T1-weighted MRI of left tibia

FIG. 10.107 Axial, Tl-weighted MRI of left tibia

FIG. 10.108 Axial, Tl-weighted MRI of left tibia.

FIG. 10.109 Axial, Tl-weighted MRI of left tibia

FIG. 10.110 Coronal, Tl-weighted MRI of left fibula

The peroneus tertius muscle is considered by some to be a distal extension of the extensor digitorum longus muscle. It arises from the anterior surface of the lower fibula, and its tendon inserts on the dorsal aspect of the base of the fifth metatarsal. This muscle functions as a weak evertor and dorsiflexor of the foot at the ankle joint (Figs. 10.100 and 10.101).

Anterior Group-Peroneus Group

Peroneus (fibularis) longus

Peroneus (fibularis) brevis

The two peroneus muscles act as plantar flexors but also stabilize the lateral ankle and longitudinal arch of the foot (Figs. 10.100 and 10.101).

The peroneus (fibularis) longus muscle is located on the lateral side of the leg arising from the tibiofibular joint, the head of the fibula, and the lateral condyle of the tibia. The peroneus longus muscle has a long belly and an even longer tendon. The tendon of the peroneus longus muscle runs in a shallow groove behind the lateral malleolus, passing below the peroneal tubercle of the calcaneus and across the lateral side of the cuboid. It then runs obliquely across the sole of the foot to insert on the base of the first metatarsal and lateral surface of the medial cuneiform (Figs. 10.100, 10.101, 10.103-10.111, and 10.114-10.116).

FIG. 10.111 Coronal, T1-weighted MRI of left lower leg

The peroneus (fibularis) brevis muscle is shorter and smaller than its counterpart. It lies under the peroneus longus, arising from the distal two-thirds of the lateral surface of the fibula. The tendon of the peroneus brevis muscle accompanies the tendon of the peroneus longus muscle in a common synovial sheath behind the lateral malleolus, with the peroneus brevis just anterior to the peroneus longus. At approximately the level of the peroneal tubercle of the calcaneus, the tendons separate into their own synovial sheath, with the peroneus brevis tendon attaching to the base of the fifth metatarsal (Figs. 10.100, 10.101, 10.104, 10.105, 10.107-10.110, and 10.114-10.116).

FIG. 10.112 Sagittal, Tl-weighted MRI of fibula

Posterior Group. The posterior group is functionally considered the flexors, which are responsible for plantar flexion of the foot. This group is subdivided into the superficial layer and the deep layer. These muscles are shown in Figs. 10.101-10.116 and described in Table 10.3.

Posterior Group-Superficial Layer

Gastrocnemius

Soleus

Plantaris

The gastrocnemius muscle is a prominent flexor of the foot and is responsible for giving the calf its shape on the back of the leg. It consists of two heads arising from the medial and lateral femoral condyles. The medial head arises from the medial supracondylar ridge and adductor tubercle on the popliteal surface of the femur. The lateral head arises just behind the lateral epicondyle on the outer surface of the lateral femoral condyle. The two heads form the lower boundaries of the popliteal fossa, and their fibers run distally, where they join the tendon of the soleus muscle to form the Achilles tendon, which inserts on the calcaneal tuberosity (Figs. 10.28 and 10.101-10.116).

The soleus muscle is a broad, flat muscle located beneath the gastrocnemius. It arises from the soleal line on the posterior tibia and the upper third of the fibula. The muscle fibers run distally and merge with the tendon of the gastrocnemius to form the superficial Achilles tendon (Figs. 10.101, 10.103, 10.104, and 10.106-10.116).

The plantaris muscle is a long, thin muscle that arises from the lowest part of the lateral supracondylar ridge, the adjacent popliteal surface, and the joint capsule. The tendon runs inferiorly between the gastrocnemius and soleus muscles, following the medial edge of the Achilles tendon to insert on the Achilles tendon or calcaneus (Figs. 10.102-10.104, 10.106, and 10.114-10.116).

Posterior Group-Deep Layer

Tibialis posterior

Flexor hallucis longus

Flexor digitorum longus

Popliteus

The tibialis posterior muscle is the deepest muscle located on the back of the leg. It arises from the superolateral surface of the posterior tibia just below the soleal line, the interosseous membrane, and the posterior surface of the fibula. The tendon of the tibialis posterior muscle passes through the malleolar groove of the tibia behind the medial malleolus to attach to the tuberosity of the navicular and plantar surface of the medial cuneiform bones (Figs. 10.104, 10.105, 10.107-10.109, 10.112, and 10.116).

FIG. 10.113 Sagittal, Tl-weighted MRI of tibia.

The flexor hallucis longus muscle is a powerful muscle located beneath the gastrocnemius and soleus muscles. It arises from the distal two-thirds of the posterior fibula, the interosseous membrane, and adjacent fascia. The tendon of the flexor hallucis longus muscle runs distally, crossing over the dorsal aspect of the ankle, through the malleolar groove of the tibia to the sole of the foot, where it inserts into the base of the distal phalanx of the first toe (Figs. 10.104, 10.105, 10.10810.110, and 10.113).

The flexor digitorum longus muscle arises from the posterior surface of the body of the tibia immediately below the soleal line. The muscle descends along the tibial side of the leg to become tendinous just above the medial malleolus. The tendon of the flexor digitorum longus muscle passes behind the medial malleolus to the sole of the foot, deep to the flexor hallucis longus, and divides into four individual tendons that insert into the bases of the distal phalanges of the second through fifth digits (Figs. 10.104, 10.105, 10.108, 10.109, 10.113, and 10.116).

The popliteus muscle is a thin triangular muscle that forms the lower floor of the popliteal fossa. It arises just below the lateral femoral epicondyle and extends obliquely to the triangular surface above the soleal (popliteal) line on the posterior tibia (Figs. 10.103, 10.106, 10.107, 10.110, 10.112, 10.113, 10.115, and 10.116).

TABLE 10.3 Muscles of the Lower Leg

Muscle	Proximal Insertion	Distal Insertion	Action
Anterior Group Extensor Group
Tibialis anterior	Lateral tibia, upper two-thirds	Medial cuneiform and first metatarsal,	Dorsiflexion of foot
		plantar surface
Extensor digitorum	Proximal fibula and interosseous	Second to fifth digits of foot	Extensor of lateral four digits at the
longus	membrane		metatarsophalangeal joints
Extensor hallucis	Anterior fibula and interosseous	Distal phalanx of first toe	Extends joints of first toe and dorsi
longus	membrane		flexion of foot
Peroneus tertius Peroneus Group	Distal fibula	Base of fifth metatarsal	Evertor and dorsiflexion of the foot
Peroneus longus	Tibiofibular joint, head of fibula, and	First metatarsal and lateral surface	Plantar flexion and stabilizer of ankle
	lateral condyle of tibia	of medial cuneiform
Peroneus brevis	Lateral surface of distal fibula	Fifth metatarsal	Plantar flexion and stabilizer of ankle
Posterior Group Superficial Layer
Gastrocnemius	Medial head: supracondylar ridge and	Both heads join soleus tendon to form	Flexor of the foot
	adductor tubercle	Achilles tendon to insert
	Lateral head: lateral epicondyle of femur	on calcaneal tuberosity
Soleus	Soleal line of tibia and proximal fibula	Achilles tendon	Flexor of the foot
Plantaris	Lateral supracondylar ridge	Medial side of Achilles tendon or calcaneus	Flexor of the foot
Deep Layer Tibialis posterior	Posterior tibia, interosseous mem-	Navicular and plantar surface of	Flexor of the foot
	brane, and posterior fibula	medial cuneiform
Flexor hallucis	Posterior fibula, interosseous mem-	Distal phalanx of first toe	Flexor of the foot
longus	brane, and adjacent fascia
Flexor digitorum	Posterior body of tibia below soleal line	Distal phalanges of second to fifth digits	Flexor of the foot
longus
Popliteus	Lateral femoral epicondyle	Posterior, proximal tibia	Flexor of the foot

ANKLE AND FOOT

Bony Anatomy

The bony anatomy of the ankle and foot includes the tarsals, metatarsals, and phalanges. The tarsals consist of seven bones: talus, calcaneus, navicular, cuboid, and three cuneiform bones. Five metatarsals and 14 phalanges make up the toes (Figs. 10.117-10.141).

Tarsals. The talus (astragalus) is the second largest tarsal bone. Together with the calcaneus, it is responsible for transmitting the entire weight of the body to the foot. The talus consists of a body, neck, and head. The body is wedge-shaped with an upper articular surface (trochlea) that is wider in front than in back (Figs. 10.117, 10.118, 10.121, and 10.122). The cartilage-covered trochlea provides articulation with the tibia and fibula (Figs. 10.121-10.128). The short, broad neck extends anteriorly to the head, which articulates with the navicular bone (Figs. 10.118 and 10.121).

The largest tarsal bone is the calcaneus, which lies beneath the talus. It has an elongated cuboid shape with a posterior surface comprising the prominence of the heel. On the medial surface of the calcaneus is a shelflike process termed the sustentaculum tali, which provides support for the talus (Figs. 10.119, 10.120, and 10.12410.126). On the plantar surface of the posterior calcaneus is the large calcaneal tuberosity for insertion of ligaments and tendons, the largest being the Achilles tendon (Figs. 10.119-10.123).

FIG. 10.117 Bones of the right foot. (A) Superior view. (B) Medial view. (C) Lateral view. (D) Arches of foot.

FIG. 10.118 (A) 3D CT of foot, superior view. (B) 3D CT of foot, medial view.

The articulation between the talus and calcaneus is termed the subtalar joint, which is composed of three articulations formed by the anterior, middle, and posterior facets (Figs. 10.119 and 10.120). The smallest of the three is the anterior facet, which can be independent of or continuous with the middle facet. The middle facet lies on a ledge of bone projecting off the medial surface of the calcaneus at the sustentaculum tali (Figs. 10.119-10.122 and 10.12410.126). This shelf and the entire middle facet joint provide weight-bearing support to the medial side of the ankle. The posterior facet joint is the largest and provides support for most of the body of the talus (Figs. 10.11910.123). Separating this facet from the middle facet is the tarsal canal. This canal contains blood vessels, fat, and the interosseous talocalcaneal ligament. The tarsal canal widens laterally to form the sinus tarsi, which contains the cervical ligament (Figs. 10.121-10.124, 10.139, and 10.140).

In addition to the talus and calcaneus, the cuboid, navicular, and three cuneiform bones make up the remaining five tarsal bones of the foot (Figs. 10.117, 10.118, and 10.129-10.141). Lateral and anterior to the calcaneus is the cuboid bone, which articulates anteriorly with the bases of the fourth and fifth metatarsal bones. The navicular bone articulates posteriorly with the talus and anteriorly with the cuneiform bones on the medial side of the foot. The three cuneiform bones— medial, intermediate, and lateral—articulate anteriorly with the first three metatarsal bones (Figs. 10.117, 10.118, and 10.129-10.132).

FIG. 10.120 (A) 3D CT of calcaneus, dorsal view. (B) 3D CT of calcaneus, lateral view.

FIG. 10.121 Sagittal section of foot.

FIG. 10.123 Sagittal CT reformat of calcaneus.

FIG. 10.124 Coronal view of subtalar joint.

FIG. 10.125 Coronal, T1-weighted MRI of talocrural joint, left ankle.

Sinus Tarsi Syndrome

Sinus tarsi syndrome is a common syndrome associated with posttraumatic lateral hindfoot pain. The interosseous ligament, when injured due to lateral ankle sprains or an inflammatory process, such as arthritis, can lead to characteristic pain over the lateral side of the ankle. Treatment may include anti-inflammatory medication, a period of immobilization, or injection of local anesthetic.

Metatarsals. The metatarsals are long, slender bones. There are five metatarsals in each foot, with each bone having a distal head, proximal base, and body or shaft in between. The heads articulate with the proximal phalanges of the toes, and the bases articulate with the tar- sals (Figs. 10.117, 10.118, 10.129-10.132, and 10.13610.141). The first metatarsal is associated with two sesamoid bones. They can be found on the medial and lateral surfaces of the metatarsal head and are embedded within the medial and lateral slips of the flexor hallucis brevis tendon.

Phalanges. Each foot has 14 phalanges—3 phalanges for each toe (proximal, middle, distal), except the great toe, which has just 2 (proximal and distal). The phalanges of the toes are shorter and stouter than their counterparts in the fingers and, like the metatarsals, have a base, body, and head (Figs. 10.117, 10.118, 10.131, 10.132, and 10.139).

Joints

The joints of the ankle and foot include the talocrural (ankle); intertarsal, tarsometatarsal, and intermetatarsal; metatarsophalangeal; and interphalangeal joints. The joints of the ankle and foot are shown in Figs. 10.118 and 10.121-10.141.

Talocrural. The talocrural joint (ankle joint) is created by the articulations between the tibia, fibula, and talus. The tibia and fibula rest on the trochlear surface of the talus to form what is commonly termed the mortise joint, which allows for dorsal and plantar flexion. Both the tibia and fibula terminate distally in projections termed malleoli, which prevent medial and lateral displacement of the talus (Figs. 10.12110.128).

Intertarsal. The intertarsal joints are created by the articulations between the tarsal bones and include the subtalar, talocalcaneonavicular, calcaneocuboid, transverse (midtarsal), cuneonavicular, intercuneiform, and the cuneocuboid joints. The subtalar joint consists of the articulation between the talus and calcaneus and was discussed under bony anatomy (Figs. 10.12110.123). The subtalar joint provides the ability to invert and evert the foot. The talocalcaneonavicular and calcaneocuboid joints combine to form the transverse or midtarsal joint. This joint provides an irregular plane across the foot extending from side to side, with the talus and calcaneus located posteriorly and the navicular and cuboid located anteriorly (Figs. 10.122 and 10.123). The transverse tarsal joint plays an important part in putting the spring in your step by acting as a shock absorber during the push-off phase of walking or running. The cuneonavicular, intercuneiform, and cuneocuboid joints contribute to the flexibility of the foot by providing a slight gliding movement between bones.

Tarsometatarsal. The tarsometatarsal joints exist between the bases of all five metatarsals and the anterior four tarsal bones (cuboid, three cuneiforms). The articulations between the tarsals and metatarsals permit only limited gliding movement between the bones (Figs. 10.121 and 10.129-10.132). The intermetatarsal joints are the articulations between the bases of the lateral four metatarsals. The intermetatarsal joints permit a small degree of gliding between metatarsals and contribute to eversion and inversion of the foot (Figs. 10.118A and 10.131).

Metatarsophalangeal. The heads of the metatarsals articulate with the bases of the proximal phalanges at the metatarsophalangeal joints (Figs. 10.118A, 10.121, 10.131, and 10.140). This articulation provides flexion and extension of the toes.

Interphalangeal. The heads of the phalanges articulate with the bases of the more distal phalanges to create the interphalangeal joints. The interphalangeal joints are hinge joints that permit plantar and dorsiflexion of the phalanges (Fig. 10.118).

Arches

The bones of the foot are arranged in transverse and longitudinal arches that provide flexibility and resilience to the foot to support the weight of the body, absorb shocks, and provide spring and lift during activity (Fig. 10.117). The longitudinal arch (plantar arch) has two parts: one on the lateral side of the foot (lateral longitudinal arch) and one on the medial side of the foot (medial longitudinal arch). The bony landmarks for the medial longitudinal arch are the head of the first metatarsal anteriorly and the calcaneal tuberosity posteriorly. On the lateral side the bony landmarks include the head of the fifth metatarsal anteriorly and the calcaneal tuberosity posteriorly. The medial longitudinal arch is more elastic and is associated with greater curvature, whereas the lateral longitudinal arch is flatter and less flexible because it makes contact with the ground. The longitudinal arches provide a firm base for support of the body in the upright position. The transverse arch is formed by the distal row of tarsal bones (cuboid, three cuneiforms) and the bases of the metatarsals that create a domed curve across the foot. The transverse arch is the major weight-bearing arch of the foot and helps distribute body weight over the base of the foot. The integrity of arches is maintained by the tarsal, tarsometatarsal, and intermetatarsal joints and their supporting ligaments (Fig. 10.117D).

FIG. 10.129 Axial CT of left ankle and foot with navicular bone.

FIG. 10.131 Axial CT scan of left ankle and foot with cuboid bone.

FIG. 10.133 Coronal CT reformat of right ankle and foot with talus.

FIG. 10.134 Coronal CT reformat of right ankle and foot with calcaneus

FIG. 10.135 Coronal CT reformat of right ankle and foot with navicular and cuboid bones.

FIG. 10.136 Coronal CT reformat of right foot with lateral cuneiform bone.

FIG. 10.137 Coronal CT reformat of right foot with cuneiform bones.

FIG. 10.138 Coronal CT reformat of right foot with metatarsals.

FIG. 10.139 Sagittal CT reformat of ankle and foot, medial aspect.

FIG. 10.140 Sagittal CT reformat of ankle and foot with talus.

Retinacula, Fascia, and Ligaments

Retinacula. As in the wrist, fascia in various regions of the ankle will thicken to form retinacula. The retinacula form sheaths for stabilizing tendons crossing over the joints of the ankle. They are called the flexor, extensor, and peroneal retinacula, after the tendons they serve (Figs. 10.142, 10.144, and 10.145). The flexor retinaculum extends between the medial malleolus and the medial tubercle of the calcaneus. It forms four tunnels, collectively called the tarsal tunnel, for the passage of the tendons of the tibialis posterior, flexor digitorum longus, and flexor hallucis lon- gus muscles, as well as the posterior tibial vessels and nerve (Fig. 10.142). The extensor retinaculum consists of two portions: an upper portion (superior extensor retinaculum) and lower portion (inferior extensor retinaculum). The superior extensor retinaculum runs horizontally between the tibia and fibula just above the ankle joint. It extends over the tendons of the tibialis anterior and extensor hal- lucis muscles. The inferior extensor retinaculum splits into two bands that extend across the dorsum of the foot, originating from the upper surface of the calcaneus and sinus tarsi. The upper band of the inferior extensor retinaculum attaches to the medial malleolus, whereas the lower band extends to the fascia on the medial side of the foot. The tendons of the extensor digitorum longus and peroneus tertius muscles run deep to the inferior extensor retinaculum (Fig. 10.142). The peroneal retinacula split into two bands, forming the superior and inferior peroneal retinacula. The superior peroneal retinaculum extends from the lateral side of the calcaneus to the posterior border of the lateral malleolus. The inferior peroneal retinaculum extends from the lateral side of the calcaneus to blend with the fibers of the inferior extensor retinaculum. The superior and inferior peroneal retinacula transmit the tendons for the peroneus brevis and peroneus longus muscles (Figs. 10.142, 10.144, and 10.145).

Tarsal Tunnel Syndrome

The tarsal tunnel is formed by the flexor retinaculum as it stretches between the medial malleolus and the calcaneus, creating a tunnel for the passage of the flexor tendons and neurovascular structures as they enter the foot. Tarsal tunnel syndrome is a condition that occurs from direct pressure on the tibial nerve as it courses through the tarsal tunnel along with the flexor tendons. Anything that causes swelling of the tissues running within or surrounding the tarsal tunnel can contribute to the condition, including inflammation, tumors, trauma, varicose veins, and diabetes. Typically, the syndrome presents as pain and/or a tingling sensation in the sole of the foot or at the site of compression. Commonly, the pain and numbness are aggravated with activity, as in long periods of standing or walking. Treatment options include eliminating the source of the compression with anti-inflammatory medication, orthotics, cortisone injections, and, if necessary, surgery.

Fascia. In addition to the retinacula, another area of thickened fascia is located on the plantar surface of the foot. The plantar fascia (aponeurosis) is approximately 80 layers thick, creating some of the thickest fascia within the human body. It begins at the inferior aspect of the calcaneus and spreads anteriorly into five separate slips that create the fibrous flexor sheaths of the toes (Figs. 10.121, 10.124, and 10.125). The plantar fascia is extremely important for maintaining the longitudinal arch of the foot.

Ligaments. Other support structures of the ankle and foot include a complex architecture of multiple ligaments that provide necessary stability. The ligaments of the ankle and foot can be identified in Figs. 10.143-10.161. The main support structures of the ankle include the deltoid ligament, lateral ligaments, spring (plantar) ligament, and interosseous ligament. The deltoid ligament provides medial support and is the strongest ligament in the ankle joint. It arises from the medial malleolus and fans out into deep and superficial bands that include the anterior and posterior tibiotalar, tibiocalcaneal, tibionavicular, and tibiospring ligaments, which insert on the talus, calcaneus, navicular bones, and spring ligament, respectively (Figs. 10.143A, 10.14510.147, 10.156, and 10.157). The lateral border of the ankle joint is strengthened by several ligaments termed the anterior talofibular, calcaneofibular, posterior talofibular, anterior tibiofibular, and posterior tibiofibular ligaments (Figs. 10.143B, C, and D; 10.144-10.148; 10.155; and 10.156). All of these ligaments originate at the fibular malleolus and insert on the adjacent bone structures. The spring (plantar) ligament is a triangular band of fibers that arises from the sustentaculum tali and attaches to the posterior surface of the navicular bone (Figs. 10.121, 10.148, and 10.149). It is an important ligament in maintaining the longitudinal arch of the foot. A strong band of tissue binding the talus to the calcaneus is the interosseous talocalcaneal ligament, which is obliquely oriented in the tarsal canal and helps limit eversion (Figs. 10.121, 10.143C, 10.147, and 10.156). The cervical ligament located in the sinus tarsi helps limit inversion of the ankle (Figs. 10.121, 10.122, 10.124, 10.143C, 10.147, 10.152, 10.153, 10.156, and 10.157).

FIG. 10.143, cont'd (C) Ligaments of right foot, lateral view. (D) Ligaments of right foot, anterior view.

FIG. 10.144 Axial, T1-weighted MRI of right ankle.

FIG. 10.146 Axial, T1-weighted MRI of right ankle.

FIG. 10.148 Axial, T1-weighted MRI of right ankle.

FIG. 10.150 Axial, Tl-weighted MRI of right ankle.

FIG. 10.151 Sagittal, T1-weighted MRI of ankle ligaments and tendons.

FIG. 10.152 Sagittal, T1-weighted MRI of ankle ligaments and tendons.

FIG. 10.153 Sagittal, T1-weighted MRI of ankle ligaments and tendons.

FIG. 10.154 Sagittal, T1-weighted MRI of ankle ligaments and tendons.

FIG. 10.156 Coronal, T1-weighted MRI of left ankle.

FIG. 10.158 Coronal, Tl-weighted MRI of left foot.

FIG. 10.159 Coronal, Tl-weighted MRI of left foot.

FIG. 10.160 Coronal, T1-weighted MRI of left foot.

FIG. 10.161 Coronal, T1-weighted MRI of left foot.

Ligaments of the Toes. The metatarsophalangeal joint is strengthened medially and laterally by strong collateral ligaments, dorsally from fibers of the extensor tendons, and underneath by the plantar plate. The plantar plate (ligament) is a firm fibrocartilaginous structure that attaches to the base of the proximal phalanges, the collateral ligaments, and the deep transverse metatarsal ligaments. The plantar plate functions as the insertion point for the tendons, ligaments, and other stabilizing structures of the metatarsophalangeal joint. The collateral ligaments extend from the heads of the metatarsals, fanning out to attach to the bases of the proximal phalanges. The heads of the second through fifth metatarsals are interconnected by the deep transverse metatarsal ligament. In a similar manner as the metatarsophalangeal joints, the interphalangeal joints are strengthened medially and laterally by collateral ligaments and on the plantar surface by the plantar plate (Figs. 10.143D and 10.162-10.164).

FIG. 10.164 Axial, T1-weighted MRI of foot.

Tendons

The musculotendinous structures of the ankle can be divided into posterior, anterior, medial, and lateral groups.

Posterior Group. The posterior group is composed of the single Achilles tendon, the largest and most powerful tendon of the body. The Achilles tendon arises from the gastrocnemius and soleus muscles and attaches to the calcaneal tuberosity on the posterior aspect of the calcaneus (Figs. 10.115, 10.121, 10.144-10.148, 10.152, and 10.153).

Anterior Group. The anterior group is made up of the tibialis anterior, extensor hallucis longus, and extensor digitorum longus tendons, which are listed medial to lateral and act to extend and dorsiflex the foot. The tibialis anterior muscle becomes tendinous at the distal tibia and attaches to the plantar and medial aspects of the first cuneiform and metatarsal bones. The tendon of the extensor hallucis longus muscle originates from the anterior fibula and inserts on the great toe. The most lateral of this group is the extensor digitorum longus tendon, which originates at the level of the lateral malleolus and inserts on the second through the fifth digits (Figs. 10.142, 10.144-10.148, 10.151, 10.153, 10.156 10.163, and 10.165).

Medial Group. The medial group is composed of the posterior tibialis tendons, flexor digitorum longus, and flexor hallucis longus tendons, which act as a group to invert and plantar flex the foot. The tibialis posterior tendon fans out in multiple strands that insert on the plantar aspect of the sustentaculum tali, navicular, medial cuneiform, and second through fourth metatarsals. Coursing posterior and lateral to the tibialis posterior tendon is the flexor digitorum longus tendon, which inserts on the second through fourth phalanges. The tendon of the flexor hallucis longus muscle curves under the sustentaculum tali and then courses along the plantar surface of the foot to insert on the great toe (Figs. 10.142, 10.144-10.148, 10.151, 10.152, 10.155 10.163, and 10.165).

Lateral Group. The two peroneus tendons, the peroneus longus and peroneus brevis, make up the lateral group and act to evert and weakly plantar flex the foot, as well as stabilize the ankle joint laterally. These two tendons share a common tendinous sheath behind the lateral malleolus. Below the malleolus, they diverge into separate tendon sheaths, with the peroneus brevis tendon inserting on the base of the fifth metatarsal and the peroneus longus tendon curving beneath the calcaneus to insert on the base of the first metatarsal and medial cuneiform bones (Figs. 10.142, 10.144-10.149, 10.15410.161, and 10.165).

Muscles of the Foot

The muscles of the foot are divided into the muscles of the dorsum and muscles of the sole of the foot. These muscles are shown in Figs. 10.147-10.167 and described in Table 10.4.

Muscles of the Dorsum of the Foot. The dorsal muscles include the extensor digitorum brevis and the extensor hallucis brevis muscles, which form a fleshy mass on the lateral part of the dorsal foot. The extensor digito- rum brevis muscle arises from the anterior, upper surface of the calcaneus and passes obliquely across the dorsum to end in three tendons that insert onto the dorsal aponeurosis of the second through fourth digits, deep to the tendons of the extensor digitorum longus muscle (Fig. 10.165). The extensor digitorum brevis muscle is responsible for dorsiflexion of the second through fourth digits. The extensor hallucis brevis muscle splits off of the extensor digitorum brevis muscle to insert on the dorsal aponeurosis of the first toe. It acts to dorsiflex the first digit.

Muscles of the Sole of the Foot. The muscles of the sole of the foot can be described by four muscular layers.

First Layer. The muscles located within the first layer are the most superficial and include, from medial to lateral, the abductor hallucis, flexor digitorum brevis, and abductor digiti minimi muscles (Figs. 10.121, 10.124, 10.164, 10.166A, and 10.167). The abductor hallucis muscle arises from the medial process of the calcaneal tuberosity and lies along the medial border of the foot to insert on the medial base of the proximal phalanx of the first digit. The flexor digitorum brevis muscle also arises from the medial process of the calcaneal tuberosity, as well as the plantar aponeurosis, to insert on both sides of the middle phalanges of the lateral four digits. The abductor digiti minimi muscle arises from the lateral process of the calcaneal tuberosity, the tuberosity of the fifth metatarsal, and the plantar aponeurosis to form the lateral margin of the foot. It inserts on the lateral side of the base of the proximal phalanx of the fifth digit.

Second Layer. This layer is located deep to the first layer and includes the quadratus plantae and lumbrical muscles (Figs. 10.121, 10.124, 10.155-10.161, 10.163, and 10.166B). The quadratus plantae muscle is a small, flat muscle that arises as two small slips from the medial and lateral margins of the plantar surface of the calcaneus and joins with the lateral side of the flexor digitorum longus tendon that continues to the distal phalanges of the lateral four digits. The four small lumbrical muscles arise from the medial surfaces of the individual tendons of the flexor digitorum longus. They insert on the medial margin of the proximal phalanges of the second through fifth digits and extend into the extensor aponeurosis.

FIG. 10.166 Muscle layers of right foot. (A) First layer. (B) Second layer. (C) Third layer. (D) Fourth layer.

Third Layer. The third layer consists of three muscles: the flexor hallucis brevis, adductor hallucis, and flexor digiti minimi brevis muscles (Figs. 10.121, 10.161, 10.164, 10.166C, and 10.167). The flexor hallucis brevis muscle arises from the medial cuneiform bone and the tibialis posterior tendon. It splits into two tendons that cover the plantar surface of the first metatarsal, extending to both sides of the base of the proximal phalanx of the first digit. The two sesamoid bones associated with the first metatarsal are embedded within the flexor hallucis brevis tendons. The adductor hallucis muscle has two heads; the oblique head arises from the cuboid and lateral cuneiform bones and the bases of the second and third metatarsals, and the transverse head arises from the deep transverse metatarsal ligament and the metatarsal joint capsule. Both heads insert on the lateral side of the base of the proximal phalanx of the first digit. The slender flexor digiti minimi brevis muscle arises from the base of the fifth metatarsal and inserts on the base of the proximal phalanx of the fifth digit.

Fourth Layer. This layer consists of the interosseous muscles (Figs. 10.121, 10.164, 10.166D, and 10.167). Three plantar and four dorsal interossei muscles are located between the metatarsal bones. The plantar interosseous muscles arise from the bases and medial surfaces of the third through fifth metatarsals and insert on the medial sides of the bases of the proximal phalanges of the third through fifth digits. The dorsal interosseous muscles are larger than their plantar counterparts, arising from adjacent surfaces of the metatarsal bones and extending to attach to the sides of the proximal phalanx and capsules of the metatarsal phalangeal joints of the second through fourth digits.

FIG. 10.167 Coronal view of muscles of foot at metatarsals.

TABLE 10.4 Muscles of the Foot

Muscle	Proximal Insertion	Distal Insertion	Action
Dorsum of Foot
Extensor digitorum brevis	Anterosuperior surface of calcaneus	Dorsal aponeurosis of second to fourth digits	Dorsiflexion of metatarsophalangeal joint of great toe and second to fourth toes
Extensor hallucis brevis Sole of Foot First Layer	Extensor digitorum brevis muscle	Base of proximal phalanx of great toe	Dorsiflexes the first digit
Abductor hallucis	Medial process of calcaneal tuberosity	Medial base of proximal phalanx of first digit	Abducts first toe from second; flexes metatarsophalangeal joint
Flexor digitorum brevis	Medial process of calcaneal tuberosity and plantar aponeurosis	Both sides of middle phalanges of second to fifth digits	Flexes metatarsophalangeal and proximal interphalangeal joints of second to fifth digits
Abductor digiti minimi	Lateral process of calcaneal tuberosity, fifth metatarsal	Proximal phalanx of fifth digit	Abducts fifth toe and flexes metatarsophalangeal joint
Second Layer Quadratus plantae	Calcaneus, plantar surface	Distal phalanges of second to fifth digits	Assists the flexor digitorum longus in flexion of toes
Lumbrical	Flexor digitorum longus tendons	Medial margins of proximal phalanges of second to fifth digits	Flexes metatarsophalangeal joints; extends proximal and distal interphalangeal joints of second to fifth digits
Third Layer
Flexor hallucis brevis	Medial cuneiform and tibialis posterior tendon	Proximal phalanx of first digit, both sesamoid bones	Flexes metatarsophalangeal joint of first toe
Adductor hallucis	Oblique head: cuboid, lateral cuneiform, and second and third metatarsals Transverse head: deep transverse metatarsal ligament and metatarsal joint capsule	Lateral side of base of the proximal phalanx of first digit	Adducts first toe toward second and flexes first toe
Flexor digiti minimi	Base of fifth metatarsal	Base of the proximal phalanx of fifth digit	Flexes metatarsophalangeal joint of fifth digit
Fourth Layer Plantar interosseous	Bases and medial surfaces of third to fifth metatarsals	Medial sides of the bases of the proximal phalanges of third to fifth digits	Adducts third to fifth toes, flexes metatarsophalangeal, and extends interphalangeal joints of third to fifth digits
Dorsal interosseous	Metatarsals	Proximal phalanx and capsules of metatarsal phalangeal joints of second to fourth digits	Abducts second to fourth digits away from midline, flexes metatarsophalangeal joints, and extends interphalangeal joints of second to fourth digits

NEUROVASCULATURE

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Femoral. Traveling vertically along the anteromedial aspect of the hip is the femoral artery. The femoral artery, an extension of the external iliac artery, enters the anterior compartment of the thigh beneath the inguinal ligament, where it is relatively superficial and easily palpable (Figs. 10.168-10.173). It descends the thigh and continues through the opening in the adductor magnus muscle as the popliteal artery in the knee. The femoral artery and its main branches, superficial and deep, supply all the compartments of the thigh, as well as the skin of the anterior abdominal wall, inguinal region, and external genitalia. The superficial branches of the femoral artery accompany the veins of the hip and include the inferior epigastric, superficial circumflex iliac, and the external pudendal arteries (Fig. 10.174). The largest deep branch of the femoral artery is the profunda femoris artery, which arises from the posterolateral aspect of the femoral artery about 4 cm below the inguinal ligament and runs distally behind the femoral artery (Figs. 10.168, 10.169, and 10.171-10.174). It passes between the vastus medialis muscle and the muscles of the adductor group. The profundus femoris gives off two large branches: the medial and lateral circumflex femoral arteries (Figs. 10.168, 10.169, 10.173, and 10.174). The branches curve around the proximal femur and hip joint to supply the muscles of the adductor group and parts of the gluteal musculature, as well as the extensors and flexors at the thigh. The terminal branches of the profunda femoris artery are the perforating arteries (3-5) near the linea aspera that pass through the adductor muscles (Figs. 10.168, 10.173, and 10.174).

Popliteal. The popliteal artery is the continuation of the femoral artery. It runs deep near the bones of the knee joint during its course through the popliteal fossa. It passes distally over the popliteus muscle and divides into the anterior and posterior tibial arteries (Figs. 10.17310.176). The popliteal artery accompanies the popliteal vein and tibial nerve. The tibial nerve is the most superficial structure in the popliteal fossa (Figs. 10.177 and 10.178). The popliteal artery supplies the surrounding muscles and forms a substantial plexus of articular branches anastomosing around the knee joint. The popliteal artery gives off branches to portions of the thigh muscles near the knee joint, dispatches the sural arteries distally to the gastrocnemius muscle, and supplies the knee joint with the anastomosing genicular arteries (lateral superior, lateral inferior, medial superior, medial inferior, and descending) (Fig. 10.174).

Anterior Tibial. The anterior tibial artery courses anteriorly at the level of the fibular head into the anterior compartment of the lower leg. It runs distally as far as the anterior side of the ankle, where it becomes the dorsalis pedis artery. The branches of the anterior tibial artery are the posterior tibial recurrent, the anterior tibial recurrent, the medial anterior malleolar, the lateral anterior malleolar, and numerous muscular branches (Figs. 10.174-10.176).

Posterior Tibial. The posterior tibial artery is usually larger than the anterior tibial artery. As it passes distally in the posterior compartment, it courses toward the medial side of the leg. The posterior tibial artery terminates by dividing into the medial and lateral plantar arteries in the foot. The peroneal (fibular) artery arises from the posterior tibial artery approximately 2 cm below the distal border of the popliteus muscle. It descends posteriorly along the medial aspect of the fibula and terminates on the lateral surface of the calcaneal tuberosity. Branches of the peroneal artery include the fibular, communicating, perforating, posterior lateral malleolar, and lateral calcaneal, as well as numerous muscular and cutaneous branches (Figs. 10.174-10.176).

FIG. 10.170 Axial, contrast-enhanced CT of right hip with femoral artery.

FIG. 10.171 Axial, contrast-enhanced CT of right hip with femoral artery and vein.

FIG. 10.172 Axial, contrast-enhanced CT of right hip with profundus femoris artery.

FIG. 10.174 Arteries of lower extremity. Left, Anterior view. Right, Posterior view.

Veins

There are two groups of veins in the lower extremity: superficial and deep. The superficial veins arise in the foot from the dorsal venous arch and merge chiefly into two main trunks, the great saphenous and the small saphenous veins. The great saphenous vein ascends the medial aspect of the leg and thigh to drain into the femoral vein near the hip joint. From the lateral side of the foot, the small saphenous vein passes posterolaterally to join the popliteal vein (Fig. 10.179).

Between these two superficial veins and the system of the deep veins are numerous deep anastomoses. The deep veins accompany their corresponding arteries and include the anterior and posterior tibial veins, which unite to form the popliteal vein on the posterior aspect of the knee. The popliteal vein becomes the superficial femoral vein in the distal thigh. As the superficial femoral vein ascends the thigh, it joins with the profunda femoris vein, which drains the thigh muscles, and becomes the femoral vein, which courses medial to the femoral artery. The femoral vein joins with the greater saphenous vein in the proximal thigh and then continues deep to the inguinal ligament as the external iliac vein (Figs. 10.40-10.48, 10.102-10.109, 10.170-10.172, 10.179, and 10.180).

FIG. 10.177 Axial, T1-weighted MRI of right knee with vessels.

FIG. 10.178 Axial, T1-weighted MRI of right knee with vessels.

FIG. 10.179 Veins of the leg. Left, Anterior view. Right, Posterior view.

FIG. 10.180 Anterior view of femoral artery and vein and great saphenous vein.

Nerves

The nerves of the lower extremity are derived from the lumbar and sacral plexuses (see Chapter 4). The femoral nerve enters the thigh beneath the inguinal ligament and divides into several superficial and deep branches to supply the anterior compartment of the thigh. The femoral nerve terminates as the saphenous nerve to innervate the skin on the medial side of the leg and foot. The obturator nerve courses through the obturator canal and immediately divides into anterior and posterior divisions to supply the medial compartment of the thigh. The sciatic nerve is the largest peripheral nerve in the body. Its branches supply the posterior compartment of the thigh and all compartments of the distal leg and foot. The sciatic nerve runs deep to the gluteus maximus muscle and descends in the midline of the thigh, where it usually divides into two terminal branches: the tibial (medial popliteal) and common peroneal (lateral popliteal) nerves (Figs. 10.40-10.45, 10.102-10.109, 10.170, 10.177, 10.178, and 10.181).

FIG. 10.181 Anterior and posterior views of right leg and foot with nerves.

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