# Lecture 11: Lower Limb - Leg and Knee ## Unit 1.3: Human Gross Anatomy I - Musculoskeletal System --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Describe the bones of the leg (tibia and fibula) and their features 2. Explain the structure, ligaments, and menisci of the knee joint 3. Identify the muscles of the anterior, lateral, and posterior compartments of the leg 4. Describe the popliteal fossa and its contents 5. Explain common knee injuries and their anatomical basis 6. Correlate compartment anatomy with neurovascular distribution --- ## Bones of the Leg The leg (anatomically defined as the region between the knee and ankle) contains two bones: the tibia and fibula. These bones have distinct functions and form important joints with each other and with adjacent bones. ### The Tibia The tibia is the large, medial bone of the leg and bears the entire weight transmitted from the femur. It is the second-longest bone in the body after the femur. The proximal tibia expands to form the medial and lateral condyles, which articulate with the corresponding femoral condyles. The medial condyle is larger. The tibial plateaus are the relatively flat superior surfaces of the condyles that receive the femoral condyles. The intercondylar eminence is a central bony projection between the plateaus that provides attachment for the cruciate ligaments and fits into the intercondylar notch of the femur. The tibial tuberosity is a prominent projection on the anterior surface, just below the knee joint. It is the site of insertion for the patellar ligament and is easily palpable. In adolescents, traction on the tuberosity's apophysis can cause Osgood-Schlatter disease. The shaft of the tibia has a triangular cross-section. The medial surface is subcutaneous throughout its length—this is the "shin," which is easily palpable and vulnerable to trauma. The lateral surface provides muscle attachments. The posterior surface features the soleal line, an oblique ridge running inferolaterally that provides origin for the soleus muscle. The sharp interosseous border along the lateral edge provides attachment for the interosseous membrane. Distally, the tibia expands slightly and projects medially as the medial malleolus, the bony prominence on the inner ankle. The lateral surface has a triangular fibular notch for articulation with the fibula at the distal tibiofibular joint. ### The Fibula The fibula is the slender lateral bone of the leg. It does not participate in the knee joint and bears no weight at the knee, but it is crucial for muscle attachment and forms an essential part of the ankle joint. The head of the fibula is the expanded proximal end that articulates with the lateral tibial condyle at the proximal tibiofibular joint. The common peroneal nerve wraps around the neck of the fibula just below the head—a location of significant clinical importance because the nerve is vulnerable to injury here. The shaft of the fibula is thin and provides extensive attachment for leg muscles. The interosseous border runs along its medial surface for attachment of the interosseous membrane. The lateral malleolus is the distal projection of the fibula, forming the bony prominence on the outer ankle. It extends approximately 1 cm more distally than the medial malleolus, creating an asymmetric mortise that adds stability to the ankle joint. ### Tibiofibular Joints The tibia and fibula articulate at three locations. The proximal tibiofibular joint is a plane synovial joint between the head of the fibula and the posteroinferior aspect of the lateral tibial condyle. The middle tibiofibular "joint" is actually a syndesmosis—the interosseous membrane, a fibrous sheet connecting the interosseous borders of both bones along most of their length. The fibers run predominantly inferolaterally (from tibia to fibula), which helps transmit forces from the fibula to the tibia. The distal tibiofibular joint is another syndesmosis, connected by anterior and posterior tibiofibular ligaments and the interosseous ligament. This strong connection is essential for maintaining the integrity of the ankle mortise; injury to this syndesmosis (a "high ankle sprain") can be more disabling than injury to the ankle ligaments themselves. Panel A: Tibia proximal expansion showing medial condyle (larger), lateral condyle with tibial plateaus, intercondylar eminence, and tibial tuberosity with patellar ligament attachment in anterior view. Panel B: Tibial shaft in cross-section showing triangular shape with medial surface (subcutaneous shin), lateral surface, posterior surface with soleal line, and interosseous border; distal tibia with medial malleolus and fibular notch. Panel C: Fibula with head (articulating with tibia), neck (common peroneal nerve vulnerability labeled), thin shaft with interosseous border, and lateral malleolus extending further distally than medial. Panel D: Interosseous membrane as semi-transparent sheet between bones with proximal and distal tibiofibular joints highlighted and scale bar included. --- ## The Patella The patella is the largest sesamoid bone in the body, developing within the quadriceps tendon. It is triangular in shape, with the apex pointing inferiorly. ### Structure and Surfaces The anterior surface is rough and subcutaneous, covered by the prepatellar bursa. The posterior surface is smooth and articular, divided into larger lateral and smaller medial facets that articulate with the patellar surface of the femur (trochlear groove). A vertical ridge separates the facets. The base of the patella is the broad superior edge where the quadriceps tendon attaches. The apex is the inferior point from which the patellar ligament descends to the tibial tuberosity. ### Function The patella serves multiple functions. It protects the anterior aspect of the knee joint from direct trauma. More importantly, it increases the mechanical advantage of the quadriceps by increasing the moment arm of the extensor mechanism—the patella holds the quadriceps tendon away from the axis of rotation, making extension more efficient. This leverage function is particularly important in the last degrees of extension. Panel A: Anterior view of patella showing triangular shape with base superiorly (quadriceps tendon attachment) and apex inferiorly (patellar ligament continuation), roughened subcutaneous surface. Panel B: Posterior view showing smooth articular surface divided by vertical ridge into larger lateral facet and smaller medial facet with articular cartilage in white. Panel C: Lateral view showing patella within quadriceps tendon and its relationship to femoral trochlea in cross-section with prepatellar bursa position indicated. Panel D: Mechanical advantage diagram showing increased moment arm for knee extension with lever arm arrows demonstrating how the patella improves quadriceps efficiency. --- ## The Knee Joint The knee is the largest and most complex joint in the body. It is classified as a modified hinge joint, though it permits not only flexion and extension but also limited rotation when flexed. ### Joint Components The knee actually consists of two articulations sharing a common joint cavity. The tibiofemoral joint is the main weight-bearing articulation between the femoral condyles and the tibial plateaus. The patellofemoral joint is the articulation between the patella and the trochlear groove of the femur. ### Articular Surfaces The femoral condyles are large, convex, curved surfaces that articulate with the relatively flat tibial plateaus. This incongruence would make for an unstable joint, but the menisci (fibrocartilage wedges) intervene to improve contact and distribute load. The patellar surface of the femur (trochlea) is a smooth groove on the anterior distal femur through which the patella glides during flexion and extension. ### Joint Capsule The fibrous capsule of the knee is thin and relatively weak—the joint relies heavily on ligaments and muscles for stability. The capsule is deficient anteriorly, where it is replaced by the patella and patellar ligament. It attaches around the margins of the femoral condyles above and the tibial margins below. The synovial membrane lines the inner surface of the fibrous capsule and extends into several recesses. The suprapatellar bursa is a large extension of the synovial cavity that extends approximately 6 cm above the patella, deep to the quadriceps. This communication means that knee joint effusions often cause swelling above the patella. Panel A: Sagittal section showing femoral condyle articulating with tibial plateau, meniscus as fibrocartilage wedge improving congruence, and patella in patellar groove anteriorly. Panel B: Joint capsule with synovial membrane (red lining) and suprapatellar bursa extending approximately 6 cm above patella under quadriceps, infrapatellar fat pad posterior to patellar ligament. Panel C: Anterior view with patella centrally in trochlea, medial and lateral femoral condyles, tibial plateaus with intercondylar eminence, and joint space outlined. Panel D: Cross-section at joint level showing two articulations (tibiofemoral and patellofemoral) with component structures labeled. --- ## The Menisci The menisci are fibrocartilaginous structures that compensate for the incongruence between the curved femoral condyles and the flat tibial plateaus. ### Structure and Attachments Each meniscus is wedge-shaped in cross-section, with a thick peripheral border and a thin free inner edge. The peripheral margin attaches to the joint capsule via the coronary ligaments. The anterior and posterior horns of each meniscus attach to the tibial plateau at specific points. ### Medial Meniscus The medial meniscus is C-shaped and larger than the lateral. Its anterior horn attaches in front of the ACL attachment, and its posterior horn attaches in front of the PCL. The peripheral margin is firmly attached to the deep fibers of the medial collateral ligament, which restricts its mobility. This relative immobility makes the medial meniscus more prone to injury when subjected to rotational forces. ### Lateral Meniscus The lateral meniscus is more circular (almost O-shaped) and smaller. It is not attached to the lateral collateral ligament—the popliteus tendon passes between the lateral meniscus and the capsule. This greater mobility makes the lateral meniscus less susceptible to injury. The transverse ligament connects the anterior horns of both menisci. ### Function The menisci perform several essential functions. They deepen the tibial plateaus, improving the conformity with the femoral condyles and enhancing stability. They distribute load over a larger area, reducing the stress on the articular cartilage. They absorb shock during weight-bearing activities. They also contribute to joint lubrication and proprioception. ### Blood Supply The blood supply to the menisci has critical implications for healing after injury. The peripheral one-third (the "red zone") receives blood supply from the perimeniscal capillary plexus and can heal after repair. The central two-thirds (the "white zone") is avascular, receiving nutrition only by diffusion from the synovial fluid, and heals poorly. Tears in the white zone often require partial meniscectomy rather than repair. Panel A: Superior view of tibial plateau showing medial meniscus (C-shaped, larger) with anterior and posterior horns and attachment to MCL indicating restricted mobility. Panel B: Lateral meniscus (O-shaped, smaller) with popliteus tendon passing between it and capsule, transverse ligament connecting anterior horns, and ACL/PCL attachment sites marked. Panel C: Cross-section of meniscus showing wedge shape with thick vascular peripheral "red zone" versus thin avascular inner "white zone" and coronary ligament attachment to capsule. Panel D: Comparison inset of C-shape versus O-shape and relative sizes of menisci with implications for injury susceptibility. --- ## Ligaments of the Knee The knee depends heavily on ligamentous support for stability. The ligaments are organized into extracapsular and intracapsular groups. ### Extracapsular Ligaments The medial collateral ligament (MCL) is a broad, flat band on the medial side of the knee. It extends from the medial femoral epicondyle to the medial surface of the tibia, approximately 5 cm below the joint line. The deep fibers of the MCL attach firmly to the medial meniscus—this connection explains why medial meniscus tears often accompany MCL injuries. The MCL resists valgus stress (force pushing the knee inward) and is the most commonly injured knee ligament, typically from a blow to the lateral side of the knee. The lateral collateral ligament (LCL) is cord-like rather than flat, extending from the lateral femoral epicondyle to the head of the fibula. Unlike the MCL, the LCL is not attached to the lateral meniscus—the popliteus tendon and joint capsule intervene. The biceps femoris tendon inserts nearby on the fibular head. The LCL resists varus stress (force pushing the knee outward). It is injured less frequently than the MCL because the opposite leg protects against varus forces. The patellar ligament (sometimes called the patellar tendon) is the continuation of the quadriceps tendon from the patella to the tibial tuberosity. The patella is a sesamoid bone embedded within this tendinous structure. The patellar ligament transmits the force of quadriceps contraction to extend the knee. The oblique popliteal ligament is an expansion of the semimembranosus tendon that crosses the posterior aspect of the knee, reinforcing the posterior capsule. The arcuate popliteal ligament is Y-shaped and reinforces the posterolateral capsule. ### Intracapsular Ligaments The cruciate ligaments are located within the joint capsule but outside the synovial cavity (they are covered by a synovial fold). They cross each other like an "X" (hence "cruciate," from the Latin for cross). The anterior cruciate ligament (ACL) attaches from the anterior intercondylar area of the tibia to the posteromedial aspect of the lateral femoral condyle. Its primary function is to prevent anterior translation of the tibia relative to the femur. It also provides rotational stability and resists hyperextension. The ACL is tested clinically with the anterior drawer test (with the knee flexed 90 degrees) and the Lachman test (with the knee flexed 20-30 degrees, which is more sensitive). A positive test shows excessive anterior tibial translation. The posterior cruciate ligament (PCL) attaches from the posterior intercondylar area of the tibia to the anterolateral aspect of the medial femoral condyle. It prevents posterior translation of the tibia. The PCL is stronger and less commonly injured than the ACL. The PCL is tested with the posterior drawer test. A classic mechanism of PCL injury is the "dashboard injury," where the proximal tibia strikes the dashboard during a motor vehicle collision, forcing it posteriorly. Panel A: Anterior view showing MCL (broad flat band, medial epicondyle to tibia with medial meniscus attachment), LCL (cord-like, lateral epicondyle to fibular head, no meniscus attachment), and patellar ligament from patella to tibial tuberosity. Panel B: Posterior view showing oblique popliteal ligament (semimembranosus expansion) and arcuate popliteal ligament (Y-shaped posterolateral) reinforcing the posterior capsule. Panel C: Sagittal section showing cruciate ligaments crossing with ACL from anterior tibial area to lateral femoral condyle and PCL from posterior tibial area to medial femoral condyle, stress resistance arrows indicated. Panel D: Clinical test positions for anterior drawer, Lachman (most sensitive for ACL), and posterior drawer tests with positive finding indicators. --- ## Bursae of the Knee Several bursae facilitate movement around the knee and reduce friction between structures. The suprapatellar bursa lies between the quadriceps tendon and the femur, extending approximately 6 cm above the patella. It communicates with the knee joint cavity, so joint effusions typically extend into this bursa, causing suprapatellar swelling. The prepatellar bursa lies between the patella and the overlying skin. It is not connected to the joint cavity. Chronic irritation from kneeling produces prepatellar bursitis, historically called "housemaid's knee." The superficial infrapatellar bursa lies between the tibial tuberosity and the skin. Irritation from kneeling in an upright position (as during prayer) causes inflammation here—hence "clergyman's knee." The deep infrapatellar bursa lies between the patellar ligament and the upper tibia. It may communicate with the joint cavity. The pes anserine bursa lies between the pes anserinus (the combined insertion of sartorius, gracilis, and semitendinosus) and the tibia, just below the medial joint line. Pes anserine bursitis causes medial knee pain that may be mistaken for medial compartment pathology. The semimembranosus bursa lies between the semimembranosus tendon and the medial head of the gastrocnemius, posterior to the medial femoral condyle. It typically communicates with the knee joint through a valve-like opening. Distension of this bursa produces a Baker's cyst (popliteal cyst), which presents as a swelling in the popliteal fossa. Baker's cysts are usually secondary to knee pathology that causes effusion; the increased intra-articular pressure forces fluid into the bursa. Panel A: Sagittal section showing suprapatellar bursa extending superiorly under quadriceps (communicates with joint cavity) and prepatellar bursa superficial to patella (housemaid's knee when inflamed, no joint communication). Panel B: Superficial infrapatellar bursa over tibial tuberosity (clergyman's knee) and deep infrapatellar bursa between patellar ligament and tibia. Panel C: Posterior view showing semimembranosus/popliteal bursa between semimembranosus and medial gastrocnemius head (Baker's cyst when distended, communicates with joint). Panel D: Pes anserine bursa location on medial view below medial joint line under pes anserinus tendons, each bursa color-coded with clinical significance noted. --- ## Movements at the Knee The knee functions primarily as a hinge, permitting flexion and extension, but it also allows limited rotation when flexed. Flexion brings the posterior leg toward the posterior thigh, with a range of approximately 135 degrees (limited by contact between calf and thigh). The primary flexors are the hamstrings, assisted by gastrocnemius, gracilis, sartorius, and popliteus. Extension returns the leg to the straight position. The quadriceps is the sole extensor, with the vastus medialis particularly important for the terminal degrees of extension. When the knee is flexed, the collateral ligaments become slack, permitting limited rotation. Internal rotation (medial rotation) is performed by the popliteus, semimembranosus, and semitendinosus. External rotation (lateral rotation) is performed by the biceps femoris. ### The Screw-Home Mechanism The knee employs a "screw-home" locking mechanism for terminal extension. During the last 15-20 degrees of extension, the femur rotates medially on the tibia (or the tibia rotates laterally on the femur if the foot is fixed). This rotation occurs because the medial femoral condyle is larger than the lateral, so the lateral condyle completes its motion first while the medial condyle continues to roll backward. This locking mechanism creates a stable, extended position that can be maintained with minimal muscular effort—important for standing. To initiate flexion from the locked position, the knee must first be "unlocked." The popliteus muscle performs this function by internally rotating the femur on the fixed tibia (or externally rotating the tibia on the fixed femur), reversing the screw-home motion. --- ## The Popliteal Fossa The popliteal fossa is a diamond-shaped space at the posterior aspect of the knee, containing important neurovascular structures. ### Boundaries The superolateral boundary is formed by the biceps femoris muscle and tendon. The superomedial boundary is formed by the semitendinosus and semimembranosus muscles. The inferomedial boundary is the medial head of the gastrocnemius. The inferolateral boundary is the lateral head of the gastrocnemius and the plantaris muscle. The floor of the fossa consists of the posterior surface of the distal femur, the posterior capsule of the knee joint, and the popliteus muscle. The roof is formed by the popliteal fascia (a continuation of the fascia lata). ### Contents The contents of the popliteal fossa, from superficial to deep, are the tibial nerve, the popliteal vein, and the popliteal artery. The mnemonic "SPAN" (from lateral to medial: Sciatic branches, Popliteal vessels, Artery, Nerve) can help, though it must be remembered that the tibial nerve is actually the most superficial structure. The tibial nerve is a terminal branch of the sciatic nerve. It enters the fossa at the apex of the upper diamond and descends vertically through the center to exit between the heads of the gastrocnemius. The popliteal vein lies deep to the tibial nerve. It receives the small saphenous vein (which pierces the popliteal fascia to join it) and several genicular veins. The popliteal artery is the deepest structure, lying directly on the bone and joint capsule. This position makes it vulnerable to injury in posterior knee dislocations and difficult to palpate. The popliteal artery gives off genicular branches that form an anastomosis around the knee. The common peroneal nerve enters the fossa along the medial border of the biceps femoris tendon. It then winds around the neck of the fibula (outside the fossa proper), where it is vulnerable to injury. The small saphenous vein and popliteal lymph nodes are also found in the fossa. Panel A: Posterior view of popliteal fossa diamond shape with superolateral boundary (biceps femoris), superomedial boundary (semitendinosus and semimembranosus), inferolateral boundary (lateral gastrocnemius and plantaris), inferomedial boundary (medial gastrocnemius). Panel B: Contents in layers showing tibial nerve (yellow, most superficial), popliteal vein (blue, deeper, with small saphenous vein joining), and popliteal artery (red, deepest against bone). Panel C: Common peroneal nerve (green) along biceps tendon wrapping around fibular neck, floor consisting of femur, joint capsule, and popliteus, with genicular arterial branches indicated. Panel D: Cross-section at mid-fossa level showing depth relationships of contents with Baker's cyst common location marked posteromedially. --- ## Muscles of the Leg The leg contains three compartments separated by the tibia, fibula, interosseous membrane, and intermuscular septa. Each compartment has characteristic functions and nerve supply. ### Anterior Compartment The anterior compartment lies between the lateral surface of the tibia and the anterior surface of the interosseous membrane and fibula. All muscles in this compartment are innervated by the deep peroneal (fibular) nerve (L4-S1) and perform dorsiflexion (moving the foot upward at the ankle). The tibialis anterior is the largest and most medial muscle of the compartment. It originates from the lateral surface of the tibia and the interosseous membrane, and its tendon crosses the ankle to insert onto the medial cuneiform and the base of the first metatarsal. It dorsiflexes the ankle and inverts the foot. Its tendon is visible and palpable on the anterior ankle during dorsiflexion. The extensor hallucis longus lies deep to the tibialis anterior and extensor digitorum longus. It originates from the middle fibula and interosseous membrane, inserting onto the distal phalanx of the great toe. It extends the great toe and assists dorsiflexion. The extensor digitorum longus is the most lateral muscle of the compartment. It originates from the lateral tibial condyle, the fibula, and the interosseous membrane. Its four tendons insert into the extensor expansions of the lateral four toes. It extends the toes and assists dorsiflexion. The peroneus (fibularis) tertius is essentially the most lateral part of the extensor digitorum longus. It originates from the distal fibula and inserts onto the base of the fifth metatarsal. It dorsiflexes and everts the foot. ### Lateral Compartment The lateral compartment occupies the space between the fibula and the lateral intermuscular septum. Both muscles are innervated by the superficial peroneal (fibular) nerve (L5-S2) and primarily perform eversion. The peroneus (fibularis) longus is the larger, more superficial muscle. It originates from the lateral aspect of the proximal fibula and the head of the fibula. Its long tendon passes behind the lateral malleolus, crosses the sole of the foot obliquely from lateral to medial, and inserts onto the medial cuneiform and the base of the first metatarsal—the same insertions as the tibialis anterior. This arrangement creates a stirrup that supports the transverse arch of the foot. The fibularis longus everts the foot and assists plantarflexion. The peroneus (fibularis) brevis is smaller and lies deep to the longus. It originates from the distal two-thirds of the lateral fibula. Its tendon passes behind the lateral malleolus and inserts onto the tuberosity of the fifth metatarsal. It everts the foot and assists plantarflexion. Panel A: Anterior compartment cross-section showing boundaries (tibia medially, interosseous membrane posteriorly) with tibialis anterior (largest, blue) and extensor hallucis longus (deep, green) from medial to lateral. Panel B: Extensor digitorum longus (red) to extensor expansions of toes 2-5 and fibularis tertius (orange) to MT5 base, deep peroneal nerve (yellow) running between muscles, tendons crossing ankle with extensor retinaculum. Panel C: Lateral compartment showing fibularis longus (superficial, purple, tendon crossing sole to medial cuneiform/MT1) and fibularis brevis (deep, pink, tendon to MT5 tuberosity), both passing behind lateral malleolus. Panel D: Superficial peroneal nerve (green) between lateral compartment muscles with cross-section showing compartment positions and nerve locations. --- ### Posterior Compartment The posterior compartment is the largest compartment of the leg and is divided into superficial and deep groups by the deep transverse fascia. All muscles are innervated by the tibial nerve (S1-S2) and perform plantarflexion. The superficial group contains the powerful plantarflexors. The gastrocnemius is the most superficial muscle, forming the visible bulk of the calf. It has two heads: the medial head arises from the posterior surface of the medial femoral condyle, and the lateral head arises from the lateral femoral condyle. The muscle bellies unite and join with the soleus to form the calcaneal (Achilles) tendon, which inserts onto the posterior surface of the calcaneus. Because the gastrocnemius crosses both the knee and ankle, it can flex the knee as well as plantarflex the ankle. It is most effective as a plantarflexor when the knee is extended. The soleus is a broad, flat muscle lying deep to the gastrocnemius. It originates from the posterior surfaces of the tibia (soleal line) and fibula and joins the gastrocnemius to form the Achilles tendon. Unlike the gastrocnemius, the soleus crosses only the ankle joint. It is a slow-twitch muscle that is important for maintaining posture during standing. Together, the gastrocnemius and soleus are called the triceps surae ("three-headed calf muscle"). The plantaris is a small muscle with a long, thin tendon. It originates from the lateral supracondylar ridge of the femur and its tendon runs between the gastrocnemius and soleus to insert on the calcaneus medial to the Achilles tendon. The plantaris is absent in approximately 10% of individuals and has little functional significance; its tendon is sometimes harvested for surgical reconstruction elsewhere. The deep group contains four muscles lying on the posterior surface of the tibia, fibula, and interosseous membrane. The popliteus is a flat, triangular muscle at the back of the knee. It originates from the lateral surface of the lateral femoral condyle (within the knee joint capsule) and inserts onto the posterior tibia above the soleal line. Its primary function is to "unlock" the knee by internally rotating the femur on the fixed tibia, initiating flexion from the fully extended position. The tibialis posterior is the deepest muscle of the posterior compartment, lying between the flexor digitorum longus and the flexor hallucis longus. It originates from the posterior surfaces of the tibia, fibula, and interosseous membrane. Its tendon passes behind the medial malleolus and inserts onto the navicular, cuneiforms, and the bases of metatarsals 2-4. The tibialis posterior is the primary invertor of the foot and also assists plantarflexion. Dysfunction of this muscle leads to adult-acquired flatfoot. The flexor digitorum longus lies medially, originating from the posterior tibia. Its tendon passes behind the medial malleolus and divides in the sole to insert onto the distal phalanges of the lateral four toes. It flexes the toes and assists plantarflexion and inversion. The flexor hallucis longus is the most lateral of the deep muscles. It originates from the posterior fibula and interosseous membrane. Its tendon passes behind the medial malleolus (in a groove between the two processes of the posterior talus) and inserts onto the distal phalanx of the great toe. It is a powerful flexor of the great toe (important for push-off during walking) and assists plantarflexion. The tendons of the flexor digitorum longus and flexor hallucis longus cross in the sole of the foot at the "knot of Henry," where they exchange fibers. Panel A: Superficial posterior compartment with gastrocnemius (two heads from femoral condyles, red) and soleus (flat, blue) uniting to form Achilles tendon inserting on calcaneus, and plantaris (thin with long tendon, green), triceps surae labeled. Panel B: Deep layer with popliteus (triangular, at knee, "unlocks knee") and tibialis posterior (central, yellow, to navicular and cuneiforms, "primary invertor"), deep transverse fascia separating layers. Panel C: Flexor digitorum longus (medial, orange, from tibia to distal phalanges of toes 2-5) and flexor hallucis longus (lateral, purple, from fibula to distal phalanx of great toe) with tendons passing behind medial malleolus. Panel D: Knot of Henry inset showing crossing of FDL and FHL in the sole, tibial nerve descending between compartment layers. --- ## Nerves of the Leg The leg receives its nerve supply from branches of the sciatic nerve. ### Common Peroneal Nerve The common peroneal (fibular) nerve is a terminal branch of the sciatic nerve that separates from the tibial nerve in the popliteal fossa or distal thigh. It runs along the medial border of the biceps femoris, then winds around the neck of the fibula in a superficial, subcutaneous position before dividing into its terminal branches. This location around the fibular neck makes the common peroneal nerve extremely vulnerable to injury. Fractures of the fibular neck, pressure from leg casts, habitual leg crossing, and prolonged squatting can all compress the nerve. The deep peroneal nerve is the motor branch to the anterior compartment. It passes through the anterior compartment, traveling with the anterior tibial artery, and continues onto the dorsum of the foot. It provides sensory innervation to a small area of skin in the first web space between the great toe and the second toe. The superficial peroneal nerve is the motor branch to the lateral compartment. It descends in the lateral compartment and then pierces the deep fascia in the distal leg to become cutaneous, supplying most of the dorsum of the foot. Injury to the common peroneal nerve causes paralysis of all muscles in the anterior and lateral compartments. The clinical result is "foot drop"—inability to dorsiflex the ankle. Patients develop a characteristic "steppage gait," lifting the leg high during the swing phase to prevent the toes from dragging on the ground. Sensory loss occurs over the lateral leg (superficial peroneal territory) and the first web space (deep peroneal territory). ### Tibial Nerve The tibial nerve is the larger terminal branch of the sciatic nerve. It descends through the popliteal fossa, passes deep to the soleus (in company with the posterior tibial artery), and continues through the posterior compartment, supplying all the posterior leg muscles. At the ankle, the tibial nerve passes behind the medial malleolus through the tarsal tunnel (deep to the flexor retinaculum) to enter the foot. Within the tarsal tunnel, it gives off the medial calcaneal branches (sensory to the heel) and then divides into the medial and lateral plantar nerves. Panel A: Posterior view showing sciatic nerve dividing into common peroneal (green) and tibial (yellow) nerves, common peroneal along biceps femoris wrapping around fibular neck (vulnerability zone in red). Panel B: Common peroneal dividing into deep peroneal (to anterior compartment) and superficial peroneal (to lateral compartment), tibial nerve descending through popliteal fossa deep to soleus with posterior tibial artery to tarsal tunnel. Panel C: Sensory distribution map showing superficial peroneal territory (lateral leg, most dorsal foot), deep peroneal (first web space), and tibial (plantar foot) with cross-section at mid-leg showing nerve positions. Panel D: Foot drop illustration showing inability to dorsiflex and steppage gait pattern diagram from common peroneal nerve injury. --- ## Clinical Correlations ### ACL Injury Anterior cruciate ligament tears are among the most common serious knee injuries, particularly in athletes involved in sports requiring pivoting, cutting, and sudden deceleration (soccer, basketball, skiing). The typical mechanism is a non-contact injury: the patient plants the foot, pivots, and feels a "pop" in the knee. Immediate swelling (hemarthrosis) develops within hours as the torn ligament bleeds into the joint. The patient may feel the knee "give way" and usually cannot continue activity. Physical examination reveals a positive Lachman test (anterior tibial translation with the knee flexed 20-30 degrees) and often a positive anterior drawer test. The "unhappy triad" (O'Donoghue's triad) describes the combination of ACL tear, medial meniscus tear, and MCL injury—though contemporary evidence suggests the lateral meniscus may be more commonly involved. Treatment depends on activity level and functional demands. Many patients, especially athletes, undergo ACL reconstruction using tendon grafts (patellar tendon, hamstring tendons, or allograft). ### Meniscal Tears Meniscal tears typically result from twisting injuries with the foot planted and the knee partially flexed. The medial meniscus is more commonly torn than the lateral because of its attachment to the MCL, which reduces its mobility. Patients report pain along the joint line, mechanical symptoms (catching, locking, or giving way), and often swelling (though less acute than with ACL injury). A "locked" knee that cannot fully extend suggests a displaced bucket-handle tear. McMurray's test (rotational stress on the flexed knee) may be positive, with a palpable click or pain along the joint line. Treatment depends on the location and pattern of the tear. Tears in the peripheral "red zone" may heal with repair. Tears in the avascular "white zone" are typically treated with partial meniscectomy. ### Compartment Syndrome Compartment syndrome occurs when increased pressure within a closed fascial compartment compromises circulation to the muscles and nerves within that compartment. In the leg, the anterior compartment is most commonly affected. Causes include fractures (especially of the tibia), crush injuries, prolonged limb compression, vascular injury, and tight casts. The pathophysiology involves a vicious cycle: swelling increases compartment pressure, which compromises venous outflow, causing further swelling. The classic findings are the "5 P's": pain out of proportion to the injury (especially with passive stretch of the compartment muscles), pressure (the compartment feels tense), paresthesias (as nerves become ischemic), pallor, and pulselessness—though the last two are late findings and indicate severe ischemia. Treatment is emergent fasciotomy—surgical release of the compartment fascia to relieve pressure. Delay in treatment can lead to irreversible muscle necrosis (Volkmann's ischemic contracture) and nerve damage. ### Baker's Cyst A Baker's cyst (popliteal cyst) is distension of the semimembranosus bursa, which communicates with the knee joint. It presents as a swelling in the popliteal fossa, typically secondary to knee pathology causing effusion (osteoarthritis, rheumatoid arthritis, meniscal tears). A one-way valve mechanism allows fluid to enter the bursa but impedes return, causing progressive distension. The cyst may rupture, causing sudden calf pain and swelling that can mimic deep vein thrombosis—an important differential diagnosis. ### Deep Vein Thrombosis Deep vein thrombosis (DVT) involves clot formation in the deep veins of the leg, commonly the popliteal or femoral veins. Risk factors include immobility, surgery, malignancy, and hypercoagulable states. Patients present with calf pain, swelling, warmth, and tenderness. Homans' sign (calf pain with passive dorsiflexion) is neither sensitive nor specific. The major concern is pulmonary embolism if the clot dislodges and travels to the lungs. Panel A: ACL injury showing pivot mechanism with planted foot, immediate swelling, Lachman test position, and unhappy triad components (ACL tear, meniscus tear, MCL injury) labeled on knee diagram. Panel B: Meniscal tear patterns including bucket-handle (causing locking), radial, and longitudinal tears with McMurray test position; anterior compartment syndrome cross-section showing swollen compartment with compressed vessels and nerves and 5 P's listed. Panel C: Baker's cyst in posterior view showing distended semimembranosus bursa with joint communication through one-way valve mechanism and rupture pattern indicated. Panel D: DVT showing swollen calf with clot in popliteal vein cross-section, embolization risk arrow to lungs, fasciotomy incision sites on leg outline, and risk factors listed. --- ## Summary The tibia is the weight-bearing bone of the leg, with a subcutaneous medial surface (the shin) and a distal projection forming the medial malleolus. The fibula provides muscle attachment and forms the lateral malleolus; the common peroneal nerve is vulnerable at its neck. The knee joint is stabilized by the menisci (fibrocartilaginous shock absorbers), the collateral ligaments (resisting varus and valgus stress), and the cruciate ligaments (resisting anteroposterior translation). The medial meniscus is attached to the MCL and more commonly injured than the lateral. The ACL prevents anterior tibial translation; the PCL prevents posterior tibial translation. The popliteal fossa contains the tibial nerve (superficial), popliteal vein, and popliteal artery (deepest). The common peroneal nerve lies laterally along the biceps tendon. The leg has three compartments. The anterior compartment (deep peroneal nerve) contains the dorsiflexors and toe extensors. The lateral compartment (superficial peroneal nerve) contains the evertors. The posterior compartment (tibial nerve) contains the plantarflexors and toe flexors. The gastrocnemius and soleus form the triceps surae, inserting via the Achilles tendon. Common peroneal nerve injury causes foot drop and steppage gait. Compartment syndrome is a surgical emergency requiring fasciotomy. --- ## Key Terms | Term | Definition | |------|------------| | Meniscus | Fibrocartilaginous wedge in the knee that improves congruence and absorbs shock | | ACL | Anterior cruciate ligament; prevents anterior tibial translation | | Popliteal fossa | Diamond-shaped space posterior to the knee containing major neurovascular structures | | Foot drop | Inability to dorsiflex the ankle, typically from common peroneal nerve injury | | Compartment syndrome | Elevated pressure within a closed fascial compartment causing tissue ischemia | | Achilles tendon | Common tendon of the gastrocnemius and soleus inserting on the calcaneus | --- *This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*