Knee Joint Anatomy
The knee joint is a synovial joint between the distal end of the femur, proximal end of the tibia and posterior aspect of the patella (Fig. 6.73). It is a hinge joint with a wide range of flexion and extension and limited lateral and medial rotation. The joint is relatively superficial anteriorly, medially and laterally where some of its bony features are palpable. Posteriorly, the joint is inaccessible, being deeply placed in the floor of the popliteal fossa (Fig. 6.74).
Anteriorly, the patellar ligament (Fig. 6.75) passes from the apex of the patella to the tibial tubercle. It is easily palpable and is used clinically for the stretch reflex (knee jerk, L3 & L4).
The medial and lateral condyles of the femur have articular surfaces, covered by hyaline cartilage, which extend over their inferior and posterior aspects and articulate with the respective condyles on the tibia (Fig. 6.73). On the front of the femur lies the patellar articular surface, while posteriorly a deep intercondylar fossa separates the two condyles. The articular areas on the tibial condyles are separated by the intercondylar eminence, the lateral articular area being flatter and smaller than the medial area (Fig. 6.73).
The capsule is thin anteriorly and posteriorly but is reinforced on each side by strong collateral ligaments. On the sides of the femur, the capsule attachment extends up to the epicondyles. Posteriorly, it attaches along the superior margins of the condyles (Figs 6.76 & 6.78) and the intercondylar line, while anteriorly it extends proximally (Figs 6.75, 6.79 & 6.81) to accommodate the synovial membrane that forms the suprapatellar bursa (pouch).
On the sides of the tibia, the capsule attaches close to the articular margins. Those parts of the capsule on each side of the joint that loosely anchor the outer borders of the menisci to the tibia are called the coronary ligaments. Posteriorly, the capsular attachment inclines downwards to include the posterior part of the intercondylar area, while anteriorly the attachment deviates inferiorly as far as the tibial tubercle. On the patella, the capsule is attached close to the articular margins.
Posteriorly, part of the insertion of semimembranosus forms the oblique popliteal ligament, which passes upwards and laterally (Figs 6.76 & 6.77), reinforcing the capsule. Where the posterior part of the capsule is pierced by popliteus, it thickens to form the arcuate ligament (Fig. 6.76).
On the medial side of the joint, the thick tibial collateral ligament (Fig. 6.79) broadens as it descends from the medial femoral epicondyle to the upper part of the subcutaneous surface of the tibia. Its deep aspect is attached to the outer margin of the medial meniscus (Fig. 6.78), which diminishes mobility of the meniscus, making it more susceptible to tears. On the lateral side the cord like fibular collateral ligament (Fig. 6.80) descends from the lateral epicondyle of the femur to the styloid process and head of the fibula, separated from the lateral meniscus by the popliteus tendon (Fig. 6.78).
The intracapsular ligaments comprise the anterior and posterior cruciate ligaments and the meniscofemoral ligament.
The cruciate ligaments (Fig. 6.78) are named according to their attachment to the intercondylar eminence of the tibia (Fig. 6.82). The anterior ligament passes upwards, backwards and laterally to attach to the medial surface of the lateral condyle of the femur.
The posterior ligament passes upwards, forwards and medially (Fig. 6.78) to attach to the lateral surface of the medial femoral condyle. The meniscofemoral ligament is adjacent to the posterior cruciate ligament (Fig. 6.77) and attaches the posterior border of the lateral meniscus close to the femoral attachment of the posterior cruciate ligament. It stabilizes the meniscus during rotation of the femur on the tibia. Although within the capsule, the cruciate and meniscofemoral ligaments are covered by reflections of synovial membrane and are therefore not within the synovial cavity.
The medial and lateral menisci are Cshaped (Fig. 6.82) with their anterior and posterior horns attached to the intercondylar eminence of the tibia and their outer borders to the joint capsule (coronary ligaments). The menisci differ in size and shape, the medial being narrower though slightly larger, so that its horns embrace those of the lateral meniscus. Also, the medial is attached to the medial collateral ligament and in cross section is deeper than the lateral meniscus. A transverse ligament (Fig. 6.82) connects the anterior horn of the medial meniscus with the anterior aspect of the lateral meniscus.
Infrapatellar fat pad
Deep to the patellar ligament is a quantity of fat (Fig. 6.74), which bulges the synovial membrane into the interior of the joint. Folds of synovial membrane, the alar folds (Fig. 6.81), extend on either side from the main pad. Another fold, the ligamentum mucosum, lies in the midline between the anterior part of the intercondylar notch and the lower margin of the patella.
Synovial membrane and bursae
Synovial membrane lines the interior of the capsule but does not cover the menisci. On the femur, it is attached to the margins of the intercondylar notch and covers the front and sides of the cruciate ligaments. Synovial membrane also covers the infrapatellar fat pad and the tendon of popliteus.
The suprapatellar bursa (Fig. 6.81; Fig. 6.74) is a large pouch of synovial membrane, passing a hand’s breadth proximal to the upper border of the patella, deep to quadriceps. Part of vastus intermedius attaches to it. Since the bursa is continuous with the synovial cavity of the joint, it provides a route for injecting fluid into or withdrawing fluid from the joint. After injuries to the joint, fluid accumulates (effusion) in the suprapatellar bursa, causing typical fullness around the knee and the basis for the patellartap test. Several other bursae lie near the knee joint and may enlarge, causing swelling. Bursae between the capsule and the two heads of gastrocnemius often communicate with the knee joint, while that beneath the medial head may also communicate with the overlying semimembranosus bursa. Other bursae, which do not communicate with the joint, are the pre and infrapatellar bursae beneath the skin covering the patella and patellar ligament, respectively. These may also enlarge.
From its tibial attachment, the popliteus passes upwards and laterally (Fig. 6.76), penetrating the posterior aspect of the capsule of the knee joint deep to the arcuate ligament. Within the joint, its tendon, covered by synovial membrane, attaches to the posterior border of the lateral meniscus and to the femur (Fig. 6.77) immediatelybelow the lateral epicondyle. Popliteus is supplied by the tibial nerve and its actions are considered below.
The principal movements of the knee joint are flexion and extension. Flexion is produced mainly by the hamstrings (semimembranosus, semitendinosus and biceps) assisted by the two heads of gastrocnemius. Extension is produced by quadriceps femoris acting through the patellar ligament. Gluteus maximus, acting through the iliotibial tract, maintains stability of the knee in the extended position.
Because of the shape of the articular surfaces, the femur rotates medially during the later stages of extension. The lateral condyle and meniscus (moving in unison because of the meniscofemoral ligament) glide forwards on the lateral tibial condyle, while the medial condyle completes its movement of extension on the medial meniscus. Full extension is achieved with completion of medial rotation and further movement is prevented by tension in the collateral and oblique posterior ligaments.
During the early stages of flexion, lateral rotation of the femur on the tibia is produced by popliteus, which also pulls the lateral meniscus posteriorly. During flexion and extension the patella glides over the patellar surface of the femur.
Slight active rotation of the tibia on the femur can occur when the knee is in a flexed but nonweightbearing position. Sartorius, gracilis and semitendinosus rotate medially, while biceps femoris rotates laterally.
The knee joint is very stable. The most important factors are muscle tone, especially in quadriceps and the ligaments. The cruciate ligaments stabilize the femur on the tibia, preventing excessive anteroposterior movement. The collateral ligaments assist medial and lateral stability, while the iliotibial tract stabilizes the knee during extension. All of these ligaments, together with the oblique posterior ligament, prevent hyperextension. Cruciate and collateral ligament injuries together with meniscal tears commonly occur in sports, particularly following twisting movements, during which the foot is anchored to the ground.
Owing to angulation of the femur relative to the tibia, contraction of quadriceps femoris tends to displace the patella laterally. This displacement is prevented, first by the lowest fibres of vastus medialis, which insert into the medial patellar border and whose active contraction resists lateral movement of the patella, and second by the large size and prominence of the lateral femoral condyle, making lateral patellar movement mechanically difficult.
Occasionally, the lateral femoral condyle fails to develop normally, resulting in patellar instability.
Branches from the femoral, obturator and sciatic nerves supply the joint, sensory fibres from the femoral nerve travelling with the branches to the vasti and sartorius. Genicular branches from the tibial and common fibular divisions of the sciatic nerve, together with fibres from the posterior division of the obturator nerve, also supply the joint.
The knee joint receives its blood supply from the extensive genicular anastomosis derived mainly from branches of the popliteal, anterior and posterior tibial arteries (Fig. 1.27). Dislocation of the knee may damage the popliteal artery and seriously compromise the blood supply to the leg and foot. It may also damage the tibial and common peroneal (fibular) nerves causing loss of sensation and movement below the level of the knee.