The Knee Joint

The very complicated knee joint is composed of the distal femur, proximal tibia, and patella. Although the fibula does not participate directly in the mechanics of the knee joint, some muscles that cross the knee attach to the fibula. The movements produced at the knee joint are flexing (bending), extending (straightening), and medial and lateral rotation of the lower leg in relation to the thigh.

The lateral rotation movement can occur when the knee is bent. The more the knee is bent, the easier it is to rotate the lower leg and the foot. During flexion, the femur rolls back on the tibia and then glides on the same spot on the tibia. The rolling movement continues until the anterior cruciate ligament is completely stretched, after which movement of gliding is initiated.

The anterior cruciate ligament thus prevents movement when the lower leg is moved forward in relation to the thigh. A common injury in soccer is rupture of the anterior cruciate ligaments, which can happen when a player's foot is blocked and the lower leg rotates medially. The posterior cruciate ligament is injured when the lower leg is pressed backward or when the knee is severely overstretched.

Gluteus maximus

(Infrapiriform foramen) Obturator internus Sacrotuberous ligament Obturator internus Ischial tuberosity

Adductor magnus

Gracilis Semitendinosus Biceps femoris, long head

Semimembranosus Semitendinosus, tendon Semimembranosus, tendon Gastrocnemius, medial head

Gracilis Semitendinosus Biceps femoris, long head

Semimembranosus Semitendinosus, tendon Semimembranosus, tendon Gastrocnemius, medial head

Vastus lateralis Biceps femoris, short head

Biceps femoris, long head Popliteal artery Gastrocnemius, lateral head

Figure 12-4 Muscles of the thigh, posterior view.

Iliac crest

Gluteus medius

Gluteus minimus (Suprapiriform foramen) Piriformis Gemellus superior Tensor fasciae latae Gemellus inferior Quadratus femoris Gluteus medius Greater trochanter Trochanteric bursa of gluteus maximus

Gluteus maximus Adductor minimus

Adductor magnus

Vastus lateralis Biceps femoris, short head

Biceps femoris, long head Popliteal artery Gastrocnemius, lateral head

Figure 12-4 Muscles of the thigh, posterior view.

The function of these two collateral ligaments is to prevent sideways bending of the knee. They are taut when the knee is stretched and slack when the knee is bent. This means, for example, that the lower leg can be rotated laterally until the ligaments are taut again. The lower leg usually cannot rotate as much medially as it can laterally because the cruciate ligaments in the joint twist around each other during medial rotation and thereby block the movement (Fig. 12-5).

The surface of the lower end of the femur is elliptical, and the upper extremity of the tibia is flat. Therefore there would be very little contact between their surfaces if the cartilage were not shaped to receive the end of the femur. The undersides of the menisci are plain, like the surface of the tibia. As a consequence, the stress to which the knee is subjected can be distributed over a relatively large area. In flexion and extension of the knee joint, the menisci glide to suit the form of the condyles of the femur. Because the medial meniscus fuses with the medial collateral ligament, it is very easily injured by being subjected to excessive stress while in unusual positions.

Injury to the menisci resulting from rotational strain on a bent weight-bearing knee is very common. If there is a sudden pitching in the knee joint during lateral rotation of the lower leg, the medial ligament stretches and can thereby tear the meniscus, which is locked between the femur and tibia. Because of this, movements that strain the medial collateral ligament should be avoided.

Lateral condyle Lateral meniscus

Anterior ligament of fibular head Head of fibula

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