Knee Joint

The knee joint is a synovial modified-hinge joint, in which condyles of the femur and tibia articulate as does the patella and patellar surface of the femur. The fibula is not part of the knee joint.

Patella



The patella is the largest sesamoid bone. It has circular-triangular shape and embeds in the tendon of quadriceps femoris, anterior to the knee joint.

Menisci



These are crescent shaped fibro-cartilage structures that act as shock absorbers. They sit on grooves on the articular surfaces of the tibial condyles. The medial meniscus is larger and "C" shaped, the lateral meniscus is more of an "O" shape. The menisci are attached to the tibial intercondylar area by their horns and small coronary ligaments.

Cruciate ligaments



These are intracapsular ligaments and there are two:
• The anterior cruciate ligament crosses from the front of the intercondylar area of the tibia to the medial side of the lateral femoral condyle. This ligament prevents hyper-extension and stops forward slip of the tibia on the femur.
• The posterior cruciate ligament crosses from the back of the intercondylar area of the tibia to the lateral side of the medial condyle. It becomes taut in hyper-flexion and resists posterior slip of the tibia on the femur.

Collateral ligaments



• The medial (tibial) collateral ligament consists of superficial and deep parts. The superficial component is attached above to the femoral epicondyle and below to the subcutaneous surface of the tibia. The deep component is firmly attached to the medial meniscus.
• The lateral (fibular) collateral ligament is attached to the femoral epicondyle above and, along with biceps femoris, to the head of the fibula below. Unlike the medial collateral ligament it lies away from the capsule and meniscus.

The collateral ligaments are taut in full extension, limiting extension and aiding in locking.

Capsule



The capsule is attached to the margins of the articular surfaces except anteriorly where it dips downwards. In the anterior part of the capsule there is a large opening through which the synovial membrane is continuous with the suprapatellar bursa. This bursa extends superiorly ~5cm above the patella between the femur and quadriceps. Posteriorly the capsule communicates with another bursa under the medial head of gastrocnemius and often, through it, with the bursa of semimembranosus. Posterolaterally another opening in the capsule permits the passage of the tendon of popliteus.

Behind the knee the oblique popliteal ligament, a reflected extension from the semimembranosus tendon, strengthens the capsule. Anteriorly the capsule is reinforced by the ligamentum patellae and the patellar retinacula. The latter are reflected fibrous expansions arising from vastus lateralis and medialis muscles which blend with the capsule anteriorly

Blood supply



Supplied by the rich anastomosis formed by the genicular branches of the popliteal artery.

Nerve supply: innervated by branches of the femoral, tibial, common peroneal and obturator nerves.

Movements at the knee

• Flexion: semimebranosus, semitendinosus, biceps, gracilis, sartorius (gastrocnemius, plantaris, popliteus)
• Extension: quadriceps femoris, iliotibial tract (gluteus maximum, tensior fascia latae)
• Internal rotation (with knee fixed): semimebranosus, semitendinosus, gracilis, sartorius
• External rotation (with knee fixed): Biceps
  

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