The Knee-Joint, the largest in the body, despite its exposed position is rarely dislocated owing to its great strength. This strength is due chiefly to the muscles, fasciae, and ligaments, including the internal ones, and to a less degree to the lateral breadth of the articulation. The movement of the knee-joint is not that of a hinge, but consists rather of a gliding of the head of the tibia on the. femoral condyles, combined with rotation, outwards in extension and inwards on flexion. In full extension the limb becomes locked, and no rotation is permitted. This position tends to be assumed in standing, the weight of the body falling in front of the centre of the joint, and tending to produce overextension. In front the quadriceps, containing the patella and ending in the ligamentum patellae which is attached to the tubercle of the tibia, replaces the capsular ligament. Laterally also tendinous expansions from the vasti reinforce the capsule which extends laterally from either margin of the patella and patellar ligament, but is non-existent above the patella, the synovial membrane being practically in contact with the quadriceps.
The patella is developed in the quadriceps tendon, the ossine centre appearing about the third year, and ossification being complete by the fifteenth year. Its anterior surface is very superficial, being covered by skin, superficial and deep fasciae, and a thin expansion of the quadriceps tendon, which has an important bearing upon fracture of the bone. The anterior surface is slightly convex, and perforated by numerous vascular foramina, the patella being richly supplied with blood from the anastomotica, anterior tibial recurrent, and inferior articular of the popliteal, and hence bleeding freely, as a rule, after fracture.
The upper margin of the bone is thick, the quadriceps being inserted into the anterior two-thirds, while a small portion behind this, which is devoid of cartilage, is covered by synovial membrane. The lateral margins and apex are narrow, the former receiving tendinous insertions from the vasti, while the apex gives insertion to the ligamentum patellae. The tendinous insertions of the vasti, also termed lateral patellar ligaments, if not ruptured, play an important part in minimizing separation of the fragments when fracture occurs. Save for the small portion on the posterior surface of the apex which is covered by synovial membrane, the whole posterior surface of the patella is covered with articular cartilage, and is unequally divided by a vertical ridge into a small internal and large external portions, each of which is again divided into three by a couple of faint transverse ridges, while a seventh small vertical area may be present near the inner margin. On section, both vertically and horizontally the patella is roughly triangular, and the vertical triangularity is of importance in the mechanism of fracture.