Of the Lateral Dislocations of the patella, that outwards is the more common, owing to the tendency of the quadriceps when contracted to pull the patella and ligamentum patellae into,a straight line, instead of following the angle formed by the femur and tibia. Normally this tendency is corrected by the prominence of the external condyle. The dislocation generally occurs when the limb is extended, is due to muscular action, and is incomplete. It is occasionally met with in cases of genu valgum. A dislocation of the patella on its edge, the inner edge projecting forwards, may also occur.
Congenital absence of the patella has occurred, and may be associated with a striking deformity, genu recurvatum, rn which the knees bend backwards, the child standing on the calves of its legs.
Antero-laterally the capsule of the joint is strengthened, in addition to the lateral patellar ligaments, by the ilio-tibial band of fascia lata externally, and the sartorius and semimembranosus internally. Laterally it is strengthened by the lateral ligaments, of which the internal is flattened, and extends from the tuberosity of the internal condyle to the upper end of the inner border of the tibial shaft, while it is attached between the bones to the inner semilunar cartilage. It is crossed superficially by the tendons of the sartorius, gracilis and semitendinosus, a bursa intervening, while the tendon of the semimembranosus extends beneath its lower and posterior border, and still lower the inferior internal articular vessels pass under it. The external lateral ligament is rounded, and extends from the tuberosity of the external condyle to the head of the fibula, just in front of the styloid process. In front of it passes an expansion of the ilio-tibial band ; the tendon of the biceps splits to enclose it ; and the tendon of the popliteus and inferior external articular vessels pass under it. Posteriorly the capsule is more defined,, extending from the upper border of the condyles and intercondylar notch to the head of the tibia. It is reinforced by an oblique posterior ligament (ligament of Winslow), derived from the semimembranosus tendon, extending from the inner and lower part of the joint to the outer condyle. The capsule is overlaid by the heads of the gastrocnemius, and is pierced under the inner head by the communication between the gastrocnemius bursa and the joint, and is also perforated by the azygos vessels and popliteus tendon. The internal ligaments consist of the two crucial ligaments, which are attached below, in front of, and behind the tibial spine, the anterior ascending to the posterior and inner aspect of the external condyle, and the posterior to the anterior and external aspect of the interna; condyle. Of these ligaments, the anterior resists extension and inward rotation, or forward displacement of the tibia.