Accidental dislocations are as numerous almost as the joints themselves; horses, by the nature of their employments and the dangers to which they are exposed, incur injuries which dislocate and damage the joints of the limbs, and less frequently those of the spine and other parts of the skeleton. The liability to dislocation is much greater in some joints than in others. This depends partly on the form and character of the joint itself, and partly also on the strength of the ligaments and muscles which unite and support it. The rarity of dislocation of the hip is due to the depth of the cup into which the head of the femur fits, and the strength of the ligaments and muscles which bind them together. The relative frequency of luxation of the patella may be ascribed to the facility with which the bone is under some circumstances enabled to glide over the outer small ridge of the femur upon which it plays.
Luxation of certain bones, notably the patella, is sometimes due to extreme debility and weakness, while in others it follows upon contraction of the tendons induced by hard work, sprains, and other injuries, familiar examples of which are to be seen every day in our streets. "Upright" is the term used to denote such cases of partial dislocation occurring in the region of the pastern; but they are not infrequently seen in a condition in which the lower extremity of the canon-bone projects beyond the level of the articular surface of the suffraginis, or upper pastern-bone.