The nature of the instability of the hip joint in dogs with dysplasia of the hip was investigated. In the experimental canine model, instability of the joint, as shown by subluxation of the femoral head on a roentgenogram, was accompanied by an increase in the volume of the synovial fluid and of the ligamentum teres and, therefore, an increase in the intra-articular volume. Injection of a buffered solution of hyaluronic acid into non-subluxated hip joints in situ increased subluxation, whereas withdrawal of synovial fluid from one joint with severe subluxation reduced the degree of subluxation. The withdrawal of varying amounts of synovial fluid from excised but intact joints resulted in a decrease in the lateral displacement of the femoral heads. Injection of the hyaluronic acid solution into excised joints resulted in increased lateral displacement of the femoral heads. The results suggest that the joint instability associated with canine hip dysplasia is related to increased intra-articular volume.

Clinical Relevance: Instability of the hip joint (abnormal laxity) is a feature of hip dysplasia in dogs and congenital dislocation of the hip in children. The canine disease can provide a valuable model in which to examine the pathogenesis of hip instability. The model may be useful to test prophylactic procedures to prevent the joint instability and thus ameliorate disease.