Canine hip dysplasia is a common orthopedic disease affecting millions of dogs in the United States alone. Canine hip dysplasia affects many breeds of dogs, but is of particular high incidence in dogs large in size. Included in the more than 80 canine breeds affected are German shepherd, Newfoundland, Old English sheepdog, English bulldog, Labrador retriever, other retriever breeds, Irish setter, Great Dane, and St. Bernard. The disease is characterized by laxity and incongruity of the hip joint resulting in degeneration of joint tissues. Osteoarthritis develops as a result of the abnormal positioning of the head of the femur in relation to the joint due to laxity, and resulting in the erosion of joint cartilage, and inflammation of the synovium. The development in a dog of osteoarthritis in a dysplastic hip joint is a benchmark sign of canine hip dysplasia.
Canine hip dysplasia is believed to be hereditary disease of complex genetic and environmental bases. Clinical symptoms range from mild hip joint discomfort to severe and debilitating; and the disease is the most common cause of rear-end lameness. However, in most cases the clinical symptoms appear in affected dogs much later in life than initiation of the disease. In that regard, canine hip dysplasia typically develops during the first 6 to 12 months of age, whereas symptoms may develop years later. There are several reasons why early diagnosis (e.g., less than one year of age) of canine hip dysplasia is desirable and useful. For example, a diagnostic procedure that can accurately and sensitively identify affected dogs at an early age could enable dog breeders to reduce the prevalence of the disease by choosing not to breed affected dogs. Additionally, there are some treatment options available for dogs affected by canine hip dysplasia. One such treatment is a surgical procedure, known as BOP (Biocompatible Osteoconductive Polymer) Shelf Arthroplasty, in which a polymer is used to rebuild the defective hips of dysplastic dogs so that subluxation is then prevented. Another treatment relates to management of the physical activity and weight (by diet and choice of exercise) of the affected dog in attempts to limit strain on the affected hip joints.
A current method for detecting hip dysplasia, disclosed in U.S. Pat. No. 5,482,055, is based on compression-distraction stress radiography. This method measures joint laxity that is related to the presence of absence of osteoarthritis (Smith et al., 1993, Am. J. Vet. Res. 54:1021-42; Smith et al., 1990, J. Am. Vet. Med. Assoc. 196:59-70). This stress method elicits passive laxity, which is the maximal lateral displacement of the femoral heads that occurs when a force is used to distract the hip joint. An adjustable device, a "distractor", is used to reveal passive laxity, and to quantitate lateral displacement of the femoral head by a measurement called the "distraction index". This index is used as a measure of passive laxity and as a predictive value, wherein below a certain value it is unlikely that the dog will develop osteoarthritis. For example, in Labrador retrievers, a distraction index of 0.3 is indicative of tight hip joints, and an index of&gt;0.7 is indicative of loose hip joints (passive laxity). Thus, as the distraction index increases from 0.3 to 0.7, the probability of the development of osteoarthritis increases. However, a problem arises in that the outcome for Labradors having a distraction index in this range cannot reliably be predicted (Smith et al., 1993, supra; Lust et al., 1993, Am. J. Vet. Res. 54:1990-9). Likely reasons include the other components of the hip joint structure, such as acetabular and femoral head conformation, contribute to functional joint stability and prevent the conversion of intermediate degrees of passive laxity into hip subluxation during ambulation.
Thus, there remains a need for methods which can be used for early diagnosis of dogs affected with canine hip dysplasia, and a need for methods by which functional laxity can be measured in detecting the presence or absence of canine hip dysplasia.