The canine hip is composed of the femoral head (ball), acetabulum (socket), and the surrounding soft tissues (joint capsule, round ligament within the joint, and muscles). The femoral head and acetabulum are covered with a thin smooth layer of cartilage that functions to provide cushioning and gliding surface between the two bones.

Canine hip dysplasia (CHD) is the most prevalent developmental disorder of the hip joint, usually bilaterally, which involves both the soft and hard tissues. In the early phases, dysplasia is initially characterized by laxity of the joint and the surrounding soft tissues. As the disease progresses, the joint components begin to undergo malformation and partial dislocation (subluxation). As a result the forces from weight bearing are no longer distributed over a wide surface area, but become more focally concentrated which causes excessive wearing on the cartilage and underlying bone. Remodeling changes and osteoarthritis ensue.

CHD is a multifactorial condition that has a genetic basis and the pattern of inheritance indicates that more than one gene is involved. Other factors that can influence the development of the hip joint are the size of the dog, growth rate, body type, muscle mass, neuromuscular alterations, and endocrine and vitamin imbalances.

Clinical Presentation:

The history and clinical signs of CHD may vary with the age of the patient. In some instances, there may be no signs appreciated by the owners. In the young dogs, (4-12 months of age), the condition is manifested by a progressive reduction in activity associated with soreness in the hindlimbs. There are usually signs of difficulty rising, decreased willingness to walk, run, jump and climb stairs, and lameness that the dog initially warms out of, but worsens with heavy activity, especially on the following day. There is often a "bunny-hopping" type of hindlimb gait when walked quickly. Occasionally an audible and palpable click can be appreciated when the dog walks. The muscles of the pelvic limbs are usually poorly developed. The older dogs present a different clinical picture because they suffer from pain due to chronic degenerative disease, which resulted from dysplastic changes. Lameness after exercise, reluctance to climb stairs, muscle atrophy, audible or palpable grinding, and restricted joint motion are common signs. The dog prefers to sit rather than stand and has difficulty rising.

Diagnosis:

The diagnosis is based on the age, breed, clinical course, physical evaluation and radiographs. In many instances a complete orthopedic evaluation will also include anesthesia for a more thorough evaluation. Joint laxity, crepitus and decreased range of motion are the most frequent findings. Radiographic evaluation is essential in establishing a positive diagnosis.

The standard view for evaluating the hips is the limb extended position. This technique, employed by the Orthopedic Foundation for Animals (OFA), is used to evaluate for existing subluxation and arthritis. The OFA has established a seven-grade system for categorizing hips. Excellent, good, fair, or near normal are determined to be within the range of "normal" radiographically. Those with dysplasia are characterized as mild, moderate and severe. The dog must be over 2 years of age to apply these gradations.

Although preliminary evaluation can be performed, a high incidence of false negative results can occur: 89% at 6-10 weeks of age; 56% at 16-18 weeks age; The accuracy is reported to be between 70-89% between 12-18 months of age, and between 85-95% at 2 years of age.

A newer technique referred to as stress radiography (distractive index (DI) or PennHIP®) is useful for predicting CHD based on laxity. In some cases laxity cannot be appreciated with the hip extended projection (OFA style radiographs). Stress radiographs are acquired with the hips in both a compressed position and distracted position. A distraction index is calculated from these views to quantitate the degree of passive laxity present. It is used to compared an individual dog’s hips to other members of the same breed, and can be used to predict the likelihood of developing degenerative joint disease secondary to hip laxity. This technique supports the observations made over several decades that "tighter hips are better hips". Individual logistic regression curves that predict the risk of developing degenerative joint disease are being developed for different breeds because some breeds tend to be more laxity tolerant or laxity sensitive than other breeds. Statistical models indicate that the distractive index at an age as young as 4 months was the best predictor of degenerative joint disease probability and that the strength of the prediction improved when the analysis was done at 6 months or one year of age. Current recommendations are to have follow-up radiographs taken at 6 months or one year of age to confirm the 16 week laxity.

Summary:

The architecture of the newborn hip is predetermined by a genetic blue print. Therefore, as expected, breeding disease free dogs will be more likely to produce dogs with normal hips, while the offspring of dogs with hip dysplasia have an increased risk of developing the disease. The OFA style radiographs are useful for looking at existing disease while the distractive index radiographs are useful for predicting disease at a younger age.

A number of other conditions in the young and older animal may be mistaken for CHD and subsequent arthritis. These include other orthopedic (panosteitis, osteochondrosis, trauma, injury to the cruciate ligaments, arthritis, neoplasia) and neurologic conditions. It is apparent that a thorough orthopedic, neurologic and radiographic evaluation be performed before the diagnosis of CHD is established. Once the diagnosis of CHD is confirmed, treatment can then be planned around the age of the patient, expected activity level, comfort level, severity of the condition, and finances.

Both medical and surgical treatment will be discussed in the future.