Hip dysplasia is an abnormal formation of the hip socket that, in its more severe form, can eventually cause crippling lameness and painful arthritis of the joints. It is a polygenic trait, that is afected by enviromental factors in the production of the final phenotype. It can be found in many animals and occasionally in humans, but is most commonly associated with dogs, and is common in many dog breeds, particularly the larger breeds.

Hip dysplasia is one of the most studied veterinary conditions in dogs, and the most common single cause of arthritis of the hips. In humans it occurs at a rate of about 1 births per thousand (0.1%)

Dogs might exhibit signs of stiffness or soreness after rising from rest, reluctance to exercise, bunny-hopping or other abnormal gait (legs move more together when running rather than swinging alternately), lameness, pain, reluctance to stand on rear legs, jump up, or climb stairs, subluxation or dislocation of the hip joint, or wasting away of the muscle mass in the hip area. Radiographs (X-rays) often confirm the presence of hip dysplasia, but radiographic features may not be present until two years of age in some dogs. Moreover, many affected dogs do not show clinical signs, but some dogs manifest the problem before seven months of age, while others do not show it until well into adulthood.

In part this is because the underlying hip problem may be mild or severe, may be worsening or stable, and the body may be more or less able to keep the joint in repair well enough to cope. Also, different animals have different pain tolerances and different weights, and use their bodies differently, so a light dog who only walks, will have a different joint use than a more heavy or very active dog. Some dogs will have a problem early on, others may never have a real problem at all.

Each case must be treated on its own merits, and a range of treatment options exist.