Canine hip dysplasia may be defined as a mismatch of skeletal growth and the development of the supporting muscle mass, leading to progressive
laxity and subluxation of the hip joint and resultant degenerative joint disease.
Hip dysplasia stems from a combination of genetic and environmental factors. It definitely is inherited, but dysplastic parents
can produce normal offspring and vice-versa.
It usually is seen clinically in large breeds (German Shepherds, Golden Retrievers, Labradors, Rottweilers, etc.), but some
large breeds are less frequently affected (Great Danes, Greyhounds, Dobermans). Dysplasia is sometimes described in small
breeds and even in cats, but in these it is seldom a clinical disease. Hip dysplasia tends to affect the largest and heaviest
individuals of the breed and litter, and may therefore be exacerbated by intensive nutrition and overeating.
Many other factors have been associated with hip dysplasia, such as hormonal influences, conformation and intrinsic muscle
disease, but their exact contribution is uncertain.
Hip dysplasia is a developmental, not a congenital, disease. In other words, the puppy's hip joints are normal at birth. As
the puppy matures, however, the muscles and other associated soft-tissue structures are unable to maintain the stability of
the hip joint, producing progressive laxity and subluxation. This leads to incongruency of joint surfaces that produces failure
of the normal development and growth of articular cartilage, remodeling and malformation of the femoral head and acetabulum.
The resultant abnormal weight bearing produces increased stress and damage to portions of the femoral head and acetabulum,
along with stretching and tearing of the joint capsule. Moderately to severely affected dogs may develop signs by 4 months
to 6 months of age. Signs commonly reported are: exercise intolerance, a "bunny hopping" gait, reluctance to rise or to climb
stairs and lameness. Affected dogs may present with single-limb lameness, even though bilateral dysplasia is present in more
than 90 percent of the cases.
Examination findings include: conformational abnormalities – squared-off hind quarters due to dorso laterally displaced greater
trochanters, hind-limb atrophy and laxity and subluxation of the hip, which may be elicited in the relaxed-dog (Ortolani)
test. Radiographs will show mild to severe subluxation of the hips (Figure1).
Remodeling changes may appear before 1 year of age in severely affected dogs. There is a wide range of clinical presentations
in both the severity of the lesions and severity of clinical signs, yet there is not necessarily a correlation between the
severity of the radio graphic lesions and severity of clinical signs.
As the dog matures, clinical signs usually lessen in severity, often by 16 months to 18 months of age. The coxo femoral
instability produces hypertrophy of the joint capsule, laying down fibrous tissue and remodeling of the joint that has the
effect of stabilizing the joint and reducing lameness. A patient may remain relatively asymptomatic for several years, even
in some cases for the rest of its life. But the degenerative processes present in the joint continue as the dog ages, usually
producing a recurrence of lameness in middle age.
The degree of lameness is not necessarily proportional to severity of radiographic signs, however, and some dogs with severe
degenerative changes will function fairly well as house pets, while others will be severely affected. Radiographs reveal remodeling
and periarticular changes consistent with arthritis.
Figure 1: Radiograph of a 6-month-old Golden Retriever. Note the severe subluxation of both femoral heads., Figure 2: Radiograph
of the same dog in Figure 1, six weeks after a right-sided pelvic osteotomy. Note the deeply seated right femoral head compared
to the still markedly subluxated left femoral head.