The Classic Comprehensive Handbook of Cat Care
(POLYDACTYLY, PATELLAR LUXATION, HIP
The incidence of abnormalities of the appendicular skeleton in cats historically has been rare. The most commonly recognized abnormality has been the formation of extra complete or partially formed digits on each affected foot, particularly the front feet. This condition is called polydactyly and is caused by the action of a single dominant gene. Polydactyly is usually of no serious physical consequence to an affected cat. However, some cats have as many as four nearly normal extra digits on the affected feet and in these there is a tendency for the toenails on the extra toes to grow into the digital pads causing discomfort and infection if the claws are not kept trimmed.
As the popularity of purebred cats has risen so has the incidence of more serious skeletal diseases. Patellar luxation (dislocation of the kneecap) was first recognized in the Devon Rex breed, but is now seen in others as well. Hip dysplasia, a deformity of the hip joint in which the joint socket is abnormally shallow and the head of the femur is malformed, has also been diagnosed in lines of purebred cats.
Both patellar luxation and hip dysplasia are diseases that are strongly genetically influenced in dogs and appear to be similarly influenced in cats.
Most cats affected with such skeletal abnormalities are not significantly impaired in their day-to-day function. However, all affected individuals are predisposed to develop osteoarthritis (degenerative joint disease), and markedly affected animals may have problems with lameness throughout their lives.
Dog breeders who minimize the importance of skeletal abnormalities that seem to cause no obvious problems for their animals are responsible for the persistence of these problems in dog breeds. The time to stop the establishment of similar crippling, genetically influenced diseases in cats is before the problem becomes as prevalent as it is in purebred dogs. Be sure that any kitten you acquire (purebred or random bred) is found normal on repeated physical examination through his or her skeletal growth period and be sure that any cat selected for breeding has a normal musculoskeletal system.