Patellar luxation

Definition:

The kneecap or "patella" is a small bone located forward to the stifle joint in animals (or to the knee in Humans). It is connected to ligaments and tendons which assure its vertical motion during the stifle flexion and extension. Patella luxates when it leaves the limb's axis (i.e. the trochlear sulcus), laterally or medially during a movement. In the cat, luxating patella is generally medial and uni or bilateral.

Importance:

Unlike dogs, patellar luxation is poorly documented and poorly known in cats. But this disorder seems to be more frequent in Devon Rex cats than in other breed cats.

Origin:

Today, it is unknown if a genetic incidence exists in this affection, but it's widely suspected. Thus, preventing programs are inexistent for the moment.

Clinical features:

The disease is congenital: the animals born with the disorder which can be detected on kittens and young cats.
Patellar luxation is almost always associated with a poor conformation of the quadriceps muscle and of the extremities of tibia and femur, which are closely related.
Clinically, the cat doesn't lame in the majority of the cases. Rarely, it can have an intermittent weight-bearing lameness when the patella luxates beyond the trochlear ridge (the limb is kept flexed for only one or two step(s)).

The veterinary practitioner often make the diagnosis as an incidental finding on physical examination: he is able to luxate voluntarily the patella. Various grades exists (I to IV) depending on the severity of the disorder. Most of the cases are grade I (the patella can be luxated, but it spontaneously reduces when pressure is released. Flexion and extension of the joint are normal. Spontaneous luxation during normal joint motion rarely occurs).
Over the years, joint lesions develop and lead to degenerative arthritis of the stifle joint, which eventually causes lameness and pain in older animals.

Treatment:

Either conservative or surgical treatment is possible. But surgical treatment must be undertaken only on grade II and more. Secondary degenerative arthropathy is controlled with anti-inflammatory, specific diet, and in severe case, with intraarticular injection or surgery. Prognosis is generally good for grade I to III, and guarded for grade IV.

Prevention:

The veterinarian should systematically test patellar luxation on kittens during the physical examination of vaccinations. The precautionary principle should be to prevent reproduction of the affected cats.

 

Source: Luxating patellae in Devon rex cats, Prior JE, vet record (1985), aug 17, 154-155.