Patellar Luxation and Patellar Fracture in Cats

Patellar luxation is less common in cats than in dogs. Patellar luxation can be traumatic in origin or occur as a sequel to fracture malunion. Most of the reported cases are developmental. Certain breeds, such as Abyssinian and Devon Rex cats have been reported as having a higher prevalence for patellar luxation, but other breeds and non pedigree cats are affected as well. As many as 38% of Abyssinian cats had loose patellas during palpation in one study. The condition has also been documented in two British shorthair littermates. Patellar luxation in cats seems to have a hereditary component, but the exact mode of inheritance is unclear. Prevalence and grade of luxation are thought to be modified by several factors, implicating a polygenetic trait as in dogs. Patella luxation can also be associated with hip dysplasia.

The patella usually luxates medially, and both stifle joints are often affected. Affected cats are usually relatively young at the time of presentation. One study however reported a median age of 3.3 years, which is considerably older than the median age for dogs presenting with patella luxation. Cats have a relatively wider patella and more physiological laxity of the patella than dogs; the patella can be manually moved onto the trochlear ridge of the femoral condyles in many normal cats. Therefore, grading systems developed for dogs should be used with caution. Table 1 provides a classification system for patella luxation in the cat. Most cats have grade A and B luxations, grade D luxations are rare reported anatomical changes include a shallow trochlear groove and medial displacement of the tibial tuberosity. Gross conformational changes are generally not present. Secondary osteoarthritis is often absent or mild, but older cats especially can have more severe degenerative changes in their stifle joints. Cartilage erosions may be seen retropatellar and on the medial condylar ridge.  [...]