The extent to which a cat can crawl, walk, run, or leap depends on the structural soundness of its stifles, the joints in its rear legs. Each stifle houses the same components as those that constitute the human knee joint: the lower end of the thigh bone (femur); the upper end of the shin bone (tibia); and the upper end of a smaller bone (fibula), which is located adjacent to the tibia. The joints also contain muscles, which enable movement, and various other soft tissues: ligaments, which connect the bones and provide stability to the knee; tendons, which connect the bones to the muscles in the knee; and structures called menisci, which function as shock absorbers.

Of course, this elaborate and considerably delicate arrangement needs to be protected from injury. And this protection is supplied by the patella, a thick, triangular shield of bone covering the front of the stifle. In an anatomically intact animal, the patella—or kneecap—is firmly attached to ligaments and tendons within the knee joint and slides smoothly up and down along a groove (trochlea) in the lower part of the femur as a cat’s knee bends and straightens.

Unfortunately, either blunt physical trauma or a congenital defect may result in a condition referred to as a luxating patella, which may seriously compromise the ability of this shield to carry out its protective function. In this condition, says Ursula Krotscheck, DVM, assistant professor of small animal surgery at Cornell University’s College of Veterinary Medicine, “The kneecap becomes dislocated, either toward the inside or outside of the leg. The typical clinical signs include lameness, skipping on the affected leg, and an unwillingness to jump.”

If caused by physical trauma, the clinical signs of a luxating patella will become suddenly evident. If, on the other hand, the cause is a genetically inherited defect, the signs will slowly emerge and gradually evolve into a lameness as an affected cat grows older. The disorder is usually diagnosed by means of a physical exam and then confirmed by x-ray.

“Many animals can get along all right with the condition,” she notes. “However, we grade patients on a scale of one to four.” In grade one—the least severe—the patella can be manually moved out of position but returns easily to its normal position when released; in grade four, the patella is “out all the time” and cannot be manually repositioned; in grades two and three, the condition ranges in severity between these two extremes.”

“This grading scale helps us determine whether or not the animal is a good candidate for surgery,” explains Dr. Krotscheck. “For grades three and four, we’ll usually recommend surgery. Without it, the condition can lead to chronic lameness and, potentially, an inability to use the affected leg. Surgery is less likely for grades one and two. Those cats have an intermittent lameness and will occasionally hop or skip. Some animals may even learn how to pop the patella back into place.”

If surgery is performed, the objective will be to permanently restore the patella to its proper position in the femoral groove and adjust the tension of the tissues that are supposed to be holding the patella in place. In cats with grades one, two, and three luxating patellas, Dr. Krotscheck notes, surgery is usually very successful, with little chance of recurrence. In cats with a grade four luxation, the chance of recurrence is approximately 50 percent.”

Following surgery, she advises, a cat’s owner needs to keep the animal quiet, pay attention to any bandaging, and, under the guidance of a veterinarian, try to help restore the animal’s normal range of patellar motion by means of mild exercise of the knee joint and the appropriate use of warm and cold packing.