Some cats suffer from a congenital defect called patellar luxation. This condition allows the kneecaps to slip off the front of the joint probably to the inside of the joint. Many toy breed dogs have this problem because they tend to be bow-legged. The problem is unusual in cats but equally debilitating. Several reconstructive procedures are incorporated into the surgical repair of these "trick knees" to stabilize the patella (kneecap) in the proper position over the front of the joint: The groove in which the patella is supposed to rest is deepened, and a new lateral patellar ligament is created to anchor the patella in its normal position.
Some veterinary general practitioners are accomplished orthopedic surgeons, but if you would like a second opinion about the surgery, ask your veterinarian to refer you to a board-certified veterinary orthopedic surgeon. Surgery is a viable alternative to continued abnormal stresses in the knees.