It's quite possible that your kitty has experienced a luxating patella, or slipped kneecap, without you ever witnessing it. Although luxating patellas often affect small dogs, they usually limp so their owners notice it. Cats often don't limp if their kneecap slips, but many go back in place themselves.
Located in his hind legs, your cat's patellas, or kneecaps, should be in grooves at the end of his femurs, or thigh bones. If the patella luxates, or dislocates, it pops out of that groove. In some breeds, including the Abyssinian, this is a genetic condition. It occurs in other cats if they're born with excess bone curvature. For the majority of cats, the issue starts with too shallow a groove to properly hold the kneecap. There are four grades of patellar luxation, ranging from mild to severe. If your cat is mildly affected, it's not that he can heal himself, but that you might never be aware of a problem. If he's seriously affected, surgery is necessary.
If your cat is limping or seems off as he moves, he could have a luxated patella. Even if he's not limping, keep an eye on him if he's much less active than usual or isn't jumping up on his favorite windowsill or other preferred perches. He might appear to "skip" on that hind leg. While slipped kneecaps are relatively rare in felines, they often occur because of trauma. Even if you didn't witness his accident, sudden severe limping in the back legs usually results from trauma.
Your vet can feel that the kneecap is out of place. She'll confirm it with X-rays. Depending on the grading scale, she might recommend rest or surgery. Cats with grades three or four luxated patellas are generally surgical candidates. Without it, they may eventually lose the use of the leg. The surgery consists of putting the patella back into its groove, by making the groove deeper and tightening the surrounding tissues and the joint capsule to help keep the kneecap in place. Even after surgery, it's possible for the kneecap to slip again.
Kitty, Heal Thyself
While you should certainly take your cat to the vet if he shows any signs of a slipped kneecap, it's possible that your cat has his own way of dealing with the issue. If you've ever seen him kick his leg out quickly to one side for no obvious reason, that could be his way of getting his kneecap back in place. If you see him do this, take him to the vet even if he appears to walk normally. The kneecap keeps slipping, your cat continues kicking, and all the while the tissues around his kneecap deteriorate and your cat becomes more vulnerable to arthritis.
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