What it is
The kneecap is a small bone at the front of the knee that glides along a groove in the thigh bone as the leg bends and straightens. If the groove is too shallow or the ligaments holding the kneecap are too loose, it slides out to the side, and for a moment the leg no longer works properly.
The condition can be present from birth and inherited, but a kneecap can also slip after an injury. One or both back legs may be affected.
Cats adapt well, so a mild case is easy to miss: the kneecap can pop back by itself and the cat carries on playing. Still, every slip rubs the joint, so a strange hop or a new reluctance to jump is worth showing to a vet.
Signs you may notice
Mid-stride a cat suddenly skips along on three legs for a few steps with a back leg held up, then walks on as if nothing happened. It may also avoid jumping or move stiffly.
When to see a vet
Book a visit if your cat keeps lifting a back leg, limps or avoids jumping. If it suddenly will not put any weight on the leg, see a vet as soon as possible.
This is general information, not veterinary advice. For your own cat's health, tests and treatment, see a vet.