Luxating patella, and the skip that gives it away

What is slipping

The kneecap sits in a groove at the end of the femur and slides up and down it as the leg bends. In a luxating patella it comes out of that groove sideways, usually toward the inside of the leg.

The cause is rarely the kneecap itself. It is the alignment of everything holding it: a groove too shallow to keep it in, or a thigh and shin that are slightly rotated relative to each other, so the tendon pulls the kneecap off to one side every time the leg straightens.

That is why it is so common in small breeds and why it is so often on both sides. It is a conformation problem, present from the way the leg is built rather than acquired from an injury.

Cats get it too, and the Abyssinian and Devon Rex are the ones it is documented in.

The sign people miss

The classic presentation is a dog that is walking normally, skips, holds one hind leg up for a stride or two, and then puts it down and continues as if nothing occurred.

What has happened is that the kneecap slipped out, the leg locked, and a few hops shook it back in. It does not look like pain because the dog is not limping afterwards, and it gets described as a funny little hop for years.

Other signs: sitting with a leg stuck out to the side, a reluctance to jump onto furniture, an intermittent bunny-hop with both hind legs, and in bad cases a crouched stance behind.

It is diagnosed by a vet feeling the knee, and it takes seconds. Any dog doing the skipping step is worth mentioning at the next appointment rather than waiting for a limp that may never come.

The four grades

Luxating patella is graded one to four, and the grade decides everything about what happens next.

Grade 1: the kneecap can be pushed out by hand and returns on its own. Usually no signs at all.

Grade 2: it luxates occasionally by itself, and goes back when the dog extends the leg or a hand replaces it. This is the skipping step, and it is the commonest grade to be diagnosed.

Grade 3: it sits out of the groove most of the time and can be pushed back, but comes straight out again.

Grade 4: permanently out and cannot be replaced by hand. The dog stands crouched and walks with the hocks turned out.

Grades one and two are frequently managed without surgery. Three and four are usually surgical, and earlier is better in a young dog, because a kneecap riding outside its groove wears the groove shallower and makes the problem progressive.

What it leads to if left

Two things, and the second is the one people do not expect.

Arthritis, from a joint surface being ground in a way it was not designed for. The same weight control, controlled exercise and pain relief apply as for any arthritic joint.

And cruciate ligament rupture. A knee that is unstable in one direction loads the cruciate abnormally, and dogs with luxating patellas rupture cruciates at a markedly higher rate. That is a bigger, more expensive problem than the patella, and it is a large part of why grade three and four are treated rather than watched.

Buying and living with it

It is heritable and it is examinable, so ask. Breeders in the affected breeds can have patellas assessed and recorded — the OFA runs a patella database in the United States, and the BVA and Kennel Club scheme covers it in Britain. A breeder of Chihuahuas, Pomeranians, Yorkshire terriers, toy poodles or French bulldogs who has never heard of patella grading is telling you something.

Keep the dog lean. Weight is the one variable an owner fully controls and it changes how hard a loose knee has to work.

Discourage jumping down from height — sofas, beds, the boot of a car — and use steps or a ramp for a dog that does it constantly.

Non-slip flooring matters more than it sounds. A small dog scrabbling on laminate is loading exactly the joint that cannot cope with it.

Build the muscle around the knee with steady lead walking rather than hard turns and ball chasing.