This is a very common orthopedic disease of small dogs, although it can occur in dogs of any size. Toy poodles, Yorkies, Pomeranians, Chihuahuas, and Boston Terriers are some of the most commonly affected breeds. The patella, or knee cap, normally sits in a groove along the front of the knee joint, but in some dogs the groove is not deep enough and the knee cap slips out of place. Most of the time the patella will luxate towards the inside of the leg, known as medial patella luxation (MPL). Sometimes it will luxate laterally, or towards the outside of the leg. Lateral luxation is more likely to occur in a larger dog.
Mildly affected dogs will frequently have few to no clinical signs. Often this is an incidental finding on physical exam that the owner was unaware of. The classic clinical sign is a dog who is suddenly lame and carrying a leg, only to seemingly be magically cured after a few steps. What happens is the patella suddenly slips out causing difficulty straightening the leg due to the tendons becoming displaced. Once the patella slips back in the dog can ambulate normally again.
Patella luxation has varying degrees of severity and is graded on scale of 1-4. A grade 1 luxation can be manually luxated but will pop back into place when the pressure is released. A grade 2 luxation can be manually luxated but stays out of place when pressure is released. In a grade 3 luxation the patella is out most of the time but can be pushed back into place. In a grade 4 luxation the patella is permanently out of place and can't be pushed back in.
Although the actual act of luxating isn't very painful, over time repeated luxation and misalignment can cause arthritis and malformation of the leg bones. So, what to do? There are several surgeries that will correct this malformation but surgery is not a blanket recommendation for all affected dogs. Most dogs with grade 1 and 2 luxations will go on to live their life with minimal issues. The occasional flare-up can be treated with rest and anti-inflammatories on an as needed basis. Joint supplements such as glucosamine and chondroitin can also help. Dogs with persistent lameness or those with severe luxations are candidates for surgery so that the deformity can be corrected before causing secondary issues.
This disease does have a genetic component and affected dogs, even if non-clinical and mild should not be used for breeding. Half of all affected dogs will have bilateral involvement, but the grades can be different on each side. In my experience large breed dogs are more likely to have clinical lameness than small breed dogs-but that is generally true of most orthopedic diseases. Cats can also have patella luxations but rarely show clinical signs.
If you are curious, yes, Lyric does have bilateral grade 2 luxations. She is rarely bothered by them-when they pop out she is able to stretch her leg in such a way as to pop them back in and has never limped. One of the well-known orthopedic surgeons who taught me in school recommended I not pursue repair as long as she is not bothered by them, even as an agility dog. Any surgery involving a joint will induce arthritis so you always need to weigh how much arthritis a dog will get if surgery is not pursued, as this can vary with individual dogs and various conditions. I do keep her, and my other dogs, on a glucosamine/chondroitin supplement.