If you have a small dog -- and they don't come much smaller than Chihuahuas -- there's a chance he could suffer from slipped kneecaps at some point. Formally known as patellar luxation, a slipped kneecap can so seriously affect a dog that it requires surgery to correct the problem. Mildly affected Chihuahuas are predisposed to arthritis.
Based on health tests submitted by breeders and owners to the Orthopedic Foundation for Animals from 1974 to 2009, the Chihuahua ranks 22nd of 93 breeds in the OFA's database for patellar luxation, according to the Purina Pro Club Chihuahua newsletter. Overall, 5.5 percent of Chis are affected with some degree of patellar luxation, overwhelming of a genetic rather than traumatic origin. While that might not sound like a lot, it's a fairly high instance for a genetic disease. The actual figure might be higher: Data submitters are more likely to be conscientious owners and breeders, the ones least likely to breed dogs with known hereditary issues.
At the front of your dog's knee, the patella bone rests in a groove in the end of the thigh bone on the hind legs. This allows the kneecap to move up and down when the knee, or stifle, bends. When the patella luxates, the kneecap falls out of the groove, generally on the inside. That cause a dog's leg to lock up. In mild cases, the patella pops back into the groove, but in more serious cases and as time goes on, that doesn't happen. In half of all cases, both knees are affected.
Symptoms of luxating patellas in a Chihuahua generally appear by the time the dog is 2 years old. Past that age, you're unlikely to have a problem. Your little guy might hold a hind leg up when running, possibly yelping in pain. If you see your dog frequently extending a rear limb, he could be trying to get his knee back in place. As the condition gets worse, your dog seems lame most of the time. Puppies suffering from patellar luxation appear bowlegged in the back; the condition becomes more extreme as they age.
Severity of luxating patellas is graded on a scale from 1 to 4. Grade 1 luxation signifies a mild condition that pops into place on its own; grade 4 luxation indicates a kneecap outside the groove that can't be manually put back. Generally, dogs with luxation of grade 2 or higher are considered surgical candidates. During surgery, the groove is deepened so the patella won't slip out. If your dog's legs have bowed, the surgeon will correct the condition by cutting the bone to change alignment, a procedure known as osteotomy. After surgery, your dog's activities will be restricted for a while, but he should be back to normal within three months.
Since a genetic factor is linked to Chihuahuas with patellar luxation, the Chihuahua Club of America encourages breeders to test for the disorder. If you're purchasing a Chi, ask the breeder about the parents' health history, including information about kneecaps. Don't breed a dog with a high grade of patellar luxation; breeding those with mild luxation is controversial but not uncommon. If you do breed a Chi with low-grade luxation, pair the dog with one with patella issues.
Jane Meggitt has been a writer for more than 20 years. In addition to reporting for a major newspaper chain, she has been published in "Horse News," "Suburban Classic," "Hoof Beats," "Equine Journal" and other publications. She has a Bachelor of Arts in English from New York University and an Associate of Arts from the American Academy of Dramatics Arts, New York City.