According to the Chihuahua Club of America, up to three of every four Chihuahuas have patellas that dislocate or "luxate." Several genetic traits combine to cause luxating patellas, so they are not easily removed from the breed's genetic lines. Fortunately, many dogs with luxating patellas require no treatment at all.
Subluxation and Luxation of the Patella
A luxating patella occurs when the dog’s kneecap, the bone known as the patella, moves from its place in the joint. According to the Whole Dog website, the dog’s knee might luxate due to weakness in the tendons or ligaments that hold the kneecap in place or due to the groove in which the kneecap is seated being too shallow. It is possible that an injury to a dog’s leg will cause luxation of the patella, but most likely there was a genetic predisposition for the knee to be displaced.
Luxating patellas are graded by severity, from I to IV. The first three grades are considered "subluxations," because the patella moves out of place but can return to its groove. The final grade is considered a complete luxation, because the patella leaves its groove and cannot be seated again.
Grades of Patellar Subluxation
Grade I subluxation occurs when the patella slips in and out of place when manipulated by hand. Grade II subluxation occurs when the patella occasionally moves out of place on its own and will move back on its own or when moved by hand. Grade III subluxation causes repeated or continuing lameness, due to the frequency with which the patella slips. Grade IV luxation occurs when the patella moves out of place and can’t be put back into place.If your Chihuahua runs with one hind leg held under her stomach or if she occasionally hops like a rabbit instead of with a normal four-beat gait, have your dog’s knees examined by a veterinarian.
No Treatment
Grade I luxation does not require treatment. Knees with Grade I luxation are essentially healthy knees that show symptoms only on physical exam. As a dog with Grade I luxation ages, the patella may deteriorate into a Grade II patella.
Medical Treatment
Luxating patellas are sometimes treated with crate rest, limited activity and drugs. Each of these treatments has mixed or limited success and none is curative. If your Chihuahua has a Grade II luxation of the patella, her veterinarian might ask you to keep her from running or jumping. The vet may also subscribe a non-steroidal anti-inflammatory drug (NSAID) like Rimadyl. In some cases, your dog’s veterinarian may suggest a steroidal drug instead. In addition to these drugs, some evidence exists that glucosamine and chondroitin taken in pill or liquid form aid in the medical treatment of lower-grade luxations. Glucosamine and chondroitin are readily available to pet owners without prescription.
Surgical Treatment
If your Chihuahua is in constant pain from her knee moving in and out of place (Grade III) or if she is unable to extend her leg at all (Grade IV), she'll need corrective surgery. Surgery can deepen the groove in which the patella sits. Luxation can also be corrected by surgically reconstructing the soft tissues -- the muscles and tendons -- that attach the patella to the tibia (leg bone) or by removing the end of the tibia and reattaching it in a more beneficial position. Each of these surgeries is serious and difficult, requiring up to two months for recovery. However, about 90 percent of all dogs that have corrective surgery will have a very good outcome.