This is the scientific terminology for slipping kneecaps, a condition that occurs in the hind legs. Patellar luxation is often considered congenital in toy breeds because conditions to predispose toward it are present at birth, and it can often be seen as soon as puppies start walking. This is thought to be a hereditary condition.
The luxation can range from occasional, with the kneecap in correct position more often than not (grade 1), to permanent, with the kneecap never where it belongs (grade 4). Dogs with a kneecap out of joint may move with a skipping action or carry the affected leg so it doesn't touch the ground.
Puppies may show signs of patellar luxation from the time they begin walking, including failing to straighten the leg and carrying it oddly. Such early signs usually indicate a more severe problem, grade 3 or 4. Those with less severe abnormalities may show an abnormal gait from time to time, but they quickly revert to normal.
Veterinary attention usually isn't sought until the dog is older and the unusual gait is appearing more frequently. Individuals with a grade 1 condition present at birth may not show signs until their senior years, when years of minor trauma have weakened the soft tissues and caused increasing joint pain.
Watch the hind legs of a dog in movement. If it looks like the dog is skipping, the dog is probably carrying the affected limb and only putting it down every two or three steps. You could also see the hock (the prominent joint at the top of the section of leg perpendicular to the floor) flexed out while the knee turns in and the foot points in, forming a pigeon-toed position. In severe cases, the dog may not use the leg at all.
Several organizations exist to help breeders and others track hereditary diseases in dogs. The Orthopedic Foundation of America (OFA) began with hip dysplasia, but now tracks eye disorders and a variety of skeletal disorders. CERF (Canine Eye Registration Foundation) specializes in disorders of the eye.
The veterinarian will watch your dog move, checking for the symptoms previously described. She will also manipulate the joint, feeling for the patella pushing out of position and for looseness in the medial collateral ligament, which should help hold the kneecap in place. The way the patella acts when being manipulated can usually determine the grade of the condition.
In cases where a low-grade condition suddenly worsens due to overexertion or injury, the dog may suddenly be unable to stand. This can appear to be a neurological disorder, but both hind legs should be checked for patellar luxation.
Treatment and Prognosis
With grade 1 or 2 conditions, the dog may be functional and able to move about normally most of the time. It will be up to you and your veterinarian to discuss whether you should adopt a watch and wait plan, keeping vigilant for any worsening of the condition, or elect immediate surgery.
Though patellar luxation is common in Yorkshire terriers, the Orthopedic Foundation of America (OFA) has no database on the breed because fewer than fifty evaluations have been submitted in the past twenty years.
The veterinary orthopedist has a variety of surgical choices. The groove in which the patella sits can be deepened, the tendons and muscles can be realigned through well-placed incisions or sutures, and bones can be somewhat rebuilt. Ligaments and meniscus should be examined for any injuries, and repaired if necessary, during surgery. Prognosis for complete recovery is good in dogs with grade 1 or 2 conditions. More severe disorders can require more extreme surgical options.