When the patella is out of place (luxated) it usually can't return to its normal position until the quadraceps muscle relaxes, which causes the leg to straighten at the knee. The dog will often hop for a few steps with the leg held straight forward until the patella pops back into place. As the patella pops over the ridge of the trochlear groove, it may hurt, so some dogs may yelp. sometimes the patella may not pop back into place on its own. Four grades of patellar luxation severity are described.

*Grade 1: The dog may occasionally skip, holding one hind leg forward for a step or two. The patella usually stays in place unless it is manually shifted out of position. It returns to its correct position easily

*Grade 2: The dog often holds the affected leg up when running or walking, but the patella usually slides back into position by itself. When the leg is manipulated it has a grinding feeling.

*Grade 3: The patella is permanently out of position. Even when the patella is manually placed back in position, it doesn't stay long. The dog will sometimes us the affected leg.

*Grade 4: The patella is always out of position and cannot be replaced manually. The dog never puts weight on the leg. 

Petellar luxation can occur in one or both hind legs, and the patella canbe displaced toward the inside or outside. In the Yorkshire Terrier is almost always displaced toward the inside, which gives the leg a bowed appearance. It's usually present by 4 to 6 months of age, although the symptoms may go unnoticed for several years. It gets gradually worse with age because every time the patella pops out of position it stretches the surrounding tissues that are needed to hold down the edge of the trochlear groove. The abnormal wear can lead to arthritic changes, which is one reason it's important to implement treatment early. 

You may be able to slow the progress of Grade 1 or 2 luxation by keeping the dog at a trim weight, by building the muscles of the rear with steady, moderate exercise such as walking ( especially up hills), and perhaps by giving glucosamine supplements. Surgery to reconstruct the soft tissue surrounding the patella may provide permanent relief if done in these early stages. Many veterinarians consider surgery for Grades 1 or 2 and possibly 3, to be overkill, however, pointing out the discomfort, expense, and possible arthritic aftereffects of such surgery.

The above information is an excerpt from The Yorkshire Terrier Handbook written by D. Caroline, Ph.D. . This book holds an abundance of valuable information for the Yorkie owner. It can be purchased at your local Borders Book store.