Mandy has begun to limp on her right rear leg. The year-old toy poodle’s lameness at first was intermittent, occurring maybe one or two times a day. Mandy would put weight on the leg but not in full support. Two weeks later, Mandy is walking on three legs.

Nicole, after searching the Internet, has decided Mandy likely has a luxating patella. She wants to know how to address the problem.

There is certainly a possibility that Mandy has a luxating patella. However, I do not believe that is the problem.

Luxating patella refers to a problem with the knee cap or patella not wanting to stay within its normal alignment. The patella is a bone contained within the tendon that is formed by the confluence of the quadriceps muscle. It is the tendon the doctor taps to test reflexes in your leg during a physical. The patella normally rides within a groove formed at the end of the upper leg bone, the femur. There are two ridges on either side of this groove that hold the patella in place. In cases of patellar luxation, the patella rides outside the groove. This can be caused by one or several structural problems in the knee. This is most common in smaller-breed dogs, Yorkshire terriers and poodles being higher-incidence breeds.

There are dogs that have luxating patellas without any symptoms, meaning that they don’t limp. Others will hold the affected leg up when the patella luxates and then use it again properly when it falls back into the groove. Some dogs have a pronounced lameness, owing to the pain caused by the patella riding outside the groove. Patients that have a persistent lameness with this condition usually need surgery.

Another consideration for Mandy’s lameness is Legg-Perthes, a disorder that affects the hip joint. This disease occurs when there is a loss of blood supply nourishing the head of the femur (thigh bone) where it fits into the socket of the pelvis. Sometimes this occurs with trauma and other times without apparent incident.

Without blood, the bone tissue begins to die off. This process is progressive and progressively more painful. I think Mandy may be dealing with this problem, as luxating patellas do not generally progress as described by Nicole. Incidentally, poodles are a high-incidence breed for this disease also.

Radiographs will show a mottled appearance to the part of the femur that fits into the hip socket (called the femoral head), which occurs with Legg-Perthes. Surgery to remove the femoral head will eliminate the source of pain. These patients adapt quite well and usually return to normal function of the affected leg.