Patella Luxation (Dislocating Kneecap)

What is a luxating patella?
A luxating, or dislocating, kneecap is a condition in which bony abnormalities result in distortion of the knee joint so that the forces applied to the kneecap cause it to hop out of a groove (trochlea) located at the bottom of the thigh bone.

The abnormalities of the knee that allow the kneecap to dislocate include a too-shallow trochlear groove; a twisted shin bone resulting in a misaligned tibial crest; torn (or stretched) soft tissues that normally hold the kneecap in place; and in some cases significant bowing or twisting of the bottom of the femur bone. Dislocation of the kneecap also causes the shin bone to turn inward, which may cause the cranial cruciate ligament (CrCL) to tear. In fact, about 20 to 25% of the dogs that have a patellar luxation sustain an injury to this main-stabilizing ligament of the knee. Clinical signs of a dislocating kneecap include lameness, intermittent skipping gait, intermittent crying out or unwillingness to jump on elevated surfaces.

The diagnosis is made on physical examination, in which the surgeon can feel the kneecap dislocate out of place. In some cases, X-rays of the knee and thigh bone will be made to evaluate for twisting of the femur bone. This is most commonly indicated in large breed dogs. Other diagnostic tests that will be completed prior to surgery including a complete blood count and chemistry profile to ensure your pet is healthy enough to undergo general anesthesia.

The goal of surgery is to correct the anatomy of the knee so that the kneecap will remain in the trochlea. If the trochlea is shallow, the area is surgically deepened to create a groove that will capture the kneecap. Next, the alignment of the patellar ligament is assessed, and if not in line with the trochlear groove it must be moved over. This is accomplished by cutting the front of the tibia bone, moving the ligament, and securing the bone with pins. The pins are permanently left in place under normal circumstances. As a final step, the stretched soft tissues are trimmed and stitched together to pull the kneecap back into the trochlear groove.

In large breed dogs, a bowed thigh bone, which contributes to a dislocating kneecap, needs to be corrected. This is done by cutting, straightening and supporting the bone with a metal plate and screws.

About 90% of dogs that undergo this surgery will make a full recovery. Dogs that have a severe case of patellar luxation may have a recurrence of the condition, necessitating a second operation.

For more information on this subject, speak to the veterinarian who is treating your pet.