What Is It?

The patella, more commonly known as the kneecap, is a small bone that sits in a groove, called the trochlear groove, at the bottom of the femur. The patella is held onto the tibia by the patellar tendon, and is attached at the top by the quadriceps muscle of the thigh. When the quadriceps muscle contracts, it pulls the stifle (knee) into extension.

A patellar luxation occurs when the patella slides out of its place within the trochlear groove. In a medial patellar luxation, the patella is displaced to the medial, or inner, side of the stifle. Lateral luxations can also occur, but are less common.

Who Gets Them?

MPLs can occur in dogs of any age, breed, or gender. However, small and toy-breed dogs are most frequently affected.

What Are The Signs?

Most dogs with a medial patellar ligament have an intermittent lameness, but are still weight-bearing. They often have a sporadic skipping gait, or will hold the leg in flexion for a couple steps. Depending on the degree of luxation, they are able to “pop” the patella back in place. Over time, signs can become more apparent as the animal gains weight, the luxation becomes permanent, and/or the articular cartilage erodes. In very severe cases, the dog may walk in a crouched position because they are unable to fully extend the stifle because the patella remains dislocated. The instability in the stifle can also lead to a cruciate ligament rupture or a hip luxation, since other structures in the limb have to compensate.

How Is It Diagnosed?

It is diagnosed based on clinical signs, and the ability of the veterinarian to manually dislocate the patella on physical exam. Radiographs can be used to visualize a dislocated patella. Patellar luxations are graded on a scale of 1 to 4.

A grade 1 means that the patella becomes dislocated intermittently, and the dog favors the leg occasionally. In these cases, the patella is easily popped back into place.

In a grade 2, the luxation occurs more frequently than a grade 1, but lameness is still intermittent and mild. Dogs with a grade 2 luxation can live with it for many years, but over time, the slipping in an out of the patella can cause erosion of the bone.

In both grades 3 and 4, the patella is permanently dislocated, and the dog cannot “pop” it back into place, and it coincides with a twisting if the tibia, and a shallow or even absent trochlear groove. These dogs are constantly lame.

Why Did This Happen To My dog?

Most medial patellar luxations are considered congenital – they occur early on in life, and are not associated with trauma. In most cases, the condition results from having a trochlear groove that is too shallow, or even absent. In some dogs, MPLs are caused by a congenital or developmental misalignment of the femur or tibia. Patients that have a chronic history of lameness, namely those with hip dysplasia or cranial cruciate ligament deficiency, are predisposed to this. Other causes include tightness of the quadriceps muscle, an elongated patellar ligament, or trauma.

How Is It Treated?

Grade 1 patellar luxations that do not cause clinical signs should be monitored, but usually to not warrant surgical correction. Surgery is considered in grades 2, 3, and 4. If the cause of the luxation is that the trochlear groove is too shallow or is misshapen, it can be deepened surgically. Another technique to correct an MPL is by loosening the soft tissue structures on the medial side, while tightening the opposite side so that the tendency for the patella to dislocate medially is alleviated. Dogs that have a malalignment and/or twisting of the tibia or femur must have corrective surgery to fix the underlying misalignment, because just adjusting the soft tissue structures or deepening the trochlear groove alone will not correct the problem and will not prevent re-luxation.

Can It Be Prevented?

Many breeds are predisposed to congenital medial patellar luxations. Therefore, dogs diagnosed with patellar luxations should not be bred.

What Is The Prognosis For My Dog?

Overall, the prognosis for patients undergoing surgical correction of a grade 1, 2, or 3 patellar luxation is excellent for return to normal limb function. The prognosis is less favorable in large dogs, especially when the patellar luxation is combined with other conditions, such as hip dysplasia. Degenerative joint disease is expected, and it progresses despite treatment but will be less severe than without surgical repair. Weight management and appropriate exercises will help minimize the progression of DJD. Prognosis for patients with a grade 4 patellar luxation is guarded, because many of these dogs require multiple surgeries, and some require extensive surgical correction.

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