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. (show anatomy of knee)
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.
MPLs can occur in dogs of any age, breed, or gender. However, small and toy-breed dogs are most
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.
A medial patellar luxation 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.
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 medial patellar luxations. Other causes of
MPLs include tightness of the quadriceps muscle, an elongated patellar ligament, or trauma.
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
malignment 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 reluxation.
Many breeds are predisposed to congenital medial patellar luxations. Therefore, dogs diagnosed with
patellar luxations should not be bred.
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.