Medial patella luxation (MPL) is one of the most common orthopedic conditions seen in the canine population (80-90% compared to lateral luxation depending on the source), and it is more common in small breed dogs including miniature and toy poodles, Yorkshire terriers, Pomeranians, Chihuahuas, Boston terriers, Pekingese, and cavalier King Charles spaniels. The majority of the cases is due to congenital abnormalities, although it can be caused by traumatic events as well, and females have been reported to be 1.5x more likely to develop it. Due to its congenital nature it is also often seen bilaterally. MPL also predisposes the patient to have a rupture of the cranial cruciate ligament.

Diagnosis:

The hallmark of MPL is the patient presenting with a intermittent skipping gait. That limb becomes non-weightbearing during the luxation phase and returns to weight bearing once the patella reduces, commonly when the dog kicks his/her leg back into extension. Although not as often, patients may also present with a constant lameness on the hind limb. These patients are usually middle aged or older and upon questioning the owners will likely describe that the patient had a long term history of intermittent lameness and skipping gait. In this scenario the clinician also needs to consider a possible rupture of the cruciate ligament. Patella luxation is diagnosed by physical examination; however, it is also recommended to take x-rays to assess for degenerative changes and to evaluate bony conformation.

The severity of the MPL is classified from grades 1 to 4:

I: patella can be manually dislocated but it reduces once released

 II: patella remains luxated until it is manually reduced or until the dog extends the stifle joint and rotates the tibia to reduce it

III: patella remains luxated most of the time but it can be manually reduced with the joint in extension (stifle flexion results in luxation)

IV: patella is permanently luxated and cannot be manually repositioned. The femoral trochlear groove is shallow or absent and there is displacement of the quadriceps muscle group

Treatment:

Grades I and asymptomatic grades II can respond very successfully to conservative management with rehab; however, grades III and IV will more than likely require surgery. However, each case can be different and a consult with a veterinary surgeon and/or canine rehab therapist is always recommended. The decision to pursue surgery should take into consideration the frequency and severity of the lameness, any functional limitations, and the opportunity to decrease the likelihood of tearing the cruciate ligament. There are a few different surgical procedures available depending on the exact cause of the luxation (conformational vs shallow groove vs tibia internal rotation), and to discuss those into detail would go beyond my scope as a physical therapist and in-depth knowledge on the subject. However, next week I will discuss more about the rehabilitation approach for conservative management and post-op patients. 

~TheK9PT