Patellar luxation in the dog is almost as common as ACL rupture. In this condition, the patella (kneecap), normally located in the trochlear groove, will come out of the groove towards the inside of the knee, making the knee appear to lock up in flexion. Though it rarely presents as an acute problem, it can cause as much damage as an ACL rupture does, or eventually end up with an ACL rupture. It is more common in smaller dogs, and often involves both knees. It presents clinically in 4 separate (though not entirely distinct) stages, each requiring different repair techniques.
GRADE I – In this instance, the patella barely luxates out of the trochlear groove and will return on its own; this stage does not require surgical intervention unless it is clinically indicated for your dog. This is a normal finding in most cats.
GRADE II – In this case, the patella easily comes out of the trochlear groove, and is easily returned with slight manual pressure. These patients will frequently skip when they run, and usually require surgical correction.
GRADE III – In grade III, the patella spends more time out of the trochlear groove than in it. When manually pushed back into the groove, it comes back out. These cases will frequently have scar tissue medially, and appear to have a crooked leg (some actually do). These patients need more involved surgical repair to improve the luxation.
GRADE IV – We rarely see cases get to the state of grade IV. The patella is permanently out of the groove, and cannot be forced back in. Most of these dogs will require serious surgical intervention to correct them, including some corrections to the degree of “crookedness” present in the knee.
Surgical Correction (Medial Luxation)
Surgery requires a number of procedures usually taken in a stepwise fashion to achieve stability of the knee throughout range of motion. Radiographs must be taken to assess the straightness of the limb as well as the condition of the hip joints and knees.
STEP 1 – Lateral approach to the knee, with a medial release of the scar tissue. The joint should be inspected for the presence of cranial cruciate ligament (CCL) tearing. The medial release must be completed above the patella to be sufficient; the knee should be imbricated (tightened) on the lateral aspect.
STEP 2 – Inspect the trochlear groove; is it too shallow? It should accommodate at least 50% of the depth of the patella, if not it requires a technique to deepen it. We can now repair the trochlear groove using an implant called RidgeStop(TM). This lessens the amount of painful surgery your pet must undergo to correct this situation.
STEP 3 – Transposition of the tibial tuberosity; this technique is used to realign the patellar ligament so that when the quadriceps muscle contracts, the patella stays in the groove. Less dogs require this step if they have the RidgeStop(TM) implant used in their knee or if they also require TTA or MMP surgery. TPLO can also be used to correct MPL but it is more difficult to perform.
STEP 4 – Release of the rectus femoris muscle is sometimes required in smaller animals, as this muscle sometimes pulls the patella out to begin with. There are 3 other muscles that will take over function of this muscle, so it is not crucial that it stay intact, thought we usually attach it to the top of the femur.
Reasons For Failure
Patellar luxation repair has a failure rate anywhere from 15-20% depending on the severity of the condition and the timing of repair. The more severely affected knees have higher failure/complication rates associated with surgical intervention. Your dog should take about 4-6 weeks of cage rest to heal, and should be walking normally. There is osteoarthritis present in the knee prior to surgery, which surgery cannot cure. It is important to keep in mind that as your dog gets older with MPL, the likelihood of cranial cruciate tear concurrently is much higher, particularly after just 3 years of age.
More recently, we have been provided with a new implant called the Ridge Stop(R). This implant is placed in the knee joint to keep the patella from sliding out of the groove. The recovery is a bit slow, but it requires less invasive procedures to correct several of the problems that occur with patella luxation. The only real disadvantages to RidgeStop are cost and potential for infection.