Patella is the medical term for the kneecap. The patella in dogs serves the same purpose as in people, namely to aid the quadriceps muscles (thigh muscles) in the extension of the knee. Normally the patella lies in the trochlear groove of the distal femur within the tendon of the of the quadriceps muscles (See Figure 1). While in the correct anatomic position it glides very smoothly and non-painfully during the flexion and extension of the knee. Luxation of the patella simply means that it "pops out " of the trochlear groove. In the case of a medial patella luxation it "pops out" to the inside of the knee.

Medial patella luxation can be congenital (present at birth) or acquired. In miniature and small breeds of dogs the condition is generally present at birth and has a lot to do with the confirmation of their rear limbs. Most of these dogs have a "bow-legged" stance, which results in the muscles of their thighs pulling their patella to the inside of their knees. This abnormality is aggravated when the tibial crest (the point the quadriceps tendon attaches to on the lower limb) is internally rotated and the trochlear groove is shallower than normal.

After a period of time the chronic luxation of the patella results in abnormal wear on the cartilage of the knee and underside of the patella. This can result in inflammation, arthritis and chronic pain. Initially most dogs can pop the kneecap back in after luxation, but eventually the luxation may become too severe.

There are different grades of patella luxation (See Figure 2). Prognosis for correction is somewhat dependent on the grade.

 

 

 

 

 

 

 

 

 

 

 

The different grades are:

GRADE I The patella can be forcibly luxated, but does not generally result in pain or is in need of correction.
GRADE II The patella luxates more easily, but stays in the groove the majority of the time. This grade may need correction.
GRADE III The patella is generally out of the groove most of the time, but can be replaced manually. This grade usually requires corrective surgery.
GRADE IV The patella is luxated permanently and cannot be replaced in the groove. This grade is in need of surgical correction but prognosis for successful correction is not as good.

Surgical correction usually consists of a combination of three different procedures. The first consists of deepening the trochlear groove via a wedge osteotomy (See Figure 3). This allows the patella to be seated deeply in the groove at the distal end of the femur. The deeper the groove the less likely the patella is to luxate. The second procedure is a tibial crest transposition (See Figure 4). With this procedure an osteotomy is performed on the bony attachment point of the quadriceps on the tibia. The tibial crest is moved in the opposite direction of the luxation and reattached to the bone with a pin. This realigns the muscle pull straight up a down the leg and corrects the tendency of the patella to luxate medially because of the bow legged confirmation. The third procedure is a joint capsulectomy . A segment of the lateral (outside) portion of the joint capsule is removed. This tightens the joint capsule so that there is no unnecessary slack that would allow the patella to drift medially. One or all of these procedures may be performed depending on what is needed to correct your dog's problem.

After the surgery the leg will be bandaged for a variable amount of time depending on your doctor. Once the bandage has been removed you can start physical therapy as prescribed by the surgeon. In most case use of the limb is encourage. However, certain activities such as running, jumping or playing must be avoided for 6 - 8 weeks. At the end of 6 - 8 weeks an x-ray should be taken to make sure the tibial crest has healed.