Article Written by Dr. Daniel A. Degner, Board-certified Veterinary Surgeon (DACVS)
Dislocation of the kneecap is painful and results in lameness
Evaluation of limb alignment may be recommended, especially in large breed dogs to assess the need to correct this; failure to address this can result in increased failure rates
Concurrent cruciate ligament injury may be present in dogs that have a dislocating kneecap, therefore this may also need to be addressed
Prognosis generally is very good - the higher the grade of the patellar luxation, the greater the failure rate unless more corrective surgeries are done
The patella (commonly known as the kneecap) normally rides in a groove at the bottom of the femur at the level of the knee joint in a groove called the trochlear groove; Fig 1 and 2 show a front view of the knee joint; Fig 1 demonstrates the patella in the groove, where as Fig 2 demonstrates the knee cap dislocated out out of the groove (P=patella; F=femoropatellar ligaments which hold the patella in the groove; PL=patellar ligament; G=trochlear groove that the patella rides in). Fig 3 demonstrates a skyline view of the trochlear groove and the patella - you are looking down the thigh or femur bone toward the knee joint; take note of the deep groove that is found in a normal animal.
Patellar luxation is caused by congenital abnormality usually at the level of the hip joint and results in abnormal forces on the kneecap, which cause it to eventually ride outside of the groove. The groove becomes very shallow and the attachment of the ligament of the patella may be malpositioned on the tibia bone. If the patellar luxation occurs in immature animals, the tibia and femur bones become twisted.
Below left is a an illustration of a normal limb with normal alignment of the femur bone, tibia bone, quadriceps muscle group (red), patella (brown) and the attachment of the patellar ligament (grey) to the tibia. Below left is a dog that has a deformity of the femur bone which causes the hind limb to be "bow-legged" and has resulted in the patellar luxation.
a skipping gait
stiffness of the hind limb
Some pets show only a single sign, whereas others show many signs of the condition
Failure to treat the condition could lead progressive debilitating arthritis of the joint
If the groove that the patella rides in is shallow or misshapen, it is surgically deepened; we usually use an advanced technique to perform this called the block osteotomy
If the attachment of the patellar ligament to the tibia, called the tibial crest, is in the wrong position, it is repositioned
The capsule of the knee joint is tightened; this tightens the femoropatellar ligament
In some dogs the femur bone is fairly twisted and needs to be cut and realigned
A support bandage is usually not used after surgery so that physiotherapy can be started soon after surgery
Below is an illustration of a front view of the knee joint before and after surgery; take note that there is no groove for the patella to ride in and the patella is dislocated to the side of the knee joint; the tibial crest (brown upside down tear shape) which is out of position is cut off of the tibia bone and moved over to realign the patellar ligament; the illustration below on the right shows the deepened groove with the patella seated in it and the tibial crest moved over to a better position on the tibia.
By 10 to 14 days after the surgery your pet should be touching the toes to the ground at a walk
By 2 to 3 months after surgery your pet should be using the limb well
If your pet does not follow a normal progression of recovery, the surgeon should be notified
Surgery has approximately a 90% success rate. Success is defined as the return of good function of the limb
Unfortunately surgery will not remove the arthritis that may already be present in the knee. As a result, your pet may have some stiffness of the limb in the mornings or after laying down for a nap. In addition, your pet may have some lameness after heavy exercise
By having the surgery done earlier, the chance of developing significant arthritis is decreased
Dogs that have a higher grade of patellar luxation may have increased risk for reluxation of the patella
Large breed dogs that have patellar luxation may have increased risk for reluxation of the patella if a corrective femoral osteotomy is not performed
There is an inherent risk of anesthetic death with any procedure requiring anesthesia, however, this is very small
Infection of the surgical site, although not common, can occur
If exercise is not minimized for 8 weeks after the surgery, breakdown of the repair may occur, thus requiring a second surgery