Introduction
- 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, whereas Fig 2 demonstrates the knee cap dislocated 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.
Clinical signs
- Lameness
- Intermittent skipping gait
- Pain
- 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 to progressive debilitating arthritis of the joint
Surgery
- 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. The illustration on the right demonstrates elevation of the cartilage/bone plate. I now use a modified procedure in which the cartilage/bone plate is left attached to the soft tissues (periosteum) at the top of the groove, thus minimizing the risk that the cartilage/bone plate will become displaced in the joint.
- Additional bone is removed from the raw bone bed and the cartilage/bone plate is replaced creating a deep groove for the patella to ride in. The benefit of the block osteotomy is that it uniformly deepens the groove from top to bottom and preserves the cartilage that the patella rides on.
- The soft tissues along the side of the patella usually are stretched and are tightened to provide additional support to keep the patella in the trochlear groove.
- The femur bone may be twisted in some dogs which worsens the condition of the luxating patella. I find that dogs that have a greater than 14 degree bowing (varus) of the thigh bone should have this surgically corrected. Special alignment x-rays are usually taken in large breed dogs to check for this problem.
- A support bandage is usually not used after surgery so that rehabilitation therapy can be started soon after surgery
Aftercare
- Provide analgesics and nonsteroidal anti-inflammatory medication as directed by your pet’s surgeon
- Excessive activity must be limited for about 2 to 3 months
- Rehabilitation therapy is very important
Convalescence
- 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
Prognosis
- 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
Potential complications
- In the event of an uncommon reaction to anesthesia, death may occur
- Infection of the surgical site, although not common, can occur
- Pin migration or pin breakage
- Seroma formation over the pins
- If exercise is not minimized for 8 weeks after the surgery, breakdown of the repair may occur, thus requiring a second surgery