What is a luxating patella?
The patella, or knee cap, should be located in the center of the knee joint.
The term "luxating" means out of place or dislocated. Therefore, a luxating
patella is a knee cap that moves out of its normal location.
What causes this to occur?
The muscles of the thigh attach directly or indirectly to the top of the knee
cap. There is a ligament, called the patellar ligament, which runs from the
bottom of the knee cap to a point on the tibia just below the knee joint. When
the thigh muscles contract, the force is transmitted through the patella and
through the patellar ligament and results in extension (straightening) of the
knee joint. The patella stays in the center of the leg because the point of
attachment of the patellar ligament is on the midline and because the patella
slides in a groove on the lower end of the femur (the bone between the knee
and the hip).
The patella luxates because the point of attachment of the patellar ligament
is not on the midline of the tibia. It is almost always located too far medial
(toward the middle of the body). As the thigh muscles contract, force pulls
against the groove on the inner side of the femur. After several months or years
of this abnormal movement, the inner side of the groove wears down or the ligament
stretches and the patella is free to move out of the groove or dislocate. When
this occurs, the dog has difficulty bearing weight on the leg. It may learn
how to kick the leg and snap the patella back into its normal location. However,
because the groove is gone, it dislocates again easily.
Does a luxating patella cause any long-term problems for my dog?
Some dogs can tolerate this problem for many years, some for all of their
lives. However, this anatomical abnormality present weakness predisposes the
patella to other injuries, especially torn cruciate ligaments. With advancing
age, the joint may become arthritic and painful.
Can a luxating patella be corrected?
Surgery should be performed if your dog has a persistent lameness or if other
knee injuries occur secondary to the luxation.
Surgical repair is a three step process.
1. The point of attachment of the patellar ligament is moved and transplanted
surgically to its proper location to correct the incorrect alignment.
2. The groove in the femur is deepened so the patella will stay in place.
3. The capsule around the joint is tightened. This last step is important
because the joint capsule will have stretched during the period of luxation.
If the surgery is performed before arthritis occurs, the prognosis is excellent.
Your dog should regain full use of its leg. However, if arthritis has already
occurred, the joint will still be somewhat painful, especially in cold weather.
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