The patella ligament is a structure than runs over the cranial aspect (the front) of the stifle joint (knee joint). The patella ligament has a bone — the patella, that most people know as the knee-cap. This ligament and bone track up and down in a V-shaped groove, called a trochlea, from the base of the femur to the tibial crest (the front and top of the shin-bone). The patella and patella ligament form a critical part of normal stifle flexion and extension.
In some individuals, the patella ligaments can move abnormally side to side out over the ridges of its trochlear groove. This is called luxation, and is generally due to abnormal limb conformation. In many cases, the tibial tuberosity (where the patella ligament attaches to the tibia) is positioned too far to one side, which pulls the ligament over the edge of the trochlear groove. In some cases, the trochlear groove may be too shallow, allowing the patella ligament to easily slip over the ridges.
Every time the patella slips over the trochlear ridges, it causes inflammation and abrasion to the cartilage within the joint. Over time, this inflammation can cause pain and result in early-onset arthritis.
As the patella and patella ligament are part of normal knee movement when walking or running, patella luxation causes the knee to be fixed in flexion (bend).
The patella ligament may luxate medially (towards the inside of the leg), or laterally (towards the outside of the leg) depending on the individual’s conformation. Medial patella luxation is much more common than lateral patella luxation. Patella luxation is most common in many small breeds of dog, but can occur in dogs and cats of any size.