Lateral patellar luxation, unlike medial patellar luxationoccurs almost exclusively in large-breed dogs. However, large-breed dogs can also experience patella luxation to the medial aspect of the femur.
Let’s break down the term. Lateral comes from the Latin lateralus, meaning “belonging to the side.” Therefore, when something is lateral on the body, it is relatively further toward the side or outside of the body than whatever it is being compared to.
Patella is also Latin, and comes from the diminutive form of patina, which means “shallow dish,” designating the shape of a dissected patella. You know the “patella” as the “kneecap.” Patellar is the adjective form of the noun patella.
Luxation comes from the Latin word luxus meaning “dislocated.”
Therefore, in lateral patellar luxation (LPL), the kneecap dislocates from its normal position in the patellar groove (also called the trochlear groove or trochlear sulcus) of the femur (thighbone) and rides over the ridge of the groove to reside toward the outside of the body.
Much of LPL can be attributed to genetic malformation. Most commonly, affected dogs are born with anteversion of the femur. To understand anteversion, picture the top of the femur (thighbone) twisted in an opposite direction from the bottom of the bone. With the top twisted “out” (laterally) and the bottom twisted “in” (medially), the kneecap is naturally pulled toward the outside. Anteversion, alone, may be sufficient to cause the patella to luxate, or there may be other contributing factors, described below.
Another mutation that occurs in many of the dogs affected is a shallow femoral groove. A normal femoral groove is lined with healthy cartilage and deep enough that the patellas ride up and down, nestled firmly in place. A shallow groove, however, allows the patella to easily move in and out of place. Combined with forces pulling the kneecap to the outside of the leg, the patella may be forced to luxate even during normal daily activity.
Of course, LPL can also occur after trauma, such as a dog being hit by a car.
Luxation caused by trauma may require only that the kneecap be moved back into place. If its associated ligaments have been slightly stretched, a temporary bandage may be placed to allow structures to regain their normal size and position. On the other hand, if trauma has resulted in broken bones and/or torn ligaments, major surgical repair may be required.
Some femoral grooves are so shallow as to provide absolutely no control for the patella. In this case, a wedge is surgically removed from the groove, preserving the vitally-important cartilage. Additional bone is removed, then the wedge is returned to its original location. The patella can continue to ride on smooth cartilage, but now it will rest in a new, deeper femoral groove.
If anteversion is also part of the problem, crooked femurs may require surgically cutting and straightening of the femur, allowing it to heal supported by orthopaedic plates and bone grafts.
WHY IS TREATMENT NECESSARY?
Severity of patellar luxation is measured by Grades I–IV. A Grade I luxating patella patient may experience kneecap dislocation infrequently and almost never have sufficient pain to cause limping.
Grade II luxation patients may have an occasional “skip” in the gait and infrequent limping as the patella sometimes dislocates, then moves back into the femoral groove on its own.
Grade III patients “skip” frequently and commonly present to the veterinarian with a carrying-leg (non-weight-bearing) lameness.
Grade IV dogs suffer from almost-permanent dislocation of the patella and usually walk with a crouched gait because of the inability to stand up straight. Doing so would require the leg(s) be fully extended, which is too painful for the dog whose kneecaps are out of place. Dogs with Grade IV luxation frequently have advanced arthritis of the knees.
Most lateral patellar luxation patients who are symptomatic will exhibit signs of limping or holding up the affected leg. Abnormal stretching of the ligaments that attach to the top and bottom of the kneecap is painful, and adds to increased laxity in the joint. Over time, all of the surrounding structures of the knee become looser.
Every time the patella moves over the ridge of the femoral groove, damage is done to the cartilage. Such damage causes osteoarthritis. Eventually, the cartilage will be damaged beyond repair, and the patient is doomed to a life of painful arthritis. While treatments have advanced for arthritis, the affected dog is never as comfortable, healthy and active as a dog with a normal knee joint.
The vast majority of lateral patellar luxation patients who have surgery will be near-normal if surgery is performed prior to the formation of significant arthritis.
See you Monday, Dr. Randolph.