Medial patellar luxation in dogs is very common in those under twenty pounds.
Let’s break down the term. Medial comes from the Latin medianus, meaning “middle.” Therefore, when something is medial on the body, it is relatively further toward the middle or midline 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 medial patellar luxation, 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 middle of the body.
Medial patellar luxation (MPL) in dogs is a genetic malformation, and occurs because of two primary abnormalities.
Breed predisposition especially exists in chihuahuas, Yorkshire terriers, toy and miniature poodles, Boston terriers and Pomeranians. Female dogs are half-again more likely to suffer from the condition than males.
Of course, MPL can also occur after trauma, such as a dog being hit by a car.
Chondrodysplasia in dogs may result in legs sufficiently crooked to pull patellas out of place because of poor alignment.
Medial deviation of the tibial crest is one of the more common causes. Run your index finger across your kneecap, down your leg until you touch your shin. That bone is the tibia. A normal patellar ligament will attach on the very front of the tibia. However, if breed-related genetic variations cause the patellar ligament to attach to the medial (inside) surface of the tibia instead, the kneecap is constantly pulled in that direction. If the wrong combination of factors comes together, the kneecap can be pulled out of the patellar groove by this misalignment.
Another mutation that occurs in many of the breeds listed above 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.
Treatment of medial patellar luxation depends on the cause.
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.
Severe chondrodysplasia resulting in extremely crooked femurs may require surgically cutting and straightening of the femur, allowing it to heal supported by orthopaedic plates.
Most medial patellar luxation, however, is repaired by addressing the tibial crest and femoral groove.
Femoral crest transposition is a procedure in which the incorrectly-placed bony attachment of the distal patellar ligament is removed, a “bed” of fresh bone is prepared on the front of the tibia (shin) and orthopaedic appliances are used to reattach it. In some cases, alignment repair alone will solve the patient’s problem.
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.
WHY IS TREATMENT NECESSARY?
Degree 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.
A few very, very small patients with patellar luxation experience no pain or interference with function.
Most medial 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.
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 well-off as a dog with a normal knee joint.
The vast majority of medial patellar luxation patients who have surgery will be near-normal if surgery is performed prior to the formation of significant arthritis.
See you tomorrow, Dr. Randolph.