Summary of the Condition Luxating Patellas

Jeff Poling

It should be noted, right off the bat, that this information should serve as a starting point for research, not as definitive answers. Every time I gather knew information, I come up with new questions as I digest it, especially when two sources seem to contradict each other. I will not call Spot's orthopedist to answer those questions as I feel I cannot monopolize the man's time on issues that do not bear directly on Spot's recovery. So I present here the results of my research insofar as I understand the facts and their interpretations.

Spot has grade 2 bilateral medial patellar luxation. Medial patellar luxation is a condition where the patella dislocates (luxates) to the inside, toward the body. This is the most common condition, occurring in 75% to 80% of all cases, according to the Orthopedic Foundation for Animals (OFA), and is almost always the condition suffered by toys. Lateral luxation, where the patella dislocates to the outside, away from the body, is much rarer, and is most often not seen before age 5 when it exists in toys.

Patellar luxation is degenerative. Once it starts, cartilage and bone tissue are broken down until damage and lameness occurs. Because it is degenerative, a dog can start out showing little or no symptoms, only to later be diagnosed as a full grade 2 or worse (see for an explanation of grades; suffice to say, the higher the number the worse it is). This does, I believe, explain why it took so long to diagnose Spot: there were no indications of the condition until it had worn down his tissue to the point of being obvious. It should be noted here that an orthopedic vet was able to find the condition in Spot's right leg, where sufficient breakdown had not yet occurred to reveal a problem to his regular vet. That is why examination by an orthopedic vet, rather than a regular vet, may be indicated. This is especially true as sometimes, as in Spot's case, the normal symptoms do not occur (see for the normal symptoms), or if no symptoms occur at all because the dog is a couch potato.

Of interesting note is whether or not the depth of the trochlear groove causes patellar luxation. I have heard from many a breeder that deep grooves are proof of solid knees in a dog, and that shallow grooves cause patellar luxation. According to Spot's orthopedic vet, neither is true (obviously, since Spot has deep grooves yet still has the condition). In fact, patellar luxation causes shallow grooves, not the other way around. While a dog can be born with deep grooves, a dog with shallow grooves, that has all its pieces and parts in the right place, will end up with deep grooves as the normal motion of the patella deepens the groove.

Patellar luxation is caused entirely by misalignment of the muscles and ligaments between the femur, patella, and tibia. When in proper place, they all pull in the same plane and the patella rides properly in its groove. However, if a misalignment occurs somewhere in this system, the patella will have forces applied out of its normal plane of motion, and will be pulled right out of the groove, no matter how deep the groove is. As noted above, the condition is degenerative, and things will steadily get worse. In Spot's case, it appears he was born with, or his patellas rode normally long enough to develop, deep grooves, but as he developed, the misalignment of his bones got worse until luxation occurred, which proceeded to get worse as degeneration set in. This also explains why it took so long to diagnose, as it got worse over time.

The misalignment can be caused by trauma, but whether it is heritable is another question. Although the luxation may not be congenital (meaning "present at birth," not genetic or inherited), the abnormalities that lead to it later (again, it is a degenerative condition) generally, but not always, are. The OFA states that medial patellar luxation should be considered heritable, while the heritability of lateral patellar luxation is unknown. Spot's doctor stated that the heritability of medial patellar luxation should also be listed as unknown. The reason for this unknown factor is that patellar luxation is developmental. Some dogs may have a "genetic predisposition" that can make a dog susceptible to developing luxating patellas. This possibly refers to inherited skeletal abnormalities, such as torsion of one or both of the leg bones, or similar issues. My own personal take is that this "genetic predisposition" does exist and is at the root of at least some cases, as Spot's half-brother by his sire also has the condition, with the same symptomology. However, patellar luxation can also be due to other developmental factors, such as improper exercise, malnutrition, or anything that can impact bone, muscle and ligament growth at any time during the dog's life. As such, there are many things that can cause the condition, not just inherited factors. Clearing up any genetic factors in all dogs will not eliminate the condition as trauma and other factors can still cause it. It also means that if scientists ever fully understand the developmental issues that lead to patellar luxation, which they currently do not, they may be able to prevent it even in dogs that have "genetic predisposition."

It is hard to conceive a tougher nut to crack. The facts that patellar luxation is a degenerative condition, one that, unless it is due to severe torsion of the bones, may not appear on x-ray, may have multiple genetic and environmental causes, and may not show up symptomatically until many years down the line, after a particular dog has been bred, just complicate the bejeebers out of things.

Copyright © 2002 by Jeff Poling.
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Revised: December 31, 2001; New: December 31, 2001