Question of the Week: Floating Patella

Q: I have a 2-year-old mini stud with a left hind leg that locks up on him at times. If we walk him backwards, it lets go and he walks and runs fine. The vet diagnosed him with floating patella. We have never heard of it, and she also said it could be passed on to any colts. Could you please explain what floating patella is, and if it would be passed on to my horse’s foals? Can it be treated?

A: A floating patella is also known as a luxating patella, or “upward fixation of the patella”, and is a condition seen in equines, canines, and humans, where the patella (or kneecap) is moved out of its patellar grove, the anatomical location at the end of the femur where the patella normally sits in the stifle joint. Normally, the patella is held in place by three patellar ligaments and when these become somewhat slack, they are unable to hold the patella in its natural place, allowing it to slip out. While this sounds painful, it really isn’t, but it does severely hamper the horse’s ability to move his hind leg in a normal fashion. What results with a luxated patella is a leg that looks “locked up” and stiff. When the patella is outside of its normal position, the stifle joint loses its flexing capabilities but normal locomotion resumes as soon as the patella pops back in place, which is usually within seconds or minutes.

Luxating patellas are most commonly seen in younger horses and can be the result of weak quadriceps. This large group of muscles in the hind leg is responsible for holding the patellar ligaments tight so when these muscles are weak, the ligaments that hold the patella in place are weak as well, pre-disposing the horse to a mobile patella. Some horses only suffer from a floating patella in one hind leg, while others have bilateral, or two-sided, luxating patellas.

The most commonly recommended treatment for luxating patellas is strength conditioning with focus on the hind legs. An emphasis on hill work can help strength your horse’s quadriceps, which in turn will help keep those patellar ligaments tight. A more invasive procedure involves surgically tightening one of the patellar ligaments by nicking it to create the formation of scar tissue, which is tighter and less elastic than normal tissue. In recent decades this has fallen out of favor, but may be the only option for a horse, particularly a riding horse, that has severe laxity in those ligaments.

Unfortunately, luxating patellas are a genetic defect and these genes will be passed from your stud to his offspring. Studies have shown luxating patellas to be a recessive trait, meaning not all offspring will exhibit the clinical problem, but will still harbor the gene and therefore silently pass the trait on to their offspring.

— Anna O’Brien, DVM

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