What does it mean when a horse has locking stifles or a locking patella?

We were at a barrel race last week and my mare suddenly did not want to move. We initially thought that she may be tying up. The vet came out and said that since she wasn’t sweating, she wasn’t likely tying up. After a brief exam, he pushed her hind end a bit, and then she began to move, explaining that her stifles were “locked”. He was in a hurry and did not have time to discuss the issue at length. Could you please discuss “locking stifles?”

The actual term for this condition is “upward fixation of the patella.” According to Adams’ Lameness in Horses “upward fixation of the patella (UFP) occurs when the medial patellar ligament becomes caught over the medial trochlear ridge. If it becomes fixed in that position, the hind limb cannot be flexed.”

This condition is more likely to occur in a horse with straight hind limb conformation or upright stifles. Though UFP often presents in one limb, both limbs are likely affected. In addition to the conformation of the horse, another significant factor is the fitness level (under conditioned) of the horse. With the out of shape horse, there can be a laxity in the patellar ligament, allowing for the upward fixation. This fixation can occur in different severities, leading to varied levels of lameness. There are multiple treatment options available, but it is best to consult a veterinarian for each individual case.

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