A"Popped knees" is a layman’s term for enlargement in the front of the carpal joints of a horse. The knee looks like it has "popped out." Sometimes this is accompanied by marked lameness, sometimes not. The enlargement at the front of the knee usually is from the enlargement of the synovial structures–the three tendons that go over the front of the knees (flexor carpi radialis, flexor carpi ulnaris, and extensor carpi radialis), two joints, and one bursa between the tendon and the joints. Inflammation in any of those structures will cause an enlargement to the front of the knee and therefore give the appearance that the knee has "popped."

Causes vary. Most commonly, popped knees occur from a chip in the joint in racehorses or horses which perform at high speed. Infection, developmental abnormalities in young foals which rupture their tendons in front of the knee, and direct trauma to the knee are other causes.

It’s important to identify which structures are inflamed, as treatment can vary. Sometimes all three structures can be involved, sometimes just one