How does the patellar locking mechanism stabilize the horse's stifle in extension?

How does the patellar locking mechanism stabilize the horse's stifle in extension?

• A. The medial patellar ligament tightens to lock the patella.
• B. The femur rotates to lock the stifle.
• C. Medial patellar ligament engages over the medial trochlear ridge when the stifle is extended, locking the patella and maintaining the stifle in extension with minimal muscle effort.
• D. The patellar ligament is unaffected by the stifle position.

Answer: (C)

The stay in extension is achieved by a passive locking action of the medial patellar ligament. As the stifle extends, the patella slides so the medial patellar ligament hooks over the medial trochlear ridge of the femur. This engagement tightens the ligament and prevents the patella from moving, effectively locking the extensor mechanism in place. With the ligaments supporting the joint, the stifle can stay extended with only minimal muscle effort, which is exactly how a horse can stand for long periods. The lock is released when the limb flexes or weight shifts and the ligament disengages from the ridge.