In horses, which feature contributes to the stay apparatus by enabling passive stabilization?

In horses, which feature contributes to the stay apparatus by enabling passive stabilization?

• A. Prominent patellar locking mechanism.
• B. Flexor tendon sheath expansion.
• C. Navicular tuberosities shape.
• D. Proximal phalanx duplex.

Answer: (A)

The stay apparatus relies on a locking mechanism at the stifle that provides passive stabilization. When the knee extends, the patella engages and locks over the medial trochlear ridge, aided by the associated ligaments, so the limb remains extended with the quadriceps relatively relaxed. This lock prevents the joint from collapsing into flexion, allowing the horse to stand quietly and bear weight without continuous muscular effort. Other features don’t create this locking effect—they contribute to other aspects of limb biomechanics but not to the passive stabilization provided by the patellar lock.