If the stifle is flexed and the hock does not flex with it, there is a problem with which structure?

• A. Navicular ligaments
• B. Cranial tibial tendon
• C. Seasmoidean ligaments
• D. Peroneus tertius

Answer: (D)

The stay apparatus relies on a reciprocal mechanism that links the stifle and the hock so their movements are coordinated. The peroneus tertius, along with the cranial tibial tendon, forms this connection, transmitting motion between the two joints. When the stifle is flexed, the normal function is for the hock to flex as well, because the tendon linkage allows that movement to pass through both joints.

If you observe the stifle flexing while the hock does not follow suit, that disruption points to a failure in the reciprocal mechanism. The most common culprit is a rupture or severe damage to the peroneus tertius, which breaks the continuity needed to transfer motion from the stifle to the hock. Without the peroneus tertius functioning, the stifle can move independently of the hock, producing the described sign.

Navicular or sesamoidean ligaments affect other parts of the limb and joints and wouldn’t explain this specific lack of hock flexion in response to stifle flexion. The cranial tibial tendon is part of the same reciprocal system, but clinically this particular sign most classically indicates peroneus tertius disruption.