Which of the following is a common differential diagnosis for stay apparatus-related lameness that involves the distal fetlock ligaments?

• A. Navicular disease
• B. Distal sesamoidean ligament injury
• C. SDFT/DDFT tendinopathy
• D. Osteoarthritis of the hock

Answer: (B)

Stay apparatus relies on ligaments to hold the limb in a weight-bearing position with minimal muscular effort. The distal sesamoidean ligaments form a key part of this system by anchoring the navicular (distal sesamoid) apparatus to the pastern and distal phalanx, helping stabilize the fetlock joint during loading. When these ligaments are injured, the stability of the fetlock is compromised, leading to pain and lameness centered in the distal fetlock region that is characteristic of stay apparatus–related lameness. This makes distal sesamoidean ligament injury the most typical differential in this context.

Navicular disease can involve the navicular apparatus but is not specifically about the distal fetlock ligaments. Tendinopathy of the SDFT/DDFT affects tendons rather than ligaments. Osteoarthritis of the hock concerns a different joint altogether.