What is proximal suspensory desmitis (PSD) and its relevance to the stay apparatus?

• A. Inflammation or degeneration of the proximal suspensory ligament that disrupts the stay mechanism and causes lameness, especially after work.
• B. Inflammation of the digital cushion causing altered gait.
• C. A fracture of the proximal cannon bone.
• D. A tear of the dorsal Y-ligament.

Answer: (A)

Proximal suspensory desmitis refers to inflammation or degeneration of the proximal suspensory ligament, a major component of the stay apparatus. In a healthy stay apparatus, the suspensory ligament, along with the other supportive structures, helps hold the fetlock in extension so the horse can bear weight with minimal muscular effort. When the proximal suspensory ligament is damaged, its ability to resist fetlock flexion under load is reduced, so the limb becomes painful and the horse shows lameness that often becomes more evident after work when the ligament is stressed. The other options point to problems with different structures (digital cushion, bone fracture, or a different ligament) and do not describe the injury to the proximal suspensory ligament that disrupts the stay mechanism.