Some Aspects of the Pathophysiology of Subchondral Bone Injury of the Fetlock Joint

Alterations to the structure and the density of subchondral bone (SCB) is known to contribute to many of the pathological conditions that occur in the metacarpophalangeal joints (MCPJs) and the metatarsophalangeal joints (MTPJs) of the horse. In general, the most severe SCB changes and the most severe pathology are found in the racing horse; however, the joint injuries found in event, showjumping and endurance horses may also be related to underlying subchondral bone pathology.

Biomechanical features
In order to understand the aetiology of the SCB alterations, an appreciation of the biomechanics of the MCPJ during the stance phase of the stride is necessary. At impact, the MCPJ is in a position with the third metacarpal bone (McIII) and proximal phalanx (P1) dorsal cortices in alignment, or a neutral position. From impact to mid-stance, the MCPJ rapidly hyperextends as a result of the force of McIII pushing distally. Initially, the superficial digital flexor tendon (SDFT), deep digital flexor tendon (DDFT) and suspensory ligaments elongate to absorb the energy as McIII is driven distally; however, these tendons and ligaments rapidly undergo strain that approximates their elastic limit at mid-stance. At the limit of the stretch of the soft tissues, the MCPJ has reached maximum extension with the dorsoproximal margin of P1 impinging on the dorsodistal margin of McIII. Concurrently, the proximal sesamoid bones are pulled tightly against the distal palmar/plantar condyles of McIII due to the tensile forces on the suspensory ligament branches and the continued downward force from McIII. By examining the relationship of the bones in this position of fetlock joint hyperextension, an appreciation of the location of the highest loads in the joint can be obtained (Brama 2001; Easton and Kawcak 2007). […]