Experimental model of synovitis/capsulitis in the equine metacarpophalangeal joint

To develop and define a model of acute synovitis/capsulitis in the equine metacarpophalangeal joint (fetlock) to study clinical effects of diagnostic or therapeutic procedures.

5 adult Standardbreds.

Polyvinyl alcohol foam particles were injected into the left front fetlock of horses; the right front fetlock was used as a control. Horses were examined clinically and for lameness on a regular basis. Blood samples were taken to measure routine variables. Synovial fluid samples were collected from both fetlocks, and macroscopic, microscopic, and biochemical variables were measured. The effects of local diagnostic nerve blocks and intra-articular anesthesia of the fetlock on ensuing lameness were determined on days 2 and 3. After macroscopic evaluation at necropsy, histologic examination was performed on the fetlock cartilage and capsule.

Changes in synovial fluid variables included increased TNF-alpha and matrix metalloproteinases values and in other indicators of acute inflammation. With respect to application of local nerve blocks, lameness decreased most after a combination of metacarpal and palmar nerve blocks, whereas lameness could only partially be blocked by intra-articular anesthesia. At necropsy, mild chronic inflammation of the synovium and deeper layers of the joint capsule was found; polyvinyl alcohol particles had been incorporated into the joint capsule. There was no evidence of articular cartilage damage.

Injection of polyvinyl alcohol foam particles induces transient synovitis/capsulitis in the fetlock, mimicking clinical joint disorders. The model does not seriously affect the horse's long-term wellbeing. Such model serves well for studying the effect of diagnostic or therapeutic procedures.