“Vanilla” was an 8 year old crossbred weighting approximately 500 kg with an unknown history - she was kept in a pasture with other horses and used occasionally for easy riding - palpation of her hind feet showed a firm swelling bilaterally that was not painful and typical for a “high ringbone” - flexion of the feet was limited - dorsoplantar and lateral radiographs were made of both feet in the field with a portable x-ray unit which explains the lack of radiographic quality. This is a classic case of the final stage of a “high ringbone” that is bilaterally symmetrical - It is a degenerative condition of the pastern joint (distal P-1 and proximal P2) - The pattern of new bone formation is dependent on these joints being high load/low motion joints - In “high ringbone”, the apposing joint surfaces are set under sustained compression and the articular cartilage begins to undergo full thickness necrosis - simultaneously, static loading of the subchondral bone stimulates bone remodeling units to arise within the apposing subchondral bone plates - Cortical bone remodeling units consist of “cutting cones” of osteoclasts that lead “filling cones” of osteogenic granulation tissue - These penetrate the necrotic articular cartilage at sites of full thickness necrosis of the articular cartilage and bridge the joint space between apposing subchondral bone plates - Subsequently, osteogenic granulation tissue of the filling cones arising from apposing subchondral bony plates fuse and form bony bridges connecting the opposing subchondral bone surfaces - These “spot welds” stabilize the joint until the joint is ankylosed converting a highload/low motion joint into a highload/NO motion joint.“High ringbone” is a painful disorder characterized early by the production of enthesophytes forming in the fibrotic joint capsule insertion lines and the insertion of the common digital extensor tendon - What begins early as a periarticular disease that is painful soon becomes articular with limited motion and is much less painful - Often the disorder is found in horses running at high speeds who are required to make abrupt stops, acute turns, and twisting motions - The naturally limited range of motion of the joint is lost initially because of the pain - The resulting complete loss of joint movement permits normal weight-bearing - This joint disease produces several patterns of radiographic change - The first is characterized by enthesophyte and osteophyte formation - This is followed by joint space narrowing with destruction of the subchondral bone - Finally, the bony ankylosis leads to joint fusion as seen in this horse.These enlarged views of the joint space are presented with differing contrast and density and show the disappearance of the normal subchondral bone and the radiolucent joint spaces following the bony ankylosis - Notice the process is slightly advanced in one foot when compared with the opposite. It was interesting that “vanilla” was pasture-sound and could run with the other horses without showing clinical signs - She was ultimately euthanized because of an acute colic.
