“Chocolate” was an 8 year old crossbred equine weighting approximately 500 kg with an unknown clinical history – at this time, 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 - flexion of the joints in 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 decrease in radiographic quality and the multiple film artifacts. Dorsoplantar and lateral radiographs of the left hind foot Dorsoplantar and lateral radiographs of the right hind foot Radiographic findings
- The bones in the feet remain in good alignment, however, the proximal interphalaneal joints have a mature bony ankylosis with marked periosteal new bone bridging the joints
- This produces several patterns of radiographic change - the first is characterized by enthesophyte formation that 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 – (changes are more mature on the left)
- Soft tissue swelling is minimal
- The metatarso-phalangeal joints and the distal interphalangel joints and the navicular bones appear within normal limits on these studies that are oblique
Radiographic diagnosis
- This is a classic case of the final stages of a “high ringbone” (noninflammatory ankylosing arthrosis) that is bilaterally symmetrical - It is a degenerative condition of the pastern joint (proximal interphalangeal joint)
- 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 apposing subchondral bone plates
Comments
- Cortical bone remodeling units consisting 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 fuses and forms bony bridges that connect the opposing subchondral bone surfaces - These “spot welds” stabilize the joint until the joint is fully ankylosed converting a highload/low motion joint into a highload/NO motion joint.
- “High ringbone” in an early phase is a painful disorder characterized 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 of the joint that 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 periarticular new bone - the resulting complete loss of joint movement often permits near-normal weightbearing as was seen with “Chocolate”.
These enlarged views of the joint spaces 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 “chocolate” was pasture-sound and could run with the other horses without showing clinical signs - she was untimately euthanized because of an acute colic.
