Horses with osteoarthritis (OA) of the distal interphalangeal joint (DIP joint), or coffin joint, can be unilaterally or bilaterally affected. Forelimbs tend to be more commonly affected than hindlimbs. A wide range of clinical signs are possible, from mild (shortened stride, poor performance) to severe lameness.
Horses with osteoarthritis can have varied responses to flexion of the distal limb, with some showing no positive response and others showing exacerbation of lameness. On musculoskeletal examination, horses with osteoarthritis of the DIP joint can have thickening or fibrosis dorsally just proximal to the coronary band or palpable joint effusion. However, these findings alone are not pathognomonic.
Lameness can be localized by use of either perineural anesthesia of the palmar/plantar digital nerves (pastern level or base of the proximal sesamoid bones) or intra-articular anesthesia of the DIP joint. However, diffusion of local anesthetic using either method can improve lameness originating from other areas of the foot or pastern; therefore, careful interpretation of the block and diagnostic images is important.
Radiography and ultrasonography can be used to evaluate the DIP joint for signs of osteoarthritis, including bone proliferation at the joint margin or at the attachment of the joint capsule, osseous fragments within the joint, or synovial invaginations on the distal margin of the navicular bone.
Nuclear scintigraphy can be helpful in some cases to identify increased bone turnover in the region of the DIP joint. CT and MRI can be used to evaluate the subchondral bone and the cartilage of the joint. Diagnostic arthroscopy can also be used to examine the cartilage surface of portions of the dorsal and palmar aspects of the joint.
Osteoarthritis in the DIP joint can be treated with systemic NSAIDs or chondroprotectants; intra-articular injection with corticosteroids, viscosupplements, biologics, or a combination of these; extracorporeal shock wave therapy; or shoeing changes.
The prognosis for return to function in cases of osteoarthritis of the DIP joint in horses is guarded to fair, depending on the severity of disease. The longterm prognosis depends on how well the patient responds to treatment and how quickly further degeneration occurs.
See Regional Anesthesia in Horses for discussion of specific anesthetic techniques used to localize lameness.
