Arthritis is chronic inflammation of a joint, resulting in the progressive degradation of articular cartilage and the underlying bone. In horses the condition most often affects high-load joints — the fetlock, the hock (where bone spavin forms), and the pastern articulations — where repetitive stress and concussive forces accelerate cartilage loss. Once cartilage thins, bone contacts bone, producing pain, heat, swelling, and reduced range of motion.
The most clinically significant form in the horse is degenerative joint disease (DJD), which develops gradually through normal wear in aging horses or more rapidly following injury, poor conformation, or sustained overwork. The pastern joint is the site of ringbone, and the distal hock joints produce the arthritic syndrome known as bone spavin — both are recognized forms of DJD. A horse with low-grade arthritis may show only intermittent stiffness or shortened stride at first; as the joint space narrows, lameness becomes consistent and identifiable at a specific joint on flexion testing.
Management centers on reducing inflammation and preserving joint function. NSAIDs such as phenylbutazone control pain. Intra-articular corticosteroid injections, hyaluronic acid, and newer biologics such as IRAP (interleukin-1 receptor antagonist protein) are used to slow cartilage breakdown and reduce synovial inflammation. Regular, appropriate exercise on good footing maintains joint fluid distribution and slows progression better than rest alone. A skilled farrier plays a direct role — corrective shoeing that reduces rotational forces on affected joints can extend a horse’s working life considerably. The condition is not cured; it is managed.
Further reading: Arthritis on Wikipedia; Arthritis at Britannica.