Osteoarthritis of the proximal interphalangeal joint (high ringbone) is a common cause of lameness in many types of horses used for a variety of disciplines. The osteoarthritic process can start as a single traumatic episode or as a result of “wear and tear,” or overuse. Other causes of osteoarthritis of this joint are infection and developmental orthopedic disease. The proximal interphalangeal joint is a lower motion joint that is somewhat unforgiving to high loads placed upon it. Osteoarthritis is characterized by cartilage loss and periarticular new bone formation. In the chronic, progressive osteoarthritic pastern, lameness is typically subtle at first and becomes more noticeable as the disease progresses. Radiographic findings may include periarticular new bone formation, subchondral lysis and/or sclerosis, and loss of joint space (typically the medial aspect of the joint). In the early stages of disease, diagnostic analgesia is typically required to localize the lameness. In addition, a positive response may be seen on lower limb flexion. Oral or intra-articular anti-inflammatory medication may relieve the signs of lameness temporarily. The use of intra-articular ethanol injection into the proximal interphalangeal joint has been described. If surgical fusion is not an option, multiple injections were successful in some horses for facilitated ankyloses. Surgical arthrodesis of the pastern joint is frequently required to successfully restore performance. Typically, this involves a combination of plate(s) and screw(s). In a retrospective study of 53 horses that had a proximal interphalangeal joint arthrodesis using a combination plate-screw technique (81% forelimb, 95% hindlimb), 87% of horses with follow-up were used as intended. Deterioration of articular cartilage within the metacarpal/tarsophalangeal joint is a common injury in racehorses and can lead to development of periarticular osteophytes, enthesophytes, and joint space collapse. In young, training horses, periosteal bone can form on the dorsal aspect of the distal metacarpus and the proximal aspect of the proximal phalanx, often involving the joint capsule (osselets). Osteoarthritis is often secondary to a primary abnormality such as a chip fracture or osteochondrosis (see Osteochondrosis of the Stifle in Horses). Treatment typically has two goals: reducing pain and minimizing further joint deterioration. The judicious use of anti-inflammatories (eg, NSAIDs, corticosteroids) can provide pain relief. Newer biologic therapies (platelet-rich plasma, stem-cell therapy, interleukin-1 receptor antagonist protein) may help slow down or arrest the progression of osteoarthritis. In severe, advanced cases, arthrodesis of the joint is necessary to provide pain relief.