The physis is the term for the growth plate of a bone. Three distinct areas are seen:

• metaphysis: the side of the growth plate attached to the long part of a bone
• epiphysis: the side of the growth plate attached to the joint surface of the bone
• physis: the tissue between the metaphysis and epiphysis

The physis is a common cause of growth abnormalities and lameness in young horses. The term epiphysitis is usually used but this may be a poor term as the epiphysis may not be primarily involved and the cause of problems may not be a inflammatory disease. In fact frequently it is the metaphysis adjacent to the physis that is effected. This article discusses the clinical signs, causes, diagnosis, treatment, and prognosis of these conditions.

Accurately, physitis should indicate inflammation, with signs of heat, swelling, and pain on palpation of the physis. The disease may first exhibit itself as a firm enlargement of the distal cannons (metacarpals / metatarsals) or distal forearm (radius). The cannons are most frequently effected prior to 6 months of age and the forearm before 2 years of age. The swellings should be warm and tender to pressure. Lameness is usually evident and the horse displaying an increasingly upright conformation. Irregular growth along the plate may result in angular deformities of the leg.

Apparent physitis with thickening of the tissue surrounding the growth plate, apparent visually, but with no lameness, no heat, no evidence of pain, and no radiographic abnormalities, is more accurately a physiologic response of the physis to weight bearing. This thickening of the physis that occurs without associated inflammation will sometimes unduly worry owners, farm managers, and trainers.