Developmental orthopedic disease (DOD) remains one of the top concerns of breeders worldwide. Not one condition, but rather a series of related syndromes, DOD encompasses anything that contributes to poor skeletal development in foals: angular limb deformities, osteochondrosis, osteochondritis dessicans (OCD), contracted tendons, cervical malformations, subchondral bone cysts, club foot, and physitis among them.

C. Wayne McIlwraith, BVSc, PhD, Diplomate ACVS, of Colorado State University, one of the United States’ leading orthopedic researchers, notes that there is a great deal of confusion in this area of study over terminology.

“Osteochondrosis, OCD, metabolic bone disease…all of these are terms, like DOD, that have been used to describe the overall syndrome,” said McIlwraith. “And all cases of DOD are not the same. We need to get more specific. It’s important to differentiate between clinical signs (like filling of the joints) and radiographic signs.

“A single cause for all types of DOD is unlikely, however much we’d like it to be there. As we continue to study these problems, it becomes more clear that some of our initial conceptions were incorrect.”

Physitis, or physeal dysplasia, is one of the many manifestations of DOD–essentially, it’s an inflammation of the growth plates in long bones of a growing foal. Physitis was formerly known as epiphysitis–which most horsemen had just learned to pronounce when the terminology was changed! McIlwraith explains: “(Epiphysitis) was not an accurate description of where the problem was–the epiphysis is the site of secondary ossification in a bone, while the physis is the actual growth plate.”

When we focus in on physitis, it’s helpful first of all to understand how bones grow and develop in a foal.

Normal Bone Development

Development of the skeleton of a