Equine Developmental Orthopedic Disease Study Guide

Overview and Clinical Importance

Developmental Orthopedic Disease (DOD) is an umbrella term encompassing a group of non-infectious orthopedic conditions affecting the growing skeleton of young horses. The term was coined in 1986 by the American Quarter Horse Association to describe skeletal abnormalities arising from disturbances in bone and cartilage development. DOD represents one of the most economically significant disease complexes in the equine industry, with radiographic surveys suggesting that 40-60% or more of foals may be affected by one or more manifestations.

The primary conditions under the DOD umbrella include osteochondrosis (OC), osteochondritis dissecans (OCD), physitis (epiphysitis), subchondral bone cysts, angular limb deformities, flexural limb deformities, and cervical vertebral malformation (wobbler syndrome). This study guide focuses on the three most commonly tested conditions for NAVLE: osteochondrosis/OCD, physitis, and subchondral bone cysts.

Pathophysiology of Developmental Orthopedic Disease

Endochondral Ossification

Understanding DOD requires knowledge of endochondral ossification, the process by which long bones form. In this process, a cartilage template (anlage) is systematically replaced by bone tissue. Chondrocytes in the growth plate undergo a coordinated sequence of proliferation, hypertrophy, matrix calcification, apoptosis, and replacement by osteoblasts. The process occurs in the physis (growth plate), the articular-epiphyseal cartilage complex (AECC), and cuboidal bones of the carpus and tarsus.