This study consisted of 4 parts. First, an epidemiologic study of dorsal metacarpal disease in 48 2-year-old Thoroughbred horses in race training was conducted. Over a 6 week period, 65% of the sample and 91% of those horses in uninterrupted training developed clinical signs of acute metacarpal disease. Dorsal metacarpal disease was associated with an increase in the level of exercise. Most horses were affected bilaterally.

Second, a retrospective radiographic study of 165 third metacarpal bones of 123 Thoroughbred horses was conducted. Periosteal bony proliferation was seen in 98% of the metacarpi. Incomplete fractures of the dorsal cortex of the third metacarpal bone were seen in 24% and 38% of 2 and 3-year-old horses, respectively. Fractures occurred in 35% and 13% of the left and right metacarpi, respectively. Fractures were present on the dorsal or dorsolateral aspect of the third metacarpus, and were located in the distal two-thirds and the distal third of the dorsal cortex of the left and right forelegs, respectively. Mean fracture angle with the periosteal surface was 30 degrees (+ 7.5). Endosteal bony proliferation often accompanied an incomplete fracture.

Third, age-related changes in the porosity and mineralization of the dorsal cortex of the mid-diaphysis of the third metacarpal bone was studied in horses 2 months to 12 years of age. Mean bone porosities were highest in foals and decreased through approximately 7 years of age. Intracortical porosities were highest (5.1 to 5.9%) in young horses (1 to 3 years of age). Seventeen percent increases were observed in volumetric and specific bone mineral contents from young horses to horses 3 years and older.

Fourth, age-related histologic features of the dorsal cortex were studied in horses 2 months to 12 years of age. Apposition of plexiform bone on the periosteal cortical surface was seen in foals. Secondary osteonal remodeling was first seen in the woven bone between primary osteons at 5 months of age. Bone of 1 and 2-year-old horses was characterized by the presence of numerous cortical bone remodeling units which created resorption spaces and forming secondary osteons of low mineral density. Remodeling was initially confined to circumferential laminae of woven bone in the outer cortex. Bone of older horses was extensively remodeled by secondary osteons.