Preexisting lesions associated with complete diaphyseal fractures of the third metacarpal bone in 12 Thoroughbred racehorses.

We characterized features of complete diaphyseal fractures of third metacarpal bones in Thoroughbred racehorses. Given that stress fractures are known to occur in the third metacarpal bone, an additional aim was to determine if complete fractures are associated with signs of a preexisting incomplete stress fracture. Bilateral metacarpi from 12 Thoroughbred racehorses euthanized because of complete unilateral metacarpal diaphyseal fracture were examined visually and radiographically. Open, comminuted, transverse or short oblique fractures occurred in the middle of the diaphysis or supracondylar region. Periosteal surface discoloration and bone callus formation contiguous with the fracture line were present in fractured bones. All contralateral intact metacarpi had gross anatomic lesions, and 10 had radiographic abnormalities similar to those observed on fractured metacarpi. Catastrophic metacarpal fractures occurred in racehorses with bilateral evidence of preexisting bone injury.