Idiopathic new bone formation in the femoral shafts of a cynomolgus monkey (Macaca fascicularis).

A 6.5-y-old cynomolgus monkey was referred to the Veterinary Medical Teaching Hospital at Chungnam National University for suspected bone fracture. The monkey had been reared singly in a cage at a laboratory facility. An animal caretaker incidentally found a bone fragment protruding through the skin of the right leg. Radiographic examination revealed 2 new bone fragments clearly distinguishable from the original femurs; the fragments seemed to be inserted into both femurs. One of the new bone fragments was easily separated surgically from the right femur. Although the bone fragment consisted of a medullary cavity and bone cortex, the periosteal structure was incomplete. New bone formation in nonhuman primates, as manifested in this case, has been reported previously. However, growth of an additional long bone from the original, penetrating the skin without motional disturbance, has not been reported previously.