Periosteal stress-induced reactions resembling stress fractures. A radiologic and histologic study in dogs.

An external callus is always associated with so-called stress fractures, but a fracture line cannot always be demonstrated by radiologic means. In such instances it is assumed that an undisplaced fracture or microfracture must nevertheless exist for an external callus to form. In this experimental study, 18 beagles were immobilized in a shoulder spica for periods of time ranging from 6 to 32 weeks and then remobilized. At the time of sacrifice, which varied from four to 28 weeks after remobilization was started, eight showed on radiographs an external fusiform bone formation on the distal metacarpal metaphysis during the remobilization period without evidence of a fracture line. Serial histologic examination, in seven metacarpi, also failed to reveal the presence of any break in the bone's continuity. Although this does not exclude the accumulation of microdamage (mechanical fatigue) at such sites, there is sufficient evidence that the circumscribed periosteal reaction occurred at a site of increased stress in the absence of an actual fracture. Consequently, this condition is called a periosteal stress-induced reaction.