Early fracture callus in the diaphysis of human long bones. Histologic and ultrastructural study.

The medullary callus and the periosteal callus of fractured long bones were studied in 26 adults undergoing open reduction and internal fixation of closed diaphyseal fractures that occurred 1 to 21 days before surgery. In the 1st week after fracture, a progressive increase was observed in the number of polymorphic mesenchymal cells in the medullary callus and of fibroblast-like cells in the periosteum, where the first calcification foci were seen 7 days after injury. In the 2nd week after fracture, the medullary callus presented numerous mesenchymal cells, fibroblasts, and newly formed capillaries, whereas the inner periosteal layer showed many osteoblast-like cells. New bone trabeculae were first seen in the periosteal callus 12 days after injury. In the 3rd week after fracture, new trabecular bone appeared in the medullary callus. Cartilage also became apparent in the medullary and periosteal callus but remained limited in amount. Calcification within cartilage was first observed in the periosteum 18 days after fracture. The process of fracture healing in long bones in humans is similar, though not identical, to that described for long bones in laboratory animals.