Histological and ultrastructural study of early fracture callus in human long and short bones.

We studied the tissue filling the medullary canal (medullary callus) and the periosteal callus of fractured long bones in 26 adults undergoing open reduction and internal fixation of closed diaphyseal fractures which had occurred one to 21 days before " surgery. In the first 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 at seven days after injury. In the second week after fracture, the medullary callus presented numerous mesenchymal cells, fibroblasts and newly formed capillaries. whereas the inner periosteal layer showed many osteoblast. New bone trabeculae were first seen in the periosteal callus 12 days after injury. In the third week after fracture, new membranous bone appeared in the medullary callus. Cartilage also became apparent both in the medullary and periosteal callus, but remained limited in amount. Enchondral calcification was first observed in the Periosteum 18 days after fracture. Specimens of long bones have been compared with healing tissue drawn from fractures of patellae. The process of fracture healing in long bones in man is slower than short bones and it is similar, though not identical, to that described for long bones in laboratory animals.