Physeal fractures, part I: histologic features of bone, cartilage, and bar formation in a small animal model.

Physeal fractures and the formation of physeal bars can pose significant problems in skeletal development for the injured, growing child. Regrettably, little experimental attention has been directed toward this clinical disturbance. The current study documents early histologic changes (days 2-6) and subsequent alterations (day 21) following a physeal fracture in the rat proximal tibia model. The fracture plane was usually contained within the physis but could involve many regions of the physis. In some instances, the fracture plane extended to the physeal epiphyseal border. When the fracture was contained within the physis, healing was uneventful. However, when the fracture extended through the physis to the epiphyseal physeal border, there was greater physeal disorganization and formation of vertical septa leading to physeal bars. Physeal bars appeared to form at sites of vertical fibrotic septa into which marrow cells, osteoclasts, and osteoblasts had migrated. Bar formation mediated by primary osteogenesis (rather than by endochondral bone formation) followed. This study examines the changes in the histologic features of the rat proximal tibial physis, epiphysis, and metaphysis after a physeal fracture and identifies key factors associated with physeal bar formation.