A fracture occurs when the continuity of a bone is broken and local blood supply is interrupted. If the overlying soft tissues are also injured, fracture healing may be delayed or disrupted, particularly in anatomic regions with decreased vascular networks such as the tibial diaphysis. Bone is unique in its ability to regenerate itself. Healing occurs via reactivation of embryologic processes resulting in the formation of bone not scar. Fracture healing can be described in three conceptual stages. An understanding of the timing and mechanisms associated with each stage is important in planning fracture treatment. The stages are:
InflammationThis begins immediately after bone injury with the formation of a local hematoma or fibrin clot. There is local cell death where vessel disruption has resulted in ischemia - usually at the very ends of the fractured bone. Over the course of the next few days, this area becomes infiltrated by inflammatory cells and is characterized by local swelling and warmth. The inflammatory cells release lysosomal enzymes and other mediators that attract pluripotent cells to the area; they also act to remove necrotic tissue. Fibroblasts, mesenchymal cells and osteoprogenitor cells appear and may transform nearby tissues. The fracture is tender and may be grossly mobile to physical examination at this stage. Inflammation is at its peak 48 hours after a fracture.
RepairThe reparative phase begins a few days after the injury with the arrival of mesenchymal cells able to differentiate into fibroblasts, chondroblasts and osteoblasts. The repair phase persists for several months; it can be divided into two distinct phases: soft and hard callus formation.
- "Soft callus" formation lasts for approximately six weeks from the time of injury. During this preliminary stage of repair, pain and swelling subside and bony fragments become united by fibrous and cartilagenous tissue. Woven bone is formed. While this creates some stability, the fracture may still angulate at this stage if not held with stable external support, such as a cast or external fixator, or internal support provided by plates, screws or intramedullary devices.
- "Hard callus" formation - During this second stage of repair, woven bone is transformed into lamellar bone. This takes approximately three months.
RemodelingRemodeling is the process by which bone is removed in tiny increments and then replaced by new bone. After a fracture, remodeling may continue for months or even years. The adult human skeleton continuously replaces itself at rate of 10-18% per year. The rate of remodeling is accelerated in children and during fracture repair. In addition to being an essential part of fracture healing, remodeling plays an important role in calcium homeostasis. During the remodeling phase the woven bone is converted to lamellar bone and the medullary canal is reconstituted. During this phase, bone responds to loading characteristics according to Wolff's law. Some angular deformity may correct during this stage in children with sufficient growth remaining (up to 5o per year of growth remaining).