Fractures are the most common pathologic conditions affecting bone. The following qualifiers describe fracture types and affect treatment:
Simple: the overlying skin is intact
Compound: the bone communicates with the skin surface
Comminuted: the bone is fragmented
Displaced: the ends of the bone at the fracture site are not aligned
Stress: a slowly developing fracture that follows a period of increased physical activity in which the bone is subjected to repetitive loads
Greenstick: extending only partially through the bone, common in infants when bones are soft
Pathologic: involving bone weakened by an underlying disease process, such as a tumor
Fracture repair involves regulated expression of a multitude of genes and can be separated into overlapping stages with particular molecular, biochemical, histologic, and biomechanical features.
Immediately after fracture, rupture of blood vessels results in a hematoma, which fills the fracture gap and surrounds the area of bone injury (
Fig. 21.12 ). The clotted blood provides a fibrin mesh, sealing off the fracture site and creating a scaffold for the influx of inflammatory cells and the ingrowth of fibroblasts and new capillaries. Simultaneously, degranulated platelets and migrating inflammatory cells release PDGF, TGF-β, FGF, and other factors that activate osteoprogenitor cells in the periosteum, medullary cavity, and surrounding soft tissues and stimulate osteoclastic and osteoblastic activity. By the end of the first week, a mass of predominantly uncalcified tissue— called soft callus or procallus—provides anchorage between the ends of the fractured bones. After approximately 2 weeks, the soft callus is transformed into a bony callus. The
Fig.21.12Thereactiontoafracturebeginswithanorganizinghematoma.Withintwoweeks,thetwoendsofthebonearebridgedbyafibrinmesh-workinwhichosteoclasts,osteoblasts,andchondrocytesdifferentiatefromprecursors.Thesecellsproducecartilageandbonematrix,which,withadequateimmobilization,remodelsintonormallamellarbone.
http://ebooksmedicine.net activated osteoprogenitor cells deposit woven bone. In some cases, the activated mesenchymal cells in the soft tissues and bone surrounding the fracture line also differentiate into chondrocytes that make fibrocartilage and hyaline cartilage. The newly formed cartilage along the fracture line undergoes endochondral ossification, forming a contiguous network of bone with newly deposited bone trabeculae in the medulla and beneath the periosteum. In this fashion, the fractured ends are bridged (
Fig. 21.12
As the callus matures and is subjected to weight-bearing forces, portions that are not physically stressed are resorbed. This remodeling reduces the size of the callus until the shape and outline of the fractured bone are reestablished as lamellar bone. The healing process is complete with restoration of the medullary cavity.
The sequence of events in the healing of a fracture can be easily impeded or blocked. Displaced and comminuted fractures frequently result in some deformity. Inadequate immobilization permits movement of the callus and prevents its normal maturation, resulting in delayed union or nonunion. If a nonunion persists, the malformed callus undergoes cystic degeneration, and the luminal surface may become lined by synovial-like cells, creating a false joint or pseudoarthrosis. Infection of the fracture site, especially common in open fractures, is a serious obstacle to healing. Malnutrition and skeletal dysplasia also hinder fracture healing.