The physiology of fracture healing constitutes the basis for many decisions in the emergency department. The judgment as to whether an angulated fracture requires reduction or can be left to heal as is, the choice of treatment modality in relation to the patient's age, and the prognosis for regaining function or being left with residual deformity all require familiarity with the short- and long-term aspects of the healing process.
Fracture healing can be described in terms of three phases—the inflammatory, reparative, and remodeling— each of which gradually blends into the next. 2
When a fracture occurs, the microscopic vessels crossing the fracture line are severed, depriving the damaged bone ends of their blood supply. In the ensuing hours and days, the bone ends necrose, triggering a classic inflammatory response. This early phase is brief but creates the tissue environment for the most predominant aspect of fracture healing: the reparative phase.
Soon, granulation tissue begins to infiltrate the area. Within this tissue are specialized cells capable of forming collagen, cartilage, and bone, the ingredients of callus, which gradually surrounds the fractured ends and stabilizes them. With time, the callus becomes more densely mineralized.
Meanwhile, the necrotic edges of the fragments are removed by osteoclasts, cells whose specific function is to resorb bone. That is why some "hairline" fractures do not appear on x-ray until days after injury. Initially invisible, the diagnostic fracture line appears only after necrotic bone has been resorbed from the area.
The final phase of bone healing, the remodeling phase, is the longest, often lasting years. Remodeling is the tendency of bone to gradually regain its original shape and contour. During this phase, the superfluous portions of callus are resorbed, and new bone is laid down along natural lines of stress. These internal layers, easily visible in x-rays of normal bone, are the bony trabeculae. Formation of trabecular bone is a physiologically efficient process, providing maximum strength relative to the amount of bone material used.
The anticipated success of remodeling is related to a number of factors. Young children have a greater capacity for remodeling than adults do. Accordingly, their potential for residual deformity is less, other circumstances being equal. Remodeling is also related to the magnitude and direction of unreduced angulation and to the fracture's location along the bone. Specific predictors of satisfactory remodeling include youth, proximity of the fracture to the end of the bone (but not involving the epiphyseal plate), and direction of angulation coinciding with the plane of natural joint motion.
Clinical decisions regarding the aggressiveness of fracture reduction are directly linked to a knowledge of this physiology. Angulation near the end of a long bone, for example, is more acceptable than angulation near the midshaft. Dorsal or volar angulation at the wrist has a better prognosis than ulnar or radial angulation because the natural plane of wrist motion is dorsal-volar. Mild angulation in a 2-year-old may be left to remodel on its own, whereas the same angulation in an adult may require correction.
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