Clinically, fracture healing is complete when bone strength at the fracture site has been restored to normal and the marrow space is re-formed. This may occur as soon as 6 weeks after fracture. Radiographic evidence of healing may also be seen as early as 6 weeks after fracture. Biologically, a fracture is considered healed when all regenerative processes at the fracture site have ceased. A bone scan may show increased metabolic activity at the fracture site for months or years while remodeling continues.
Primary union of bone does not occur normally in nature, but it can be induced artificially with internal fixation and rigid immobilization along with anatomic alignment (see Plate 7-25). Primary cortical union is not true bone regeneration but rather recruitment of normal remodeling processes to bridge the fracture gap. Compared with inductive callus healing, the process of primary union is extremely slow.
Complete immobilization inhibits the formation of inductive callus but promotes primary cortical union. When a rigid compression plate is affixed to a fracture, the necrotic cortical bone at the fracture site is not resorbed as in the normal process of inductive callus healing. Rather, the dead bone is recanalized by new haversian systems with mature osteons, as occurs in normal bone remodeling. New bone also arises from endosteum to bridge the fracture gap. Revascularization occurs from adjacent medullary vessels.
Unlike inductive callus formation, which includes both external bridging callus and late medullary callus, primary union of cortical bone cannot effectively bridge gaps at a fracture site. For primary union to occur, the fracture gap must be obliterated by perfect apposition and compression of the fractured bone ends. In addition, movement at the fracture site must be minimal. The bone’s ability to heal by primary union depends on rigid immobilization, which enables the fragile medullary vessels to recanalize the necrotic bone and cross the fracture site. When the bone is healed, tolerance to total rigidity is excellent. The major disadvantages of primary union are its slowness compared with inductive callus formation and the need for artificial stability with rigid internal fixation to be maintained for a long period of time.
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