Bone Fractures

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Caused by physical trauma, bone fracture is one of the most common abnormalities of bone.
The degree of fracture can vary widely from a simple crack in the cortical bone to a complex multiple fracture with fragmentation and displacement of the bone pieces, associated with severe damage to the surrounding soft tissues and, sometimes, exposure of the bone fragments to the exterior through a large gaping wound (open 'compound' fracture).

For a fracture to heal efficiently it is important that all conditions are optimal; there are many factors that interfere with the satisfactory healing of fractures.

(a) Due to tearing of blood vessels in the medullary cavity, cortex and periosteum, haematoma forms at the site of fracture. The periosteum is stripped from the bone surface.

(b) Organization of the haematoma is associated with the migration of neutrophils and macrophages into the fracture haematoma; these cells phagocytose the haematoma and necrotic debris. This is followed by ingrowth of capillaries and fibroblasts from surrounding tissue, producing fibrovascular granulation tissue. New osteoprogenitor cells develop from mesenchymal precursor cells.

(c) Osteoblasts derived from osteoprogenitor cells migrate into the granulation tissue and differentiate into osteoid-synthesizing units, which proceed to deposit large quantities of osteoid collagen in a haphazard way, producing a woven bone pattern. External callus bridges the fracture site outside the bone and, if there is a significant gap between the bone ends, it may include cartilage. Internal callus bridges the fracture in the medullary cavity and rarely contains cartilage. When bone ends are closely apposed, direct ossification between fractured ends occurs.

(d) Callus is usually well-established by the third week after fracture, but initial bony union is by woven bone, which is mechanically weak. Re-modelling of callus occurs once the defect between the two bone ends is bridged by bony callus, taking many months. A combination of osteoclastic erosion and organized osteoblastic osteoid synthesis removes surplus calcified callus, replacing the inefficient bulky woven bone with compact organized lamellar bone.

(e) With time, re-modelling creates new lamellar trabecular bone, which is orientated in a direction determined by the stresses to which the bone is exposed with mobilization. Even with re-modelling, cortical irregularity and minor marrow space fibrosis persist at the site of fracture.
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