Bone Remodeling

Bone remodeling is the continuous cycle in which osteoclasts resorb mineralized matrix and osteoblasts deposit new matrix in its place, so that an adult skeleton replaces a few percent of its mass each year. Bone is vascularized, innervated, metabolically active tissue and the body's main calcium reservoir — not the dry scaffolding of an articulated museum skeleton. The dry-skeleton model misleads accordingly: dry bone either is or is not broken, while living bone is a tissue whose blood supply can be interrupted (see Watershed Blood Supply) and whose metabolism can be slowed.

Mechanically, bone is a composite. Type I collagen supplies tensile toughness; hydroxyapatite mineral supplies compressive stiffness. Demineralized bone is rubbery and deproteinated bone is chalky; neither alone survives loading. Remodeling is load-sensitive: trabeculae align to habitual stress, which is why immobilization costs density and why graded loading is part of rehabilitation.

Union proceeds in overlapping stages. Hematoma and inflammation fill the gap within hours and recruit cells. Soft callus, fibrocartilage bridging the fragments, forms over weeks and confers rough stability. Hard callus replaces it with woven bone visible on radiographs. Remodeling then converts woven bone to lamellar bone along load lines over months to years.

Imaging lags the biology, so a radiograph is a proxy and not Ground Truth for union. When a stage stalls the result is Fracture Nonunion, and the cause is normally mechanical or vascular rather than obscure, which is where the demand for a Plausible Mechanism earns its keep. The methods described in Fracture Fixation are best read as attempts to supply each stage the conditions it needs.