Bone Healing Stages
Bone healing progresses through four distinct stages. Understanding this process explains why fractures fail to heal and guides interventions to promote union.
✦ The Mnemonic
"Healing Follows Bony Remodelling"
Haematoma · Fibrocartilaginous callus · Bony (hard) callus · Remodelling — four stages
Clinical Breakdown
Primary vs secondary healing: secondary (endochondral) healing through the four stages above requires some movement — this is why casts and intramedullary nails are used. Primary (direct) healing occurs only when fracture surfaces are in absolute contact (rigid plate fixation, gap <0.01 mm) — no callus visible on X-ray; Haversian remodelling crosses directly.
Wolff's law: bone remodels along the lines of mechanical stress applied to it. This is why a healed fracture eventually becomes indistinguishable from the original bone, and why bone density increases in athletes but decreases during immobilisation or spaceflight.
Factors impairing healing: systemic (age, malnutrition, corticosteroids, diabetes, NSAIDs, smoking, anaemia); local (infection, poor blood supply, gap at fracture site, interposed soft tissue, pathological bone, inadequate immobilisation). Smoking is the most modifiable systemic factor — it prolongs time to union by approximately 60%.
⭐ Clinical Pearl
Non-union is failure of fracture healing by 6 months. Types: hypertrophic (adequate blood supply; insufficient stability — 'elephant foot' callus on X-ray) or atrophic (inadequate blood supply — no callus). Treatment: hypertrophic → fixation; atrophic → bone graft + fixation. BMP (bone morphogenetic protein) can be used to augment healing in atrophic non-unions.