Phases of Bone Healing and Bone Remodelling Cycle
DNB Orthopaedics • October 2023 • Paper I • Question 8 • 10 Marks [3+7]
Introduction
Bone is unique among tissues because it can heal by regeneration rather than scar formation. Fracture healing depends on the interaction of hematoma, inflammatory mediators, vascularity, progenitor cells, mechanical stability and remodeling.
Healing may occur by:
- Primary/direct bone healing under absolute stability with minimal fracture gap.
- Secondary/indirect bone healing under relative stability, with callus formation.
Most fractures heal by secondary fracture healing, which proceeds through inflammation, soft callus, hard callus and remodeling.
A. Phases of Bone Healing [3 Marks]
The classical phases of secondary fracture healing are:
FRACTURE ↓ 1. HEMATOMA + INFLAMMATORY PHASE ↓ 2. REPARATIVE PHASE • Soft callus • Hard callus ↓ 3. REMODELLING PHASE
1. Hematoma and Inflammatory Phase
Immediately following fracture, disruption of medullary and periosteal blood vessels results in formation of a fracture hematoma.
Local hypoxia and tissue necrosis trigger an inflammatory response.
Important Events
- Platelet aggregation and clot formation
- Release of inflammatory cytokines
- Recruitment of neutrophils and macrophages
- Removal of necrotic tissue
- Recruitment of mesenchymal progenitor cells
- Angiogenesis begins
Important Mediators
- IL-1
- IL-6
- TNF-α
- PDGF
- TGF-β
- BMPs