A diaphyseal forearm fracture is fixed with rigid compression plating achieving anatomical reduction and interfragmentary compression. Six weeks later, histology of the healing site shows direct Haversian remodelling across the fracture line with no callus formation. This pattern of healing occurs because rigid fixation provides:
- A Excessive micromotion stimulating periosteal callus
- B Complete absence of vascularity at the fracture site
- C Interfragmentary strain below the tolerance of granulation tissue ✓
- D Cartilage template formation before ossification
Correct answer: C. Interfragmentary strain below the tolerance of granulation tissue
Explanation
Primary or direct bone healing occurs when absolute stability reduces interfragmentary strain below what granulation tissue can tolerate, so neither fibrous tissue nor cartilage forms. Osteoclasts cut across the fracture line and osteoblasts lay down lamellar bone directly through cutting cones. It is slower than secondary healing and produces no visible callus, which is why absence of callus after rigid plating does not imply non-union.
Reference: AO Principles of Fracture Management, 3rd ed.
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