Fracture Healing: How Callus Remodels and Excess Bone Disappears
After fracture, a callus (woven bone + cartilage) forms, stabilizing fragments. Over months, it remodels to original lamellar bone.
Remodeling Phases
- Inflammatory (Days 1–7): Hematoma, cytokines, MSC recruitment.
- Soft Callus (Days 7–21): Fibrocartilage, woven bone.
- Hard Callus (Weeks 3–8): Woven bone mineralized; bridging visible on X-ray.
- Remodeling (Months–Years): Woven → lamellar bone; excess resorbed.
Mechanisms of Excess Bone Removal
- BMU (Basic Multicellular Unit) Remodeling: Osteoclasts resorb tunnel → osteoblasts lay lamellar bone (Haversian systems).
- Targeted Resorption: Mechanical strain sensors (osteocytes) direct resorption where stress is low (excess callus) and formation where stress is high.
- Modeling vs. Remodeling: Modeling changes bone shape (growth); remodeling renews existing bone. Both active in fracture healing.
- Mechanical Loading: Weight-bearing stimulates osteocytes → Wnt/β-catenin → bone formation on tension side, resorption on compression side → restores anatomy.
Clinical Correlate
- Early weight-bearing (per surgeon) accelerates remodeling.
- Nonunion: failed bridging; risk factors: smoking, NSAIDs, diabetes, poor fixation.
- Malunion: healed in wrong position; may need osteotomy.
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