A 32-year-old male labourer presents to the orthopaedic clinic 6 weeks after sustaining a closed fracture of the mid-shaft humerus. Clinical examination reveals mild swelling and tenderness at the fracture site, but no gross deformity. Plain radiographs show early callus formation bridging the fracture ends, with some resorption of the fracture margins. The patient reports pain on active movement but no neurovascular deficit. Which stage of fracture healing is most likely occurring at this point?

Explanation

## Fracture Healing Timeline and Phases **Key Point:** Fracture healing progresses through four overlapping phases: inflammation (0–3 weeks), soft callus (2–12 weeks), hard callus (3–12 months), and remodelling (months to years). The timing and radiological findings determine the phase. ### Phase Identification at 6 Weeks At 6 weeks post-fracture: - The inflammatory phase (dominated by haematoma and fibrin clot) is largely complete. - Early callus formation is visible on radiographs — this is the **soft callus stage**. - Endochondral ossification (cartilage → bone) is the primary mechanism of callus formation. - The fracture margins show resorption due to osteoclastic activity. - Clinical signs: mild swelling, tenderness, pain on movement (still healing, not yet rigid). ### Soft Callus vs Hard Callus | Feature | Soft Callus (2–12 weeks) | Hard Callus (3–12 months) | |---------|--------------------------|---------------------------| | **Composition** | Fibrocartilage, hyaline cartilage | Woven bone, lamellar bone | | **Radiographic appearance** | Early bridging callus, fuzzy margins | Dense, well-defined callus | | **Mechanical strength** | Weak, not weight-bearing | Strong, can bear load | | **Osteoclastic activity** | Minimal | Prominent (remodelling) | | **Clinical stability** | Unstable, pain on movement | Stable, minimal pain | **High-Yield:** At 6 weeks, the presence of early callus bridging with fuzzy radiographic margins and ongoing pain on movement is pathognomonic for the **soft callus phase**. This is when endochondral ossification is most active. **Clinical Pearl:** Soft callus is not yet mechanically competent — the fracture remains unstable and requires continued immobilization or protection. Weight-bearing should be avoided until hard callus forms (typically 8–12 weeks for long bones). ### Why Not Other Phases? - **Inflammatory phase (0–3 weeks):** Would show haematoma and minimal callus; at 6 weeks, inflammation has largely resolved and callus is already forming. - **Hard callus (3–12 months):** Radiographs would show dense, well-defined callus with resorption of fracture margins; soft callus is still present at 6 weeks. - **Remodelling (months to years):** Occurs after hard callus is established; the fracture would be clinically stable and pain-free. **Mnemonic:** **FSHC** — **F**ibrocartilage (soft), **S**oft callus, **H**ard callus, **C**omplete remodelling. Soft callus is the second major phase and is characterized by cartilage and early bone formation.