## Fracture Healing — Inflammatory Phase ### Timeline and Cellular Events The inflammatory phase of fracture healing occurs within the first **24–72 hours** following injury. During this phase, the primary cellular response involves recruitment and activation of mesenchymal stem cells (MSCs) and fibroblasts, which begin the process of organizing the fracture hematoma and laying down the initial fibrin scaffold. ### Role of Each Cell Type | Cell Type | Phase | Function | |-----------|-------|----------| | Mesenchymal stem cells & fibroblasts | Inflammatory (0–72 hrs) | Organize hematoma, lay fibrin scaffold, initiate repair | | Osteoblasts | Soft callus & hard callus (1–12 weeks) | Produce bone matrix (osteoid), mineralization | | Chondrocytes | Soft callus (1–3 weeks) | Produce cartilage in endochondral ossification pathway | | Osteoclasts | Remodeling (weeks–months) | Remove excess bone, restore normal architecture | ### Key Point: **MSCs and fibroblasts dominate the inflammatory phase and are essential for hematoma organization and early matrix deposition.** Osteoblasts become prominent later, during the soft and hard callus phases. ### High-Yield: Remember the **sequence**: Inflammation → Soft callus (cartilage) → Hard callus (woven bone) → Remodeling (lamellar bone). Each phase has its own dominant cell type. ### Clinical Pearl: Delayed recruitment of MSCs (e.g., in severe soft-tissue injury, infection, or poor blood supply) compromises early fracture healing and increases nonunion risk. 
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