Hematoma Formation
🛠️Immediately after fracture, torn vessels bleed into the gap, creating a hematoma that acts as a provisional scaffold and seals the site.
🛠️The hematoma triggers an acute inflammatory response with platelet-derived factors and cytokines (IL-1, TNF-α) that recruit mesenchymal/osteoprogenitor cells.
🛠️Hypoxia within the clot induces VEGF, promoting angiogenesis, which is essential for subsequent repair stages.
🛠️Neutrophils and macrophages debride necrotic tissue, clearing the way for granulation tissue.
Fibrocartilaginous (Soft) Callus
🛠️Within days, granulation tissue matures into a soft callus composed of fibroblasts, chondroblasts, and early collagen (type III) bridging the fracture.
🛠️Periosteal and endosteal progenitors differentiate toward fibrocartilage, providing initial stability but low mechanical strength.
🛠️New capillary ingrowth sustains the metabolically active repair zone, supporting cell proliferation and matrix deposition.
🛠️Micromotion is tolerated at this stage; excessive motion, however, risks nonunion by preventing maturation.
Bony (Hard) Callus
🛠️Over weeks, the soft callus is replaced by woven bone via endochondral ossification (cartilage → bone) and areas of intramembranous ossification at well-vascularized surfaces.
🛠️Osteoblasts lay down woven bone rapidly, increasing mechanical strength and allowing progressive load-bearing under guidance of clinical protocols.
🛠️Mineralization intensifies (calcium phosphate deposition), converting the flexible bridge into a rigid bony callus.
🛠️Radiographically, this correlates with callus visibility and the beginning of cortical continuity across the fracture line.
Remodeling
🛠️Across months to years, osteoclasts resorb disorganized woven bone, while osteoblasts rebuild lamellar bone along lines of stress (Wolff’s law).
🛠️The medullary cavity is reconstituted; cortical thickness, trabecular orientation, and joint congruity are gradually restored.
🛠️Mechanical properties (stiffness, toughness) approach pre-injury levels as the callus thins and reshapes.
🛠️Systemic factors (e.g., nutrition, smoking status, endocrine balance) and local factors (stability, perfusion) modulate remodeling quality and pace.