Look a little closer
A broken bone does not simply glue two pieces together. Blood and immune cells establish a temporary repair site, a cartilage bridge becomes immature bone, and prolonged remodeling restores a structure closer to the original.
Immediately after fracture, torn vessels bleed and create a clot called a fracture hematoma. Inflammatory cells remove damaged material and release signals that recruit progenitor cells and new blood vessels.
Fibrous tissue and cartilage then form a soft callus around the gap, reducing motion temporarily. It is not yet strong enough for normal loading but provides a biological scaffold between fragments.
With adequate blood supply and suitable stability, cartilage mineralizes and osteoblasts produce immature woven bone, forming a hard callus. This can appear as thickening around the fracture line on an X-ray.
During remodeling, osteoclasts remove excess bone while osteoblasts arrange stronger lamellar bone along patterns of mechanical load. This can continue for months or years after the break appears united.
Excessive motion, poor blood supply or infection can delay healing or cause nonunion. Casts and fixation do not manufacture bone; they provide alignment and stability for cells to bridge the gap, so fractures require medical diagnosis and follow-up.
