How do broken bones heal?

When a bone breaks, blood vessels tear and a clot called a haematoma forms around the fracture within hours. Over the next days, inflammation brings in cells that clear debris and begin repair. Within about two to three weeks, a soft callus of cartilage and fibrous tissue bridges the broken ends. Bone-building cells called osteoblasts then turn it into a hard callus of woven bone over the following weeks. Finally, over months to years, osteoclasts and osteoblasts remodel the bone back toward its original shape and strength, which is why casts hold the bone still in the right position.