When bones are fractured, the body immediately initiates the first phase of healing, the reactive phase. Ruptured blood vessels gather at the site of the break and form a clot. This clot contains fibroblasts, which are connective tissue cells that produce collagen proteins. When this clot forms, it lays the groundwork for what will be a full-scale restoration of the bone.
In a few days, the broken ends of the bone produce new blood vessels that grow into the clot that now bridges the separation in bone caused by the fracture. White blood cells arrive with these new blood vessels and begin carting away unneeded material from the site of the break. Now, the fibroblasts begin to multiply and secrete collagen fibers which form a matrix that replaces the blood clot.
In the reparative phase, specialized cells -- osteoprogenitor cells -- located in the periosteal membrane that covers most of the bone begin transforming into different types of needed cells. Some of these cells -- chondroblasts -- produce cartilage, while others -- osteoblasts -- produce uncalcified bone called callus. The new cartilage and callus bridge the separated pieces of bone, and the cartilage begins to ossify into trabecular bone.
In the third phase, the remodeling phase, osteoclasts begin removing the trabecular bone while osteoblasts replace it with compact bone. Once this phase is finished, the fractured bone has healed. (For more on the healing process, try How do broken bones heal?)
Sometimes when bones break they have to be realigned so that the fractured ends line up correctly for healing. All pressure must be taken off the bone, so that the ends that are attempting to fuse back together don't move out of position. Surgery might need to be performed to hold the bone fragments together with metal plates, rods or screws. Not only do these devices secure the bone's position for healing, they provide a starter bridge for the calcium deposits that will begin accumulating along the healing area.
Next, we'll learn about what happens when two bones meet in the night (or day).