But we're ahead of ourselves. There are five phase of fracture healing:
- Haematoma formation - almost inevitably, the forces that broke the bone also break blood vessels. As a result, the break becomes surrounded by a haematoma.
- Granulation tissue formation - within hours of the fracture, the haematoma begins to be reabsorbed and it is simultaneously replaced by an inflammatory infiltrate. With time, this acute inflammatory infiltrate develops into a chronic one, with fibroblasts and small new vessel formation. In addition, the cells under the periosteum and medulla proliferate.
- Callus formation - the predominant cell population changes to osteoblasts (which lay down new bone) and osteoclasts (which reabsorb old bone). The resulting cellular tissue, with enlarging islands of new bone, is called a callus.
- Consolidation - the bone laid down quickly in the callus is called woven bone, but in the consolidation stage it is replaced by the stronger lamellar bone. This process may take months.
- Remodelling - the healed bone's lamellae align themselves predominantly in the direction of the forces acting on the bone, and excessive bits of bone are reabsorbed.
There is a nice picture of the stages here.
This whole process takes an average of 6-8 weeks for upper limb fractures and 12-16 weeks for lower limb fractures, but there is a lot of variation in these figures. The biggest factor determining healing time is age - children heal much quicker (and much better) than adults.