Bone Graft: How is bone graft attached to the broken bone?

After the small ground up bone fragments are inserted between the fracture fragments they get bathed in blood and tissue fluid. This clots to form a blood clot/bone graft slurry. By the normal process of healing the blood clot turns into scar tissue but now it has lots of bone fragments in it. (so it is stiffer than the former scar tissue). The scar binds the original fracture fragments together. The bone graft fragments in the the scar still have some living bone cells and they start laying down bone mineral in the scar tissue as soon as they have re-established a blood supply. Bone Metabolic Protein in the graft tissue also stimulates the differentiation of bone cells out of fibroblasts. Some of the larger bone graft fragments are too large to be nourished just with tissue fluid and die. They still add to the structural stiffness of the healing area. Once a blood supply is re-established the dead fragments are used as a scaffolding for new bone and at the same time eaten up by bone remodelling cells. This process of simultaneous new bone deposition and erosion of the old is called "creeping substitution".
There are some "ifs" to this process but if all goes well the scar tissue is ossified quickly (6- 8 weeks) and the fracture is united.
If we are talking about structural bone graft (ie larger blocks of bone) the graft is fixed in place with internal fixation. Then creeping substitution replaces the dead bone of the graft with living bone (we hope). If the graft is alive with its blood supply intact it heals just like a fracture.
Myles Clough MD OS Jan 3 2002