Answer:
Follows the same sequence as fracture healing:
1) Induction: Activation of host osteoblasts and differentiation of primitive mesenchymal cells into chondroblasts and osteoblasts.
2) Inflammation: Graft invaded by PMN’s and its cellular elements are degraded. Neurovascularization and mesenchymal proliferatrion follow. Small avascular autografts can become vascularized within 4-5 days.
3) Soft tissue callous formation: The cellular matrix of the invading granulation tissue becomes more dense and the vascularity increases. Osteoclasts continue to remove dead bone, while chondroblasts deposit a new matrix of chondroid on the old bone; this begins to calcify. In cortical bone there is a preferential removal of necrotic Haversian systems rather than lamellae leading to an increased porosity of the graft.
4) Hard callus formation: Osteoclasts continue to remove dead bone and also begin degrading calcified cartilage, while osteoblasts lay down membranous bone to replace it.
5) Remodeling: Graft is remodelled into lamellar bone and a medullary canal is established.