Principles of Bone Healing

Iain H. Kalfas, MD, FACS Department of Neurosurgery, Section of Spinal Surgery, Cleveland Clinic Foundation, Cleveland, Ohio

Neurosurg Focus. 2001;10(4) 

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Bone Grafts

The two types of bone grafts frequently used in spinal fusion are autografts and allografts. Autograft bone is transplanted from another part of the recipient's body. Allograft bone is transplanted from genetically nonidentical members of the same species. Both types of bone grafts are commonly used in spine surgery.

The ideal bone graft should be: 1) osteoinductive and conductive; 2) biomechanically stable; 3) disease free; and 4) contain minimal antigenic factors. These features are all present with autograft bone. The disadvantages of autografts include the need for a separate incision for harvesting, increased operating time and blood loss, the risk of donor-site complications, and the frequent insufficient quantity of bone graft.[13,30]

The advantage of allograft bone is that it avoids the morbidity associated with donor-site complications and is readily available in the desired configuration and quantity. The disadvantages of allograft include delayed vascular penetration, slow bone formation, accelerated bone resorption, and delayed or incomplete graft incorporation.[1,5,14,16] In general, allograft bone has a higher incidence of nonunion or delayed union than autograft.[1,7,8,9,15,33] Allografts are osteoconductive but are only weakly osteoinductive. Although transmission of infection and lack of histocompatibility are potential problems with allograft bone, improved tissue-banking standards have greatly reduced their incidence.

Bone grafts can also be classified according to their structural anatomy: cortical or cancellous. Cortical bone has fewer osteoblasts and osteocytes, less surface area per unit weight, and contributes a barrier to vascular ingrowth and remodeling compared with cancellous bone. The advantage of cortical bone is its superior structural strength.

The initial remodeling response to cortical bone is resorptive as osteoclastic activity predominates. Cortical grafts progressively weaken with time because of this bone resorption as well as slow, incomplete remodeling. Conversely, cancellous bone becomes progressively stronger because of its ability to induce early, rapid, new bone formation.

When selecting a bone graft, the spine surgeon needs to consider the specific structural and biological demands that will be placed on the graft. If the graft is placed anteriorly in a compressive mode, cortical bone, either autogenic or allogenic, will be required. If placed posteriorly as a graft under tension with lower demands for structural support but also a lower probability of early vascular ingrowth, a cancellous autograft is preferred.

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