Abstract
Autograft – Since before modern surgical techniques were described, ancient Greeks new of the possibilities for bone to grow after fracture. Studying open fractures, often post mortem, they new of the importance of both the “amount and integrity of bone architecture” that was necessary for two ends of a bone to heal. More recently, modern spinal surgical techniques, many pioneered by surgeons such as John Moe MD, use the same knowledge that for the intentional arthrodesis of two or more bony spinal levels there requires a certain amount and quality of bone – both capturing osteoinductive and osteoconductive properties.
Autograft can be harvested in many ways for spinal arthrodesis and can be taken from iliac crest, tibia or fibula, and from local vertebral sources. Often requiring a separate skin and/or fascial incision, morbidities such as pain, neurovascular injury, infection, blood loss, haematoma, seroma, and fracture can plague the technique. Limited quantities, especially in children, can also be an issue with autograft. Cancellous or cortico-cancellous structural grafts can be milled and used for posterolateral fusion, interbody fusion, and can be mixed with other graft substitutes/expanders. Morbidity profile aside, autograft still remains the gold standard for spinal arthrodesis with regards “ideal properties” of bone grafts.
Allograft – Currently, allograft is the most common substitute for autograft bone in spinal fusion. Allograft is primarily osteoconductive, with minimal osteoinductive potential. Avoidance of donor site morbidity, quantity issues, and surgical time saving are all features of allograft. Increased costs and potential for infection are negative issues. Preparation can vary and fresh unprocessed grafts are no longer used. Freeze drying (lyophilization) involves drying of the grafts before freezing at sub zero temperatures, and the technique reduces immunogenicity, though upon rehydration, structural strength is lost by around 50%. Low dose radiation (< 20kGy) can also be used to process the grafts, as can ethylene oxide, yet both techniques also reduce mechanical strength of the trabecular architecture. With adequate donor screening and tissue processing, the risk of developing HIV from an allograft is estimated to be less than 1 in a million.
Incorporation of allograft is similar to that of autograft, though the process takes more time. Allograft cancellous particles provide a larger surface area and therefore incorporate faster. Studies suggest that mulched allograft femoral heads provides as good a fusion rate in posterior spinal surgeries for children with scoliosis as does the use of autograft. Combination of osteoinductive agents (BMP etc) with allograft is now possible and will likely enhance its further use. Structural fibular allografts in cervical interbody fusion and femoral ring allografts in lumbar interbody fusion have been well described and have very high rates of fusion.
The abstracts were prepared by Assoc Prof Bruce McPhee. Correspondence should be addressed to him at the Division of Orthopaedics, The University of Queensland, Clinical Sciences Building, Royal Brisbane Hospital, Herston, Brisbane, 4029, Australia.
