The effect of bone graft geometry on spinal fusion vertebral stresses

Spinal fusion is a frequent surgical technique in which the success is uncertain due to post-operative changes in the biomechanics of the spine. Bone grafts are good candidates for disc and vertebra substitutes due to their similar bone properties and their good osteogenic properties. However, the effect of the anatomic harvest location of the bone graft on the load transfer is unknown. A physiologic three-dimensional (3D) finite element model of a lumbar spine was modified to model spinal fusion with a fixator and a bone graft. Bone grafts were taken either from the femur, the tibia, or from the fibula in a configuration of three or six fragments. The configurations were submitted to physiological loadings, and strain and stress distributions were calculated within the vertebrae, the fixator and the bone grafts. Quantitative differences were found from one type of bone graft to another. It was found that fibula bone grafts provided better stability by carrying a large part of the load. However, femoral and tibial bone grafts provided a more similar strain distribution within the vertebrae compared to the physiologic model. For tibial bone grafts, load transfer was found to be sensitive to the orientation used during the surgery. The use of a femoral bone graft to replace one vertebra and two intervertebral discs was found to give a better biomechanical function than using a tibial or fibula bone graft. This surgical technique is proposed to be beneficial in the case of severe spinal trauma providing good interface is obtained between the bone graft and the vertebrae.