The Effect of Fusion on the Adjacent Segment Motion in a Lumbar Spine

Back Pain is one of the most prevalent work related injuries in the United States. It has been shown that heavy physical work, and injuries in addition to other physiological conditions cause back pain. In cases of severe back pain, surgeries are conducted and grafts are implanted at sites where disc material is removed. Thus it is important to understand how these grafts affect the mechanical response of the spine to loading in postoperative conditions. A finite element model of the lumbar spine was constructed to study the biomechanical response due to loading. The model was used to conduct the investigation of the change in motion at the segment adjacent to the fusion. The study was repeated with the inclusion of adjacent segment degenerated disease. From the current study, it was concluded that the behavior of the fused segments was similar in both single level and two level fusion cases. The percentage reduction in motion was also similar in both single level and two level fusions either with normal spine or the spine having ASDD. The effect of fusion on the motions of adjacent segments was also similar in single level fusion and two level fusions. The motions at the adjacent segment were affected by the grades of degeneration at that particular segment adjacent to the fusions. The following conclusions are achieved from the current study: 1) The fusion with graft in both normal bone and with ASDD produces decrease in motion at the level of fusion and increase in motion at the adjacent segments when compared to the motion at normal lumbar spinal motion segment. This conclusion was applicable for both single level and two level fusions. 2) When the fusion was modeled with the spine having ASDD, the motion at that adjacent segment decreased with the increasing grades of degeneration. This conclusion was applicable for both single level and two level fusions.