Effect of Fusion Methods on Adjacent Segment Biomechanics in a Lumbar Spine with Preexisting Conditions .

Low back pain is the second leading reason for visits to the doctors and is caused by various structural issues. Fusion surgery is a method to treat back pain, however, it does not go without consequence. Nonetheless, increasing clinical demand, evolving surgical methods, and patient data on outcomes complicate the future of fusion surgeries. During fusion surgery, motion is eliminated at the level and the adjacent segments undergo compensatory changes due to the “fused” vertebrae. Adjacent segment disease, ASD, which may be caused by fusion surgery, is degenerative changes at a spinal level adjacent to a surgically treated level. Excessive loading and motion experienced at the adjacent levels are examples of indicators that could lead to ASD. Yet, others believe that ASD is caused by natural history, such as disc degeneration or osteoporosis in the spine. A finite element program along with CT scans were used to create a three dimensional, finite element model of a lumbar spine. This model was validated with in vivo and in vitro results from literature to confirm its credibility. Afterwards, the model of the lumbar spine was modified to analyze various risk factors in single and multiple levels and their respective adjacent segment biomechanics for three principal loading conditions: three grades of disc degeneration, three different types of fusion techniques, osteoporosis, and their many combinations. The results of this study are presented and rank the risk factors, separately and in combination, in their likelihood of causing ASD. When considering preexisting conditions, results show that severe disc degeneration poses a larger risk in developing ASD than osteoporosis. In addition, rigid fusion techniques, such as posterior instrumentation with a bony fusion, increases the likelihood of ASD more than bony fusion or posterior instrumentation alone. Lastly, fusion techniques or preexisting conditions affecting multiple levels generate larger adjacent segment biomechanics and cause a higher risk of ASD than a single affected level. These findings will help orthopedic surgeons make better-informed decisions on surgeries based on the best outcomes for the patient.