Spinal fixation and fusion has been adopted as a common procedure in spinal surgery. However, the degeneration as a result of parafixation may produce clinically significant problems. Previous studies have already shown increased loads, mobility, and intradiscal pressure at the parafixed segments. These may hasten the degeneration and instability of the neighboring segments. However, controversy remains regarding the correlation between surgical fixation and degeneration owing to parafixation. This study tried to elucidate their relationship by analyzing the in vivo roentgenograms of the post-fusion lumbar spine to measure the motion distribution of the fixed and adjacent unfused spinal segments. In addition, a mathematical model was developed to investigate the effects of implant fixation on the kinematics and mechanics at the parafixation segments. The current radiological studies demonstrated that spinal fixation resulted in the redistribution of intersegmental mobility. The loss of flexion mobility at the fixed segments was unequally compensated for by the increased mobility at all adjacent free segments. This mechanical model can predict the redistribution patterns of mobility and stress at the parafixation segments. This redistribution depends on the fixation levels, implant rigidity, and subject predisposition. Additionally, such compensation was more marked in the tri-segmental than in the two-segment fixation, especially among patients who underwent greater spinal load. This study concluded that the increased flexural rigidity at the fixed segment leads to the compensated kinematic and mechanical demands upon the unfixed adjacent segments. Accordingly, the cumulative effects of such increased mobility and loadings on the adjacent segments could be logically postulated to be the principal causes of accelerated degeneration of the adjacent lumbar segments, both upper and lower portions, subsequent to fusion surgery.
