Effects of short anterior lumbar interbody fusion on biomechanics of neighboring unfused segments

Study Design. Segmental mobility and intradiscal pressure were measured and the data compared in six cadaveric lumbar spine specimens before and after in vitro simulated single level L4-5 and double level L4-5-S1 anterior interbody fusions. Objective. The experimental objective was to study the biomechanical effects of single level L4-5 and double level L4-5-S1 anterior interbody fusions on the neighboring unfused segments. Summary of Background Data. The relationship between the local rigidity created by fusion mass and accelerated degeneration reported at the neighboring unfused intervertebral discs is not clear. Methods. Six cadaveric lumbar spine specimens were biomechanically tested in flexion and extension. Segmental mobility and intradiscal pressure of the specimens were measured before and after in vitro simulation of single level L4-5 and double level L4-5-S1 anterior interbody fusions. Results. The mobility of the motion segments immediately above and below an L4-5 fusion was increased in flexion. When the L5-S1 segment was also fused, the loss of segmental motion in both flexion and extension at the L4-5 and L5-S1 were compensated for by increased motion in all levels above the fusion. In both flexion and extension, the intradiscal pressures after a single level L4-5 fusion and this increase was even more marked after a double level L4-5-S1 fusion. Conclusions. There is no evidence that the neighboring unfused segments are loaded beyond their physiological limits due to the fusion. However, the neighboring unfused segments have to work more frequently toward the extremes of their functional ranges of motion after fusion and these effects will be more marked after a double level L4-5-S1 fusion.