Investigation of Alterations in the Lumbar Disc Biomechanics at the Adjacent Segments After Spinal Fusion Using a CombinedIn VivoandIn SilicoApproach

The development of adjacent segment degeneration (ASD) is a major concern after lumbar spinal fusion surgery, but the causative mechanisms remain unclear. This study used a combinedin vivoandin silicomethod to investigate the changes of anatomical dimensions and biomechanical responses of the adjacent segment (L3-4) after spinal fusion (L4-S1) in five patients under weight-bearing upright standing conditions. Thein vivoadjacent disc height changes before and after fusion were measured using a dual fluoroscopic imaging system (DFIS), and the measuredin vivointervertebral positions and orientations were used as displacement boundary conditions of the patient-specific three-dimensional (3D) finite element (FE) disc models to simulate the biomechanical responses of adjacent discs to fusion of the diseased segments. Our data (represented by medians and 95% confidence intervals) showed that a significant decrease by - 0.8 (- 1.2, - 0.4) mm (p< 0.05) in the adjacent disc heights occurred at the posterior region after fusion. The significant increases in disc tissue strains and stresses, 0.32 (0.21, 0.43) mm/mm (p< 0.05) and 1.70 (1.07, 3.60) MPa (p< 0.05), respectively, after fusion were found in the posterolateral portions of the outermost annular lamella. The intradiscal pressure of the adjacent disc was significantly increased by 0.29 (0.13, 0.47) MPa after fusion (p< 0.05). This study demonstrated that fusion could cause alterations in adjacent disc biomechanics, and the combinedin vivo and in silicomethod could be a valuable tool for the quantitative assessment of ASD after fusion.

Automated summary

This study used a combined in vivo and in silico method to investigate changes in anatomical dimensions and biomechanical responses of adjacent segments after spinal fusion surgery. The results showed significant decreases in adjacent disc heights, increases in disc tissue strains and stresses, and higher intradiscal pressures after fusion. Fusion surgery can alter the biomechanics of adjacent discs, highlighting the importance of quantitative assessment of adjacent segment degeneration post-fusion.