The influence of intrinsic disc degeneration of the adjacent segments on its stress distribution after one-level lumbar fusion

The presence of degeneration alters the stress within that particular degenerated disc. Therefore, the state of degeneration of the disc at the level adjacent to a single level fusion would influence the rate of subsequent further degeneration of that same adjacent disc. Therefore, the purpose of this study was to investigate the biomechanical effect of the disc degeneration at the adjacent segment on the stress of the corresponding segment after one segment lumbar fusion using validated finite element (FE) models. Three validated L2-L5 FE models with different grades of disc degeneration (normal, mild, moderate) at the L2-L5 were simulated. Based on the intact models that represented each grade of disc degeneration, the matched L3-L4 instrumented fusion models were developed. Therefore, three intact models and three matched fusion models were simulated. Under the 400-N pre-compressive follower loading, 7.5 Nm moments simulating flexion, extension, lateral bending, and torsion in five steps were imposed on the L2 superior endplate of three intact models. The hybrid loading protocol was utilized for the fusion models. The intradiscal pressure and the maximal von Mises stress of the annulus fibrosus in the L2-L3 and L4-L5 segments were analyzed and compared between fusion and intact models with each grade of disc degeneration. Disc degeneration led to an increase of maximal von Mises stress of annulus fibrosus at this degenerated disc, related to disc degeneration under flexion, extension, lateral bending and torsion moments. Furthermore, the fusion also increased a maximal von Mises stress of annulus fibrous at both proximal and distal adjacent segments under hybrid protocol. The increases of annular stress were greater at the proximal adjacent segment than those at distal adjacent segment. However, the contribution of the fusion surgery to the increment rate of annulus fibrosus stress at the adjacent segment did not increase in relation with disc degeneration states at the adjacent segment. The present study demonstrates disc degeneration per se and fusion can cause the increase of disc stress at the adjacent segment. However, they seem not to act synergistically. Therefore, the proper decision of fusion extent needs comprehensive individual considerations about the degree of facet degeneration, canal stenosis and patient's activity.