Background: The increase in the number of anterior lumbar interbody fusions being performed carries with it the potential for the long-term complication of adjacent segmental degeneration. While its exact mechanism remains uncertain, adjacent segment degeneration has become much more widespread. Using a nonlinear, three-dimensional finite element model to analyze and compare the biomechanical influence of anterior lumbar interbody fusion and lumbar disc degeneration on the superior adjacent intervertebral disc, we attempt to determine if anterior lumbar interbody fusion aggravates adjacent segment degeneration.
Methods: A normal three-dimensional non-linear finite element model of L3-5 has been developed. Three different grades of disc degeneration models (mild, moderate, severe) and one anterior lumbar interbody fusion model were developed by changing either the geometry or associated material properties of the L4-5 segment. The 800 N pre-compressive loading plus 10 Nm moments simulating flexion, extension, lateral bending and axial rotation in five steps was imposed on the L3 superior endplate of all models. The intradiscal pressure, intersegmental rotation range and Tresca stress of the annulus fibrosus in the L3-4 segment were investigated.
Results: The intradiscal pressure, intersegmental rotation range and Tresca stress of the L3-4 segment in the fusion model are higher than in the normal model and different degeneration models under all motion directions. The intradiscal pressures in the three degenerative models are higher than in the normal model in flexion, extension and lateral bending, whereas in axial rotation, the value of the mild degeneration model is lower. The intersegmental rotation ranges in the three degenerative models are higher than in the normal model in flexion and extension. The values for the mild degeneration model in lateral bending and all the degeneration models in axial rotation are lower than in the normal model. The Tresca stresses are higher in the three degenerative models than in the normal model.
Conclusion: Anterior lumbar interbody fusion has more adverse biomechanical influence than disc degeneration on the adjacent upper disc and may aggravate the adjacent upper segmental degeneration.