Analysis of stress distribution in lumbar interbody fusion

Study Design. A 2-dimensional axisymmetric finite element model of an intervertebral segment was used to investigate the stress patterns in the adjacent vertebrae of fused spinal segment incorporating 4 common cage designs. The same was used to study the effect of maturation of bone graft on stress distribution pattern. Objectives. To study and compare the stress distribution patterns in a normal spinal segment and in the adjacent vertebrae of a fused spinal segment. The effect of bone graft incorporation around the mesh cage was also investigated. Summary of Background Data. Lumbar fusion surgery is thought to relieve discogenic low back pain by eliminating the abnormal intersegmental movement at the level of disc degeneration. Successful spinal fusion does not guarantee symptomatic pain relief. Discogenic pain is also known to be associated with an abnormal load transmission pattern across the degenerate disc. We hypothesized that the lumbar interbody fusion results in relief of discogenic pain by normalizing the load distribution pattern. Methods. We used a 2-dimensional axisymmetric finite element model of an intervertebral segment to investigate the stress patterns in the vertebrae adjacent to a fused spinal segment incorporating 4 common cage designs: ( 1) anterior lumbar interbody fusion, ( 2) posterior lumbar interbody fusion rectangular, ( 3) posterior lumbar interbody fusion threaded, and ( 4) mesh cage. Results. High stress concentrations and abnormal overall stress patterns were noted for all the cage designs studied. The anterior lumbar interbody fusion cage with its larger contact area showed the least abnormal stress magnitude in comparison with the other cages. Incorporation of bone in and around the mesh cage increased the area of contact and decreased the abnormal high stresses. The spine fusion model representing final bony healing showed restoration of near physiologic stress pattern. Conclusions. Interbody fusion cages with larger area of contact between cage and vertebral endplate produces a lower stress distribution pattern. A successful bony fusion restores near physiologic stress distribution pattern. Restoration of near normal load distribution pattern may become an important aim of surgery for discogenic low back pain.