Effects of Fusion Mass Density and Fusion Location on the Strength of a Lumbar Interbody Fusion

The location and elastic modulus of a fusion mass are important factors for clinical assessment of the adequacy of interbody fusion. Various finite element models of the L3-L4 motion segment were built using the geometry from cross-sectional CT images of the lumbar spine. By applying a compressive load to the superior endplate of the superior vertebral body, the cumulative reaction force and maximum principal strain value could be computed. Using these values and the compressive failure strain for vertebral cancellous bone, the maximum sustainable load before failure could be computed. It was consistently noted that as the density of the fusion increased, the maximum load increased. Furthermore, when the density of the fusion mass became equal to or greater than the vertebral bodies, failure was seen in the vertebral body whereas for lesser densities failure was seen in the fusion. As the fusion was laterally displaced, failure load decreased, and as the fusion was posteriorly displaced, the failure load increased. In conclusion, the results validated the importance of clinical assessment of both fusion density and fusion location.