Facet Joint Biomechanics at the Treated and Adjacent Levels After Total Disc Replacement

Study Design. Biomechanical study using human cadaveric lumbar spines. Objective. To evaluate effects of total disc replacement (TDR) on spine biomechanics at the treated and adjacent levels. Summary of Background Data. Previous studies on spine biomechanics after TDR were focused on facet forces and range of motion and report contradictory results. Characterization of contact pressure, peak contact pressure, force, and peak force before and after TDR may lead to a better understanding of facet joint function and may aid in prediction of long-term outcomes after TDR. Methods. Seven fresh-frozen human cadaveric lumbar spines were potted at T12 and L5 and installed in a 6 degrees of freedom displacement-controlled testing system. Displacements of 15 degrees flexion/extension, 10 degrees right/ left bending, and 10 degrees right/ left axial rotation were applied. Contact pressure, peak contact pressure, force, peak force, and contact area for each facet joint were recorded at L2-L3 and L3-L4 both before and after TDR at L3-L4. The data were analyzed with analysis of variance and t tests. Results. Axial rotation had the most impact on contact pressure, peak contact pressure, force, peak force, and contact area in intact spines. During lateral bending and axial rotation, TDR resulted in a significant increase in facet forces at the level of treatment and a decrease in contact pressure, peak contact pressure, and peak force at the level superior to the TDR. With flexion/extension, there was a decrease in peak contact pressure and peak contact force at the superior level. Conclusion. Our study demonstrates that rotation is the most demanding motion for the spine. We also found an increase in facet forces at the treated level after TDR. We are the first to show a decrease in several biomechanical parameters after TDR at the adjacent superior level. In general, our findings suggest there is an increase in loading of the facet joints at the level of disc implantation and an overall unloading effect at the level above.