Biomechanical Changes at Adjacent Segments Following Anterior Lumbar Interbody Fusion Using Tapered Cages

Study Design. A biomechanical evaluation of anterior cages in a calf lumbar spine model. Objectives. To determine changes in spinal motion and intradiscal pressures at immediately adjacent lumbar motion segments following anterior insertion of tapered cages. Summary of Background Data. Stand-alone anterior lumbar interbody fusion (ALIF) is an effective approach in the treatment of discogenic low back pain. A tapered lumbar (LT) cage design attempts to restore physiologic lordosis and sagittal balance. We are not aware of any previous biomechanical evaluation of the effects of LT cages on adjacent motion segments. Methods. Nine fresh calf spines (L2–L5) were procured for the study. Pure moments (up to 8.5 Nm) in flexion, extension, and lateral bending were applied to the L2 vertebra in five steps through a nonconstrained loading system. With each step of loading, three-dimensional rotation at three intervertebral disc levels was obtained through a three-camera motion analysis system, and intradiscal pressures within the nucleus pulposus of the two nonoperated discs were measured with miniature transducers. The spines were tested initially intact and following paired anterior LT cage insertion. Results. Following ALIF, small to moderate increase in motion was found at both adjacent segments in flexion (superior: 12.5%, P < 0.05; inferior: 11.3%, P < 0.02) and lateral bending (superior: 7.8%, P < 0.02; inferior: 6.6%, P < 0.02). An increase in intradiscal pressure was noted at the superior adjacent segment under flexion (21%, P < 0.01) and lateral bending (16%, P < 0.03). Intradiscal pressure changes at the inferior adjacent level were not significant. Conclusions. Statistically significant changes in intradiscal pressures and motion were found at the adjacent levels following a single-level stand-alone ALIF procedure using paired LT cages.