Introduction: Anterior lumbar interbody fusion procedures often utilize supplemental posterior fixation. “Stand-alone” fusion implants attempt to alleviate the need for supplemental posterior instrumentation. A biomechanical study was conducted to assess the stability of a lumbar interbody cage with, and without, supplemental posterior fixation. In addition, the effect of the stabilization on the adjacent, non-fixated segment was analyzed.

Methods: Non-destructive biomechanical testing was performed on nineteen healthy cadaver spine segments. Specimens were tested in six degrees of motion and a maximum pure bending moment of 10Nm was applied. Specimens were evaluated in the following sequence: Intact, cage, cage ± facet bolts, and cage ± pedicle screws. Non-constrained motion was measured at both the index and adjacent levels.

Results: The index levels were L2-L3 and L5-S1. The cage alone provided a significant decrease in motion at the L2-L3 level but not at L5-S1. At L2-L3, cage+pedicle screws decreased motion more effectively than cage+facet bolts, both the supplemented constructs outperformed intact (p < 0.05). At L5-S1, both posterior fixation systems outperformed the intact (p < 0.05). There was no significant difference between the cage + pedicle screws and the cage + facet bolts at L5-S1. Furthermore, the adjacent segments for each level (L1-L2 and L4-L5) had no significantly increased motion, compared to intact, for all six degrees of motion tested.

Conclusions: The stand-alone cage was much more effective at L2-L3, than at L5-S1, in limiting motion. At L5-S1, supplemental fixation was needed to provide adequate stabilization. No abnormal motion was identified at the adjacent, normal segments, for the stand-alone or the circumferential constructs.

Patient Care: Although abnormal motion has been previously identified at the adjacent segments abutting the instrumented index segments, throughout this study, no abnormal motion was identified at the adjacent segments (L1-L2 and L4-L5). The decreased stabilization of the lumbosacral region (L5-S1) compared with the lumbar region (L2-L3), irrespective of supplemental fixation, supports the clinical findings of higher pseudarthrosis rates at L5-S1. This further supports the concept that AP surgery may be more necessary at the L5-S1 level than at the L2-L3 level.

Learning Objectives: By the conclusion of this session, participants should be able to: 1) Describe the importance of understanding adjacent segment degeneration following interbody fusions, 2) Discuss, in small groups whether circumferential surgeries increase the risk of developing adjacent segment disease.3) Identify that that combined anterior/posterior surgery may be more necessary at the L5-S1 level than at the L2-L3 level.

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