Feasibility of Intraoperative Anterior Reduction for the Treatment of Grade II Spondylolisthesis: A Biomechanical Investigation

BACKGROUND: Spondylolisthesis is a common cause of lower back pain, often accompanied by spinal stenosis, disc degeneration, and altered biomechanics. Reduction is achieved anteriorly through anterior lumbar interbody fusion (ALIF) or posteriorly through pedicle screws/rods. ALIF allows for favorable biomechanics for fusion but has limited reduction capability in high-grade slips. Anterior reduction allows for increased slip reduction without relying on spacer impaction but has only been explored in grade I spondylolistheses. The objective of this study was to evaluate the reduction capacity of an integrated ALIF spacer with a novel built-in anterior reduction mechanism in a simulated grade II spondylolisthesis model. METHODS: Spondylolistheses were simulated in 5 fresh-frozen cadaveric lumbar segments (L4-L5) with slips at the high end of the grade II range (50% ± 5% anterior slip) via a combination of partial discectomy, facet joint loosening, and dynamic shear. Specimens were treated with ALIF spacers with a built-in anterior reduction mechanism under physiological load, and incremental reduction was recorded during each procedural step. RESULTS: = 0.05 vs insertion). CONCLUSIONS: Each procedural step incrementally contributed to slip reduction. The majority of correction was attributable to the anterior reduction mechanism, demonstrating a potential benefit over the traditional ALIF approach. Future clinical testing is necessary to determine the impact on patient outcomes. CLINICAL RELEVANCE: Slip reduction of higher-grade spondylolistheses allows for more anatomical alignment and favorable biomechanics for fusion. The ability to achieve such reduction via an anterior approach minimizes the risks of pedicle screw pullout from posterior approaches and still allows for circumferential fixation via posterior pedicle screws.