Introduction: Lateral lumbar interbody fusion (LLIF) is a useful less invasive option for many spinal conditions especially in high-risk populations. The complication profile is well-described and is unique to the procedure. LLIF is routinely performed as a stand-alone operation, and radiographic subsidence of the graft is a common occurance. However, the clinical impact of subsidence remains unclear. The aim of this study is to examine this relationship.

Methods: Retrospective review of prospective data for stand-alone LLIF operations over a seven-year period. 297 patients were included. Imaging studies were examined to evaluate graft subsidence, which was graded according to the criteria described by Marchi et al. Subsidence of 0-25% was grade 0, 25-50% grade 1, 50-75% grade 2 and > 75% grade 3 (grades 0 and 1 = low-grade, grades 2 and 3 = high-grade). Additional variables included levels fused, DEXA T-score, body mass index and demographic information. The data were analyzed using univariate and multivariate regression analysis.

Results: 34 of 297 patients (11.4%) developed radiographic subsidence. 18 patients (6.1%) required revision surgery. The mean subsidence grade for patients requiring revision surgery was 2.44 versus 0.94 for those without revision surgery (p<0.01). All revision surgery patients had high-grade subsidence. Seven of 18 patients (38.9%) requiring revision surgery suffered a vertebral body fracture. High-grade subsidence was the only significant predictor of need for revision surgery (p<0.05; OR 12, 95%CI 1.29-13.6). High-grade subsidence remained a significant predictor of revision surgery despite adjustment for other variables (OR 14.4; 95%CI 1.30-15.9)

Conclusions: High-grade graft subsidence occurred in 7.4% of patients undergoing stand-alone LLIF, and high-grade subsidence was strongly associated with need for revision surgery. Though the overall rate of revision surgery was low, placement of supplementary instrumentation up-front in high-risk patients may circumvent potentially negative clinical sequelae associated with an open operation.

Patient Care: This study provides lateral access surgeons with important information regarding the negative consequences of graft subsidence in patients undergoing stand-alone LLIF. Further, it may influence the surgeon's decision to supplement LLIF cages with posterior instrumentation in high-risk patients. It also allows the surgeon to better counsel patients regarding the risks and benefits of performing a stand-alone LLIF procedure.

Learning Objectives: 1. To describe the potentially negative clinical effects of graft subsidence following stand-alone LLIF. 2. To evaluate the risk factors associated with graft subsidence and need for revision surgery after stand-alone LLIF.

References: 1. Marchi L, Abdala N, Oliveira L, Amaral R, Coutinho E, Pimenta L. Radiographic and clinical evaluation of cage subsidence after stand-alone lateral interbody fusion. J Neurosurg Spine. 2013;19(1):110–118. 2. Le TV, Baaj AA, Dakwar E, Burkett CJ, Murray G, Smith DA, Uribe JS. Subsidence of polyetheretherketone intervertebral cages in minimally invasive lateral retroperitoneal transpsoas lumbar interbody fusion. Spine (Phila Pa 1976). 2012;37(14):1268–1273. 3. Tempel ZJ, Gandhoke GS, Okonkwo DO, Kanter AS. Impaired bone mineral density as a predictor of graft subsidence following minimally invasive transpsoas lateral lumbar interbody fusion. Eur Spine J. 2015;24(Suppl 3):S414–S419.