Determinants of lateral fusion in single-level oblique lateral lumbar interbody fusion: a retrospective analysis of fusion patterns and clinical outcomes

Study Design: Retrospective cohort study.Purpose: This study aimed to (1) determine the incidence of lateral fusion following single-level oblique lateral interbody fusion (OLIF); (2) identify risk factors associated with the development of lateral fusion; (3) evaluate the effect of different fusion patterns on interbody cage subsidence rates; and (4) assess whether fusion patterns influence postoperative clinical outcomes.Overview of Literature: Fusion characteristics following OLIF differ from those seen in conventional transforaminal lumbar interbody fusion, most notably due to lateral fusion marked by extra-vertebral bony bridging (EVB). EVB may develop early postoperatively, suggesting a potential mechanism for early interbody fusion.Methods: This retrospective cohort study included 153 single-level OLIF cases between January 2016 and December 2023. Postoperative computed tomography was used to classify patients into central fusion, lateral fusion, and non-fusion groups. Demographic, surgical, and radiographic parameters—including osteophyte grade, Hounsfield unit (HU) values, and cage positioning—were analyzed to identify factors affecting fusion. Cage subsidence and clinical outcomes (Oswestry Disability Index [ODI], Visual Analog Scale) were compared across groups.Results: Lateral fusion occurred in 39.9% of cases, central in 56.9%, and non-fusion in 3.2%. Preoperative osteophytes and higher HU values were associated with lateral fusion (p<0.001). OLIF with standalone cages (OLIF-SA) had a significantly higher lateral fusion rate than OLIF with posterior screw fixation (OLIF-PS) (p=0.002). Smoking was a significant risk factor for non-fusion (p=0.005). No significant difference in cage subsidence was observed between central and lateral fusion, but non-fusion showed more severe subsidence. Clinical outcomes improved across fusion groups, though non-fusion cases had worse ODI scores at follow-up.Conclusions: Lateral fusion is a distinct OLIF feature influenced by osteophytes, bone density, and fixation type. It does not negatively affect cage subsidence or outcomes, but solid fusion remains essential for recovery. These findings enhance understanding of OLIF fusion and may guide surgical planning.