Risk Factors for Nonunion Following Posterior Lumbar Interbody Fusion at L5-S1: Importance of Bilateral Bicortical Purchase of S1 Pedicle Screws.

Study DesignA retrospective comparative study.ObjectiveThis study explored the factors influencing L5-S1 fusion success following posterior lumbar interbody fusion (PLIF) and identified risk factors for symptomatic nonunion.MethodsThe cohort comprised 134 patients who underwent single- or multiple-level lumbar fusion, including PLIF at L5-S1, due to degenerative spine disease. Radiographic fusion was assessed using 1-year postoperative CT scans. Demographic, clinical (VAS, ODI, and EQ-5D), and radiological data were compared based on fusion status using univariate and multivariate logistic regression analyses. Subgroup analysis was performed in patients exhibiting nonunion with or without symptoms.ResultsFusion was achieved in 98 (73.1%) out of 134 patients, whereas 36 (26.9%) patients exhibited nonunion at the 1-year follow-up. Factors associated with nonunion included a higher body mass index [BMI; = .020; 95% confidence interval (CI): .702, .971] and longer fusion levels ( = .032; 95% CI: .345, .952). Bilateral bicortical purchase of S1 pedicle screws significantly improved fusion outcomes ( = .014; 95% CI: 1.281, 9.047). Among the 36 nonunion cases, symptomatic patients showed significantly worse clinical assessment in terms of VAS, ODI, and every domain of EQ-5D. However, radiological parameters exhibited no differences based on the presence of symptoms.ConclusionThe radiological nonunion rate at L5-S1 was 27%. Higher BMI and longer fusion levels were identified as risk factors, whereas bicortical screw placement at S1 emerged as a protective factor against L5-S1 nonunion. Therefore, bilateral anterior cortical purchase of S1 pedicle screws is recommended, particularly in patients undergoing longer fusion procedures.