P137. Fusion length requiring spinopelvic fixation in lumbosacral fusion with anterior column support at L5-S1: assessment of fusion status using CT

BACKGROUND CONTEXT The lumbosacral (LS) junction has a higher nonunion rate than other lumbar segments, especially in long-level fusion. Nonunion at L5–S1 would result in low back pain, spinal imbalance, and poor surgical outcomes. Although anterior column support at L5–S1 has been recommended to prevent nonunion, no previous study has evaluated the fusion length requiring additional spinopelvic fixation (SPF) in LS fusion with anterior column support at L5–S1. PURPOSE To determine the number of fused levels requiring SPF in LS fusion with anterior column support at L5–S1 by assessing fusion status using computed tomography (CT) depending on fusion length. STUDY DESIGN/SETTING Retrospective case-control study PATIENT SAMPLE Consecutive patients who underwent instrumented LS fusion with interbody fusion at L5–S1 without additional augmentation and LS CT > 1 year, postoperatively. OUTCOME MEASURES Patient demographics (sex, age, body weight, height, and body mass index), preoperative bone mineral density (BMD), current smoking status, comorbidities (diabetes mellitus, hypertension, and liver disease), degree of atrophy and fat infiltration of the paraspinal muscles, number of levels fused, and radiological parameters (pelvic incidence [PI], lumbar lordosis [LL], pelvic tilt, sacral slope, PI-LL mismatch, and sagittal vertical axis). METHODS The L5–S1 interbody fusion status was classified into a 4-grade system using LS CT performed at a minimum of 1-year postoperatively as follows: grade 1, fusion with complete remodeling and trabeculae across the disc space; grade 2, intact graft partially remodeled and incorporated without lucent lines; grade 3, intact graft with a definite lucent line between the graft and adjacent endplates; and grade 4, absence of fusion with resorption of the graft material. Grades 1 and 2 were defined as bone union and grades 3 and 4 as nonunion. After evaluating the fusion status of L5–S1 interbody fusion using CT, patients were divided into two groups: those with union and those with nonunion at L5–S1. The number of fused levels were classified into six categories (1 to ≥ 6). The fusion rates of L5–S1 interbody fusion were assessed based on the number of fused segments. Binary logistic regression analyses were performed to identify risk factors for nonunion at L5–S1. RESULTS Fusion rates of L5-S1 interbody fusion were 94.9%, 90.3%, 80.0%, 50.0%, 52.6%, and 43.5% for fusion of 1, 2, 3, 4, 5, and ≥ 6 levels, respectively. The number of spinal levels fused ≥ 4 (p < 0.001), low preoperative BMD (adjusted odds ratio [aOR] 0.676, p = 0.038), and postoperative PI-LL mismatch (aOR 1.034, p = 0.040) were identified as significant risk factors for nonunion of L5–S1 interbody fusion according to multivariate logistic regression analysis. CONCLUSIONS Exhibiting ≥ 4 fused spinal levels, low preoperative BMD, and postoperative PI-LL mismatch were identified as independent risk factors for nonunion of anterior column support at L5–S1 in LS fusion without additional fixation. Therefore, SPF should be considered in LS fusion extending to or above L2 to prevent LS junctional nonunion. FDA Device/Drug Status This abstract does not discuss or include any applicable devices or drugs. The lumbosacral (LS) junction has a higher nonunion rate than other lumbar segments, especially in long-level fusion. Nonunion at L5–S1 would result in low back pain, spinal imbalance, and poor surgical outcomes. Although anterior column support at L5–S1 has been recommended to prevent nonunion, no previous study has evaluated the fusion length requiring additional spinopelvic fixation (SPF) in LS fusion with anterior column support at L5–S1. To determine the number of fused levels requiring SPF in LS fusion with anterior column support at L5–S1 by assessing fusion status using computed tomography (CT) depending on fusion length. Retrospective case-control study Consecutive patients who underwent instrumented LS fusion with interbody fusion at L5–S1 without additional augmentation and LS CT > 1 year, postoperatively. Patient demographics (sex, age, body weight, height, and body mass index), preoperative bone mineral density (BMD), current smoking status, comorbidities (diabetes mellitus, hypertension, and liver disease), degree of atrophy and fat infiltration of the paraspinal muscles, number of levels fused, and radiological parameters (pelvic incidence [PI], lumbar lordosis [LL], pelvic tilt, sacral slope, PI-LL mismatch, and sagittal vertical axis). The L5–S1 interbody fusion status was classified into a 4-grade system using LS CT performed at a minimum of 1-year postoperatively as follows: grade 1, fusion with complete remodeling and trabeculae across the disc space; grade 2, intact graft partially remodeled and incorporated without lucent lines; grade 3, intact graft with a definite lucent line between the graft and adjacent endplates; and grade 4, absence of fusion with resorption of the graft material. Grades 1 and 2 were defined as bone union and grades 3 and 4 as nonunion. After evaluating the fusion status of L5–S1 interbody fusion using CT, patients were divided into two groups: those with union and those with nonunion at L5–S1. The number of fused levels were classified into six categories (1 to ≥ 6). The fusion rates of L5–S1 interbody fusion were assessed based on the number of fused segments. Binary logistic regression analyses were performed to identify risk factors for nonunion at L5–S1. Fusion rates of L5-S1 interbody fusion were 94.9%, 90.3%, 80.0%, 50.0%, 52.6%, and 43.5% for fusion of 1, 2, 3, 4, 5, and ≥ 6 levels, respectively. The number of spinal levels fused ≥ 4 (p < 0.001), low preoperative BMD (adjusted odds ratio [aOR] 0.676, p = 0.038), and postoperative PI-LL mismatch (aOR 1.034, p = 0.040) were identified as significant risk factors for nonunion of L5–S1 interbody fusion according to multivariate logistic regression analysis. Exhibiting ≥ 4 fused spinal levels, low preoperative BMD, and postoperative PI-LL mismatch were identified as independent risk factors for nonunion of anterior column support at L5–S1 in LS fusion without additional fixation. Therefore, SPF should be considered in LS fusion extending to or above L2 to prevent LS junctional nonunion.