The effect of paraspinal sarcopenia on postoperative sagittal balance: a multivariate analysis following multilevel lumbar fusion surgery

BACKGROUND CONTEXT: Paraspinal muscles play a crucial role in maintaining lumbar spine stability and sagittal alignment. However, the impact of sarcopenia on long-term sagittal balance and clinical outcomes following multilevel posterior lumbar interbody fusion (PLIF) surgery remains underexplored. PURPOSE: To investigate the impact of paraspinal sarcopenia on long-term sagittal alignment and persistent pain/functional disability in patients undergoing multilevel PLIF; to identify risk factors associated with sagittal imbalance after lumbar fusion surgery. STUDY DESIGN: Retrospective cross-sectional study. PATIENT SAMPLE: Patients who underwent multilevel PLIF surgery at a single institution between January 2015 and December 2022. Preoperative lumbar magnetic resonance imaging, computed tomography, and full-spine radiographs were performed within 1 week before surgery and at 1 month, 6 months, and at least 2 years postoperatively, alongside outpatient and inpatient evaluations. OUTCOME MEASURES: Muscle metrics of the psoas, erector spinae, and multifidus muscles at the L3 level, including muscle cross-sectional area index (MI), fat infiltration (FI), and muscle density (MD). Spinopelvic parameters included segmental lordosis (SL), sacral slope, pelvic tilt (PT), pelvic incidence (PI), lumbar lordosis (LL), PI-LL mismatch (PI-LL), and sagittal vertical axis (SVA). Visual analog scale assessed back pain. Oswestry Disability Index evaluated clinical functional outcomes. METHODS: Participants were categorized into sarcopenic (S) and nonsarcopenic (NS) groups based on the psoas MI (a threshold of <6.36 cm²/m² for men and <3.92 cm²/m² for women). Muscle area was normalized to patient height (cm²/m²). Fat infiltration was assessed using the Goutallier grading system. MD was calculated using custom software. LL, PT, sacral slope, PI, and SVA were measured from standing lateral radiographs. Multivariate regression analysis was conducted to identify long-term risk factors for postoperative sagittal imbalance. RESULTS: This study ultimately included 213 patients. In the sarcopenic group, significant differences were observed in body mass index, with higher FI and reduced muscle indices (p<.05), though no difference in MD was detected. Preoperative PT, as well as final follow-up LL, SL, PT, PI-LL, and SVA, differed significantly between the groups (p<.05). Moreover, patients in the S group showed poorer long-term visual analog scale and Oswestry Disability Index scores (p<.05). These findings indicate that patients with paraspinal muscle reduction exhibit worse long-term outcomes, alongside long-term loss of sagittal balance. Multivariate regression analysis identified multifidus atrophy and FI, along with insufficient preoperative LL and SL, as independent risk factors for long-term postoperative sagittal imbalance, while body mass index, psoas muscle, erector spinae, and other sagittal balance parameters showed no significant association. CONCLUSIONS: This study underscores the significant impact of paraspinal sarcopenia on sagittal balance and clinical outcomes following multilevel lumbar fusion surgery. It also elucidates the relationship between postoperative sagittal imbalance and paraspinal muscle condition. A larger paraspinal MI, reduced FI, and favorable preoperative LL and SL angles are positively associated with improved spine-pelvic sagittal balance maintenance. These findings emphasize the importance of preoperative evaluation of paraspinal muscle health, nutritional status, and sagittal alignment to reduce postoperative pain and enhance long-term quality of life.