Postoperative methocarbamol, opioid requirements, and pain after extensive spine fusion surgery: a retrospective cohort study

Abstract Background Muscle relaxants such as methocarbamol are frequently combined with other analgesics after spine surgery, yet their opioid-sparing and analgesic value is uncertain. We examined how postoperative methocarbamol relates to opioid use, pain, and secondary outcomes after extensive spine fusion, using a target trial emulation. Methods This single-center retrospective cohort study used target trial emulation. Adults (N = 1,467) underwent inpatient spine fusion involving at least three vertebral levels between 2018 and August 2024. The primary exposure was methocarbamol initiated within 12 hours postoperatively (N = 845), compared with controls without methocarbamol within 48 hours (N = 622). Primary outcomes were opioid consumption (oral morphine milligram equivalents [MME]) and average pain scores (0–10) over the first 48 postoperative hours; secondary outcomes were hospital length of stay, delirium, and SpO₂/FiO₂ ratio. Inverse probability of treatment weighting and a joint noninferiority framework were used. Results Methocarbamol was associated with significantly higher 48-hour opioid consumption (147 [84, 237] vs 98 [36, 209] mg; ratio of geometric means 1.70 [95% CI 1.47–1.96]; noninferiority P = .99) and higher pain scores (adjusted mean difference 1.13 [95% CI 0.99–1.27]; noninferiority P = .97). However, the methocarbamol group had shorter length of stay (6 vs 7 days), lower delirium (12% vs 34%), and higher SpO₂/FiO₂ ratio. Conclusions After extensive spine fusion, postoperative methocarbamol was associated with higher opioid requirements and pain but shorter hospitalization, less delirium, and improved oxygenation.