Intravenous methocarbamol for acute pain after spine surgery: a target trial emulation.

Background: Skeletal muscle relaxants are often included in multimodal analgesic regimens following spine surgery, but their actual effectiveness remains unclear due to limited and inconsistent evidence. We aimed to evaluate the effectiveness of intravenous methocarbamol in reducing acute postoperative pain and opioid consumption after elective spine surgery.

Methods: This emulated target trial used electronic health record data from patients undergoing elective spine surgery (posterior spinal fusion, anterior cervical discectomy and fusion, laminectomy/laminotomy) between January 1, 2020 and December 31, 2023. The exposure was intravenous methocarbamol (≥500 mg) administered within 2 hours postoperatively compared with usual care without methocarbamol. The primary outcome was the time-weighted average pain score in the 6-hour follow-up window after treatment assignment. The secondary outcome was cumulative opioid use measured in oral morphine equivalents (OMEs) over the same period. We used 1:1 time-varying propensity score matching to adjust for confounding.

Results: The matched cohort included 1270 patients (median (IQR) age, 61 (50-68) years; 765 (60.2%) female). Administration of intravenous methocarbamol was not associated with significant reductions in postoperative pain scores (adjusted mean difference, 0.1; 95% CI, -0.1 to 0.4) or opioid consumption (adjusted mean difference, 0.6 OME; 95% CI, -1.4 to 2.5) compared with controls. Sensitivity analyses generally confirmed primary results, except the marginal structural model indicated slightly increased pain scores in the methocarbamol group (adjusted mean difference, 0.5; 95% CI, 0.3 to 0.7), though this was not clinically meaningful.

Conclusions: In this emulated target trial of elective spine surgery, intravenous methocarbamol did not reduce acute postoperative pain or opioid consumption. These findings suggest a lack of analgesic benefit from intravenous methocarbamol use after spine surgery.