Effects of Metformin on Postoperative Opioid Consumption in Patients Undergoing Posterior Lumbar Decompression and Fusion Surgery

Introduction: Spine fusion is one of the most performed orthopedic surgeries in the United States. Several animal studies have demonstrated that metformin, a first-line treatment for type 2 diabetes (T2DM) can attenuate neuropathic pain via decreased activation of microglia and suppression of inflammatory signaling. The present study aims to examine the effects of metformin on postoperative pain, surgical outcomes, and patient-reported outcome measures (PROMs).

Methods: All patients (> 18 years old) who underwent a 1-2-level posterior lumbar decompression and fusion (PLDF) between 2018-2020 were identified via Structured Query Language (SQL) search and confirmed with manual chart review. Patient demographics, surgical outcome data, and metformin use and dosage were collected. Opioid consumption data was obtained from the Pennsylvania Prescription Drug Monitoring Program. In-hospital morphine milligram equivalents (MME) and preoperative and postoperative PROMs including the mental (MCS) and physical (PCS) component scores of short-form 12, Oswestry Disability Index (ODI), visual analogue scale (VAS) back pain, and VAS leg pain were collected. Statistical analysis was performed and a p-value < 0.05 was statistically significant. Linear and Poisson regressions were also conducted.

Results: Of the 149 diabetic patients who underwent a 1-2 level PLDF, 109 (73.2%) patients were taking metformin. There were no differences in MMEs in the 30 days prior to surgery or the 30 days after surgery between groups. Patients on metformin had a lower Elixhauser (2.12 ± 1.32 vs. 2.98 ± 1.49; p=0.001) but were otherwise similar in demographic and surgical characteristics. Patients on metformin had lower postoperative MMEs at 30-90 days (32 ± 55.9 vs. 57.9 ± 68.4; p=0.008) and 90-365 days (66 ± 180 vs. 139 ± 269; p=0.013), shorter hospital LOS (3.93 ± 3.02 vs. 4.52 ± 1.57; p=0.002) and improved more from pre-to postoperative VAS leg pain (-2.60 ± 3.80 vs. -5.67 ± 2.41; p=0.010). Linear regression analyses identified metformin use as independently predictive of fewer postoperative MMEs (Estimate -21.58, CI -42.43 to -0.72; p=0.045).

Conclusion : Our study demonstrates that metformin may reduce postoperative opioid consumption in T2DM patients undergoing spine surgery. This study suggests a strategy for better postoperative pain management, potentially reducing opioid dependence and tolerance, and improving overall patient outcomes.