The Optimal Effective Concentration of Spinal Anesthesia for Interlaminar Endoscopic Lumbar Discectomy: An Approach Based on the Biased Coin Design

Purpose: To determine the optimal spinal anesthesia concentration for interlaminar endoscopic lumbar discectomy (IELD), in order to deliver precise and individualized anesthesia for this procedure. Methods: Fifty-four patients scheduled for IELD were randomized via a biased coin design to receive spinal anesthesia with different ropivacaine doses. The initial dose was 7.5 mg/3 mL. If the patient reported a negative outcome, the subsequent dose was increased by 0.75 mg. If a positive outcome was reported, subsequent dose remained unchanged (89% probability) or was decreased by 0.75 mg (11% probability). At least 45 positive outcomes were needed to estimate the MEC 90 . Outcome measures: onset and duration of surgical analgesia, postoperative pain scores, urinary retention incidence, end-of-surgery modified Bromage score, and patient satisfaction. Results: The isotonic regression estimated the MEC 90 of ropivacaine for spinal anesthesia in IELD at 9.474 mg/3 mL (95% CI: 8.242– 10.088); Probit regression yielded MEC 90 = 9.480 mg/3 mL (95% CI: 8.806– 10.154), MEC 95 = 9.932 mg/3 mL (95% CI: 8.957– 10.907), and MEC 99 = 10.779 mg/3 mL (95% CI: 9.122– 12.436). Analgesia onset was 5.0 min (IQR: 5.0– 7.0), lasting 100.0 min (IQR: 90.0– 110.0). Postoperative pain scores (VAS, cm) at 2, 6, 12, and 24 hours were 2.0 (1.0– 3.0), 2.0 (2.0– 2.0), 2.0 (2.0– 3.0), and 2.0 (2.0– 2.0), respectively. Bromage score at surgery end was 4.5 (IQR: 3.0– 5.0). Mild urinary retention occurred in 4 patients (7.4%); no other complications were observed. Patient satisfaction: excellent (53.7%), good (20.4%), fair (13.0%), poor (5.6%), very poor (7.4%). Ropivacaine concentration was negatively correlated with analgesia onset time ( r = − 0.228, P = 0.049) and Bromage score ( r = − 0.235, P = 0.044), but not associated with analgesia duration ( r = 0.105, P = 0.224). Conclusion: The MEC 90 of ropivacaine for spinal anesthesia in IELD was 9.474 mg/3 mL, the clinically recommended dose was 9.5 to 10.5 mg/3 mL. Plain Language Summary: Interlaminar Endoscopic Lumbar Discectomy (IELD) is a minimally invasive procedure for lumbar disc herniation; however, the optimal dosing of spinal anesthesia remains unclear. Using a biased-coin up-and-down sequential design, this study determined the effective intrathecal ropivacaine concentrations for IELD. The MEC 90 was 9.47 mg/3 mL. Sensory blockade onset occurred at approximately 5 minutes (range: 5– 7 min); duration was approximately 100 minutes (90– 110 min). Postoperative VAS scores remained stable at approximately 2 cm over 24 hours. At the end of surgery, the median Bromage score was 4.5 (range: 3– 5), indicating moderate motor blockade. Four patients (7%) experienced mild urinary retention; no other adverse events occurred. Patient satisfaction was as follows: 54% rated the anesthesia “excellent”, 20% “good”, 13% “fair”, and 13% “poor” or “very poor”. Higher ropivacaine concentrations shortened the onset time and increased the degree of motor blockade (reflected by higher Bromage scores) but did not prolong the duration of sensory blockade. These findings support evidence-based ropivacaine dosing for spinal anesthesia in IELD, thereby improving safety, efficacy, and clinical decision-making.