Abstract
<title>Abstract</title> <p>Background Prolonged analgesia after shoulder arthroscopy may also prolong motor block and interfere with early protected activity. We evaluated the analgesic and motor effects of three liposomal bupivacaine (LB) concentrations administered in a fixed 20-mL interscalene block. Methods In this single-center, randomized, double-blind exploratory trial, 147 adults undergoing elective shoulder arthroscopy were allocated equally to a 20-mL ultrasound-guided interscalene block containing LB 6.7, 8.3, or 10.0 mg/mL. After trial registration but before database lock and formal comparative analysis, the outcome framework was modified: the primary endpoint was defined as the 6–72 h activity-related area under the visual analog scale-time curve (AUC-VAS), and the key secondary endpoint was the first scheduled assessment at which blocked-side elbow-flexion strength reached Medical Research Council (MRC) grade ≥ 4. AUC-VAS was analyzed using the Kruskal-Wallis test with multiplicity-adjusted pairwise comparisons. Motor recovery was evaluated using Kaplan-Meier and log-rank methods as an assessment-time analysis. Results All 147 participants completed the 72-h follow-up. Median (Q1, Q3) AUC-VAS values were 102 (96, 114), 48 (33, 60), and 42 (33, 54) in the LB 6.7, 8.3, and 10.0 mg/mL groups, respectively (H = 98.72, df = 2, P < 0.001). Versus LB 6.7 mg/mL, Hodges-Lehmann location-shift estimates were − 54 (95% CI, − 60 to − 48) for LB 8.3 mg/mL and − 60 (95% CI, − 66 to − 54) for LB 10.0 mg/mL (adjusted P < 0.001 for both). No statistically significant difference in AUC-VAS was detected between LB 10.0 and 8.3 mg/mL (− 6; 95% CI, − 12 to 0; adjusted P = 0.236). By 72 h, 47/49, 45/49, and 25/49 participants had reached MRC grade ≥ 4, respectively; the corresponding median observed recovery times were 48, 48, and 72 h (log-rank χ² = 49.64, df = 2, P < 0.001). No predefined clinically recognized safety event was documented, although the trial was not powered to establish safety. Conclusions LB 8.3 mg/mL reduced activity-related pain burden versus LB 6.7 mg/mL. No statistically significant difference in AUC-VAS was detected between LB 8.3 and 10.0 mg/mL, whereas motor recovery was delayed in the LB 10.0 mg/mL group. Because the two higher regimens exceeded the recommended total dose and the outcome framework was modified after registration, the findings are exploratory and do not support routine higher-dose use. Trial registration Chinese Clinical Trial Registry, ChiCTR2500102951. Registered on 22 May 2025, before randomization of the first participant.</p>