Intravenous dexmedetomidine dose and analgesic outcomes after femoral nerve block for anterior cruciate ligament reconstruction: a single-centre retrospective cohort study in China.

OBJECTIVE: To retrospectively examine the association between different intravenous dexmedetomidine (DEX) doses and analgesic outcomes, safety events and 6-month functional outcomes after femoral nerve block (FNB) in patients undergoing arthroscopic anterior cruciate ligament (ACL) reconstruction and to explore the dose range associated with analgesic outcomes and observed safety events. DESIGN, SETTING AND PARTICIPANTS: In this single-centre retrospective cohort study, patients who underwent arthroscopic ACL reconstruction with FNB at Peking University Third Hospital between January and December 2024 were analysed. After 1:1:1:1 matching on a generalised propensity score (calliper 0.02 per dimension), the matched cohort comprised 148 patients (37 per group): R (ropivacaine only), RD 0.5 (ropivacaine plus intravenous DEX 0.5 μg/kg), RD 0.75 (0.75 μg/kg) and RD 1 (1 μg/kg). The primary outcome was the duration of analgesia. Secondary outcomes included resting and dynamic pain scores within 48 hours, rescue analgesic consumption, safety outcomes and 6-month sensory/muscle strength assessments. A 4-parameter logistic (4PL) model was used to examine the dose-response pattern. RESULTS: All DEX groups were associated with longer analgesic duration than group R. RD 0.75 and RD 1 had longer durations than RD 0.5 (p<0.05), with no significant difference between RD 0.75 and RD 1 (p>0.05). An exploratory 4PL sigmoidal model suggested a non-linear dose-response pattern for analgesic duration (R²=0.2545, residual SE=10.29), with a plateau phase at 0.75 μg/kg. RD 0.75 and RD 1 had significantly lower 48-hour pain AUC. Postoperative 48-hour opioid consumption showed an overall between-group difference, but no pairwise comparison remained statistically significant after Bonferroni correction. Only RD 1 showed a significantly higher bradycardia incidence than R; most patients in the DEX groups showed mild-to-moderate sedation and hypoxaemia was uncommon. No 6-month sensory deficits or intergroup muscle strength differences were detected within this cohort. CONCLUSION: Intravenous DEX (0.5-1 μg/kg) was associated with prolonged early postoperative analgesia in ACL reconstruction. Within this cohort, the 0.75 μg/kg dose showed analgesic results similar to 1 μg/kg, with a numerically lower bradycardia incidence. No neurological deficits, sensory abnormalities or between-group differences in muscle strength and motor recovery were detected at 6 months within this cohort. These findings are based on a retrospective design and reflect associations rather than causal relationships, requiring verification by prospective randomised controlled trials.