Rebound pain scores as a function of femoral nerve block duration after anterior cruciate ligament reconstruction: retrospective analysis of a prospective, randomized clinical trial.

BACKGROUND AND OBJECTIVES: Continuous perineural femoral analgesia has been reported to reduce numeric rating pain scores (NRS, scale 0-10) after anterior cruciate ligament reconstruction (ACLR). In the current study, we determined rebound pain scores in autograft ACLR outpatients after nerve block analgesia resolved. METHODS: After standardized spinal anesthesia and perioperative multimodal analgesia, patients received a femoral perineural catheter and 50 hours of saline or levobupivacaine. All patients received levobupivacaine (30 mL of 0.25% as a bolus) before the infusion. Patients completed a pain diary for 6 days, indicating serial NRS scores and perceptions of when nerve block analgesia resolved. Block duration and rebound pain scores were computed. RESULTS: Data from 84 participants' pain diaries were analyzed. Patients receiving saline infusion reported mean nerve block duration of 37 hours versus 59 hours for patients receiving the levobupivacaine infusion (P < .001). Mean rebound pain scores increased by 2.0 (95% confidence interval, 1.6-2.4). Based on the computations used to derive block duration and rebound pain scores, each hour of additional block duration was predictive of a 0.03-unit reduction in rebound pain scores. CONCLUSIONS: In an anesthesia care protocol consisting of spinal anesthesia and multimodal analgesia during and after autograft ACL reconstruction, approximately 33 hours of additional nerve block duration were required to reduce rebound pain scores by one unit. Further study is required to determine rebound pain score differences when other local anesthetics and anesthetic/analgesic plans are being used and when other surgeries are being performed.