Minimum Effective Analgesic Concentration of Ropivacaine in Saphenous Block Guided by Ultrasound for Knee Arthroscopic Meniscectomy: Randomized, Double-Blind Study

BACKGROUND: After knee surgery, analgesia should be effective for mobilization and discharge. AIM OF THE STUDY: The primary objective of this study was to achieve the lowest effective analgesic concentration (MEC50 and MEC90) of ropivacaine for saphenous nerve block in arthroscopic meniscectomy. The secondary objective was to determine whether the block causes muscle weakness in the postoperative period. METHODS: The study was randomized, comparative, and double-blind. Fifty-one patients between 18 and 65 years old of both sexes, ASA I or II, who underwent knee arthroscopic meniscectomy at São Domingos Hospital were included. Patients underwent saphenous nerve block with 10 mL of ropivacaine administered by using the up-and-down method. The ropivacaine concentration was determined based on the previous patient’s response (a biased-coin up-down design sequential method). If a patient had a negative response, the concentration of ropivacaine was increased by 0.05% in the next patient; if the response was positive, the next patient was randomized to be administered the same concentration of ropivacaine or a 0.05% lower concentration. Successful block was defined as pain <4 during 6 h. Patients underwent general anesthesia with 30 µg/kg alfentanil and propofol and maintenance with propofol, and, if necessary, remifentanil was administered. Postoperative analgesia was complemented with dipyrone, and if necessary, tramadol (100 mg) could be used. The following parameters were assessed: the success of the block; pain intensity after 2, 4, and 6 h; the consumption of remifentanil; time to the first analgesic supplementation; percent of patients who needed analgesics during 6h; and muscle strength. RESULTS: The MEC50 of ropivacaine was 0.36%, and the MEC90 was 0.477%. The block was successful in 45 patients. CONCLUSION: Saphenous block with 10 mL of 0.36% ropivacaine provides adequate analgesia for outpatient meniscectomy.