Comparison of continuous femoral nerve block (CFNB/SA) and continuous femoral nerve block with mini-dose spinal morphine (CFNB/SAMO) for postoperative analgesia after total knee arthroplasty (TKA): a randomized controlled study

BACKGROUND: Unsatisfactory analgesia for major knee surgery with femoral nerve block (FNB) alone was reported and the additional benefit of sciatic block to continuous femoral nerve block (CFNB) was not conclusive. The aim of the present study was to find the benefit of the additional mini-dose spinal morphine (0.035 mg) to CFNB for postoperative pain control and to compare their associated side effects after total knee arthroplasty (TKA). METHODS: After written informed consent and with Institutional Ethics Committee approval, 68 American Society of Anesthesiologists (ASA) Physical Status I-III patients scheduled for elective unilateral TKA under spinal anesthesia (SA) were included in the present prospective, randomized controlled study. The patients were allocated into two groups. CFNB was placed in all patients by the inguinal paravascular approach with 20 ml of 0.25 % levobupivacaine. Group I (named CFNB/SA group), SA was administered with 2.8 ml levobupivacaine and Group II (named CFNB/SAMO group), SA with 2.8 ml levobupivacaine plus morphine 0.035 mg. At Post Anesthesia Care Unit (PACU), pain and other adverse effects were recorded. Pain was assessed by visual analog scale (VAS) 0-10. Tramadol 50 mg intravenous (IV) was given if the VAS > 4. In the ward, all patients were maintained by continuous femoral infusion of 0.125 % levobupivacaine rate 7 ml/hr and then reduced to 5 ml/hr if VAS ≤3. RESULTS: Patient's demographics data in each group were not different. At post-operative (PO) 12-24 h, the VAS scores were significantly lesser in the CFNB/SAMO group. Cumulative tramadol IV requirement for PO48h were also significantly lesser in the CFNB/SAMO group. Nausea, vomiting and numbness were significantly greater in the CFNB/SAMO group during early postoperative period (PO1-6 h). CONCLUSION: Though in some patients CFNB was inadequate, a mini-dose of intrathecal morphine (0.035 mg) in addition to CFNB was found to be effective with minimal side effects. TRIAL REGISTRATION: Thai Clinical Trial Registry (identifier: TCTR20150609003 , date of registration: 6 June 2015).