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Background and Objectives: Total knee arthroplasty often results in marked postoperative pain. A recent meta-analysis supports the use of femoral nerve block or alternatively spinal injection of morphine plus local anaesthetic for post-operative analgesia. On the other hand, the use of intrathecal morphine may be associated with a large number of distressing side effects (itching, urinary retention, nausea and vomiting, delayed respiratory depression). The aim of this study was to compare the effectiveness of femoral nerve block and low dose intrathecal morphine in post-operative analgesia after primary unilateral total knee arthroplasty. Material and Methods: Fifty-two consecutive patients scheduled for primary unilateral total knee arthroplasty were allocated to the intrathecal morphine group (ITM group) or to the femoral nerve block group (FNB group). In ITM group a subarachnoid puncture was performed at the L3-L4 inter-vertebral space with hyperbaric bupivacaine 15 mg plus 100 mcg of preservative-free morphine. Patients allocated to the FNB group received a single-injection ultrasound-assisted femoral nerve block with ropivacaine 0.75% 25 ml before the spinal injection of hyperbaric bupivacaine 15 mg. All patients received postoperative patient-controlled- analgesia (PCA) morphine, using a 1- mg bolus and a 5-minute lockout period. Data were analyzed using Student t test or two-way analysis of variance (ANOVA) for repeated measures with time and treatment as the 2 factors. Post hoc comparisons were performed by Bonferroni test. Statistical significance for all test was a p value <0.05. Results: Patient characteristics were similar between the 2 groups. We found a statistically significant differences in postoperative pain between the two groups: ITM group had the lower visual analogic pain score (VAS) values. Morphine consumption was lower in the ITM group: average consumption within the first 6 hours was 0.9 mg in IT group compared to 3.1 mg in FNB group; at 12 h 4.2 mg vs 6.3 mg; at 24 h 6.9 mg vs 10.3 mg; at 48 h 9.7 mg vs 13.6 mg. However, the difference in the opiate consumption was not statistically different (p value =0.06). Thirteen patients in ITM group experienced itching, only 5 in FNB group. We did not find any difference in the two treatment groups in the use of antiemetic and antipruritic medication. No cases of respiratory depression was recorded. Conclusions: Our results show that low dose of intrathecal morphine may be safe and more efficient than single-shot femoral nerve block for post-operative analgesia after total knee arthroplasty. [ABSTRACT FROM AUTHOR]