BACKGROUND: Unilateral spinal epidural anesthesia with ropivacaine can reduce the perioperative pain in the elderly undergoing hip or knee arthroplasty, but choosing which concentration of small dose hypobaric ropivacaine is an issue of concern.OBJECTIVE: To explore the anesthetic and analgesic effects of unilateral spinal epidural anesthesia with different concentrations of low dose hypobaric ropivacaine in senile knee or hip replacement.METHODS: Ninety patients undergoing knee or hip replacement in the Henan Province Hospital of TCM from August 2014 to December 2015 were enrolled, and randomized into 0.15%, 0.5% and 0.25% ropivacaine groups (n=30 per group), followed by spinal epidural anesthesia at L3-4 or L2-3 lumbar interspace, respectively.RESULTS AND CONCLUSION: (1) The order of the flat subsided time and motor block recovery time was as follows:0.15% ropivacaine group < 0.25% ropivacaine group < 0.5% ropivacaine group; the onset time of motor block was longest in the 0.25% ropivacaine group, followed by 0.5% ropivacaine group, and shortest in the 0.15% ropivacaine group (all P < 0.05). (2) The order of the visual analogue scale scores at 1, 5, and 10 hours after anesthesia and 24 hours postoperatively was as follows: 0.5% ropivacaine group < 0.25% ropivacaine group < 0.15% ropivacaine group (P < 0.05). (3) The change levels of heart rate and blood pressure were highest in the 0.5% ropivacaine group, followed by 0.15% ropivacaine group, and lowest in the 0.25% ropivacaine group. (4) The incidence of adverse events was highest in the 0.5% ropivacaine group, followed by 0.15% ropivacaine group, and lowest in the 0.25% ropivacaine group (P < 0.05). (5) To conclude, 3 mL of 0.25% ropivacaine exhibits desired anesthetic effect.