Single femoral nerve blockade combined with patient-controlled intravenous analgesia are used for postoperative analgesia for patients after TKA in the hospital of the investigators. Although this method provides acceptable analgesia, the incidence of opioid-associated side effects is relatively high. Low-dose epidural morphine is commonly used in postoperative analgesia after cesarean section, and the effect of single dose morphine lasts more than 20 hours, with low incidences of itching, nausea, vomiting, and respiratory depression. The investigators hypothesize that, for patients undergoing TKA, the addition of low-dose epidural morphine to single femoral nerve block and intravenous opioids can improve the postoperative analgesia, reduce the consumption of intravenous opioids and decrease opioid-associated side effects.