Continuous femoral 3-in-1 block alone is insufficient for the treatment of severe pain after total knee replacement (TKR). Intrathecal (IT) morphine provides effective postoperative analgesia but may result in many side effects. The optimal dose of spinal morphine when combined with continuous 3-in-1 block after TKR is not known.

Patients were randomized to receive IT morphine in five groups (n = 20 per group): 1) 0.0 mg, 2) 0.05 mg, 3) 0.1 mg, 4) 0.15 mg, and 5) 0.2 mg. All patients received continuous 3-in-1 block performed with 20 ml of 0.25% bupivacaine, followed by a continuous infusion of 0.125% bupivacaine at the rate of 2 ml/h plus PCA boluses of 1 ml with a lockout of 10 minutes. The intensity of pain at rest and on movement of the knee was assessed by using a visual analog scale for the first two postoperative days.

All treatment groups produced effective pain relief and decreased cumulative femoral PCA bolus use of 0.125% bupivacaine compared with control, respectively (P < 0.05); however, there were no significant differences among the treatment groups. The incidence of vomiting was significantly more frequent with 0.1-0.2 mg IT morphine groups compared with control, respectively (P < 0.05). The rate of administration of antipruritic medication was increased as IT morphine dose increased (P < 0.05).

Use of 0.05 mg IT morphine would appear to provide the optimal balance between pain relief and adverse effects following TKR.