Continuous posterior tibial perineural infusion : effect on postoperative opioid requirements after below knee amputation

Aims: Analgesia for below knee amputation using a perineural infusion technique avoids the complications of epidural analgesia. The population presenting for this operation often have poor immunocompetence and coagulopathies that are relative contraindications for major regional analgesia. In addition, a reduction in opioid use and thus potential for reduction in opioid side-effects is probably desirable. The aim of this study was to test whether a perineural infusion of bupivacaine reduced the use of opioids after below knee amputation. A second aim of this study was to test whether there was an effect of this technique on the development of phantom limb pain at six months postoperatively. Methods: 24 patients scheduled for below knee amputation were randomised prospectively into two groups. The test group received a 10ml bolus of Bupivacaine via an intraoperatively placed perineural catheter along the posterior tibial nerve at the conclusion of surgery and a continuous perineural infusion for two days postoperatively; the control group received Normal Saline according to the same regime. Both groups had free access to morphine via a patient controlled analgesia machine. Patients and pain evaluators were blinded as to group membership. General anaesthesia was provided for all but three patients; two patients in the test group and one in the control group had spinal anaesthesia. To test the safety of the technique, Bupivacaine levels were assayed in another group of five patients receiving perineural bupivacaine on the first postoperative day after the conclusion of randomisation. Pain assessment was performed using the short form McGill pain questionnaire (SFMPQ) preoperatively, at one week and three months. All patients were visited by the an acute pain service team in the postoperative period. Results: Two test subjects and one control subject were withdrawn at day one for postoperative confusion, leaving 10 Bupivacaine and 11 control subjects for assessment. There was significantly less morphine used by the test group at 48 hours. SFMPQ scores preoperatively and at 7 days were not significantly different between the two groups. There were 9 patients in each group for assessment of phantom pain at 6 months. There was a low incidence of phantom sensation of 6/12 at this time. 4 of these belonged to the Bupivacaine group. No patients in either group reported significant pain or sensations at 6 months. The study thus lacked power to determine whether the intervention affected the development of phantom pain. There were no complications attributable to the use of perineural catheters. Plasma levels of Bupivacaine ranged from 0.7 to 5.45 micrograms/ml in 5 subjects with bupivacaine perineural infusions; none of these subjects displayed signs of bupivacaine toxicity. Conclusion: Posterior tibial nerve perineural infusion of bupivacaine is a simple intraoperative procedure performed during below knee amputation. It was associated with a significant reduction in oipoid use at 48 hours in this study