Sciatic nerve block and the improvement of femoral nerve block analgesia after total knee replacement

EDITOR: Femoral nerve block is frequently used for pain control after total knee replacement (TKR) [1-5]. Although one study concluded that a supplemental sciatic nerve block did not improve further the analgesic efficacy [6], our findings disagree. We report the results of an open clinical observational study made in patients undergoing TKR under spinal anaesthesia with a standardized protocol for postoperative analgesia carried out in our institution. Pain relief was provided by local anaesthetic administered via an indwelling femoral catheter after surgery in all patients. Sciatic block was performed as the first rescue analgesic choice after observing the effects of femoral block and localizing the residual pain. After receiving full information on postoperative analgesic management, we observed 40 consecutive patients undergoing TKR using a single injection or continuous spinal anaesthesia with bupivacaine. Before spinal anaesthesia, in all patients, a catheter (Contiplex® D Set; Braun, Melsungen, Germany) was introduced into the femoral sheath using a nerve-stimulation technique. Its position was verified by loss of cold sensation with ether drops applied in the saphenous nerve area (internal side of calf) 10-15 min after injection of lidocaine 1% 15 mL. All patients were instructed (a) to use a patient-controlled analgesia (PCA) device (Abbott, Baar, Switzerland) via the femoral catheter, (b) to quantify the intensity of pain on a visual analogue scale (VAS) ranging from 0 to 10 cm, and (c) to localize the pain as predominantly (a) at the anterior and/or internal (femoral and obturator nerve) or (b) the posterior and/or external (sciatic nerve) knee area. As soon as the pain score of the operated knee at rest was >3 cm, ropivacaine 0.2% 20 mL was injected using a PCA pump via the femoral catheter. Nurses repeated the dose if necessary after 15 min until an adequate block of the femoral nerve was obtained. This was defined as the loss of cold sensation at the anterior mid-thigh side and a decrease of pain intensity at the anterior and/or internal knee area. Following this treatment, the patients in whom the pain score decreased to ≤3 cm and remained within this value after complete regression of spinal anaesthesia were allocated to a 'femoral-only' group. On the other hand, if the residual pain was rated >3 cm after a clinically successful femoral block and/or if the pain score decreased to ≤3 cm - but increased to >3 cm after regression of spinal anaesthesia and the pain was localized predominantly in the posterior and/or external knee area - a sciatic block with 20-30 mL bupivacaine 0.5% with epinephrine 1:200 000 was performed using the anterior approach [7]. When the pain score decreased to ≤3 cm after this single block, the patients were allocated to a 'femoral-sciatic' group. Analgesia was provided for a further 48 h at least with femoral PCA (ropivacaine 0.2% 200 mL, bolus 20 mL, lookout time 120 min) and subcutaneous (s.c.) rescue morphine (0.1 mg kg−1, maximum six doses every 24 h). Pain scores before and after each block, and at 12 and 24 h after the femoral block, were recorded together with the rescue morphine requirements. Mean ± SD, or median, with ranges are given, and data were compared with an unpaired t-test or χ2-test as required. P < 0.05 was considered as significant. The cold test suggested the correct position of the femoral catheter (complete loss of cold sensation) in 34 of the 40 patients observed. The test was doubtful in five subjects (cold sensation decreased but not absent) and was negative in one patient (normal cold sensation). A new catheter was introduced in this patient immediately after surgery. For technical reasons, four of 40 patients were not considered for further analysis. Among the 36 remaining patients, according to previously described criteria, 12 (33%) needed femoral block only (femoral-only group) whereas 24 (67%) required an additional sciatic block (femoral-sciatic group). Patients characteristics such as age (72 ± 6 versus 77 ± 8 yr), height (164 ± 8 versus 159 ± 10 cm), weight (78 ± 12 versus 74 ± 16 kg), female/male ratio (7/5 versus 17/7) and ASA physical status were comparable between the femoral-only and femoral-sciatic groups, respectively. The main data for both groups are presented in Table 1. The interval between femoral injection and sciatic block was 87 ± 41 min. Five patients in the femoral-only group (42%) and six in femoral-sciatic group (25%) received morphine during the first 24 h after surgery.Table 1: Pain scores (cm) and rescue morphine requirements for both groups.The results of this observational study indicate that sciatic nerve block was useful in improving postoperative analgesia after TKR in two-thirds of patients previously receiving femoral nerve block only. The efficacy of the sciatic block is demonstrated by a clinically significant reduction in pain score from 7.33 ± 1.76 to 0.22 ± 0.42 cm (Table 1). These findings are not in agreement with previously published data reporting no analgesia benefit after TKR of the combination of sciatic and femoral nerve blocks [6]. This difference can be explained by the different technique used. In the previous study, the blocks were performed before resolution of spinal anaesthesia. In addition, all patients received ketorolac i.v. at fixed intervals and an intravenous (i.v.) morphine PCA device was available, which could have influenced the intensity of pain. In our report, the sciatic block was performed as a first-choice treatment for established pain, the rescue morphine being given on demand afterwards. Others studies, investigating the postoperative analgesic effects of peripheral nerve blocks after TKR surgery, compared continuous femoral block to opioids by the systemic route [1-5] and/or epidural analgesia [3,4]. In three studies [1,2,5], analgesia obtained by femoral block alone appeared to be insufficient, since clinically significant doses of rescue systemic opioids were required on the day of surgery. Two other studies [3,4] reported satisfactory analgesia with femoral block alone: the need for rescue systemic opioid was clinically irrelevant. However, in these studies [3,4], surgery was performed under general anaesthesia with sufentanil as an analgesic, and postoperative femoral catheter analgesia was provided with a mixture of bupivacaine, sufentanil and clonidine [3], or lidocaine, morphine and clonidine [4]. It is possible that the analgesics given during surgery, and the systemic effects of analgesics administrated in the femoral sheath in the postoperative period, could have decreased the intensity of the pain originating from the sciatic nerve area. In the present observation, femoral block alone provided sufficient analgesia in only one-third of patients. Our results show that to obtain satisfactory analgesia with peripheral nerve blocks after TKR, a femoral block needs to be accompanied by a sciatic block in two-thirds of patients. Acknowledgement Part of our work was presented as a poster at the 8th European Society of Anaesthesiologists Annual Meeting, 1-4 April 2000, Vienna, Austria. A. Weber R. Fournier E. Van Gessel Z. Gamulin Departments of Anaesthesiology; Pharmacology and Surgical Intensive Care; University Hospitals; Geneva, Switzerland