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From the University of Santiago de Compostela, Department of Anesthesiology, Hospital Clínico Universitario de Santiago, Spain.
Address correspondence and reprint requests to Manuel Taboada Muñiz, Department of Anesthesiology, Hospital Clínico Universitario de Santiago, Travesía da Choupana s/n. 15706 Santiago de Compostela, Spain. Address e-mail to manutabo{at}yahoo.es.
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Abstract |
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Introduction |
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The purpose of the present study was to compare two different approaches (subgluteal and posterior popliteal) for continuous postoperative analgesia using stimulating catheters.
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METHODS |
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Patients in the subgluteal group were placed in Sim’s position (1). A line was drawn from the midpoint of the greater trochanter to the ischial tuberosity. A second perpendicular line was drawn from the midpoint of the previous line and extended caudally for 4 cm. This point represented the site of needle insertion. A 10 cm, 19-gauge short-beveled stimulating needle (Pajunk, Medizintechnologie, Geisingen, Germany), connected to a nerve stimulator (Pajunk, Medizintechnologie), was inserted maintaining 30–45 degrees angle to the skin. Initially, the stimulating current delivered was 1.5 mA, and it was progressively decreased to <0.5 mA with a stimulation of 0.1 ms at 2 Hz. Plantar flexion of the foot was the evoked motor response elicited in all patients. After identification of the tibial nerve with a stimulating intensity <0.5 mA, the stimulating catheter was advanced 4–5 cm past the needle tip while maintaining plantar flexion with intensity <0.5 mA.
Patients in the popliteal group were placed in the prone position. Nine cm above the popliteal crease and 1 cm lateral to the midline was the point of needle insertion. The same stimulating needle was inserted maintaining 30–45 degree angle to the skin, and catheter advancement was then performed as previously described.
Catheter position was considered acceptable when plantar flexion was elicited at a current output via the catheter <0.5 mA. The number of advances attempted was recorded. For the surgical block, mepivacaine 1.5% was administered in divided doses (maximum, 30 mL). Sensory and motor block were evaluated in the sciatic nerve distributions every 5 min. If the block was not adequate for surgery after 30 min, general anesthesia was induced.
After surgery, the stimulating catheter was connected to an electronic patient-controlled analgesia (PCA) pump (CADD-Legacy 6300, Deltec, Inc. St. Paul, MN), set to deliver a continuous infusion of 3 mL/h of 0.0625% levobupivacaine with a bolus of 3 mL every 20 min. Patients also received 30 mg of ketorolac IV every 8 h. In case of insufficient pain control, 100 mg of tramadol was administered IV every 6 h. The degree of pain was evaluated at 6, 12, and 20–24 h postoperatively with a 100-mm visual analog scale (VAS). At 24 h, the worst pain score and the average pain score during that day, the total consumption of the local anesthetic, the number of incremental doses delivered by the PCA, and the need for rescue tramadol were assessed.
A power analysis estimated that 22 patients per group would be required to detect a possible difference of 30 points in VAS score between the 2 approaches with a two-tailed 
error of 5% and a ß error of 10%. Six more patients were included in each group to allow for possible dropouts.
Statistical analysis was performed by using the Statistical Package for the Social Sciences (SPSS for Windows, version 10.0; SPSS Inc., Chicago, IL). Data distribution was first evaluated using the Kolmogorov-Smirnov test. Continuous variables between groups were compared using either two-sampled Student’s t-test or the Mann-Whitney U-test, according to the data distribution. Discrete variables between groups were compared using a 
2 or Fisher’s exact test when numbers were small. A P value of < 0.05 was considered statistically significant. Continuous variables were presented as mean ± sd except for VAS scores (presented as median with 25th-75th percentiles); qualitative data were presented as numbers (%).
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RESULTS |
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No differences were observed between groups with respect to VAS (Table 2). However, the consumption of local anesthetic was larger in group popliteal compared with group subgluteal (Table 1) (P < 0.05). One patient in each group had complete motor block of the operated foot at the end of the study because they administered continuous bolus doses by the PCA pump.
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Two patients in the popliteal group had insufficient postoperative analgesia for technical reasons with the catheter. One patient had insufficient analgesia because of removal of the catheter before the end of the study, and the other patient had insufficient analgesia because of partial displacement of the catheter with oozing of local anesthetic at the puncture site. These two patients were withdrawn from data analysis of the local anesthetic used.
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DISCUSSION |
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An explanation for the different amount of local anesthetic necessary to produce adequate postoperative analgesia must be sought in the sciatic anatomy. Computed tomography scans showed that there were 2 separate trunks in 27% of subjects at 20 cm and in 90% at 30 cm distal to the greater trochanter (15). The cross-sectional areas of the perineural space measured at 20 and 30 cm were 1.8 and 5.6 cm2 respectively, and the space between the trunks was filled with adipose tissue and blood vessels. This anatomy may explain the larger volume of local anesthetic necessary for continuous sciatic block with the distal approach as compared to the more proximal approach. At the popliteal level, the two nerve trunks are separated (16,17); fat and layers of connective tissue make it difficult for the local anesthetic to diffuse the distance. A larger volume of local anesthetic administered through the stimulating catheter is necessary to block the two components of the sciatic nerve at the popliteal level compared with the subgluteal level, where the two sciatic nerve components are closer. In the present investigation, placement of the stimulating catheter required more attempts to obtain an adequate catheter position with the popliteal group compared with the subgluteal group. Anatomy may also explain these results.
We report similar postoperative pain relief with the stimulating catheters at both injection sites. di Benedetto et al. (10) reported nearly equivalent postoperative analgesia in continuous sciatic nerve block after foot surgery after both posterior popliteal and subgluteal approaches. However, in the study by di Benedetto et al. (10) no differences in overall local anesthetic consumption were seen. A difference in the catheters used (nonstimulating catheters), different local anesthetic and concentrations administered might explain the deviating results of the two investigations.
Previous studies have compared the efficacy of stimulating versus nonstimulating catheters for continuous analgesia with reduced amounts of dilute local anesthetic solutions (11–14,18,19). Rodríguez et al. (18), in a recent study, evaluated using a posterior popliteal approach and stimulating and nonstimulating catheters for postoperative analgesia after hallux valgus repair and reported an increased overall quality of analgesia using stimulating catheters. Casati et al. (19) in a similar study observed less local anesthetic consumption, less need for postoperative rescue opioid analgesics, and shorter onset time of surgical block when a stimulating catheter was used.
In conclusion, the results of the present randomized study demonstrate that continuous sciatic nerve block with stimulating catheters was effective using two different approaches (posterior popliteal and subgluteal approaches). However, stimulating catheters inserted more proximally reduced the consumption of local anesthetics during the first 24 hours after surgery compared with stimulating catheters inserted more distally. The anatomy at the subgluteal level appears to favor the administration of smaller amounts of local anesthetic.
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Footnotes |
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  M. Taboada, J. Rodriguez, M. Bermudez, C. Valino, B. Ulloa, F. Aneiros, F. Gude, J. Cortes, J. Alvarez, and P. G. Atanassoff A "New" Automated Bolus Technique for Continuous Popliteal Block: A Prospective, Randomized Comparison with a Continuous Infusion Technique Anesth. Analg., October 1, 2008; 107(4): 1433 - 1437. [Abstract] [Full Text] [PDF]
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 Q. H. De Tran, A. Clemente, and R. J. Finlayson A review of approaches and techniques for lower extremity nerve blocks: [Un bilan des approches et techniques pour les blocs nerveux du membre inferieur] Can J Anesth, November 1, 2007; 54(11): 922 - 934. [Abstract] [Full Text] [PDF]
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