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From the Department of Anesthesiology and Intensive Care, University Hospital of Toulouse, University Paul Sabatier, Toulouse, France.
Address correspondence and reprint requests to: Dr. Antoine Pianezza, MD, Department of Anesthesiology and Intensive Care, University Hospital of Toulouse, Rangueil Hospital, Orthopedic section, 1, Avenue Jean Poulhès, Toulouse, France. Address e-mail to a.pianezza{at}wanadoo.fr.
Abstract
BACKGROUND: The lateral sciatic mid-femoral block (LSMF), proved to be reliable, safe, and effective on both branches of the sciatic nerve with a single injection. However, we do not know which component of the sciatic nerve (the tibial [T] or the common peroneal [CP]) produces a better success rate when performing a LSMF with a single injection technique. In this prospective study we compared the success rate of the T motor response with the CP motor response.
METHODS: Ninety-five patients undergoing ankle or foot surgery were enrolled. Thirty milliliters of 0.475% ropivacaine was injected at the first evoked motor response, either T or CP, with a minimal intensity between 0.3 and 0.5 mA.
RESULTS: Seventy-two patients were included in group T and 23 in group CP. The block was considered a success when a complete sensory block of the sciatic nerve was obtained. The success rate was 90% (65) for the T response and 70% (16) for the CP response (P < 0.05).Time to perform the block (CP: 4.5 ± 3 min vs T: 4.5 ± 4 min; P = NS) as well as sensory and motor onset times were not significantly different between groups. No complications were observed in either group.
CONCLUSION: We conclude that the evoked motor response of the T branch is associated with a higher success rate than a CP response using the modified LSMF block.
For foot and ankle surgery, alternate approaches to sciatic nerve block have already been described and proven to be efficient (1,2). The lateral sciatic mid-femoral block (LSMF) has proven to be as reliable as the posterior popliteal fossa approach (3), achieving adequate blockade of both branches (common peroneal: CP and tibial: T) of the sciatic nerve with a single injection (4), with the advantage of supine patient position. However, the evoked motor response, T or CP, which provides the best success rate with a single injection technique is unknown. The aim of this study was to compare success rates of T or CP motor response when performing a modified LSMF with a single injection.
METHODS
After approval by our local ethical committee, a prospective study was conducted for 18 mo on all patients who underwent orthopedic ankle or foot surgery. All patients gave informed consent. Patients with any contraindication to regional anesthesia were not included.
After installation of standard monitoring, sufentanil (0.1 µg/kg) was given IV 5 min before the procedure. An experienced senior anesthesiologist performed all blocks using a nerve stimulator (BRAUN® Stimuplex® HNS11, Mesungen, Germany) set at 100 µs, 1.5 mA, and 1 Hz, and an insulated, graduated needle (Uniplex 100 mm 21G; Pajunk®, Germany).
The patients were supine with a pillow placed under the popliteal fossa. The leg was maintained in neutral position without rotation. The landmarks (i.e., the segment joining the middle of the inguinal ligament and the upper part of the patella, and the groove between muscles biceps femoris and vastus lateralis of the quadriceps) were identified and marked. The puncture site corresponded to the projection of the middle segment on the groove (Fig. 1). After skin disinfection and local anesthesia of the skin with 3 mL of lidocaine 1%, the insulated needle was inserted perpendicular to the skin plane until obtaining plantar flexion/inversion (T) or dorsiflexion/eversion (CP) of the foot at 0.3–0.5 mA. If no response was elicited, the needle was withdrawn to the skin, and directed posteriorly with about 5° angulation. If no response was elicited, the needle was redirected anteriorly at about 5° angulation above the original insertion orientation. According to the type of motor response, the patients were divided into two groups: the T group (tibial response) or the CP group (common peroneal response). Thirty milliliters of ropivacaine 0.475% was then slowly injected. If necessary, a saphenous nerve block was performed, as described by Chassery et al. (5).
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The block was considered successful when a complete sciatic sensory block was obtained using a scale from 0 (normal sensation) to 2 (complete loss of sensation). The onset time of sensory and motor blockade was evaluated at 5, 15, 30, 45, and 60 min. If the block was unsuccessful, patients received general anesthesia. All patients had a tourniquet positioned on the calf. Each patient was followed-up by the surgeon postoperatively at least once to identify complications.
Before the trial, and based on the study of Toboada et al. (6) the sample size was evaluated. A power calculation for a 40% difference in the success rate with a probability level of 0.05 and power of 0.80 (1-ß) yielded a sample size of 20 patients for each group. Because of the study design (nonrandomized) we had enrolled 95 patients to obtain 23 patients in the CP group. Statistical analyses were performed using the Statview® software (version 5.0, SAS Institute Inc, Cary, NC). Data are presented as mean ± sd or percent. 
2 test, ANOVA and Student’s t-test were performed as appropriate. P < 0.05 was considered statistically significant.
RESULTS
Ninety-five patients were included, 23 in the CP group and 72 in the T group. The sciatic nerve was identified in all patients. Demographic and surgical data are shown in Table 1.
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The overall success rate was 86% (T = 90% vs CP = 70%; P < 0.05). Twenty-six percent (6) of patients had a complete T motor block in the CP group versus 68% (49) in the T group (P < 0.05), and 52% (12) had a complete CP motor block in the CP group versus 67% (48) in the T group (P < 0.05). The intensity of stimulation was respectively: 0.42 ± 0.03 mA (T) and 0.43 ± 0.04 mA (CP) (P = NS).
The motor response was found at 7.5 ± 1.5 cm in the CP group, and at 7.6 ± 1.6 cm in the T group (P = NS). Seventeen patients required an anterior redirection of the insulated needle to obtain a motor response, 42 required a posterior redirection, and 36 needed no redirection. Thirty-six percent of patients required a saphenous nerve block. Time to perform the block was not significantly different between groups (CP: 4.5 ± 3 min vs T: 4.5 ± 4 min; P = NS). There was no difference in onset of sensory and motor block (Figs. 2 and 3). No clinical complications occurred during the study.
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Our results demonstrated that the success rate was significantly better after motor response of the T than the CP nerve with LSMF, as previously reported for Labat’s and the lateral popliteal techniques (6,7). This success rate of 90% is comparable with the posterior popliteal approach (92%)8 and higher than the lateral popliteal technique with a single injection (54%). (9) Factors which could affect the block’s success rate, such as the concentration and volume of the anesthetic solution (10), the number of stimulations (9,11), or the intensity of nerve stimulation (12) were similar in both groups. Therefore, only the evoked motor response difference explains this success rates’ gap. The T response is more effective than CP response in LSMF, probably because of the larger thickness (13) of the T component.
The first response was T in 76% of cases, despite a lateral approach with a LSMF. Several hypotheses could explain this result. First, T and CP nerves may not lie in the same horizontal plane at the mid-femoral level. Second, the T nerve is larger (13). And third, the pillow placed under the thigh could have modified the anatomical position of the two branches of the sciatic nerve.
This study was neither blinded nor randomized. Indeed, other studies concerning the evoked motor response in sciatic nerve block were randomized (6,7). However, the study design closely resembles real practice, in that most practitioners are interested in the easiest and most rapid approach associated with success.
In conclusion, the evoked motor response of the T branch predicts a higher success rate than a CP response for this modified LSMF.
Footnotes
Accepted for publication April 23, 2007.
REFERENCES
This article has been cited by other articles:
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  D. Q.H. Tran, L. Munoz, G. Russo, and R. J. Finlayson Ultrasonography and stimulating perineural catheters for nerve blocks: a review of the evidence: [L'echographie et les catheters perineuraux stimulants pour les blocs nerveux : une synthese des donnees probantes] Can J Anesth, July 1, 2008; 55(7): 447 - 457. [Abstract] [Full Text] [PDF]
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