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*Department of Anesthesiology and Intensive Care, Hôpital Antoine Béclère, Clamart cedex;
Department of Anesthesiology and Intensive Care, Hôpital Central, Nancy cedex; and
Department of Orthopedic Surgery, Hôpital Jeanne d’Arc, Toul cedex, France
Address correspondence and reprint requests to Dr. Hervé Bouaziz, Hôpital Central, 54035 Nancy cedex, France.
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Abstract |
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Implications: A better success rate is achieved with a double stimulation technique than with a single injection for the sciatic nerve block via the lateral approach at the popliteal fossa when 20 mL of local anesthetics is used.
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Introduction |
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More recently, Zetlaoui and Bouaziz (5) reported a high success rate of the lateral approach to sciatic nerve block at the popliteal fossa for foot surgery when 10 mL of local anesthetic was injected on both distal components of the sciatic nerve after they had been identified with a peripheral nerve stimulator. Moreover, Hadzic and Vloka (6) found an equivalent efficacy of both classical posterior and lateral approaches to sciatic nerve block in providing surgical block for lower extremity surgery when using 40 mL of local anesthetic. Nevertheless, no study has investigated whether a double stimulation technique, compared with a single injection technique would provide a greater success rate of the lateral approach to sciatic nerve block at the popliteal fossa when a small volume of local anesthetic (20 mL) is used. Thus, the aim of this study was to compare the success rate of the sciatic nerve block using the lateral approach at the popliteal level when only one or both components of this nerve (i.e., tibial nerve [TN] and common peroneal nerve [CPN]) are identified.
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Methods |
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Our technique has been described elsewhere (5). Briefly, patients were supine, the long axis of the foot positioned at a 90° angle to the table. The site of insertion of the needle (22-gauge insulated 50-mm needle, [Stimuplex B; Braun, Boulogne-Billancourt, France]) was the intersection of the vertical line drawn from the upper edge of the patella and the groove between the lateral border of the vastus lateralis and the tendon of the biceps femoris. Both landmarks were drawn on the skin. The insulated needle was attached to a peripheral nerve stimulator (PNS) (Stimuplex DIG; Braun) and inserted 20–30° posterior to the horizontal plane, with a slight caudal direction. Initially, the stimulating current was set between 1.5–2.0 mA, and the frequency of stimulation was set at 2 Hz. The intensity of the stimulating current was progressively decreased as the needle tip was moved closer to the nerve. The needle was considered to be close enough to the nerve when the stimulating current was 
0.5 mA. Eversion, inversion, plantar flexion, or dorsiflexion of the foot was elicited, and both CPN and TN were identified by their evoked motor responses. A dorsiflexion or eversion of the foot identified the CPN, and a plantar flexion of the foot was required to identify the TN.
In Group 1 STIM (n = 24 patients), the evoked motor response sought was an inversion of the foot. When the stimulating current was 0.5 mA, 20 mL of a local anesthetic solution (equal amount of 2% lidocaine and 0.5% bupivacaine with 1:200,000 epinephrine) was slowly injected. An elicited inversion of the foot, as recently demonstrated by Benzon et al. (7), is the best predictor of a complete sensory blockade of the foot after a sciatic nerve block at the popliteal level. Inversion is caused by the action of both the tibialis anterior muscle, which is innervated by the TN, and the tibialis anterior muscle, which is innervated by the deep peroneal nerve (DPN), a branch of the CPN. Therefore, in the case of an elicited inversion of the foot, the needle tip is located very close to both branches of the sciatic nerve or to the sciatic nerve itself before it divides into TN and CPN. Moreover, stimulation of both the DPN innervating the tibialis anterior muscle and the TN innervating the tibialis posterior muscle, might result in mild inversion of the foot. Thus, only a strong movement of inversion was considered a positive response to avoid this error. If the elicited movement was a plantar flexion of the foot, the needle was considered too medial and was redirected more laterally. When the elicited movement was a dorsiflexion or an eversion of the foot, the needle was redirected more medially.
In Group 2 STIM (n = 26 patients), the same approach was used, but both components of the sciatic nerve were identified. When the CPN was identified with a stimulating intensity 0.5 mA, 10 mL of the same local anesthetic solution was slowly injected. The intensity of the stimulating current was then turned up to 2.0 mA, and the needle was directed following the same axis to identify the TN. The needle was considered to be close to the TN when the stimulating current was 0.5 mA. Then, 10 mL of the same local anesthetic solution was slowly administered (8).
In cases of surgery involving the area of the saphenous nerve, this nerve was blocked, as described previously, with 10 mL of the same anesthetic solution (8).
Time 0 was defined as the time corresponding to the removal of the insulated needle from the skin. Sensory and motor block were then assessed every 5 min for a 45 min period by an anesthesiologist unaware of the technique used (single-blinded study). The sensory block was assessed using light touch in the peripheral sensory distributions of both distal branches of the sciatic nerve below the knee. We assessed the sensory block only in cutaneous distributions specific to either the TN (plantar side of the foot) or the different peroneal nerves (PN). In the PN sensory distribution, we tested the cutaneous area innervated by the CPN (lateral cutaneous side of the calf) and the superficial peroneal nerve (dorsal and medial aspect of the foot). The DPN distribution (between the first and second toes) was assessed only in cases of hallux valgus or surgery in this particular area. As the sural cutaneous distribution is not a specific area because its innervation depends on both TN and PN innervation, this area was used neither for the TN nor for the PN sensory block assessment. The degree of sensory block was classified as follows: 0 = normal sensation, 1 = blunted sensation (analgesia), and 2 = absence of sensation (anesthesia). We considered the PN sensory block to be complete only when both CPN and superficial peroneal nerve had a score of 2. Sensory block was considered complete when each sensory testing in TN and PN sensory distributions had a score of 2; otherwise, it was considered incomplete or absent (score of 0). Motor block was assessed for voluntary motor responses by asking the patient to plantar flex (TN) or dorsiflex (CPN) the foot and was classified as follows: 0 = normal movement, 1 = decreased movement (incomplete motor block), and 2 = no movement (complete motor block). Motor block was considered complete when motor response in both TN and PN distributions had a score of 2; otherwise it was considered incomplete or absent (score of 0). Onset time of sensory block and of motor block was defined as the interval between Time 0 and a complete block.
The success rate was defined as a complete sensory block associated with a pain free surgery. If the sensory block score was not 2 in at least one of the territories, at the end of the 45 min assessment period, the sciatic block was considered incomplete. When the sciatic nerve was not successfully blocked, at least in one of the sensory distributions of the surgical site, and if the patient experienced pain during surgical instrumentation that did not respond to incremental IV bolus of sufentanil (5 µg) and/or local anesthetic infiltration by the surgeon, general anesthesia was induced.
Based on previous studies, we hypothesized that we could observe at least a 30% difference in success rate between Group 2 STIM and Group 1 STIM (7,9,10). The calculated number of patients required was, thus, 20 in each group, using a one-sided test, to detect a 40% difference of success rate, with 
= 0.05 and ß = 0.10. Nonparametric variables (age, weight, height, intensity of stimulation, and onset time of sensory and motor block) are expressed as median and range and were compared between groups using a Mann-Whitney U-test, because they were not normally distributed. Discrete variables (sex ratio, success rate) were compared between groups using a 
2 analysis. To analyze the rescue technique, because of the small number of patients in each group, we used Fisher’s exact test. P < 0.05 was considered statistically significant.
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Results |
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General anesthesia was required (Table 3) in one patient in Group 2 STIM and in six patients in Group 1 STIM (P = 0.04, Fisher’s exact test).
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Discussion |
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Benzon et a1. (7) proposed the inversion is the best movement to elicit using a posterior approach at the popliteal fossa, as they had obtained a 100% success rate in six healthy volunteers. They attributed this result to the proximity of the needle tip to both components of the sciatic nerve. The difference in the results concerning the success rate after an elicited inversion of the foot from the present study (success after inversion, 58%) and that from Benzon et al. (7) may be explained by a difference in assessment of the sensory block, because the success was defined as sensory block to pin prick, and no surgery was performed. The explanation may also be related to the importance of volume of local anesthetic administered: 40 mL compared with 20 mL. Furthermore, Schirmek and Deusch (11) found an increasing percentage of patients with complete anesthesia after successive injections of local anesthetics, using a catheter introduced in the popliteal fossa.
Smith and Siggins (12) compared two groups of patients receiving the same total dose of local anesthetic in a sciatic nerve block at the buttock level. One group received a single injection of a high volume and low concentration of local anesthetic, whereas the other group received a low volume but high concentration. Interestingly, they demonstrated an earlier onset of the block but also a better diffusion in the different cutaneous nerve distributions when patients had received the higher concentration of local anesthetic. This study suggests that a high volume of local anesthetic for a sciatic block may not be the most important factor in predicting success rate and onset of anesthesia. Although our study did not compare different volumes and concentrations of local anesthetic, our results, with a small volume of a concentrated solution of local anesthetic inducing a high success rate in the 2 STIM group, compare favorably with the findings of Smith and Siggins (12).
In Group 2 STIM, the onset of sensory block in the PN distribution was shorter than in the TN distribution, in agreement with previous studies (5,11). The TN is larger than the CPN at the popliteal level (11). Thus, as the time to obtain a complete block is probably related to the different size of the two components of the sciatic nerve, it is not surprising that anesthesia occurs more rapidly in the PN distribution. Nevertheless, the CPN is located lateral to the TN, and, when using the lateral approach to the sciatic nerve, we "reached" the CPN first. This could also have accounted for the earlier onset of sensory block in the PN distribution. Nevertheless, except in cases of surgery restricted to the exclusive PN distribution area, the clinical benefit of such a difference of onset time of sensory block is small, because a complete sensory block in both TN and PN distributions is required for most surgical procedures.
In this lateral approach to the sciatic nerve block at the popliteal fossa, the stimulating insulated needle successively "reaches" the CPN first and then the TN with a 30° posterior angle. Therefore, the second nerve is stimulated in an area that has already received 10 mL of local anesthetic. The question of the neurologic safety of this lateral approach to the sciatic nerve block at the popliteal fossa might arise, as this technique is new (2), and, thus, lacks long-term safety assessment. Using the posterior approach on 625 surgical patients, Singelyn et al. (13) provided strong evidence for the safety of this technique. A large study to assess safety for the lateral approach to the sciatic nerve block at the popliteal fossa has not been performed. Nevertheless, we used a peripheral nerve stimulator, and the stimulating current was increased to 2 mA after the first injection of local anesthetic, before moving the needle to seek for the second nerve. This should have allowed stimulation of the previously blocked nerve. Also, the lateral approach to the sciatic nerve block at the popliteal fossa is not the only regional anesthesia technique using multiple nerve stimulation: several such techniques of blockade of the upper extremity are now considered safe (14–17), despite multiple injections of local anesthetic and needle movements in the proximity of the axillary plexus nerves.
In conclusion, multiple stimulation of both sciatic nerve components below the knee provides a better success rate when a sciatic block is performed at the popliteal level by the lateral approach when 20 mL of local anesthetic is used. The safety of this lateral approach should be addressed in large studies
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