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REGIONAL ANESTHESIA

Ultrasound Guidance for Lateral Midfemoral Sciatic Nerve Block: A Prospective, Comparative, Randomized Study

Vicente Domingo-Triadó, MD, DEAA^*, Salvador Selfa, MD, Francisco Martínez, Dolores Sánchez-Contreras, MD^*, Montserrat Reche, MD^*, Jose Tecles, MD, PhD, María T. Crespo, MD^*, Jose M. Palanca, MD, PhD^¶, and Blanca Moro, MD^*

From the *Departments of Anesthesiology, Radiology, and Traumatology, Lluís Alcanyis Hospital, Xàtiva, Spain; and the Departments of ¶Anesthesiology, and Statistics, Faculty of Mathematics, University of Valencia, Spain.

Address correspondence and reprint requests to Vicente Domingo-Triadó, MD, DEAA, Department of Anesthesiology, Hospital Lluís Alcanyís, Ctra. Xàtiva-Silla Km 2, 46800 Xàtiva, (Valencia), Spain. Address e-mail to vdt5677{at}mail.ono.es.

	Abstract

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Block of the sciatic nerve at the midfemoral level is usually performed using nerve stimulation techniques. We investigated the efficacy of ultrasound, combined with nerve stimulation, to locate and block the sciatic nerve at the lateral midfemoral level compared to nerve stimulation alone. Sixty-one patients scheduled for foot and ankle surgery were enrolled in this prospective, randomized study. Thirty patients underwent a lateral block of the sciatic nerve at the midfemoral level guided by ultrasound (group US) and 31 patients received the block without ultrasound (group ES). Once an adequate motor response was obtained using nerve stimulation, 35 mL of ropivacaine 0.5% was administered. The main end-points of the study were: number of attempts to obtain an adequate motor response, success rate of nerve location at the first attempt, quality and duration of both sensory and motor blocks, and anesthetic distribution. The success of sciatic nerve location at the first attempt was significantly more frequent in the US group than in the ES group (76.6% versus 41.9%; P < 0.001). The quality of the sensory block and the tolerance to the pneumatic tourniquet were also significantly better in the US group (P < 0.01). We conclude that ultrasound combined with nerve stimulation improved the quality of the sensory block and the tolerance to the pneumatic tourniquet, reducing the number of attempts to perform sciatic nerve block at the midfemoral level.

	Introduction

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Sciatic nerve block at the midfemoral level is usually performed using nerve stimulation (1,2). Ultrasound has recently been introduced as a method for locating peripheral nerves and applying anesthetic techniques (3–5). Accurate localization of the nerve and the needle position permits assessment of local anesthetic distribution. Previous studies have investigated the usefulness of ultrasound during brachial plexus block at the supraclavicular and infraclavicular levels (3,4,6). In the lower limb, sciatic nerve block using ultrasound has been reported at the subgluteal (7) and popliteal (8–10) levels. However, there is no randomized study comparing the utility of the procedure for blocking the sciatic nerve at the midfemoral level.

The aim of the present study was to evaluate the usefulness of ultrasound in combination with nerve stimulation to locate and block the sciatic nerve. We also analyzed the distribution of the anesthetic around the nerve and its possible correlation with the quality of the block.

	METHODS

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The study protocol was approved by our hospital ethics committee, and informed written consent was obtained from all patients. Sixty-one ASA physical status I–III patients scheduled for foot or ankle surgery were included in the study. Patients with contraindication to regional anesthesia or with neuropathy were excluded. Patients were randomly assigned to undergo a midfemoral block guided by ultrasound combined with nerve stimulation (group US) or nerve stimulation alone (group ES). The allocation was performed by a random permuted block method. Once the patient's eligibility had been determined, a closed computer program indicated group assignment.

Before the procedure, IV access was established and continuous electrocardiogram, noninvasive arterial blood pressure, and oxyhemoglobin saturation were monitored during nerve block performance and throughout surgery. Patients received IV midazolam (1–2 mg) for sedation when the anesthesiologist considered it necessary. No analgesics were administered intraoperatively. In this study, the same radiologist located the nerve by ultrasound, and the same anesthesiologist performed the block in both groups.

Midfemoral block of the sciatic nerve with ultrasound was performed with the patient supine. The lower limb was elevated on a pillow to make it possible to introduce the ultrasound scanner (Toshiba Aplio®; Toshiba Medical Systems, Tustin, CA) linear transducer 7.5–11 MHz (Toshiba®) into the back of the thigh (Fig. 1). The site of needle insertion was halfway on a line connecting the posterior edge of the greater trochanter and the lateral epicondyle at the knee joint. A 10-cm, 21-gauge insulated stimulating needle (Stimuplex A; Braun Medical, Melsungen, Germany) attached to a nerve stimulator (Stimuplex DIG; Braun Medical) was inserted perpendicularly to the skin to the proximity of the nerve guided by ultrasound. When the tip of the needle reached the nerve (1–2 mm) (Fig. 2) the nerve stimulator was turned on. Initially a current of 1 mA was used then decreased to 0.5 mA. Frequency and duration of the stimulus was 2 Hz and 300 µs, respectively (150 nC). Once the motor response had been evoked at this current and intravascular needle placement excluded, 35 mL ropivacaine 0.5% was injected on one side of the nerve with ultrasound visualization to assess its distribution.

View larger version (54K): [in this window] [in a new window]   Figure 1. Midfemoral block technique guided by ultrasound. GT = greater trochanter; FC = femoral epicondyle.

View larger version (36K): [in this window] [in a new window]   Figure 2. Stimulating needle (thick arrows) with the tip localized in perineural area. Local anesthetic (thin arrows) surrounds the sciatic nerve (N) showing the doughnut shape.

Midfemoral block of the sciatic nerve without ultrasound guidance was performed with the patient supine. Landmarks, puncture site, and technique were the same as previously described for the US group. Anesthetic spread was not assessed in this group.

Data collection included: time required to perform the block defined as the time from the first needle insertion to successful nerve location (ES group) or from the beginning of ultrasound technique to successful nerve location (US group); the number of attempts needed to locate the sciatic nerve (number of needle passes before successfully evoking an adequate motor response); the depth at which sciatic nerve was located; the diameter of sciatic nerve; the distribution of the local anesthetic around sciatic nerve (US group); the type of nerve stimulation obtained; the cutaneous regions blocked; and the degree of discomfort experienced during the procedure, on an visual analogue scale (from 0 to 10). The cutaneous regions tested were: the plantar aspect of the foot (tibial nerve), the first interdigital space (deep peroneal nerve), and the anteromedial and external dorsal area of both foot and leg (superficial peroneal nerve). Quality of nerve block was assessed by the pinprick method (25-gauge needle) by an anesthesiologist who was unaware of group randomization using a scale from 0 to 2 (0 = complete sensory block; 1 = partial sensory block; 2 = normal sensory perception). Sensory latency was defined as time from local anesthetic administration to the absence of sensitivity assessed by the pinprick method. Motor block of the foot was assessed on a modified Bromage scale (11,12) (from Bromage 1 = the full capacity for flexion and adduction of the ankle for the tibial nerve or the opposite for the peroneal nerve to Bromage 4 = a total inability to perform the relevant movement). Time to motor block was defined as the time from local anesthetic administration until slight decrease in foot movement (Bromage scale 2).

Postoperative analgesia duration was defined as the time that elapsed from the establishment of the sensory block until the first request for analgesic administration. Both postoperative analgesia and motor block were assessed hourly by an anesthesiologist who was blinded as to patient randomization. Patients were instructed to note the exact time of sensory and motor recovery of their foot and were followed by telephone interview at 24 h and in a consulting room afterwards.

Tolerance to the pneumatic tourniquet above the ankle was assessed on a scale ranging 0 to 2 (0 = good tolerance; 1 = sedation necessary; 2 = no tolerance). As part of our protocol an additional saphenous nerve block was considered when the incision extended to its innervation area. After 1 h from anesthetic administration, if the sensory block involving the surgical area was considered inadequate (score 2), patients received spinal anesthesia.

Previous studies (1,2) of sciatic nerve block using a midfemoral approach were considered to calculate the number of patients necessary for this study. We wished to detect a 25% difference in the number of attempts to perform the technique between the US group and the ES group, accepting an error of 5% and a ß error of 20%. Based on these figures, the required study size ranged from 25 to 30 patients/group.

The statistical analysis was performed using the Statistical Package for Social Sciences (SPSS for Windows, version 12.0; SPSS Inc., Chicago, IL). Data distribution was first evaluated using the Kolmogorov-Smirnov test. Continuous variables were compared between groups using either the two-sampled Student's t-test or the Mann-Whitney U-test, according to data distribution. Discrete variables were compared between groups using a ² or Fisher's exact test when numbers were small. A P value of <0.05 was considered statistically significant. Continuous variables are presented as median and range, and qualitative variables are presented as numbers (%).

	RESULTS

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Sixty-one patients were included in the study (Table 1). The successful location of the nerve at the first attempt was significantly more frequent in the US group compared with the ES group (P = 0.001) (Table 2). Furthermore, the quality of the block and the tolerance to the pneumatic tourniquet were also significantly better in the US group (P = 0.001) (Table 3). One US group patient and two ES group patients needed a spinal anesthesia as a result of block failure.

There were no significant differences in motor, sensory onset time, and time to perform the block or duration of the sensory and motor blocks (Table 2). The sciatic nerve was located at a median depth of 7 (5–9) cm in both groups, and its median diameter was 4.7 (4.0–6.7) mm evaluated by ultrasound in the US group.

All regions innervated by sciatic nerve were blocked in all patients except for four patients who achieved only partial blocks: one in the US group and three in the ES group. In one of the patients, the degree of block obtained at the surgical area was deep enough to allow surgery; the other three patients required spinal anesthesia.

Peroneal nerve stimulation was significantly more frequent in the US group than in the ES group, whereas in the ES group tibial nerve stimulation was more frequently obtained (Table 3). No correlation was found between the type of stimulation obtained and the quality of sensory or motor block. Nevertheless, the duration of the sensory block was significantly longer (P < 0.03) when tibial stimulation rather than peroneal stimulation was obtained, 19 (7–24) h versus 16 (9–24) h, regardless of patient randomization.

In the US group, the distribution of the local anesthetic around the nerve was circumferential in 22 (73.3%) of the cases (Fig. 3), superior to the nerve in 3 (10%), and inferior in 5 (16.7%). The quality of block in the circumferential distribution was good (complete sensory block) in all cases, as opposed to 87.5% of the rest of the distributions.

View larger version (41K): [in this window] [in a new window]   Figure 3. Doughnut shape. Local anesthetic (white arrows) surrounds the sciatic nerve (N) showing its characteristic doughnut shape.

One ES group patient had neuropathic pain in the innervation area of the sciatic 1 wk after the puncture. It spontaneously resolved in 10 days.

	DISCUSSION

Top Abstract Introduction METHODS RESULTS DISCUSSION REFERENCES

 
The nerve stimulation technique is viewed as the "gold standard" for locating nerves in regional anesthesia, but sometimes, despite obtaining an adequate motor response, block failure occurs. In previous studies, the success rate of sciatic nerve block at the midfemoral level varied between 50% and 77% (1,2). In our study, with the ultrasound-guided block the success rate reached 95%. Although the success rate in the ES group was similar to the US group (97% versus 94%) locating the nerve by ultrasound guidance significantly reduced the number of punctures necessary to perform the block. Moreover, 22% of patients in the ES group did not achieve a complete sensory block and required sedation for surgery. In previous studies (1,2) obtaining block of the sciatic nerve at the midfemoral level at the first attempt ranged between 45% and 48%; in this study ultrasound guidance allowed us to perform it on the first attempt in 77% of patients compared with 42% without it. Finally, in our study the frequent successful location of the sciatic nerve at the first puncture in the US group suggested that block could be performed guided by ultrasound alone. Ultrasonography may prove useful for nerve location in patients with peripheral neuropathy and alteration in motor response (10) and it also can help avoid muscle contraction pain. However, we believe that nerve stimulation should continue being an essential part of the technique, as ultrasound alone only had a 60% success rate (13). The combination of both methods is perhaps the optimal way to achieve a successful block, especially at the beginning of a learning process.

Some studies have correlated the use of ultrasound guidance with a faster onset time and a better quality of the sensory block (14,15). We did not observe a faster onset of block; however, the quality of the sensory block and tolerance to the pneumatic tourniquet in the US group were significantly better than that of the ES group. Perhaps injection of local anesthetic on both sides of the sciatic nerve would improve these variables, but it was not considered in our study because we were evaluating the distribution of local anesthetic around the nerve with only one administration.

Ultrasonography allows visualization of anatomical variations that may influence the success of the block. The sciatic nerve is covered by an aponeurotic sheath from its exit through the sciatic hole at the infrapiriform level (16). However, in 11% of patients the sciatic nerve is already branched into its tibial and peroneal nerves, both with their own aponeurotic sheath, at the infrapiriform level (17,18). This morphology may be the cause of partial blocks of the sciatic nerve with an adequate motor response. Detecting this division with ultrasound could assist the physician to modify the technique and locate the two branches independently to achieve a complete block. Although we did not report improved block quality or a faster block establishment regarding the type of response obtained, as previous studies have reported (19,20), the duration of the sensory block was significantly longer when the type of motor response obtained was plantar flexion as opposed to dorsal flexion in both groups.

One of the main drawbacks in ultrasound-guided puncture is that the needle must be on the same plane as the transducer to visualize the tip of the needle properly (3,8). Otherwise, the location of the tip is not known and a puncture could inadvertently occur. In our study this was not a problem, as the transducer could be situated on the same plane as the needle (Fig. 1). Another limitation of the technique is that a learning process in ultrasound management is required. In our study the same radiologist located the nerve by ultrasound, and the same anesthesiologist performed the block guided by ultrasound; this was intended to prevent the measurement bias introduced by the inexperience of the anesthesiologist in ultrasound location of nerves. However, at present in our institution as well as in others, a trained anesthesiologist performs the entire technique on his or her own in daily clinical practice.

In conclusion, the use of ultrasound to locate and block the sciatic nerve at the midfemoral level, combined with nerve stimulation, increases the success rate of sciatic nerve location at the first attempt and improves the quality of sensory block.

	Footnotes

 
Accepted for publication March 16, 2006.

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Top Abstract Introduction METHODS RESULTS DISCUSSION REFERENCES

 

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