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Ultrasound-Guided Anterior Approach to Sciatic Nerve Block: A Comparison with the Posterior Approach

Ota, Junichi MD; Sakura, Shinichi MD; Hara, Kaoru MD; Saito, Yoji MD

doi: 10.1213/ane.0b013e31818fc252
Analgesia: Regional Anesthesia: Research Reports
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BACKGROUND: Although the anterior approach to the sciatic nerve block has rarely been performed due to lack of reliable surface anatomical landmarks and technical difficulty, ultrasound guidance may make performance of this approach easier. In this study, we evaluated the clinical use of the ultrasound-guided anterior approach to sciatic nerve block and compared this approach with the posterior approach in adults.

METHODS: One hundred patients undergoing minor knee surgery were randomly divided into two groups to receive anterior and posterior (subgluteal) approaches to sciatic nerve block, using 1.5% mepivacaine 20 mL with epinephrine combined with femoral and lateral femoral cutaneous nerve blocks. Both approaches to sciatic nerve block were performed using a low-frequency, 5 to 2 MHz, curved array transducer. Measurements included block execution time, depth and size of the nerve, needle depth, onset time of sensory and motor blockade, and duration of the block.

RESULTS: The sciatic nerve was located significantly deeper and the needle depth was significantly greater in patients undergoing the anterior approach compared with the subgluteal approach. Both approaches were similar for execution time of sciatic nerve block, but the former took less time than the latter to perform all combinations of blocks. Although sensory block in the posterior femoral cutaneous nerve was achieved less often with the anterior approach compared with subgluteal approach (14.9% and 68.1%, respectively; P < 0.001), there were no differences in success rate, onset time or duration of blockade of the peroneal and tibial nerves between the two groups.

CONCLUSION: The anterior approach to sciatic nerve block is performed as easily and successfully as the posterior approach using ultrasound guidance.

IMPLICATIONS: The anterior approach to sciatic nerve block has rarely been performed due to lack of reliable surface anatomical landmarks and technical difficulty. The present report shows that with ultrasound guidance the anterior approach to sciatic nerve block is performed as easily and reliably as the posterior approach in patients undergoing knee surgery.

From the Department of Anesthesiology, Shimane University School of Medicine, Izumo City, Japan.

Accepted for publication July 22, 2008.

Funding and study equipment support was provided from departmental and institutional sources and SonoSite Japan, KK (Tokyo, Japan).

Presented, in part, at the Annual Meeting of the American Society of Anesthesiologists, San Francisco, CA, October 13–17, 2007.

Reprints will not be available from the author.

Address correspondence to Shinichi Sakura, MD, Department of Anesthesiology, Shimane University School of Medicine, 89-1, Enya-cho, Izumo City, 693-8501, Japan. Address e-mail to ssakura@med.shimane-u.ac.jp.

Sciatic nerve block is used to provide anesthesia or analgesia in patients undergoing surgery of the lower extremities. The anterior approach to the sciatic nerve is performed with the patient in supine position and, thus, can be advantageous in a situation in which femoral nerve block is combined. However, since the sciatic nerve runs along the posterior thigh and behind the femur, the anterior approach using surface anatomical landmarks has been associated with technical difficulty1,2 and is considered an advanced nerve block technique.3

Recently, there has been an increase in interest in the use of ultrasound for peripheral nerve blocks,4,5 including applications using the posterior approach to sciatic nerve block at the subgluteal level.6,7 The results of a volunteer study showed that the sciatic nerve can be identified at the lesser trochanter level anteriorly,8 where the anterior approach to the sciatic nerve block is also performed. However, no literature has adequately described successful use of the ultrasound-guided anterior approach to sciatic nerve block. At this level, the sciatic nerve is situated more deeply from the anterior aspect of the thigh and is less visible with ultrasound than the sciatic nerve in the subgluteal space, making ultrasound guidance with the anterior approach difficult.

In this report, we describe clinical use of the ultrasound-guided anterior approach to sciatic nerve block. In addition, we compared the quality of the block and execution time of the anterior approach with the posterior (subgluteal) approach under ultrasound guidance in patients undergoing minor knee surgery.

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METHODS

After IRB approval and written informed consent, 100 patients (ASA I and II) undergoing minor knee surgery, including arthroplasty, menisectomy or meniscal repair, were enrolled in the study. Patients with a history of diabetes mellitus, neurologic disease, infections at the site of injection or those who could not tolerate positioning for the anterior approach were excluded.

All patients fasted for approximately 8 h before entering an operating room, where an IV infusion of acetated Ringer’s solution was initiated at a rate of 1 to 3 mL · kg−1 · h−1. Standard noninvasive monitors were applied, and oxygen was administered via a facemask. Fentanyl 50 to 100 μg with or without midazolam 1 to 2 mg was given IV for anxiolysis as necessary, while ensuring that patients remained responsive to verbal cues. Patients were randomly divided by envelope into two groups to receive anterior and posterior (subgluteal) approaches to sciatic nerve block combined with femoral and lateral femoral cutaneous nerve blocks. Both approaches to sciatic nerve block were performed using a hand-held ultrasound device (MicroMaxxUltrasound System, Sonosite, Bothell, WA) with a low-frequency, 5 to 2 MHz, curved array transducer (C60e).

Patients in the anterior approach group were placed in supine position with the hip and knee on the operated side flexed and the leg externally rotated at approximately 45 degrees. The ultrasound transducer was first positioned perpendicular to the skin approximately 8 cm distal to the inguinal crease (Fig. 1). The location was then scanned by sliding and tilting the transducer until a clear transverse image of the hyperechoic sciatic nerve located posterior and medial to the lesser trochanter was obtained (Fig. 2A). After skin sterilization with an iodine-containing solution and skin infiltration with 1% mepivacaine, a short bevel 100-mm, 21-gauge insulated nerve block needle (Stimuplex A, B. Braun Melsungen AG, Germany) connected to a nerve stimulator (B. Braun Melsungen AG, Germany) was inserted parallel and inline with the ultrasound transducer covered with a sterile plastic cover and gel from anteromedial to posterolateral of the thigh while the sciatic nerve was kept in the middle of the ultrasound screen. The needle was advanced slowly under real-time ultrasound guidance until it was in close proximity to the nerve. A nerve stimulator with a pulse duration of 0.1 ms and stimulating frequency of 2 Hz was then turned on to elicit foot plantarflexion or dorsiflexion. The needle was further adjusted as needed to evoke a motor response at 0.7 mA or less. A local anesthetic solution of 20 mL of 1.5% mepivacaine with 1:400,000 epinephrine was then injected incrementally. The needle-tip was repositioned so that a circumferential spread of the solution could be produced (Fig. 2B).

Figure 1

Figure 1

Figure 2

Figure 2

Patients in the posterior approach group were placed laterally with the side to be anesthetized uppermost and the hip and the knee on the operated side flexed at approximately 45 degrees. The ultrasound transducer was positioned perpendicular to the skin on the line connecting the ischial tuberosity and greater trochanter (Fig. 3), and a clear transverse image of the hyperechoic sciatic nerve between the ischial tuberosity and greater trochanter was obtained (Fig. 4). After skin sterilization with an iodine-containing solution and skin infiltration with 1% mepivacaine, the block was conducted with a short bevel 100-mm, 21-gauge insulated nerve block needle inserted parallel and inline with the ultrasound transducer from posterolateral to anteromedial. Injection of local anesthetic solution was the same as with the anterior approach. If the sciatic nerve was not visualized with ultrasound, the patient was excluded from the study.

Figure 3

Figure 3

Figure 4

Figure 4

After sciatic nerve block, patients in both groups were placed in a supine position with both legs extended. Femoral nerve and lateral femoral cutaneous nerve blocks were also then performed under real-time ultrasound guidance using a high-frequency, 13 to 6 MHz, linear array transducer (HFL38, Sonosite, Bothell, WA) as described elsewhere.9,10 Local anesthetic solutions used for the former and latter blocks were 0.5% ropivacaine 15 to 20 mL and 1% mepivacaine 5 to 10 mL, respectively. All nerve blocks were performed by anesthesiologists (the authors) who have a similar amount of experience in each block and technique used in the study. During surgery, no additional local anesthetics were administered, but patients were sedated as requested with bolus IV injections of midazolam 1 to 2 mg or a continuous IV infusion of propofol 2 to 6 mg · kg−1 · h−1. If surgical anesthesia was deemed inadequate during surgery, bolus IV injections of fentanyl 50 μg were given at the discretion of the attending anesthesiologist. If general anesthesia was required, the patient was excluded from the study.

Measurements included the depth and the size of the sciatic nerve, the needle depth (distance from the skin to the needle tip that was confirmed to be in contact with the sciatic nerve both with the ultrasound image and nerve stimulation), block execution time for the sciatic nerve block (time from the insertion of the block needle to the end of local anesthetic injection and withdrawal of the needle), block execution time for all the blocks (time for sciatic, femoral, lateral femoral cutaneous nerve blocks from the start of positioning a patient for the first [sciatic nerve] block to the end of the third [lateral femoral cutaneous nerve] block), time required for onset of sensory and motor blocks of the sciatic nerve and duration of blockade of the sciatic nerve (time to complete recovery of sensation on the dorsal aspect of the foot and motor recovery of foot movement). Measurements of sizes and distances were conducted on the screen by an internal measuring program of the ultrasound device.

Sensory and motor blockade on the operated limb were evaluated every 5 min after injection of local anesthetic for 30 min, and again immediately and every 2 h after the completion of surgery. Sensory examination was conducted on the plantar aspect of the foot (tibial nerve), the dorsal aspect of the foot (superficial peroneal nerve), the posterolateral area of the leg (sural nerve), and the posterior area of the thigh (posterior cutaneous nerve of thigh) by pinprick (18G). Postoperative sensory examination was not conducted in the sural nerve and posterior cutaneous thigh nerve that were covered with a dressing. Sensory block was considered complete when the patient did not feel pinprick sensation. Motor block was considered complete when the patient could neither planarflex nor dorsiflex the foot. All data were collected by an investigator who was not blinded to group assignment.

Sample size was determined by a power analysis based on the ability to detect a difference of 25% in the rate of sensory block with β set at 0.2 and α set at 0.05. A minimal sample of 82 patients (41 in each group) met these criteria. To accommodate for patient dropouts, 100 patients were enrolled in the study. Data were presented as mean ± sd unless otherwise stated. Continuous variables were compared with Student’s t-test. The χ2 analysis was used to compare the number of patients whose sensory and motor blockade was completed 10, 20, and 30 min after the block. A P value <0.05 was considered to be statistically significant.

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RESULTS

Of 100 patients enrolled in the study, 94 (47 in each group) completed the study. Five patients were excluded because the sciatic nerve could not be identified with ultrasound; the remaining one patient in the anterior approach group was excluded from study due to a protocol violation (Table 1). Both groups were similar in sex, age, weight, height and physical status (Table 2).

Table 1

Table 1

Table 2

Table 2

The sciatic nerve was located significantly deeper in the anterior approach group than in the posterior approach group (Table 3). The size of the sciatic nerve was similar on the ultrasound screen in both groups. Although entire block needles could rarely be seen while being advanced in either group, the needle tip was visualized in most cases or inferred by the movement of the tissue around the needle. When the contact between the nerve and the needle tip was confirmed using both ultrasound and nerve stimulation with similar minimum electric currents between the two groups, the depth of the needle was significantly larger in the anterior approach group than in the posterior approach group. The anterior and posterior approaches were similar for block execution time for sciatic nerve block, but the former took less time than the latter to complete all the three blocks including changing positions. The circumferential spread of local anesthetic was observed after injection in all patients except 5 (4 and 1 patients in the anterior and posterior approach groups, respectively).

Table 3

Table 3

Onset of sensory blockade of the superficial peroneal, sural and tibial nerves was similar between groups (Table 4). Within 30 min after sciatic nerve block, complete sensory blockade was obtained in at least 1 of the 3 territories of the sciatic nerve in every patient except 3 (2 and 1 patients in the anterior and posterior approach groups, respectively). Both groups were similar for the percentage of patients who had complete sensory blockade in all territories of the sciatic nerve tested at 30 min. Sensory blockade in the posterior femoral cutaneous nerve was achieved significantly less often in patients in the anterior approach group than those in the posterior approach group. Onset of motor blockade was also similar between the two groups. Two (1 in each group) of the five patients in whom the circumferential spread of the local anesthetic was not observed did not show sensory blockade in any territories examined within 30 min. Intraoperative fentanyl (50–200 μg) injections were required by 7 patients in each group because of tourniquet pain and slight knee pain in two and five patients in each group, respectively. Block resolution was similar between groups. No neurological complications, such as prolonged sensory or motor dysfunction, were observed postoperatively.

Table 4

Table 4

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DISCUSSION

In this report, we describe clinical use of the anterior approach to sciatic nerve block under real-time ultrasound guidance and show that the technique can be used as easily and successfully as the posterior approach for minor knee surgery. As expected, the location of the sciatic nerve for the anterior approach was deeper than that for the posterior approach. However, the ultrasound image of the sciatic nerve was obtained in a similar percentage of patients and within a similar length of time with either approach. In addition, time for executing sciatic nerve block was similar between approaches, resulting in less time for the combination of blocks required for knee surgery when the anterior approach was chosen. The resultant quality of sensory and motor blockade was also similar except that the anterior approach rarely blocked the posterior area of the thigh.

The anterior approach to sciatic nerve block was first described by Beck11 in 1963, who used the greater trochanter as an anatomic landmark. His technique involved addressing the problem of not being able to always identify the landmark and, thus, several groups of researchers have later reported new techniques using different anatomical landmarks, such as the anterosuperior iliac spine and pubic symphysis tubercle12 or the inguinal crease and femoral artery.13 However, even with landmarks that can be easily identified, the location of the sciatic nerve varies among individuals. In addition, since the sciatic nerve in the anterior approach is deeply located, the block needle has to travel a long distance, and the needle tip can easily deviate from the target nerve. In contrast, ultrasound visualization enabled us to guide the needle to the nerve before a nerve stimulator was turned on despite that visualizing the entire needle was not possible.

A similar rate of visualization of the sciatic nerve between the two approaches was unexpected, since the nerve in the anterior approach is located significantly deeper. There were only two patients in whom the sciatic nerve, using the anterior approach, was not identified. However, the high rate of visualization may be explained by our use of relatively young and non-obese subjects enrolled in the study. In elderly patients, muscles can be atrophic, and the fascia may not be distinguishable with ultrasound.14,15 In obese patients, the nerve would have been located deeper and might have been less clearly visualized.16

The results showing that there were no differences in the onset of sensory and motor blockade of the sciatic nerve after the block between two approaches suggest that we can select the approach interchangeably for minor knee surgery. Although sensory block in the posterior femoral cutaneous nerve, which runs parallel to the sciatic nerve in the gluteal region, was rarely achieved after the anterior approach, this does not constitute a disadvantage for knee surgery in which a thigh tourniquet is used. In our study, the number of patients with intolerable tourniquet pain who required fentanyl was similar between groups. This result corresponds to clinical data showing that tourniquet pain is not affected by the presence of a posterior femoral cutaneous nerve block.17

There are some limitations to this study. First, investigators who collected data were not blinded to group assignment since most of the measurements had to be taken during and immediately after sciatic nerve block. Second, all blocks were conducted by anesthesiologists (the authors) who were well experienced in ultrasound-guided peripheral nerve blocks, including the techniques used in the study. Thus, visualization of the nerve and resultant sensory and motor blockade may be poorer in a daily clinical situation.

In conclusion, the present study showed that the anterior approach to sciatic nerve block is performed as easily and successfully as the posterior approach under ultrasound guidance in patients undergoing knee surgery.

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