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REGIONAL ANESTHESIA

The Effects of Three Different Approaches on the Onset Time of Sciatic Nerve Blocks with 0.75% Ropivacaine

Manuel Taboada, MD^*, Julián Álvarez, MD, PhD^*, Joaquín Cortés, MD, PhD^*, Jaime Rodríguez, MD, PhD^*, Susana Rabanal, MD^*, Francisco Gude, MD, Alexander Atanassoff, and Peter G. Atanassoff, MD Section Editor

*Department of Anesthesiology and Clinical Epidemiology Unit, Hospital Clínico Universitario de Santiago, Santiago de Compostela, Spain; Department of Anesthesiology, Universidad de Santiago de Compostela, Spain; and Department of Anesthesiology, Yale University School of Medicine, New Haven, Connecticut

Address correspondence and reprint requests to Manuel Taboada Muñiz, MD, Department of Anesthesiology, Hospital Clínico Universitario de Santiago, Travesía da Choupana s/n, 15706 Santiago de Compostela, Spain. Address e-mail to manutabo{at}mixmail.com

	Abstract

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We studied three different injection techniques of sciatic nerve block in terms of block onset time and efficacy with 0.75% ropivacaine. A total of 75 patients undergoing foot surgery were randomly allocated to receive sciatic nerve blockade by means of the classic posterior approach (group classic; n = 25), a modified subgluteus posterior approach (group subgluteus; n = 25), or a lateral popliteal approach (group popliteal; n = 25). All blocks were performed with the use of a nerve stimulator (stimulation frequency, 2 Hz; intensity, 2–0.5 mA) and 30 mL of 0.75% ropivacaine. Onset of nerve block was defined as complete loss of pinprick sensation in the sciatic nerve distribution with concomitant inability to perform plantar or dorsal flexion of the foot. In the three groups, an appropriate sciatic stimulation was elicited at <0.5 mA. The failure rate was similar in the three groups (group popliteal: 4% versus group classic: 4% versus group subgluteus: 8%). The onset of nerve block was slower in group popliteal (25 ± 5 min) compared with group classic (16 ± 4 min) and group subgluteus (17 ± 4 min; P < 0.001). There was no significant difference in the onset of nerve block between group classic and group subgluteus. No differences in the degree of pain measured at the first postoperative administration of pain medication were observed among the three groups. We conclude that the three approaches resulted in clinically acceptable anesthesia in the distribution of the sciatic nerve. The subgluteus and classic posterior approaches generated a significantly faster onset of anesthesia than the lateral popliteal approach.

IMPLICATIONS: Comparing three different approaches to the sciatic nerve with 0.75% ropivacaine, the classic and subgluteal approaches exhibited a faster onset time of sensory and motor blockade than the lateral popliteal approach.

	Introduction

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Foot surgery is often associated with severe and long-lasting postoperative pain requiring large amounts of parenteral opioids (1). The sciatic nerve block may be used alone or in combination with other peripheral nerve blocks for orthopedic procedures involving the lower limb. Several approaches to blockade of the sciatic nerve block have been described (2–7).

Ropivacaine in larger concentrations is a practical drug for lower limb nerve blocks with the advantage of long-lasting postoperative pain relief (8–10). Evaluating the use of 0.75% ropivacaine for different peripheral nerve blocks, Casati et al. (8) reported onset time characteristics similar to those of commonly used fast-onset, intermediate-duration local anesthetics, such as mepivacaine. However, various factors markedly affect the onset time of peripheral nerve blocks, including the concentration and volume of the injected anesthetic solution (8), the use of additives (11), a double-injection technique (12,13), the type of evoked motor response obtained (14,15), and the intensity of the current at which peripheral nerve stimulation is achieved (16–18).

The purpose of this prospective, randomized, blinded study was to evaluate the effects of different approaches (classic, subgluteal, and lateral popliteal) on the onset time and efficacy of sciatic nerve block performed with a single injection of the same volume of 0.75% ropivacaine.

	Methods

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The study protocol was approved by the hospital ethics committee of the University of Santiago de Compostela, and written, informed consent was obtained from each patient. Seventy-five ASA physical status I or II patients, aged 18–70 yr, scheduled for foot and ankle surgery under sciatic nerve block were enrolled in the investigation. Exclusion criteria were patient refusal, pregnancy, neurologic or neuromuscular disease, anticoagulation, and skin infection at the site of needle insertion. Patients were randomly assigned to receive the classic approach of Labat (group classic; n = 25), a modified subgluteus posterior approach (group subgluteus; n = 25), or the lateral popliteal approach (group popliteal; n = 25).

Before the nerve block, IV access was established, and continuous electrocardiogram, noninvasive blood pressure, and SpO₂ were monitored during block insertion and throughout the surgical procedure. The blocks were performed after sedation with 1–2 mg of midazolam IV. Additional doses of midazolam and fentanyl in increments of 1 mg and 50 µg, respectively, were administered when deemed necessary to alleviate any accompanying anxiety and discomfort. Because all surgeries were performed with an arterial tourniquet below the knee, a femoral nerve block was performed with 10 mL of 1.5% mepivacaine.

All peripheral nerve blocks were performed by the same anesthesiologist with an 80-mm, 22-gauge insulated stimulating needle (Pajunk; Medizintechnologie, Geisingen, Germany) attached to a nerve stimulator (Pajunk). The stimulation frequency was set at 2 Hz. The intensity of the stimulating current, initially set to deliver 2 mA, was gradually decreased to <0.5 mA while the appropriate motor response was maintained. In all patients, the targeted evoked motor response was plantar flexion of the foot.

Patients in the classic group were placed in the lateral decubitus position, with the leg to be blocked uppermost and rolled forward with the knee flexed at a 90° angle (Sims’ position). A line was drawn from the posterior superior iliac spine to the midpoint of the greater trochanter, and a second perpendicular line was drawn from the midpoint and extended caudally for 4 cm. This point represented the site of needle insertion. After local skin infiltration, the stimulating needle was inserted with a 90° angle to the skin and advanced until stimulation of the tibial nerve (plantar flexion of the foot) was obtained. If dorsiflexion was elicited as a result of stimulation of the common peroneal nerve, the needle was withdrawn and redirected 2–3 mm more deeply and medially. The position of the needle was adjusted to maintain an adequate muscular response with a stimulating current <0.5 mA; then, 30 mL of 0.75% ropivacaine was injected slowly after careful intermittent aspirations.

Patients in the subgluteus group were also placed into Sims’ position. A line was drawn from the midpoint of the greater trochanter to the ischial tuberosity. From the midpoint of this line, a second line was drawn perpendicularly and extended caudally for 4 cm. At this level, a skin depression can be palpated that represents the groove between the biceps femoris and semitendinosus muscles. This point represented the site for needle entry (19). After local skin infiltration, the stimulating needle was inserted with a 90° angle to the skin and advanced until a motor response was obtained after stimulation of the tibial nerve (plantar flexion of the foot). As with the previous approach, if dorsiflexion was elicited as a result of stimulation of the common peroneal nerve, the needle was withdrawn and redirected 2–3 mm medially. The position of the needle was adjusted to maintain an adequate motor response with a stimulating current <0.5 mA; then, the same volume of 0.75% ropivacaine was injected as previously described.

Patients receiving a lateral popliteal sciatic nerve block (group popliteal) were positioned supine with their legs extended at the knee joint. The long axis of the foot was positioned at a 90° angle to the table. The stimulating needle was inserted in a horizontal plane 11 cm cephalad to the most prominent point of the lateral femoral epicondyle (7,20). It was advanced in the groove between the biceps femoris and the vastus lateralis muscles until the femur was contacted. The needle was then withdrawn and redirected posteriorly at a 30° angle until plantar flexion was elicited. If a dorsiflexion of the foot was obtained, the needle was redirected more medially. The needle was considered to be close enough to the nerve when the motor response elicited persisted with a current intensity <0.5 mA; then, 30 mL of 0.75% ropivacaine was slowly administered.

Arterial blood pressure, heart rate, pulse oximetry, and progress of both sensory and motor blockade on the operated limb were evaluated every 5 min after local anesthetic injection by an independent blinded observer. Onset of sensory block was defined as the time to complete loss of pinprick sensation in the distribution of the tibial and common peroneal nerves. The onset of motor block was defined as the inability of the patient to perform plantar and dorsal flexion of the foot. The degree of pain during surgery was assessed with a four-point verbal rating scale score (0, no pain; 1, mild or moderate pain; 2, severe pain; and 3, unbearable pain). If a verbal rating scale score of >1 was reported by the patient, 50 µg of supplemental fentanyl was given IV. If this did not provide adequate analgesia, general anesthesia was induced. The quality of sciatic nerve block was evaluated as follows: satisfactory nerve block, no analgesia required to complete surgery; unsatisfactory nerve block, fentanyl administration required to complete surgery; and failed nerve block, general anesthesia required to complete surgery.

Postoperative analgesic medication consisted of 30 mg of ketorolac and 2 g of propacetamol administered IV at 6-h intervals if needed. Time to first request for pain medication (duration of analgesia), time to resolution of motor blockade (evaluated every hour from the end of the local anesthetic injection to resolution of motor block at the ankle), and details of any untoward event were recorded by an independent blinded observer. Patient satisfaction with the anesthetic technique was evaluated by using a two-point score: 1, satisfactory (if necessary, I would have the same anesthetic again); and 2, unsatisfactory (different anesthetic).

To calculate the required number of patients to be included in the study, we considered previous findings on sciatic nerve block with 0.75% ropivacaine (8–9). We intended to detect a 5-min difference in the onset time of sciatic nerve blockade performed with 0.75% ropivacaine between the lateral popliteal approach and the other two proximal approaches (classic and subgluteal approaches), accepting a 2-tailed alpha error of 1% and a ß error of 5%. On the basis of these figures, the required study size ranged from 20 to 25 patients per group.

Statistical analysis was performed with SPSS for Windows (Version 10; SPSS Inc., Chicago, IL). Analysis of variance with Dunnett and Scheffé tests for multiple comparisons was used to evaluate demographic data, onset and resolution of the block, and duration of postoperative analgesia. The quality of sciatic nerve block and patient satisfaction with the anesthetic technique were analyzed by contingency table and ² analyses. P < 0.05 was considered significant. Continuous variables are presented as mean ± SD, and qualitative data are presented as number (percentage).

	Results

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There were no significant differences among groups in terms of demographic data (age, weight, and height), ASA physical status, surgical times, or type of surgical procedure (Table 1). No severe untoward event was reported in any patient.

View larger version (13K): [in this window] [in a new window]   Figure 1. Survival analysis of the total time required from the completion of sciatic injection until patients were judged ready for surgery with a sciatic nerve block placed by using a popliteal approach (n = 25), subgluteus approach (n = 25), or classic approach (n = 25). The log-rank curves representing the three groups studied are significantly different (P < 0.001).

View larger version (14K): [in this window] [in a new window]   Figure 2. Total number of sciatic nerves that were satisfactory blocked, unsatisfactory blocked, or not blocked (failed block) in the three groups of patients. No differences were observed among the three groups.

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
This investigation demonstrated that performing sciatic nerve block at three different levels generated a shorter time of complete anesthesia in patients receiving the classic or subgluteal approaches as compared with a lateral approach. The quality of nerve blockade was similar in all three groups.

Various factors markedly affect the onset time of peripheral nerve blocks. These include the concentration and volume of the injected anesthetic solution (8), the use of additives (11), a double-injection technique (12,13), the type of evoked motor response obtained (14,15), and the intensity of the current at which peripheral nerve stimulation is achieved (16–18). Because all these factors were kept constant in the three groups, the site of injection may explain the time difference in completion of the regional anesthetic.

The sciatic nerve derives its fibers from the L4 to S3 spinal segments and is almost 2 cm wide at its origin near the sacral plexus. Two separate nerve trunks, the tibial nerve and the common peroneal nerve, enveloped by a fascial sheath (epineural sheath) can be distinguished from the onset (21). These two trunks leave the pelvis through the sacrosciatic foramen between the tuberosity of the ischium and the greater trochanter of the femur. The nerve becomes superficial at the end of the gluteus maximus muscle. From there, it courses down the posterior aspect of the thigh to the popliteal fossa, where the tibial and common peroneal nerves eventually separate, with the tibial nerve descending medially through the popliteal fossa into the back of the leg and the common peroneal nerve coursing laterally from the midline to pass behind the head of the fibula and lateral to the fibula neck. Vloka et al. (21) demonstrated the existence of a continuous epineural sheath surrounding the sciatic nerve and its major divisions, the tibial and common peroneal nerves. This sheath appears to form a virtual fluid conduit that communicates among the major nerve divisions in the popliteal fossa; it allows the spread of solutions injected into the sheath. The sciatic nerve is composed of independent medial and lateral divisions that are physically but not functionally joined by a common connective tissue sheath (22). The tibial and common peroneal nerves are bundled together, wrapped by a separate epineurium, and contained in a common epineural adventitia with multiple layers of connective tissue. They do not exchange nerve fibers (23). This is important to bear in mind when sciatic nerve block is performed in the popliteal fossa. Because of the described anatomy, exposure of one of these branches to the local anesthetic may be limited when the injection is made distal to the division of the two branches (22). In addition, separation of the tibial and common peroneal nerves at highly variable distances above the popliteal fossa crease is often seen (21). In some cases, the tibial nerve is located several centimeters distant from the common peroneal nerve, making it difficult or impossible for the local anesthetic to cover the distance. Additionally, the presence of fat or multiple layers of connective tissue within the popliteal space may explain the slower onset of nerve block in the lateral popliteal approach as compared with the two other approaches. To improve the success rate of popliteal blocks, some investigators have suggested a double-injection technique in which both the tibial and common peroneal nerves are separately identified and anesthetized (12,13). Others have suggested injecting a larger volume of local anesthetic to increase the spread within the epineural sheath to reach both branches (23).

In this study, 0.75% ropivacaine was used without additives. Casati et al. (8) compared the clinical properties of sciatic nerve block performed with 0.5%, 0.75%, or 1% ropivacaine with those of 2% mepivacaine. The authors suggested that 0.75% ropivacaine is the most suitable choice of local anesthetic for sciatic nerve block; it provides an onset similar to that of mepivacaine, with additional extended postoperative analgesia.

The intensity of the stimulating current, rather than the type of evoked motor response, has been suggested to be an important factor determining the quality and extent of nerve blockade (15,16). Traditional teachings suggest that the needle should be manipulated until a muscle twitch occurs between 0.5 and 1 mA (17), but increased risk of failure was observed for currents more than 0.6 to 0.8 mA. Several experts suggest that a stimulating current less than 0.5 mA should be used (18). In this study, the evoked motor response that was accepted for accurate needle placement (plantar flexion) was achieved at stimulating currents <0.5 mA.

In conclusion, the results of this prospective, randomized, double-blinded study demonstrate that the sciatic nerve can be blocked adequately and accessed at three different levels by using a classic, subgluteal, and lateral popliteal approach. The classic approach of Labat (3) and the subgluteal posterior approach generated a shorter onset time, thereby reducing the time required to achieve readiness for surgery compared with the lateral popliteal approach.

	References

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Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999