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Posterior subgluteal approach to block the sciatic nerve: description of the technique and initial clinical experiences

Di Benedetto, P.*; Casati, A.; Bertini, L.*; Fanelli, G.

European Journal of Anaesthesiology: September 2002 - Volume 19 - Issue 9 - p 682-686
Original Article
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Background and objective: A new posterior approach to the sciatic nerve in the subgluteal region was developed. We describe our clinical experiences on 135 consecutive patients.

Methods: All blocks were performed with a nerve stimulator (stimulation frequency 2 Hz; intensity from 1 reduced to ≤0.5 mA before application). A line was drawn from the greater trochanter to the ischial tuberosity of the femur; then, from the mid-point of this line, a second line was drawn perpendicularly and extended caudally for 4 cm: the end of this line represented the entry point of the needle. Sciatic stimulation was elicited at ≤0.5 mA; then ropivacaine 0.75% 20 mL was injected. An independent observer recorded the time from needle insertion to successful sciatic nerve stimulation (performance time), the depth of appropriate sciatic stimulation and the number of needle redirections, as well as the quality of nerve block, the discomfort during the procedure and patient acceptance.

Results: The performance time was 41 ± 25 s (mean ± SD) and the mean (SD) depth at which the sciatic nerve stimulation was found was 45 ± 10 mm. The median (range) number of needle redirections required to find the proper sciatic stimulation was 2 (1-5). The tibial response was observed in 77 patients (57%), while the common peroneal response was observed in 58 patients (43%). The degree of discomfort reported was very low and only 16 patients (12%) reported severe pain during placement of the block. The onset time (mean ± SD) of sensory and motor block was 7 ± 4 and 17 ± 13 min respectively, and the surgical procedure was completed with only the peripheral nerve block in 127 patients (94%). The same anaesthesia procedure was acceptable by 127 patients (94%) and only eight patients (6%) would prefer a different anaesthesia technique in the future.

Conclusions: The study demonstrated that the sciatic nerve can be easily blocked using this new posterior subgluteal approach, suggesting that it represents a safe and effective alternative to block the sciatic nerve at a proximal level, with the potential for reducing the discomfort experienced by the patient during block placement.

*CTO Roma, Department of Anaesthesiology, Rome;University of Parma, Department of Anesthesiology and Intensive Care, Parma, Italy

Correspondence to: Andrea Casati, Department of Anaesthesiology, IRCCS H. San Raffaele, Via Olgettina 60, I-20132 Milan, Italy. E-mail: casati.andrea@hsr.it; Tel: +39 02 2643 2656; Fax: +39 02 2641 2823

Accepted for publication January 2002 EJA 829

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Introduction

Sciatic nerve block is a well-established procedure for foot and ankle procedures and it is widely used alone or in combination with other peripheral nerve blocks for orthopaedic procedures involving the lower limb [1,2]. The sciatic nerve can be blocked at different levels from the parasacral region to the ankle [2-5]; however, the classical posterior approach of Labat, modified by Winnie, is undoubtedly the most frequently used. This approach requires the identification of multiple landmarks, while the stimulating needle has to pass through different layers of muscles, causing discomfort and pain to the patient [2,3]. In our clinical practice, we developed the use of a new posterior approach to the sciatic nerve in the subgluteal region. We describe this new approach in 135 consecutive patients.

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Methods

Anatomical considerations

The sciatic nerve is a mixed nerve deriving from the sacral plexus, which is formed from the ventral rami of the fourth lumbar to the fourth sacral spinal nerves [6]. It is actually made up of two separate nerves, the tibial and common peroneal, which are enclosed within a common sheath. The nerve roots entering the plexus converge to form a broad triangle passing through the inferior part of the greater sciatic foramen. The sciatic nerve then runs over the posterior aspect of the iliac bone and enters the subgluteal space below the pirifomis muscle. It passes over the obturator internus, superior and inferior gemelli, and quadratus femoris muscles, reaching the posterior part of the thigh between the greater trochanter of the femur and the ischial tuberosity. Once the sciatic nerve has entered the thigh, it runs toward the popliteal fossa, lying on the external surface of the adductor magnus, within the posterior medial compartment of the thigh. The septum intermusculare femoralis mediale and a reinforcement of the posterior fascia of the adductor magnus muscle limit this compartment. Within the popliteal fossa, the two components of the sciatic nerve clearly divide into the larger tibial division located medially, and the common peroneal nerve located laterally.

The sciatic nerve can be reached at different positions during its course from the parasacral space to the popliteal fossa, ideally identifying a 'sciatic line' that runs from the inferior border of the gluteus maximus muscle, between the greater trochanter and the ischial tuberosity, to the popliteal fossa. As soon as it enters the thigh, the sciatic nerve lies just posteriorly to the lesser trochanter in a region with connective tissue; interestingly, at this point the cutaneous landmarks can be easily identified to provide the projection of the sciatic nerve. With the patient placed laterally and the leg to be blocked rolled forward onto the flexed knee, the sulcus between the biceps femoris and semitendinosus muscles can be identified just 3 cm above the lower limit of the gluteus muscle. The depression between these two muscles goes down toward the popliteal fossa and represents the cutaneous projection of the sciatic nerve. If a line is drawn from the greater trochanter to the ischial tuberosity and a second line is drawn perpendicularly from the mid-point and extended caudally for 4 cm, we can identify the needle insertion site for a new posterior approach to the sciatic nerve (Fig. 1).

Figure 1

Figure 1

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Clinical study

After Ethics Committee approval, written informed consent was obtained from all considered patients. One hundred-and-thirty-five consecutive patients scheduled to receive a sciatic nerve block for orthopaedic procedures involving the foot were prospectively studied. Patients with contraindications to regional anaesthesia as well as those with respiratory or cardiac disease, diabetes or peripheral neuropathy, and patients receiving chronic analgesic therapy were excluded from the study.

All patients were premedicated with oral diazepam (30 mg) 30 min before placing the block. An intravenous (i.v.) cannula was placed in a forearm vein and 5 mL kg−1h−1 crystalloid infusion was given. All blocks were performed by anaesthesiologists with substantial expertise in regional anaesthesia techniques using a nerve stimulator (Plexival, Medival, Padua, Italy) and a 10 cm, 20-G, shortbevelled, Teflon®-coated stimulating needle (Vygon, Ecouen, France) [7]. The stimulation frequency was set at 2 Hz, while the intensity of the stimulating current, initially set to deliver 1 mA, was gradually decreased to <0.5 mA after the appropriate muscular response was observed. Paraesthesiae were never sought.

If indicated by the surgical procedure, a femoral nerve block was placed before the sciatic nerve block. Then the patients were placed in the lateral decubitus position, with the leg to be blocked uppermost and rolled forward with the knee flexed at 90° (Sims' position) [2]. A line was drawn from 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 point, a skin depression can be palpated, representing the groove between the biceps femoris and the lateral border of the vastus lateralis muscles. This point represented the site for needle entry (Fig. 1). The stimulating needle was inserted 90° to the skin and advanced until sciatic stimulation was observed. In all blocks, the needle position was adjusted maintaining an adequate muscular twitch with a stimulating current ≤0.5 mA (range 0.3-0.5 mA). Then ropivacaine 0.75% 20 mL was injected slowly in 5 mL increments, with careful aspiration for blood every each increment. If the sciatic nerve was not stimulated after the first needle insertion, the blocking needle was redirected with a 5° increased or decreased angle to the skin, and the manoeuvre repeated.

Standard monitoring was used throughout the study, including non-invasive arterial pressure, heart rate and pulse oximetry, while sensory and motor blocks were evaluated every 5 min after block placement to assess the adequacy of surgical anaesthesia. The patient was judged ready for surgery when a complete loss of pinprick sensation in both the tibialis and common peroneal nerve distributions - with a concomitant inability effectively to move the ankle and toes-was seen. At that point, the surgeon started the skin preparation. The following observations were noted: (1) the time from needle insertion into the skin to successful elicitation of the appropriate motor response (performance time), (2) the time from the completion of the injection to readiness to surgery (onset time), (3) the number of needle redirections required before getting the designated motor response, and (4) the depth of sciatic nerve stimulation. The use of supplementary intravenous analgesics or sedation, or both, was also recorded by the same independent observer during surgery. The nerve block was considered as (1) adequate, if neither sedation nor analgesics were required during surgery; (2) inadequate, if fentanyl 50 μg i.v. boluses were required during surgery; or (3) failed, if general anaesthesia was required during surgery.

After placement of the block, the patients were asked to grade the discomfort experienced during the procedure using a three-point scale (1: not painful; 2: moderately painful; 3: extremely painful). On the day after surgery, complete resolution of the effects of the nerve block was verified and any persistent dysaesthesia or deficit of motor function on the operated limb was noted. The presence of any neurological deficit was also evaluated 1 week after hospital discharge at the first routine postoperative orthopaedic examination.

Satisfaction with the anaesthetic technique was also evaluated 24 h after surgery using a two-point score: 1: satisfactory 'if ever operated on again in the future I would accept the same anaesthetic procedure'; and 2: unsatisfactory 'if ever operated on again in the future I would prefer a different anaesthetic technique'. Results are presented as mean (±SD) or as median (range), while categorical variables are presented as percentages.

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Results

A total of 135 patients (79 females, 56 males) were prospectively evaluated. The mean age was 44 ± 18 yr, body weight 73 ± 12 kg and height 163 ± 5 cm. Femoral nerve block was also placed in 112 patients (83%).

The time required from insertion of the needle to eliciting a correct sciatic mediated muscular twitch was 41 ± 25 s, and the mean depth at which the sciatic nerve stimulation was found was 45 ± 10 mm. The median (range) number of needle redirections required to find the proper sciatic stimulation was 2 (1-5). The tibial response, with plantar flexion of the foot and toes, was observed in 77 patients (57%), while the common peroneal response, with dorsiflexion and eversion of the foot, was observed in 58 patients (43%).

Figure 2 shows the degree of discomfort reported by the patients after block placement. The onset time (mean ± SD) of sensory and motor blocks was 7 ± 4 and 17 ± 13 min, respectively. In 127 patients (94%), the peripheral nerve block was completely successful allowing completion of surgery without the need for either fentanyl supplementation or propofol sedation. Analgesic supplementation with fentanyl was required in eight patients (6%), but six of them required propofol sedation due to a failed nerve block (4.4%).

Figure 2

Figure 2

Patient acceptance of the anaesthesia procedure was quite good: 127 patients (94%) would accept the same anaesthesia technique if required in the future for a similar surgical procedure, while only eight patients (6%) would prefer a different technique. No patient reported persistent dysaesthesia or motor function deficits at the postoperative follow-up visit.

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Discussion

Several different proximal approaches to the sciatic nerve have been described [3-5,8-11]; however, the classical posterior approach of Labat modified by Winnie is undoubtedly the most commonly used [2,10]. This approach requires multiple anatomical landmarks, which can be difficult to identify particularly in obese patients. Furthermore, this approach is often painful for the patient, mainly because of the thick layer of muscles through which the stimulating needle passes when seeking the sciatic nerve [2,12]. For this reason, we developed a new posterior approach, slightly more distal when compared with the classical posterior one, but at a level where the sciatic nerve is more superficial and covered by fewer muscular structures. The results of this prospective study demonstrate that this new subgluteal posterior approach to the sciatic nerve is easy to perform and minimally painful to the patient, with a success rate as high as 94%.

The minimal discomfort reported by our patients when placing the block is probably related to the shallower depth at which the sciatic nerve is identified compared with other approaches. In fact, correct sciatic stimulation was elicited at 4.5 cm, while the other proximal approaches require sciatic stimulation at a depth ranging from nearly 7 cm for the classical posterior approach to 10-12 cm using the anterior or high lateral approaches [5,8,11,13]. This factor, along with the few attempts required for successful nerve location, account for the very good acceptance of the anaesthetic procedure and the minimal pain observed during block placement. This new subgluteal approach could be also particularly helpful in obese patients, because the increased amount of adipose tissue in the gluteal region usually makes it quite difficult and unpredictable to identify the sciatic nerve using needles of normal length if the classical posterior approach is used. Further studies are required to compare the efficacy and ease of the new subgluteal approach with the classical posterior approach, particularly in obese patients.

The onset time of peripheral nerve blockade is mainly influenced by the clinical properties of the anaesthetic solution injected. The present investigation found a mean time for complete sensory and motor blockade of the sciatic nerve of about 17 min. This finding is consistent with data already published on the profile of peripheral nerve blocks performed with ropivacaine [14]. Similar results have been reported by other authors performing other proximal approaches to the sciatic nerve either with the same drug or with other long-acting local anaesthetic solutions, e.g. bupivacaine [5,15-17].

In conclusion, the results demonstrate that the sciatic nerve can be easily blocked using this new posterior subgluteal approach, providing minimal discomfort to the patient and a very high success rate. Further studies are required to compare this new approach to other proximal approaches to the sciatic nerve, particularly the classical posterior approach of Labat modified by Winnie. However, the results suggest that the subgluteal approach represents an effective alternative to block the sciatic nerve at a proximal level.

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References

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Keywords:

ANAESTHESIA CONDUCTION, nerve block; ANAESTHETICS, LOCAL, ropivacaine; LUMBOSACRAL PLEXUS, sciatic nerve

© 2002 European Academy of Anaesthesiology