REGIONAL ANAESTHESIA

Case Report

Ultrasound-guided sciatic nerve block: description of a newapproach at the subgluteal space

M. K. Karmakar*, W. H. Kwok, A. M. Ho, K. Tsang, P. T. Chui and T. Gin

Department of Anaesthesia and Intensive Care, The Chinese University of Hong Kong, Prince of Wales

Hospital, Shatin, NT, Hong Kong, SAR, China

*Corresponding author: Department of Anaesthesia and Intensive Care, The Chinese University of Hong Kong, Prince

of Wales Hospital, Shatin, NT, Hong Kong, SAR China. E-mail: karmakar@cuhk.edu.hk

Sciatic nerve block is frequently used for anaesthesia or analgesia during orthopaedic foot

surgery and there are several different approaches to the sciatic nerve. This report describes a

new approach to the sciatic nerve using ultrasound. Local anesthetic was injected into the

subgluteal space under ultrasound guidance which was effective in producing sciatic nerve block

in a small series of five patients. The anatomy, sonographic features, technique of identifying the

subgluteal space, and potential advantages of this approach to the sciatic nerve are discussed.

Br J Anaesth 2007; 98: 3905

Keywords: anaesthetic, techniques, regional, sciatic; nerve, nerve block, ultrasound guided,

subgluteal

Accepted for publication: November 15, 2006

Sciatic nerve block (SNB) is frequently used for anaesthesia

or analgesia during lower limb surgery. Depending on the

surgical indication, SNB can be used on its own or in

combination with an ipsilateral lumbar plexus block or

femoral nerve block for surgical anaesthesia. There are

several techniques or approaches for SNB. Most techniques

described to date rely on surface anatomical landmarks,

which can be complex. Although anatomical landmarks

provide valuable clues to the position of the sciatic nerve,

they are only surrogate markers, can vary in patients, and

may be difficult to locate in obese patients. Even nerve

stimulation that is considered the gold standard for peri-

pheral nerve localization may not always elicit a motor

response and also does not guarantee success.

Recently, there has been an increase in interest in the use

of ultrasound for peripheral nerve block and ultrasound-

guided SNB has been described.13 We have also used

ultrasound to perform SNB in both adults and children.

However, instead of performing SNB at the traditional sites

described previously (anterior, parasacral, transgluteal,

infragluteal, lateral, posterior subgluteal, infraglutealpara-

biceps, proximal thigh, or at the popliteal fossa), we have

found the subgluteal space to be an effective site for local

anesthetic injection or catheter insertion during

ultrasound-guided SNB. In this report, we describe a series

of five patients in whom SNB was successfully performed

under ultrasound guidance at the subgluteal space.

The anatomy, sonographic features, and technique of identi-

fying the subgluteal space at the level of the greater tro-

chanter and ischial tuberosity using ultrasound are also

discussed.

Case report

We performed ultrasound-guided SNB for anaesthesia in

five ASA physical status IIII patients (2764 yr old,

weight 5574 kg) who were scheduled for orthopaedic foot

surgery. No premedication was prescribed to any of these

patients. The patients were positioned laterally, with the side

to be anaesthetized uppermost and with the hip and knees

flexed (Fig. 1). The lateral prominence of the greater tro-

chanter and the ischial tuberosity were then identified, and a

line was drawn between these two landmarks using a skin

marking pen (Fig. 1). The sciatic nerve was scanned at this

location using a low-frequency, 52 MHz, curved array

probe (C60e, 52 MHz) and a Micromaxx ultrasound

system (Sonosite Inc., Bothell, WA, USA) with tissue har-

monic imaging (THI) and image capturing capabilities. A

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British Journal of Anaesthesia 98 (3): 3905 (2007)

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liberal amount of ultrasound gel was applied to the skin

over the area to be scanned for acoustic coupling and the

ultrasound probe was positioned parallel to the line pre-

viously drawn with its orientation marker directed laterally

(i.e. directed towards the greater trochanter, so as to provide

a transverse scan of the subgluteal space and the sciatic

nerve). A scout-scan (pre-intervention scan) was per-

formed to identify the sciatic nerve in the subgluteal space

and to optimize the ultrasound image before the interven-

tion. The ultrasound image was optimized by making the

following adjustments on the ultrasound unit: (a) selecting a

scanning preset, (b) setting an appropriate scanning depth,

(c) selecting the General (mid range) frequency range as

the ultrasound probe used was a broadband probe, (d) select-

ing the THI option, and (e) finally, the gain was adjusted

manually to obtain the best possible image (Fig. 2). On a

sonogram, the subgluteal space was seen as a hypoechoic

area between the hyperechoic perimysium of the gluteus

maximus and the quadratus femoris muscles (Fig. 2). It

extended from the greater trochanter laterally to the ischial

tuberosity medially. The medial limit of the subgluteal

space was difficult to see. At this level, the sciatic nerve was

seen as an oval hyperechoic nodule approximately 1.5

2 cm in diameter within the subgluteal space (Fig. 2).

Under aseptic precautions, a 100-mm, 21-gauge insu-

lated nerve block needle (Stimuplexw A, B. Braun

Melsungen AG, Germany) connected to a nerve stimulator

(Stimuplexw HNS11, B. Braun) delivering a current of1 mA at a frequency of 1 Hz was inserted in the long axis

(in plane) of the ultrasound beam (Fig. 1) and advanced

slowly towards the sciatic nerve under real-time ultrasound

guidance. As the needle was advanced in the long axis of

the ultrasound beam, it was possible to see the advancing

needle in most cases. However, when the needle could not

be seen, the position of the needle tip could only be

inferred by jiggling the needle and looking for tissue

movement on the ultrasound scan. Once the block needle

was in contact with the sciatic nerve (identified by observ-

ing nerve movement) or in the subgluteal space close to

the sciatic nerve, a positive motor response (dorsiflexion

or plantar flexion) of the foot was observed in all except

one patient (patient 5, aged 64 yr, motor response was

only obtained at a current of 2 mA). The final position of

the needle in the subgluteal space was confirmed in all

five patients by injecting 25 ml of saline through the

needle and observing a distention of the subgluteal space

[i.e. separation of the perimysium of the gluteus maximus

and quadratus femoris muscle on the ultrasound image

Fig 1 Position of the patient and the ultrasound transducer during an ultrasound-guided SNB at the subgluteal space. Also seen is an insulated nerveblock needle (100 mm, 21 gauge insulated needle, Stimuplexw A, B. Braun Melsungen AG, Germany) being inserted in the long-axis (in plane) of theultrasound beam (reproduced with permission from www.aic.cuhk.edu.hk/usgraweb).

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(Fig. 3)]. In two patients, although the needle was seen to

be in contact with the sciatic nerve and there was a posi-

tive motor response in the foot-to-nerve stimulation, the

test injection of saline only spread posterior to the perimy-

sium of the gluteus maximus muscle. This indicated that

the tip of the needle was not in the subgluteal space and

was easily rectified by advancing the needle a little further

after which the typical distention of the subgluteal space

to the saline test injection was seen on the ultrasound

image. Occasionally, a subtle pop was felt as the needle

tip traversed the perimysium of the gluteus maximus

muscle and entered the subgluteal space.

After negative aspiration through the needle, 2530 ml

of lignocaine 1% and ropivacaine 0.25% with epinephrine

1:400 000 was injected incrementally over 23 min while

observing the distribution of the local anaesthetic in real

time on the ultrasound scan. Distention of the subgluteal

space (seen in all patients, Fig. 3) and circumferential

spread of the local anaesthetic around the sciatic nerve (in

four patients, Fig. 3) was noted (see video at www.aic.

cuhk.edu.hk/usgraweb). Longitudinal scan of the sciatic

nerve in between the greater trochanter and ischial tuberos-

ity after the local anaesthetic injection also demonstrated

longitudinal spread of the local anaesthetic on either side

of the sciatic nerve. Complete anaesthesia, adequate for

surgery over the foot, developed in all patients within

1520 min after the injection of local anaesthetic. There

were no complications directly related to the SNB or to

the local anaesthetic injection, and recovery from the

anaesthesia was uneventful.

Discussion

Sciatic nerve block is frequently used for anaesthesia or

analgesia during orthopaedic foot surgery, and several differ-

ent approaches to the sciatic nerve have been described

in the literature. In this report, we have demonstrated that

the subgluteal space, where the sciatic nerve is located, is a

well-defined anatomical space and can be identified using

ultrasound at the level of the greater trochanter and ischial

tuberosity. We have also shown that local anaesthetic

injected into the subgluteal space under ultrasound guidance

is effective in producing SNB.

The technique of ultrasound-guided SNB at the subglu-

teal space, as described in this report, should be distin-

guished from, and not confused with, the technique of

posterior subgluteal SNB described by Di Benedetto and

colleagues.4 The two techniques are different: (a) ultra-

sound guided vs landmark technique and (b) local anaes-

thetic is also injected into two different locations along the

course of the sciatic nerve. While we inject the local

anaesthetic into the subgluteal space, defined as the space

between the gluteus maximus and quadratus femoris

muscle at the level of the greater trochanter and ischial

tuberosity, Di Benedetto and colleagues4 perform their

subgluteal SNB technique caudal to the inferior border of

the gluteus maximus muscle. Another approach to the

sciatic nerve that is comparable to the one that we

describe, as far as injecting the local anaesthetic into the

subgluteal space, is the lateral approach described by

Guardini and colleagues.5 However, this modified lateral

approach is performed with the patient in the supine

Fig 2 Transverse sonogram between the greater trochanter and ischial tuberosity showing the hypoechoic subgluteal space between the hyperechoicperimysium of the gluteus maximus and the quadratus femoris muscle. The sciatic nerve is seen as a hyperechoic nodule in the medial aspect of thesubgluteal space (reproduced with permission from www.aic.cuhk.edu.hk/usgraweb).

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position, using nerve stimulation, and the landmarks

described introduce, the needle along the inferior border

of the quadratus femoris muscle.5

The sciatic nerve (L4,5, S1-3) arises from the sacral

plexus and exits the pelvis through the greater sciatic

foramen, between the piriformis and the superior

gemellus muscles (Fig. 4), to enter the subgluteal space

below the piriformis muscle.4 It then descends over the

dorsum of the ischium, lying on the dorsal surface of

the gemellus superior muscle, tendon of obturator

Fig 3 Transverse sonogram of the sciatic nerve at the level of the greater trochanter and ischial tuberosity after 25 ml of local anaesthetic injection.Note the distention of the subgluteal space and circumferential spread of local anaesthetic around the sciatic nerve (reproduced with permission fromwww.aic.cuhk.edu.hk/usgraweb).

Fig 4 Relation of the sciatic nerve to the muscles of the buttock and upper thigh (reproduced with permission from www.aic.cuhk.edu.hk/usgraweb).

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internus, gemellus inferior muscle, and quadriceps

femoris muscle (in a cranial to caudal relation, Fig. 4)

before it enters the hollow between the greater trochan-

ter and the ischial tuberosity and then on to the pos-

terior compartment of the thigh. The anterior surface of

the gluteus maximus muscle covers the upper part of

the sciatic nerve and immediately distal to its lower

border (infragluteal position), the sciatic nerve is fairly

superficial. In between the greater trochanter and ischial

tuberosity, the subgluteal space is a well-defined anatom-

ical space between the anterior surface of the gluteus

maximus and the posterior surface of the quadratus

femoris muscle.5 On a sonogram, the subgluteal space is

seen as a hypoechoic area between the hyperechoic peri-

mysium of the gluteus maximus and quadratus femoris

muscles. It extends from the greater trochanter laterally

to the ischial tuberosity medially. The sciatic nerve is

seen as an oval hyperechoic nodule, approximately

1.52 cm in diameter in an average adult within the

subgluteal space (Fig. 2). The medial limit of the sub-

gluteal space is often difficult to see using ultrasound.

This may be because of the attachment of the semi-

membranosus, semitendinosus, and biceps femoris

muscles to the ischial tuberosity (Fig. 5). Other struc-

tures that are present in the subgluteal space include the

posterior cutaneous nerve of the thigh, inferior gluteal

vessels and nerve, nerve to the short and long head of

biceps femoris, the comitans artery and vein of the

sciatic nerve, and the ascending branch of the medial

circumflex femoral artery.5 The anatomical relations of

the above structures are shown in Figure 5.

Occasionally, pulsations of the inferior gluteal artery can

be seen medial to the sciatic nerve in the subgluteal

space.

Although the subgluteal space covers a wide area

anterior to the gluteus maximus muscle, we have found

that it is best seen using ultrasound at the level of the

greater trochanter and ischial tuberosity and in relation to

the quadratus femoris muscle. The quadratus femoris

muscle is a quadrangular muscle, about 4 cm in height and

attached to the posterior surface of the greater trochanter

and ischial tuberosity.5 Two lines extended laterally along

the upper and lower border of the quadratus femoris

muscle will intersect the femur about 1 cm above and

3 cm below the point of maximum lateral prominence of

the greater trochanter.5 The plane of the subgluteal space

at this level is therefore parallel to and posterior to the

plane of the quadratus femoris muscle.5 Moreover, as the

ischial tuberosity is slightly caudal and dorsal in position

relative to the greater trochanter, the plane of the subglu-

teal space is also slightly oblique (15208) to the coronalplane at this level.5

The ability to identify a potential space along the proxi-

mal course of the sciatic nerve using ultrasound is unique

and may offer several advantages over existing methods

for SNB. We have found that it is relatively simple to

insert a block needle under ultrasound guidance into the

subgluteal space. This is confirmed by injecting 23 ml of

saline through the needle and observing a distention of the

subgluteal space (i.e. separation of the perimysium of the

Fig 5 Transverse section through the gluteal region at the level of the quadratus femoris muscle showing the subgluteal space and its contents(reproduced with permission from www.aic.cuhk.edu.hk/usgraweb).

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gluteus maximus and quadratus femoris muscle, on ultra-

sound imaging). Local anaesthetic injected into the sub-

gluteal space readily spreads to and around the sciatic

nerve. It is also easy to pass a catheter into the subgluteal

space when a continuous SNB is planned (unpublished

data). In theory, the subgluteal injection should also block

the posterior cutaneous nerve of the thigh, which is an

advantage when anaesthesia over the posterior aspect of

the thigh is warranted. There are no major blood vessels in

the subgluteal space and thus vascular complications

related to the SNB may also be reduced. Currently, we use

nerve stimulation in conjunction with ultrasound imaging

for SNB at the subgluteal space, but we envisage that,

with experience, it may be possible to perform an

ultrasound-guided SNB at the subgluteal space without

using nerve stimulation. This will be particularly useful in

young children as the majority of SNB in this age group

is performed under general anaesthesia when the use of a

neuromuscular blocking agent makes nerve stimulation

impractical.

In conclusion, the subgluteal space, where the sciatic

nerve is located, is a well-defined anatomical space. It can

be identified using ultrasound at the level of the greater

trochanter and ischial tuberosity as a hypoechoic area

between the perimysium of the gluteus maximus and the

quadratus femoris muscles. Local anesthetic injected into

the subgluteal space under ultrasound guidance is effective

in producing SNB. Considering the potential advantages

of performing SNB at the subgluteal space, future studies

should compare this technique with other proximal

approaches to the sciatic nerve.

AcknowledgementThe authors would like to thank Sonosite Inc., USA for providing equip-ment support.

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