ORIGINAL COMMUNICATION

Revisiting the Surface Anatomy of the SciaticNerve in the Gluteal Region

STEPHEN S. CURRIN,1 S. ALI MIRJALILI,2 GRANT MEIKLE,1 AND MARK D. STRINGER3*1Department of Radiology, Dunedin Hospital, Dunedin, New Zealand

2Department of Anatomy with Radiology, University of Auckland, Auckland, New Zealand3Department of Paediatric Surgery, Christchurch Hospital, Christchurch, New Zealand

The surface anatomy of the sciatic nerve (SN) in the gluteal region is clinicallyimportant (e.g., intramuscular injection). Anatomy texts describe the nerve inrelation to the posterior superior iliac spine (PSIS), ischial tuberosity (IT), andgreater trochanter (GT) but descriptions are inconsistent. The surface anatomyof the SN was determined in relation to these bony landmarks using computedtomography (CT) scans in living adults. One hundred consecutive adult pelvicCT scans (36 females, mean age 76 years) were available for dual consensusanalysis. A further 19 adults (9 females, mean age 74 years) underwent pelvicCT scans in both prone and supine positions. The surface projection of the SNalong a line between the PSIS and IT and between the IT and GT was meas-ured. The SN was identified in 95% of scans at a mean of 5.261.0 cm fromthe PSIS and 11.461.1 cm from the IT. The SN was a mean of 5.860.8 cmfrom the IT and 6.261.0 cm from the GT. There were no significant differencesin mean positions of the nerve between sides and sexes. A small but clinicallyirrelevant difference in the surface marking of the SN was found betweensupine and prone positions with respect to the GT and IT but not in relation tothe PSIS and IT. In living adults, the SN lies approximately one-third of theway along a line between the PSIS and IT and half way between the GT and IT.Clin. Anat. 00:000–000, 2014. VC 2014 Wiley Periodicals, Inc.

Key words: surface anatomy; sciatic nerve; intramuscular injection; computertomography

INTRODUCTION

The sciatic nerve (SN) is the largest nerve in thebody with a diameter of �2 cm. It is formed by theventral rami of the fourth lumbar to third sacral spinalnerve roots in the sacral plexus on the anterior sur-face of piriformis. The nerve exits the pelvis via thegreater sciatic foramen, usually below piriformis, anddescends between the greater trochanter (GT) andischial tuberosity (IT) down the posterior aspect of thethigh, dividing into tibial and common fibular nervesat a variable level proximal to the knee. The SN sup-plies the hamstring muscles in the thigh and all themuscles (and most of the cutaneous sensation) belowthe knee (Standring, 2008).

An accurate understanding of the surface anatomyof the SN is imperative for safe intramuscular (IM)injection (Small, 2004) and effective SN block (Raj

et al., 1975; Franco, 2007; Robards et al., 2009). It isalso important in percutaneous drainage of deep pel-vic abscesses, even when performed under computedtomography (CT) guidance. However, contemporarydescriptions of the surface anatomy of the SN varybetween and within anatomical reference texts (Haleet al., 2010) (Table 1). A common description of thesurface anatomy of the SN is as follows: (i) from apoint midway between the posterior superior iliac

*Correspondence to: Professor Mark D. Stringer, Department ofPaediatric Surgery, Christchurch Hospital, Riccarton Avenue,Christchurch, New Zealand.E-mail: mark.stringer@cdhb.health.nz

Received 10 June 2014; Accepted 9 July 2014

Published online in Wiley Online Library (wileyonlinelibrary.com).DOI: 10.1002/ca.22449

VVC 2014 Wiley Periodicals, Inc.

Clinical Anatomy 00:00–00 (2014)

spine (PSIS) and the IT, curving laterally and inferiorlyand passing through (ii) a point midway between theIT and GT before (iii) descending in the middle of theposterior aspect of the thigh to the apex of the popli-teal fossa (Standring, 2008; Ellis and Mahadevan,2010). The description of the second of these land-marks dates back to Elements of Descriptive and Prac-tical Anatomy in 1828 (Quain, 1828) and waspresumably based on dissection findings in cadavers;the first landmark was not described in this text.

The surface anatomy of the SN in the gluteal regionhas not previously been investigated using cross-sectional imaging in living adults. The aim of thisstudy was to provide evidence-based surface mark-ings of the SN in the gluteal region.

MATERIALS AND METHODS

Consecutive supine adult CT scans of the pelviswere obtained from Dunedin Hospital and Otago Radi-ology’s imaging archive. Scans were performed usingSiemens SOMATOM Sensation 64 (Dunedin Hospital)and Siemens SOMATOM Emotion 16 (Otago Radiol-ogy) slice scanners, with slice thickness 0.6 mm (Sie-mens AG, Erlangen, Germany). Scans from DunedinHospital were performed in the supine position only,while each patient from Otago Radiology was scannedin both prone and supine positions. Scans wereincluded if the SN and the following bony landmarkswere clearly visible: PSIS, GT, and IT. Scans wereexcluded if there was a space-occupying lesion dis-torting the course of the SN or evidence of previouship joint surgery or pelvic fracture. After exclusions,100 supine scans and 19 paired prone and supine

scans were available for analysis. Supine scans fromDunedin Hospital were all enhanced with iodinatedintravenous contrast, whilst paired supine and pronescans from Otago Radiology scans were unenhanced,according to the local CT colonography protocol.

Scans were assessed by a process of dual consen-sus reporting (SC and AM) (Murphy et al., 2010) usingOsiriX imaging software (OsiriX program version 3.0,2011). Age and sex were recorded but ethnicity andbody mass index were not available. The most super-ficial points of the PSIS, IT, and GT were identified andmarked. The center of the SN was marked near itsorigin, anterior to piriformis at the lateral aspect ofthe greater sciatic foramen (Fig. 1A), and in the sub-gluteal region (Fig. 1B). Since the lines between PSISand IT and GT and IT do not lie exactly within axial,coronal, or sagittal planes but are slightly oblique, theprojected surface marking of the SN in a coronalimage (equivalent to the clinical surface marking ofthe nerve) is slightly different to the actual distance ofthe nerve from each bony landmark (Fig. 2). Bothsets of measurements (projected and actual distancesof the nerve from each bony landmark) were obtainedfor all supine scans (n5100) from Dunedin Hospital.Only projected distances of the nerve from the land-marks were obtained for scans (n519) from OtagoRadiology.

Actual Distances

Coronal and sagittal multiplanar reconstructionswere created using the axial CT slices on which the SNand bony landmarks had been identified. A noveloblique plane was then constructed between the PSISand IT and the actual distance of the center of the SN

TABLE 1. Textbook Descriptions of the Surface Markings of the Sciatic Nerve

Reference Surface markings

Gray’s Anatomy (Standring, 2008) “. . .a point midway between the posterior superior iliac spineand ischial tuberosity, curves outwards and downwardsthrough a point midway between the greater trochanterand the ischial tuberosity. . .”

Last’s Anatomy (9th edition) (McMinn, 1998)a “. . .at a point one third of the way up from the ischial tuber-osity to the posterior superior iliac spine. . .In the buttockit lies under cover of gluteus maximus midway betweenthe greater trochanter and the ischial tuberosity.”

Last’s Anatomy (12th edition) (Sinnatamby, 2011) “. . . from midpoint between ischial tuberosity and greatertrochanter. . .”

Clinical Anatomy (Ellis and Mahadevan, 2010) “. . .a line which commences at a point midway between theposterior superior iliac spine and the ischial tuberosity,curves outwards and downwards through a point midwaybetween the greater trochanter and ischial tuberosity andthen continues vertically downwards in the midline of theposterior aspect of the thigh.”

Surface Anatomy (Lumley, 2008) “midpoint of a line joining the posterior superior iliac spineto the ischial tuberosity” and “the midpoint or middle thirdof a line joining the ischial tuberosity to the upper greatertrochanter”

Clinically Oriented Anatomy (Moore et al., 2010) ” a point midway between the greater trochanter and ischialtuberosity.”

Anatomy for Diagnostic Imaging (Ryan et al., 2011) None

aThe anatomy text currently recommended by the Royal Australasian College of Surgeons.

2 Currin et al.

from each point was measured. Similarly, a novelplane was constructed between the GT and IT and theactual distance of the SN measured from each ofthese points.

Projected Distances

A 3-dimensional volume rendered image was cre-ated from axial CT slices on which the SN and bonylandmarks had been identified. The volume renderedimage was then oriented in the coronal plane so thatthe projected surface marking of the center of the SNin relation to the bony landmarks could be measured(Fig. 3).

Statistical Analysis

Data were analyzed using SPSS Statistics version20.0.0 (SPSS, Chicago, IL). Associations with age wereanalyzed using Pearson’s correlation, left and rightsides and, prone and supine positions were comparedusing a paired-samples t-test and sex differences wereanalyzed using an unpaired t-test. Intra-observerrepeatability was determined by repeating consensusmeasurements in a randomly selected sample of15 (15%) supine scans after an interval of at least4 weeks, with the observers (SC and AM) blindedto the original observations. Intraclass correlationcoefficients (ICC) were calculated and graded accord-

ing to the criteria of Landis and Koch (1977): <05poor repeatability or reproducibility; 0.00–0.205slightagreement; 0.21–0.405 fair; 0.41–0.605moderate;0.61–0.805substantial; and 0.81–1.00 almost perfect(Landis and Koch, 1977).

Ethical Approval

This study received local University ethics commit-tee approval (ref: 00909).

Fig. 2. Stacked sagittal CT scans (71-year-old male)illustrating the difference between the actual distancebetween the PSIS and IT (a) and the projected surfacemarkings of these bony points (b). (PSIS5posteriorsuperior iliac spine; IT5 ischial tuberosity).

Fig. 3. A 3-D volume rendered reconstruction dem-onstrating the SN in relation to key palpable bony land-marks. The projected distance between the GTand centerof the SN is shown in this image.

Fig. 1. Axial CT scan (0.6-mm slice) showing theposition of the SNs bilaterally (green dot) within thegreater sciatic foramina (A) and between the femur andischium (B).

Surface Anatomy of the Sciatic Nerve in the Gluteal Region 3

RESULTS

The SN was identified in 95% of supine scans fromDunedin hospital (n5100) (between PSIS and IT in97% of scans and between GT and IT in 92%). Themean age of the cohort was 76 years (range 43–91),36 (36%) were females and 64 (64%) males. The SNwas identified in all paired prone and supine scansfrom Otago Radiology (n519). The mean age of thecohort was 74 years (range 38–86), 9 (47%) werefemales and 10 (53%) males.

Actual Distances

In supine scans from Dunedin hospital (n5100),the SN was formed by contributing nerve roots at amean distance of 0.760.6 cm (range 0–2.3 cm) lat-eral to the line between the PSIS and IT. Mean distan-ces of the nerve from the PSIS and IT and from theGT and IT are shown in Table 2. The SN was a meanof 4064% (range 30–50%) of the distance along aline between the PSIS and IT; it always crossed theupper half of this line and crossed the upper third in86% of cases. The SN was a mean of 5065% (range40–70%) of the distance between the GT and IT.

Projected Distances

In supine scans from Dunedin hospital (n5100),the mean distances of the SN from the PSIS and ITand from the GT and IT on surface projection areshown in Table 3. The average location was 3065%(range 20–50%) of the way along the line betweenthe PSIS and IT; the nerve crossed the upper third ofthis line in 66% of scans and the middle third in 34%.The SN was a mean of 5065% (range 40–70%) ofthe distance between the GT and IT. In all scans it waslocated in the middle third of the line between the GTand IT.

No significant differences were found between rightand left sides (P>0.05) or with age (r50.1;P50.13).

For paired prone and supine scans from OtagoRadiology (n519), the mean distance of the SN fromthe PSIS and IT and from the GT and IT on surfaceprojections are shown in Table 4. The SN was a meanof 3064% (range 30–40%) and 40610% (range30–90%) of the distance between the PSIS and IT forsupine and prone scans, respectively; there was nosignificant difference between prone and supine scans(P50.7). The SN was a mean of 5065% (range 50–

70%) and 6065% (range 40–60%) of the distancebetween the GT and IT for supine and prone scans,respectively; there was a small but clinically irrelevantdifference between prone and supine scans (6%,P<0.05).

Reliability

ICCs for continuous data (actual and projected dis-tances) showed moderate to almost perfect repeat-ability with values ranging from 0.45 to 0.98.

DISCUSSION

CT is a useful tool for imaging the SN in the glutealregion (Lanzieri and Hilal, 1984). This is the first studyto map the course of the SN in vivo using CT andshows that contemporary descriptions of the surfaceanatomy of the SN in the gluteal region (Table 1)need to be slightly revised. Both the actual and clini-cally applicable surface projectional measurementsindicate that the course of the SN is best described ascrossing a point approximately one-third of the wayalong a line between the PSIS and the IT, curving lat-erally and inferiorly to a point midway between the GTand IT.

While no previous study has specifically investi-gated the course of the SN in the gluteal region inrelation to local bony landmarks in vivo, several clini-cal studies concerning SN block support the distal sur-face marking of the nerve midway between the GT

TABLE 2. Actual Distances of the Sciatic Nervefrom Bony Landmarks in the Gluteal Region(n5100 Supine CT Scans)

Mean distance (cm) Range (cm)

PSIS-ITFrom PSIS 6.960.9 4.5–9.5From IT 11.361.0 8.6–14.0

GT-ITFrom GT 6.461.0 3.6–8.4From IT 5.760.8 3.2–8.3

TABLE 3. Projected Surface Distances of theSciatic Nerve from Bony Landmarks in the GlutealRegion (n5100 Supine CT Scans)

Mean distance (cm) Range (cm)

PSIS-ITFrom PSIS 5.261.0 2.8–7.7From IT 11.461.1 5.2–13.8

GT-ITFrom GT 6.261.0 3.8–8.6From IT 5.860.8 3.5–7.7

TABLE 4. Projected Surface Distances of the Sci-atic Nerve from Bony Landmarks in the GlutealRegion (n519 Paired Prone and Supine CT Scans)

Mean distance (cm) Range (cm)

PSIS-ITSupine

From IT 5.761.0 4.0–7.5From PSIS 10.960.7 9.5–12.1

ProneFrom IT 4.860.7 3.5–6.5From PSIS 10.160.9 8.2–12.0

GT-ITSupine

From IT 5.560.7 4.4–7.0From GT 4.260.6 2.8–5.1

ProneFrom IT 5.960.6 4.2–6.8From GT 5.060.5 4.0–6.0

4 Currin et al.

and IT (Raj et al., 1975; Franco, 2007; Robards et al.,2009). In contrast, a single cadaver dissection studyof 18 SNs concluded that the SN was more medial,with the authors reporting that the midpoint of thenerve was located 2.860.4 cm lateral to the midpointof the IT and 4.4 cm medial to the medial aspect ofthe GT (Wadhwa et al., 2010). The authors’ use offresh cadavers dissected in the prone position maypartly explain some of the differences between theirresults and our findings but our analysis of pairedprone and supine scans showed that the relativeposition of the SN changes little between these twopositions. A more important difference between ourstudy and that of Wadhwa et al. (2010) is the pre-cise bony reference points for measurement; wechose the most superficial parts of the GT and IT asthese are more likely to represent clinically palpablelandmarks.

Minor degrees of external or internal rotation of thehip were not excluded in our study, which is a poten-tial limitation. While the SN has been shown to movewithin the sciatic notch by up to 1 cm during straightleg raise (Goddard and Reid, 1965), the effect ofminor degrees of hip rotation on the position of thenerve is unknown.

Accurate surface anatomy of the SN is clinicallyimportant for two main reasons: to reduce the risk ofiatrogenic injury during invasive procedures and tooptimize the efficacy of SN block. The size and courseof the SN in the pelvis render it vulnerable to iatro-genic injury, which may have devastating consequen-ces including paralysis and severe pain (Horowitz,1984; Kline et al., 1998; Small, 2004). A misplacedgluteal IM injection is a well-recognized cause of iatro-genic nerve injury (Small, 2004; Mishra and Stringer,2010). Although guidelines in the UK, USA, and Aus-tralia recommend that the buttock should not be usedfor IM immunizations, the gluteal region still remainsa popular IM injection site worldwide for drug adminis-tration, not only in developing countries but alsowithin advanced healthcare systems (Mishra andStringer, 2010). An accurate understanding of thecourse of the SN is essential if the dorsogluteal regionis selected for IM injection. Two methods have tradi-tionally been used. In the first, the buttock is dividedinto quadrants by a vertical line extending from thehighest point of the iliac crest and a horizontal linelocated midway between the IT and highest point ofthe iliac crest; the upper outer quadrant of the buttockis then selected for IM injection (Small, 2004; Tunstalland Shah, 2012). The second method describes aninjection site above and lateral to the midpointbetween the PSIS and GT (Craven and Hirnle, 1996).The fact that SN injuries continue to occur (Mishraand Stringer, 2010; Moore et al., 2012) suggests thatthese guidelines are not followed (Cornwall, 2011)and/or that the SN is closer to these “safe” injectionsites than previously considered. Results from ourstudy support the latter (Fig. 4).

The surface markings of the nerve are importantwhen performing a SN block for below knee surgery.Posterior approaches to SN block largely rely onstandard descriptions of the surface anatomy of theSN as it exits the gluteal region into the thigh (Raj

Fig. 4. A: Traditional surface anatomy of the SNin the gluteal region adapted from Gray’s Anatomy(2008) and (B) the surface anatomy of the nerve basedon this study of living adults. The SN is located signifi-cantly higher within the gluteal region than previouslyindicated.

Surface Anatomy of the Sciatic Nerve in the Gluteal Region 5

et al., 1975; Franco, 2007; Robards et al., 2009).Thus, parasacral approaches use the PSIS and IT aslandmarks for guiding the needle insertion point (Mor-ris et al., 1997; Franco, 2007).

Finally, another clinical intervention posing a risk ofiatrogenic injury to the SN is percutaneous transglu-teal CT-guided drainage of a pelvic abscess. Thisinvolves guiding a needle through the medial part ofthe greater sciatic foramen, immediately lateral to thesacrum. Knowledge of the surface anatomy of the SNis important when planning a safe and effective percu-taneous puncture even though the nerve can be iden-tified by the imaging technique (Butch et al., 1986;Harisinghani et al., 2002; Harisinghani et al., 2003).Moreover, CT guidance is not always available in lessprivileged healthcare systems and greater reliancemust then be placed on accurate surface anatomy.

CONCLUSION

Using cross-sectional imaging in living adults, theSN was found to be located on average about one-third of the way along a line from the PSIS to the ITand about midway between the GT and IT. The proxi-mal segment of the nerve is significantly more cepha-lad than described in most anatomy texts. Robustlandmarks for the surface anatomy of the SN shouldafford clinicians greater confidence when performingpercutaneous gluteal interventions.

ACKNOWLEDGMENT

The authors thank Robbie McPhee, Medical IllustratorandGraphic Artist, for assistancewith Figures 4a and4b.

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