The influence of patient position on ultrasound examination of the sciatic nerve in the popliteal fossa: A cross‐sectional study

Abstract

Introduction/Purpose

Ultrasound‐guided popliteal fossa sciatic nerve (PFSN) blocks are performed with patients in the supine, lateral or prone position. No known studies compare the quality of images obtained from each approach. This study examines the quality of supine and prone PFSN ultrasound images.

Methods

Thirty‐eight adult volunteers were sorted into two groups. Five regional anaesthesiologists performed ultrasound examinations of the PFSN on volunteers in supine and prone positions. Popliteal fossa sciatic nerve image quality was analysed with grayscale techniques and peer evaluation. Popliteal fossa sciatic nerve depth, distance from the popliteal crease and time until optimal imaging were recorded.

Results

The grayscale ratio of the PFSN vs. the background was 1.83 (supine) and 1.75 (prone) (P = 0.034). Similarly, the grayscale ratio of the PFSN vs. the immediately adjacent area was 1.65 (supine) and 1.55 (prone) (P = 0.004). Mean depth of the PFSN was 1.6 cm (supine) and 1.7 cm (prone) (P = 0.009). Average distance from the popliteal crease to the PFSN was 5.9 cm (supine) and 6.6 cm (prone) (P = 0.02). Mean time to acquire optimal imaging was 36 s (supine) and 47 s (prone) (P = 0.002). Observers preferred supine positioning 53.8%, prone positioning 22.5% and no preference 23.7% of the time. Observers with strong preferences preferred supine imaging in 70.9% of cases.

Conclusions

Supine ultrasound examination offered quicker identification of the PFSN, in a more superficial location, closer to the popliteal crease and with enhanced contrast to surrounding tissue, correlating with observer preferences for supine positioning. These results may influence ultrasound‐guided PFSN block success rates, especially in difficult‐to‐image patients.

Introduction

Approaches to the ultrasound‐guided sciatic nerve block in the popliteal fossa (popliteal block) include the lateral, medial and posterior approach. ¹ , ² , ³ , ⁴ , ⁵ , ⁶ The popliteal block may be performed with patients in either the prone (Figure 1a), lateral or supine (Figure 1b) position. The decision to utilise a specific approach and/or patient position is often based on provider preference and experience. In some patients, circumstances dictate that one or two of the approaches are not feasible. These limitations may be due to (1) the presence of surgical dressings or external fixators overlaying a particular insertion site; (2) difficulty modifying a patient's position from supine to prone; and (3) the presence of scar tissue, vascular structures or infected tissue in the direct pathway of the needle.

Figure 1.

Patient position for ultrasound examination. (a) Prone position for ultrasound examination. (b) Supine position for ultrasound examination. Red arrow is probe force vector, green arrow is gravity force vector. PC, popliteal crease; SN, sciatic nerve; SB, sciatic nerve bifurcation.

While different leg positions in the prone position have been compared, ⁷ no studies to date have examined the influence of patient positioning (supine vs. prone) on the ultrasound visibility of the sciatic nerve in the popliteal fossa. This study compares (1) the quality of sciatic nerve sono‐visualisation obtained in the popliteal fossa when the volunteers are supine or prone; (2) the depth from skin to the sciatic nerve proximal to the bifurcation when volunteers are supine or prone; (3) the distance from the popliteal crease to the point of optimal sciatic nerve imaging in the supine position or prone position, and the circumference of the thigh at those specific locations; and (4) the time required to obtain optimal sono‐visualisation of the sciatic nerve in the popliteal fossa in supine or prone positioning.

Based on the first author's clinical experience, it was hypothesised that supine positioning would offer faster, more accurate identification of the sciatic nerve in the popliteal fossa.

The aim of this study was to investigate the effect of patient positioning on ultrasound visualisation of the sciatic nerve in the popliteal fossa.

Methods

With approval by the University of Maryland, Baltimore (UMB) Institutional Review Board (IRB) (HP‐00049640), thirty‐eight (38) American Society of Anesthesiologists (ASA) physical status 1 and 2 adult (age > 17 years old) volunteers were recruited for the study. Written informed consent was obtained from all subjects and is on file with the University of Maryland Institutional Review Board. Demographic information was collected and included: age in years, weight in pounds, height in inches and sex. Ultrasound examinations were performed with a SonoSite X‐Porte platform (Fujifilm SonoSite Inc., Bothell, WA, USA) with a high‐frequency (xx to xx MHz) linear 50 mm X‐Porte probe. Inclusion criteria included the following: ASA physical status 1 and 2, adult (age ≥ 18 years old), volunteers. Exclusion criteria allowed exclusion of the following: ASA physical status 3 or higher, age < 18 years, anyone with altered mental status, obtundation or other conditions that hindered mental capacity to comprehend the study and volunteer to participate. Volunteer demographic data and de‐identified ultrasound images and study data were stored in password‐protected institutional servers.

The volunteers were sorted by coin flip (heads or tails) to determine the sequence in which their sciatic nerve in the popliteal fossa would be scanned. Group 1 (heads) was scanned in the supine position on each leg followed by scanning in the prone position, while Group 2 (tails) was scanned in the prone position followed by scanning in the supine position (Group 1: left leg supine, right leg supine, left leg prone, right leg prone; Group 2: left leg prone, right leg prone, left leg supine, right leg supine). Supine examinations were performed in the straight leg position with the assistance of a surgical leg rest (Figure 1b).

Five regional anaesthesiologists, each with >5 years of experience, performed the ultrasound examinations. Each examination started with identification of the popliteal artery in the popliteal crease. The ultrasound probe was then moved cranially to identify the sciatic nerve bifurcation into the tibial and common peroneal nerves (Figure 1a,b) and then moved further cranially until the sciatic nerve was identified proximal to the bifurcation (Figures 1a,b, 2a). Ultrasound images were saved demonstrating the popliteal artery in the popliteal crease, the nerve bifurcation and the sciatic nerve. The time required to obtain optimal imaging (primary end point) was recorded from the time the ultrasound probe contacted the volunteer until the best image of the sciatic nerve was obtained (Figure 2a,b).

Figure 2.

Popliteal fossa ultrasound image of sciatic nerve. (a) Unlabelled popliteal sciatic nerve scan. (b) Popliteal sciatic nerve outlined in white.

Identifying marks were placed on the volunteers' skin to identify the site of the popliteal artery in the popliteal crease (Figure 1a: PC, 1B: PC), bifurcation of the sciatic nerve (Figure 1a: SB, 1B: SB) and sciatic nerve proximal to the bifurcation (Figure 1a: SN, 1B: SN). The following secondary end points were measured and recorded: the distance between the popliteal crease and bifurcation point; distance between the popliteal crease and sciatic nerve; and thigh circumference at the level of the sciatic nerve marking. The same procedure and measurements were repeated for each of the volunteers' legs, in the prone and supine positions with the order determined by their randomisation grouping.

Statistical analysis

Ultrasound images were analysed with a grayscale technique and subjective peer evaluation. In order to achieve a statistical power level of 80 to detect an effect size (Cohen's d of 1) at a significance level of 0.05 for a two‐tailed hypothesis, a minimum total sample size of 34 was required.

Grayscale technique: (1) De‐identified sciatic nerve images, without supine or prone labelling, were identified and outlined in each image by a senior regional anaesthesiologist (PB) (Figure 2a,b). The area enclosed within the outline was graded digitally (Adobe Photoshop software) to produce a mean grayscale value of the nerve (Figure 3b). A grayscale value was also produced for the entire remaining portion of the ultrasound image, termed the background grayscale value (Figure 3c). The ratio between the nerve grayscale value and background grayscale value was computed for all supine and prone images. (2) A grayscale comparison was also performed between the sciatic nerve and the immediate adjacent structures. This was carried out by expanding the sciatic nerve outline by 50 pixels in an outward direction (Figure 3a) to create a virtual border shown in red (Figure 3c) and a virtual background (Figure 3d). The ratio between the nerve grayscale value and the virtual background grayscale value was computed for all supine and prone images.

Figure 3.

Popliteal sciatic nerve outlined, background and virtual border in red. (a) Outline of sciatic nerve in the popliteal fossa. (b) Isolated sciatic nerve for grey scale analysis. (c) Virtual border of the sciatic nerve in red. (d) Virtual background of the sciatic nerve.

Subjective peer evaluations: Subjective peer evaluations of all images were performed by 56 observers (CA1, CA2, CA3 anaesthesiology residents) from the University of Maryland and University of Rochester and five regional anaesthesia‐attending physicians from the University of Maryland. A composite pairing was made for each volunteer's ultrasound images, consisting of supine and prone images from the same leg. These composites were randomised for laterality, patient position and order of scanning (Group 1 or Group 2), and all observers were blinded. These 76 (38 volunteers, each with prone and supine ultrasound examinations of each leg) paired images of the sciatic nerve (Figure 4) were evaluated by all observers. Observers were asked to indicate which image presented a more easily identified sciatic nerve or whether there was no preference. To provide validity to the subjective peer evaluations and estimate test–retest reproducibility, one‐sixth (13) randomly selected composite pairings were repeated. Observers were blinded to the repeating of pairings.

Figure 4.

Paired images of the popliteal sciatic nerve in the supine and prone positions of the same extremity.

A ‘strong preference’ was designated if 80–100% of observers preferred one image, while ‘some preference’ was designated when 60–80% of observers preferred one image over another. Less than 60% was designated as ‘no preference’. The 13 randomly repeated composite pairings were used to estimate the reproducibility of preference and to test for ‘learning’ or a ‘fatigue’ effect. The R ² from a linear regression was used to quantify the degree of concordance, and the slope of the regression line was tested against the null hypothesis of a slope of 1. Proportions are given with exact 95% confidence limits in parentheses. The Mann–Whitney and the Kruskal–Wallis tests were used as non‐parametric tests for comparing the number of cases with an expressed preference when comparing two or more groups of observers, respectively.

The depth from the skin to the sciatic nerve, distance from the popliteal crease to the sciatic nerve and the time required to obtain the most optimal ultrasound image were recorded. The data acquired from these five analytics (grayscale of nerve vs. background, grayscale of nerve vs. virtual background, distance from skin to nerve, distance from popliteal crease to sciatic nerve and scanning time) were analysed using a two‐tail Student's t‐test (Microsoft Excel software). The correlation (Pearson's K correlation) between body mass index (BMI) and time to acquisition of the sciatic nerve for all volunteers was calculated using the Statistical Package for Social Sciences.

Ethics approval

This study was approved by the University of Maryland, Baltimore (UMB) Institutional Review Board (IRB) (HP‐00049640) for compliance with all ethical standards for this research.

Results

Demographics: 19 women and 19 men completed the study, with two volunteers scanned twice by different providers for a total of 40 left leg and 40 right leg scans (160 images). The average age was 32 years (23–55). The average BMI of study volunteers was 23.3 (standard deviation (SD) 4.7) with range of 21.2–33.6. There was no correlation between the BMI of the volunteers and the time to acquire the image of the sciatic nerve (correlation coefficient − 0.18) with a p value = 0.11.

The ratio of the grayscale of the sciatic nerve to the grayscale of the background was 1.83 [SD 0.36] and 1.75 [SD 0.33] for supine and prone, respectively (p = 0.034) (Table 1). Similarly, the ratio of the grayscale of the sciatic nerve to the grayscale of the immediately adjacent structures was 1.65 [SD 0.22] and 1.55 [SD 0.26] for supine and prone, respectively (p = 0.004) (Table 1). The mean distance from the skin to the sciatic nerve was 1.6 [SD 0.45] cm for supine and 1.7 [SD 0.45] cm for prone (p = 0.009) (Table 1). The distance between the popliteal crease and sciatic nerve was 5.9 [SD 1.95] cm in the supine position and 6.6 [SD 2.19] cm in the prone position (p = 0.02) (Table 1). The measured circumference of the thigh (cm) at the location of the sciatic nerve optimal image capture is shown in Table 1. The time required to obtain the best image of the sciatic nerve was 36 [25.3] seconds for supine and 47 [SD 30.3] seconds for prone (p = 0.002) (Table 1).

Table 1.

Measurements of time to acquire best sciatic nerve image and other secondary end points

	Thigh circumference (cm)		Distance from popliteal crease to sciatic nerve (cm)		Depth of sciatic nerve from skin (cm)		Time to acquire best image of sciatic nerve (s)		Grayscale of nerve vs. background		Grayscale of nerve vs. immediate background
	Prone	Supine	Prone	Supine	Prone	Supine	Prone	Supine	Prone	Supine	Prone	Supine
Mean	42	42	6.6	5.9	1.7	1.6	47	36	1.75	1.83	1.55	1.65
Standard Deviation	4.12	4.36	2.19	1.95	0.45	0.45	30.3	25.3	0.33	0.36	0.26	0.22
Observations	76		76		80		80		80		80
Pearson's Correlation	0.81		0.32		0.51		0.40		0.55		0.26
Hypothesised Mean Difference	0		0		0		0		0		0
Df	75		75		79		79		79		79
t Stat	0.80		−2.40		2.69		3.15		−2.16		−2.92
p (T < =t) one‐tail	0.21		0.009		0.004		0.001		0.02		0.002
t Critical one‐tail	1.67		1.67		1.66		1.66		1.66		1.66
p (T < =t) two‐tail	0.42		0.02		0.009		0.002		0.034		0.004
t Critical two‐tail	1.99		1.99		1.99		1.99		1.99		1.99

Abbreviations: cm, centimetres; sec, seconds.

Means for thigh circumference, distance from popliteal crease to sciatic nerve, depth from the skin to the sciatic nerve, time to acquire best image of the sciatic nerve, grayscale of the nerve vs. the background and grayscale of the nerve vs. the immediate background.

Observers reported cumulative preferences (strong and some preference) in 61 of 80 (76.25%) composite pair images. Supine imaging was preferred in 53.8% of cases (95% CI 42.2–65.0%), while prone positioning was preferred in 22.5% of cases (95% CI 13.9–33.25%), and 23.7% reported no preference (Table 2). If we restrict the analysis to the 55 cases where there was a strong preference for one position over the other, 70.9% (95% CI 57.1–82.4%) of our observer panellists preferred the supine ultrasound image.

Table 2.

Proportion of blinded observers who preferred prone vs. supine positioning for popliteal sciatic nerve ultrasound imaging

Prefers prone		No preference	Prefers supine
Strong	Some		Some	Strong
20.0%	2.5%	23.7%	48.8%	5.0%

Proportion of blinded observers who preferred prone vs. supine positioning for popliteal sciatic nerve ultrasound examination.

There was no significant difference in observer preferences when comparing experienced and inexperienced observers (Mann–Whitney test, p = 0.79). Experienced observers were defined as those individuals who had performed 10 or more popliteal sciatic nerve blocks prior to study participation. Likewise, there was no significant difference in the median number of cases with a preference between attending physicians and resident physicians (CA‐1, 2 or 3) (p = 0.44).

When assessing the 13 blinded repeat cases, the correlation between observers indicating a preference for supine position on the first and second attempts was R ² = 0.90. An additional model was tested with regression through the origin (i.e. a no‐intercept model), and the R ² was 0.98. The slope of the regression line between the original and repeat assessment was 1.12 (p = 0.31, standard error 0.11), showing no strength of preference change.

DISCUSSION

The data presented support the ease and feasibility of performing supine popliteal sciatic nerve blocks compared with prone patient positioning. Identification of the sciatic nerve with patients in the supine position was accomplished quicker (36 s vs. 47 s), more superficially (1.6 cm vs. 1.7 cm in depth), closer to the popliteal crease (5.9 cm vs. 6.6 cm), with greater contrasting appearance to the remaining ultrasound background image (grayscale ratio P‐value 0.034), with greater contrasting appearance to immediate adjacent tissues (grayscale ratio p value 0.004) and with a significant observer preference.

The first description of the ultrasound‐guided sciatic nerve block in the popliteal fossa was described by Sites et al. ¹ with a case series of two patients in the prone position. A year later, Minville et al. ³ published a lateral approach to the popliteal block and suggested that a lateral approach was ‘a reliable technique to block the sciatic nerve and is particularly useful when the patient is unable to lie prone’. Shortly after, Gray et al. ⁴ documented their experience performing ultrasound‐guided popliteal blocks using the lateral approach and argued that the ‘lateral approach to popliteal nerve block with ultrasound imaging has several fundamental advantages over other techniques. First, the supine position is the most convenient, if not the only, positioning option in many patients. Second, because the ultrasound transducer is remote from the point of needle entry, no sterile cover is necessary. Third, the desired needle path is parallel to the active face of the transducer, which promotes optimal needle visibility’. Others recorded a nearly identical experience with the lateral approach and similarly argued the benefits of keeping patients supine rather than turning them prone. ⁵ The medial approach was described by Taha and Ahmed ⁶ and involves a small degree of knee flexion, with or without a slight ipsilateral pelvic tilt. Still others suggested to position patients in an oblique or lateral decubitus position allowing one to scan the posterior thigh without entirely proning the patient. ⁸ Gürkan et al. ⁹ compared various leg positions in the prone position and found the ‘figure of four’ position to offer the best visibility of the sciatic nerve. Kim et al. ¹⁰ studied flexion and rotation of the hip on visibility and depth of the sciatic nerve when performing anterior sciatic nerve blockade. However, no prior study has compared the morphologic and sonographic changes that might occur to the sciatic nerve in the popliteal fossa when patients are placed in the supine vs. prone position.

Rationale for the above findings may include force vectors between the ultrasound probe and a patient's popliteal fossa. In the supine position (Figure 1b), the force vector (gravity) from the patient's leg is directed downwards into the ultrasound probe as the probes force vector pushes upwards. This is likely to cause more significant compression of the superficial tissues and decrease the depth of the sciatic nerve from the skin. In the prone position, both leg and ultrasound probe force vectors are directed downwards, which may negate some of the compressive effects and make the sciatic nerve appear deeper (Figure 1a). Likewise, tissue compression in supine examinations may allow for quicker and qualitatively better identification of the sciatic nerve compared with imaging in the prone position. This could offer an explanation to the preference demonstrated for the supine images by observers.

There are several limitations to this study. Measurement bias cannot be ruled out in the determination of the distance between the popliteal crease and the sciatic nerve. This is secondary to difficulties marking the popliteal crease in the supine position, as well as differences in skin tension between supine and prone positioning. Pressure applied by the operator to the probe upon image acquisition was not standardised nor measured. Differences in applied pressure could have influenced the measured distance from the skin to the sciatic nerve favouring one position over another. However, the validity of these findings may still have relevance as operators remained consistent when scanning a particular patient in the supine and prone positions and were instructed to mimic their clinical practice. More force may naturally be applied to the probe to obtain the optimal image of the sciatic nerve when patients are supine and will thereby demonstrate a shorter measured distance from skin to the sciatic nerve in the clinical setting as well. While all five participating attending regional anaesthesiologists were experienced in both supine and prone approaches to the sciatic nerve in the popliteal fossa, a preference bias for one approach is possible. Inherit familiarity and preference for one approach over another may lead to quicker, improved image acquisition. Data are not available to indicate each provider's preferences for patient positioning; however, anecdotal evidence indicates that supine popliteal sciatic nerve blocks are performed more commonly at this institution than in the prone position.

Despite the advantages of supine positioning found in this study, it must be realised that obtaining the ultrasound image is only a component of the nerve block procedure. Supine positioning may prove more challenging for inexperienced providers as the proceduralist has to concurrently apply upward pressure to compress the popliteal fossa, while advancing the nerve block needle. The authors utilise a sitting‐proceduralist technique with the arm resting on the bed while manoeuvring the ultrasound probe to minimise user fatigue.

CONCLUSION

This study supports the ease and feasibility of obtaining supine popliteal sciatic ultrasound images over prone patient positioning. Supine positioning offered quicker identification of the sciatic nerve, in a more superficial location, closer to the popliteal crease, and with enhanced contrast to surrounding tissue that correlated with independent observer preferences compared with the prone patient positioning. With encouraging results from this volunteer study, further studies are required to evaluate differences in block success and efficacy based on various patient positions.

Authorship statement

RES and PEB were involved in the conception, planning and design of the study; acquisition, analysis and interpretation of data; drafting and revising content; approval of final version for publication; and are accountable for all aspects of the accuracy and integrity of the study. ND was involved in the interpretation of data, drafting and revising content, approval of final version for publication and is accountable for all aspects of the accuracy and integrity of the study. SMB was involved in the statistical analysis and interpretation of the data, drafting and revising of the results section, approval of final version for publication and is accountable for all aspects of the accuracy and integrity of the study. JWS was involved in the planning and design of the study, completion of the IRB application, design of the protocol, approval of final version for publication and is accountable for all aspects of the accuracy and integrity of the study. AKG and EJH were involved in volunteer recruitment, recording of the data, preparation of the data for observer survey, analysis of the data, drafting of the manuscript, approval of final version for publication and are accountable for all aspects of the accuracy and integrity of the study.

Funding

The authors have no sources of funding to declare for this manuscript.

Conflict of interest

Dr. Samet serves as a consultant for Exo Inc., an ultrasound software portal company. No other author has any financial disclosures.

Author contributions

RES and PEB were involved in the conception, planning and design of the study; acquisition, analysis and interpretation of data; drafting and revising content; approval of final version for publication; and are accountable for all aspects of the accuracy and integrity of the study. ND was involved in the interpretation of data, drafting and revising content, approval of final version for publication and is accountable for all aspects of the accuracy and integrity of the study. SMB was involved in the statistical analysis and interpretation of the data, drafting and revising of the results section, approval of final version for publication and is accountable for all aspects of the accuracy and integrity of the study. JWS was involved in the planning and design of the study, completion of the IRB application, design of the protocol, approval of final version for publication and is accountable for all aspects of the accuracy and integrity of the study. AKG and EJH were involved in volunteer recruitment, recording of the data, preparation of the data for observer survey, analysis of the data, drafting of the manuscript, approval of final version for publication and are accountable for all aspects of the accuracy and integrity of the study.