Ultrasound Imaging Accurately Identifies the Lateral Femoral Cutaneous Nerve

Abstract

BACKGROUND: 

Anesthesia of the lateral femoral cutaneous nerve (LFCN) is useful in surgery involving the anterolateral thigh. We investigated the accuracy of ultrasound compared with anatomical landmarks in identifying the LFCN in human cadavers and volunteers.

METHODS: 

Twenty cadavers were examined. A needle was inserted targeting the LFCN with ultrasound guidance and green dye was injected. A second needle was inserted using anatomical landmarks. The LFCN was identified by dissection, and coloring of the LFCN and needle positions were evaluated. A volunteer study with 10 individuals was performed. Transdermal nerve stimulation was used to identify the LFCN bilaterally. Its position was compared with marked positions identified in advance using ultrasound and anatomical landmarks.

RESULTS: 

Sixteen of 19 needles inserted under ultrasound guidance in the cadavers were in contact with the LFCN. The median horizontal distance from the needle tip to the nerve was 0.0 mm (interquartile range [IQR], 0.0-0.0 mm). Only 1 of 19 needles inserted using anatomical landmarks was in contact with the LFCN. The median horizontal distance from the needle tip to the nerve was 18.0 mm (IQR, 11.0–23.0 mm). Sixteen of 20 marked positions made using ultrasound guidance corresponded to the identified LFCN in volunteers. The median horizontal distance from the pen-mark to the LFCN was 0.0 mm (IQR, 0.0-0.0 mm). None of the 20 marked positions made with anatomical landmarks corresponded to the LFCN. The median horizontal distance from the pen-mark to the LFCN was 15.0 mm (IQR, 10.8–20.0 mm).

CONCLUSIONS: 

Identification of the LFCN by ultrasound is technically feasible and more accurate than anatomical landmarks.

In Brief

IMPLICATIONS: This study compared the efficacy of ultrasound with anatomical landmarks in identifying the lateral femoral cutaneous nerve in cadavers and volunteers. We found identification of the nerve with ultrasound is feasible and more accurate than anatomic landmarks.

In recent years, there has been growing interest in using ultrasound guidance in regional anesthesia.^1,2 Ultrasound allows visualization of the nerves and the adjacent anatomical structures. Currently, there is a lack of knowledge on the utility of ultrasound in performing the lateral femoral cutaneous nerve (LFCN) block.

Several techniques have been described to block the LFCN, but the efficacy is usually suboptimal^3–5 as anatomical studies have shown a highly variable course of the LFCN in the inguinal region.^6–11 A blind technique, described by Brown and Schulte-Steinberg¹² and by Moore,¹³ is commonly used to block the LFCN. The technique involved injecting local anesthetics in a fan-wise manner through the fascia lata 2–3 cm medial and distal to the anterior superior iliac spine (ASIS). However, the rate of successful anesthesia has only been approximately 40%.⁴ The success rate of the LFCN block can be increased to 85% with the aid of a peripheral nerve stimulator⁴ but this technique is highly dependent on patient factors.

In this study, we investigated the accuracy and the technical feasibility of ultrasound imaging in identifying the LFCN in cadavers and volunteers, compared to the conventional technique of anatomical landmarks.

METHODS

This study was reviewed and approved by the University of British Columbia's (UBCs) ethics committees (IRB), the UBC Anesthesia and Orthopedics residency training committees, and the Vancouver Coastal Health Authority.

Cadaver Study

The embalmed cadavers were donors used within the undergraduate courses at the UBC Division of Anatomy and Cell Biology. The embalming process involved a high-pressure perfusion of 20–30 L of solution containing 72% isopropanol, 20% glycol, 4% formaldehyde, and 4% phenol into the cadavers via an arterial cannulation. The postembalming process involved a perfusion of 5–6 L of 20% Infutrace™, which allowed a noxious-free and safe environment for dissection.

Using the SonoSite MicroMaxx® (SonoSite Inc., Bothell, WA) with a 38-mm broadband (6–13 MHz) linear array transducer, an ultrasound examination of the inguinal region of 20 embalmed cadaveric hips were performed by one investigator (I.N.). The cadavers were positioned supine, and the ultrasound probe was placed on the skin in a transverse plane immediately inferior to the ASIS (Fig. 1). The insertion site of the sartorius muscle to the ASIS was identified. The probe was then swept medially and inferiorly in a sequential fashion below the inguinal ligament until two continuous hyperechoic lines were seen under the subcutaneous tissue (the fascia lata and the fascia iliaca, which were found typically about 0.5–1 cm apart). The LFCN was located in cross-section in the space between the two fascial layers. The course of the nerve was then traced by scanning the structure proximally and distally.

Figure 1.: A volunteer positioned supine with the ultrasound probe placed on the skin in a transverse plane immediately inferior to the anterior superior iliac spine (ASIS). This was the position we used to identify the lateral femoral cutaneous nerve with ultrasound in both cadavers and volunteers. IL = inguinal ligament.

The ultrasound image of the best short-axis view of the LFCN in between the two fascial layers was recorded. A 21-gauge short-bevel needle was then placed under real-time ultrasound guidance targeting the nerve with an out-of-plane approach by the investigator (I.N.) and 0.1 mL of indocyanine green was slowly injected through the needle. The needle was left in place (point A). Time taken for nerve detection was recorded, which was defined as the duration from placement of the ultrasound probe on the skin to visual identification of the LFCN. The horizontal and vertical distance of the nerve to the ASIS and its depth were measured. The horizontal distance was represented by the distance of the point at which the LFCN was identified between the two fascial layers medial to the ASIS. The vertical distance was the distance of the point at which the LFCN was identified between the two fascial layers inferior to the ASIS.

A second needle was then inserted at the conventional anatomical landmark for LFCN (point B, 2.5 cm medial and inferior to the ASIS).^12,13 To avoid potential bias during the dissection, the label of the needle at point A was randomly labeled, using a computer-generated allocation table, either “1” or “2” and the needle at point B was labeled with the alternate number.

Each cadaver was carefully dissected with manual fixation of the needles by an independent blinded investigator (N.H.). The LFCN was anatomically identified (line C) and the horizontal distance from each needle to the identified LFCN (line C) was measured and compared. The needle position at point A or B was defined as accurate if the needle tip was in contact with the LFCN.

Volunteer Study

After informed consent, 10 healthy volunteers were recruited from the UBC Anesthesia and Orthopedics residency training programs. Each subject was positioned supine and was scanned bilaterally at the inguinal region by an investigator (I.N.) using the SonoSite MicroMaxx® (SonoSite Inc., Bothell, WA) with a 38-mm broadband (6–13 MHz) linear array transducer. The LFCN was located by ultrasound in a similar fashion as described above in the cadaver study. Time taken for nerve detection was recorded, defined as the duration from placement of the ultrasound probe on the skin to visual identification of the LFCN. The horizontal and vertical distance of the nerve to the ASIS and the depth of the nerve were measured. This location (point A) was marked on the skin with a pen. A second mark (point B) was then made using the conventional anatomical landmark (2.5 cm medial and inferior to the ASIS). To avoid potential bias during the sensory stimulation, point A was randomly labeled, using a computer-generated allocation table, either “1” or “2” on the skin and point B was labeled as the alternate number. An independent blinded observer (N.H.) then applied a hand-held transdermal nerve stimulator (StimProbe, Life-Tech International, Stafford, TX) just below the inguinal ligament to elicit paresthesia in the distribution of the LFCN. We defined the location of the LFCN to be the point where the greatest paresthesia was elicited on the lateral thigh with an external current of 5 mA (point C). A longitudinal line was drawn containing point C (line C). The distances from point A perpendicular to line C and from point B perpendicular to line C were measured and compared. The marked position (point A or B) was defined as accurate if it corresponded to the point with greatest paresthesia (point C).

Study Outcomes

The primary outcome was the accuracy of ultrasound imaging compared with the accuracy of conventional anatomical landmark in identifying the LFCN, whose location was confirmed by anatomical dissection in cadavers and identified by transdermal nerve stimulation in volunteers. Accuracy was defined as the proportion of correct needle positions in cadavers and correct pen-marked positions in volunteers.

Three secondary outcomes were recorded: the difference in the horizontal distance between position found by each technique (points A and B) and the position of the LFCN (line C) based on anatomic dissection (cadaver) or transdermal nerve stimulation (volunteer); the time to identify the LFCN with ultrasound imaging, which was defined as the duration from placement of the ultrasound probe on the skin to visual identification of the LFCN; and description of the anatomical variations of the LFCN. The normal anatomical relationships of the LFCN were defined according to Moore.¹³

Statistical Analysis

No sample size calculation for this study was possible, as no description of an ultrasound-guided approach to the LFCN could be found in the published literature. Instead, a sample of 20 LFCNs was considered sufficient to generate estimates that would be useful for future sample size calculations.

Because of the nature of this pilot study, only descriptive statistics were used. Discrete data were described using proportions and percentages with their 95% confidence intervals (CIs). Normally distributed continuous data were described using means and their 95% CIs (distances) or means and their standard deviations (demographics); skewed continuous data were described using medians and their interquartile ranges (IQRs).

RESULTS

Cadaver Study

Of the 20 examined cadaveric hips, the LFCN could not be identified by dissection in one. Data from this case were excluded. The LFCNs were identified by ultrasound in the remaining 19 cadaveric hips. The nerve was identified in a transverse view near the ASIS. It was consistently positioned between the fascia lata and fascia iliaca medial to the insertion of the sartorius muscle. The LFCN appeared on ultrasound as a discrete hyperechoic, round or elliptical or lip-shaped fibrillar structure, as shown in Figure 2. It was protected by a common bed of connective tissues and fat in between the two fascial layers. The nerve emerged through the fascia lata and traveled distally in the thigh. It was important to maintain light probe pressure on the skin during scanning to avoid complete collapse of the connective tissue space between the fascia lata and the fascia iliaca. Good quality images were obtained in all cases by dynamically selecting the best section of the nerve, where the two fascial layers were clearly seen with the LFCN lying in between. On average, the best transverse view of the ultrasound image of the LFCN was obtained at 20.6 mm (95% CI, 15.2–26.1 mm) medial to and 78.4 mm (95% CI, 69.7–87.0 mm) inferior to the ASIS. The average depth of the nerve from skin was 6.0 mm (95% CI, 4.8–7.2 mm). The median time taken to identify the LFCN in cadavers using ultrasound imaging was 280 s (IQR, 170–338 s).

Figure 2.: Ultrasound image of the lateral femoral cutaneous nerve lying within the connective tissues between the fascia lata and fascia iliaca, medial to the sartorius muscle. The LFCN appeared as a hyperechoic elliptical or lip-shaped fibrillar structure on ultrasound. FI = fascia iliaca; FL = fascia lata; LFCN = lateral femoral cutaneous nerve; SM = sartorius muscle; CT = connective tissues.

Sixteen of 19 needles inserted with ultrasound guidance were in contact with the LFCNs, which were all stained green. In one case, the needle tip was found 2 mm away from the LFCN, but the distance was sufficiently close to enable the injected dye to color the nerve green. In the remaining two cases, the LFCNs were 3 and 12 mm away from the needle tips, respectively; both nerves were missed by the injected dye. Thus, the accuracy of needle placement and nerve coloring using ultrasound technique was 84.2% (95% CI, 67.8%–100%) and 89.5% (95% CI, 75.7%–100%), respectively.

Only 1 of 19 needles inserted using the anatomical landmark (point B) was found to be in contact with the LFCN, giving an accuracy of 5.3% (95% CI, 0.0%–15.3%). The median distance from point B to the LFCN was 18.0 mm (IQR, 11.0–23.0 mm). Figure 3 shows the positions of point A, point B and the anatomical courses of the LFCN found under dissection in the cadavers.

Figure 3.: Diagram showing the positions of the point located by anatomical landmark technique (open square); the points located with ultrasound guidance (open circles) and the anatomical courses of the lateral femoral cutaneous nerve found under dissection in cadavers (black diamonds and lines). ASIS = anterior superior iliac spine; IL = inguinal ligament.

Volunteer Study

All 10 subjects (6 male, 4 female; age 33.3 ± 6.9 yrs; height 174.5 ± 9.9 cm; weight 63.2 ± 12.0 kg) completed the study successfully. Twenty LFCNs were identified using both ultrasound imaging and transdermal nerve stimulation technique. Good quality ultrasound images were obtained in all cases, where the two fascial layers were clearly seen with the LFCN lying in between, similar to the cadaveric study. On average, the best transverse view of the ultrasound image of the LFCN was identified at 14.1 mm (95% CI, 10.8–17.4 mm) medial to and 50.8 mm (95% CI, 47.0–54.5 mm) inferior to the ASIS. The average depth of the nerve from skin was 6.1 mm (95% CI, 5.4–6.9 mm). The median time taken to identify the nerve with ultrasound was 22.5 s (IQR, 5–75 s).

Sixteen of 20 marked positions identified using ultrasound imaging corresponded to the LFCN position identified by the StimProbe (accuracy 80%; 95% CI, 62.5%–97.5%). The median perpendicular distance between point A and line C was 0.0 mm (IQR, 0.0–0.0 mm). On the other hand, none of the 20 pen-marked positions identified using the anatomical landmark corresponded to the LFCN position identified by the StimProbe (accuracy 0%; 95% CI, 0%–13.9%). The median perpendicular distance between point B and line C was 15.0 mm (IQR, 10.8–20.0 mm).

DISCUSSION

Our study suggested greater accuracy in identifying the LFCN in both cadavers and volunteers using the ultrasound technique compared to using anatomical landmarks. The needles inserted in cadavers and the marked positions made on volunteers using the ultrasound technique were considerably closer to the identified LFCN compared to those using the anatomical landmark. Although the LFCN was small sized, its distinguishing sonoanatomic position and echotexture made it relatively easy to be identified. Using the two fascial layers as the initial sonographic landmark to locate the LFCN was very useful. By tracing the course of the nerve proximally and distally, we could confirm that the hyperechoic round or lip-shaped structure embedded in the connective tissues between the two fascial layers was the LFCN. The nerve was lying in a very superficial layer, therefore the use of a high frequency ultrasound probe was necessary to delineate the neural structures accurately. The accuracy of locating the LFCN compared with the blind anatomical technique was enhanced with the use of the ultrasound.

Ultrasound detection of the LFCN did not require a substantial amount of time. In healthy volunteers, we identified the LFCN in less than 2 min. We required more time to identify the LFCN in cadavers due to the changes in the skin surface and subcutaneous or muscular texture that occurred with embalming; however, even the longest time (338 s) was not unduly prolonged. During the embalming process, a significant amount of fluid was infused under high pressure into the cadavers, making the skin and soft tissues thicker and stiffer.

Our results also confirm the highly variable anatomical course of the LFCN, which has been noted with previous anatomical studies.^6–11 These observations may explain, in part, the moderate success rate (40%–60%) and the slow onset of the LFCN block when the three-in-one block technique is used.^14–16 The fascia iliaca compartment block attempts to address the anatomical variability by relying on a blind field approach to anesthetize the LFCN. Although its success rate is high, possibly because of the diffusion of the local anesthetics through the fascial layer, its speed of onset is still slow.⁵ In contrast, ultrasound overcomes the anatomical variability of the LFCN by allowing us to visually trace the LFCN from the ASIS down to the inguinal region, to identify the best possible transverse view of the nerve, to visually guide the needle to the target nerve, and to visually confirm the deposition of local anesthetic around the target nerve. The accuracy of ultrasound in identifying the LFCN is encouraging from a clinical perspective. We hypothesize that ultrasound guidance may enable us to block the LFCN quickly and accurately, making this technique useful for surgical procedures of the hip or lateral thigh where selective blockade, with or without a femoral nerve block, is desirable. Since we did not actually perform blocks, the potential for ultrasound to facilitate LFCN block requires clinical confirmation.

Our study was performed on volunteers with normal or low body weight; therefore, our observations might not apply to patients with larger amounts of soft tissues. Another potential limitation of the study was the possibility of needle movement during cadaveric dissection. Maintaining a fixed needle position during cadaveric dissection may not be completely reliable and error can occur during measurement of particularly small distances. However, the investigator who performed the dissection was a qualified surgeon and was very meticulous with the manual needle fixation technique. Moreover, the investigator was blinded with the labeling of the needles and therefore this risk would have affected all needles equally.

In summary, this investigation describes a high-resolution, ultrasound-guided approach to the LFCN and its technical feasibility in localizing the nerve with good quality images. The accuracy of the technique in identifying the LFCN was higher than the accuracy from conventional anatomical landmarks. The time required to identify the nerve was minimal and acceptable for clinical practice. Further studies are required in order to prove an improved LFCN block efficacy with the ultrasound technique in the clinical setting.

ACKNOWLEDGMENTS

We thank Professor Ruth Milner for her assistance with the data analysis, the volunteers from the UBC Departments of Anesthesiology, Pharmacology and Therapeutics and Orthopedics, and the UBC Division of Anatomy and Cell Biology for providing the laboratory technicians and the cadavers.