Femoral nerve blockade is widely performed in the setting of perioperative care.1
The nerve is the largest branch of the lumbar plexus and usually consists of the roots of segments L2 to L4. It runs distally to the inguinal region, typically positioned within the two anatomic planes of the ilio-fascial space: a groove formed by the iliac and lateral psoas muscles (GIPM) posteriorly, and covered by the iliac fascia anteriorly (Fig. 1a,b).2–5 Upon arrival in the inguinal region, the femoral nerve divides into anterior (superficial) and posterior (deep) terminal branches (Fig. 1c).3,6 Using ultrasound, the femoral nerve is often visualised distal to the inguinal ligament as a hyperechoic triangular image, deep to the iliac fascia, lateral to the femoral artery and superficial to the iliopsoas muscle.7
The standard approach to blocking the femoral nerve in the inguinal region under ultrasound guidance is to spread local anaesthetic behind the iliac fascia.8,9 With the commonly used out of plane technique, this requires the needle tip to be placed precisely at the lateral side of the femoral nerve, just behind the iliac fascia.10 Because the ultrasound image of the lateral part of the femoral nerve fails to clearly delineate the lateral border, accurate positioning of the needle tip may be difficult.10,11 Another means of achieving the true location of the lateral part of the femoral nerve would involve simply identifying the GIPM and its lateral segment, previously reported to contain the nerve fibres innervating the quadriceps femoris muscle.3,6
In a preliminary study, we used an in-plane technique to place the needle tip at the lateral segment of the GIPM. We found that this allowed positioning of the needle behind the iliac fascia, close to the femoral nerve, but with a reduced risk of impaling the nerve. This led us to design this prospective observational case series. The aim of our study was (main objective) to determine the frequency of ultrasound visibility of the lateral part of the femoral nerve and GIPM, and (secondary objectives) to note the motor response to nerve electrostimulation and to observe the spread of local anaesthetic when positioning the needle tip at the lateral segment of the GIPM.
Materials and methods
The study took place at the Department of Anaesthesiology of the University Hospital of Besancon. It was conducted in accordance with STrengthening the Reporting of OBservational studies in Epidemiology (STROBE) statement (www.strobe-statement.org) and was approved by the Institutional Ethics Committee of the University Hospital of Besancon on 11 March 2009.
All patients gave informed consent, and as the study did not require significant change in our current practice, permission was given to waive written informed consent. American Society of Anesthesiologists physical status classes I to III inpatients undergoing hip or knee surgery using femoral nerve blockade and general anaesthesia were eligible to participate. Patients with clinically significant coagulopathy, infection at the injection site, allergy to local anaesthetic, cardiac conduction disease or psychiatric or neurologic illness were excluded. Upon arrival in a dedicated regional anaesthesia room, an 18-gauge intravenous (i.v.) catheter was inserted into a forearm vein, and standard monitoring was initiated. The patients laid in a recumbent position, with their lower extremity positioned without lateral rotation.
Ultrasound imaging was performed using a high-resolution mono frequency 12 MHz, 4 cm wide linear ultrasound probe (Logiq-e; GE Healthcare, Milwaukee, Wisconsin, USA), a sterile sheath and sterile gel. A scanning depth of 40 mm was selected. All ultrasound scans were performed by the same investigator (B.F.) and analysed in real time by another investigator (A.B. or J.-L.C.). For every patient, the visibility of each structure on ultrasound imaging was rated independently by each investigator using a two-point numerical scale (0, not visible; 1, visible). In case of discrepancy, the image was analysed until a consensus between investigators was obtained. First, the probe was positioned at the inguinal region in the axis of the inguinal crease (Fig. 2a,b), and then was manipulated cephalad and caudal until optimal visualisation of the anatomical planes indicating the ilio-fascial space, the iliac fascia and GIPM, was obtained (Fig. 3a,b). Direct ultrasound identification of the femoral nerve and its lateral part, defined as a triangular hyperechoic image lying on the GIPM behind the iliac fascia, were noted. Our aim was to identify the lateral segment of the GIPM on ultrasound imaging defined as a rupture of the convexity seen on the anterior aspect of iliac and psoas muscles (Fig. 3a,b). We measured the distance between the lateral segment of GIPM, and either the skin or the lateral aspect of the femoral artery (Fig. 3b). The femoral artery branching pattern (common femoral artery or bifurcation) was also noted.
Placement of the needle using ultrasound imaging
After subcutaneous infiltration with 1% lidocaine 2 ml, a 22-gauge insulated 50 mm, short-bevel needle (Stimuplex A; B Braun, Melsungen, Germany) was inserted in-plane and advanced lateral to medial in the transverse plane of the image (Fig. 2b). Care was taken to ensure that both the tip and the entire shaft of the needle remained in the path of the ultrasound beam. The needle was directed towards the femoral nerve until it made contact with the target structure defined as the lateral segment of the GIPM (Fig. 3c).
Femoral nerve electrostimulation
Electrostimulation was then begun, using a nerve stimulator (HNS 12; B-Braun, Melsungen, Germany) connected to the needle, delivering a current intensity of 0.6 mA, 1 Hz frequency and 0.1 ms pulse width. When a clear motor response was seen in any of the quadriceps femoris muscles (rectus femoris, vastus lateralis, intermedius or medialis), without movement of the needle tip, the stimulating threshold was noted.
Local anaesthetic injection
Ropivacaine 0.2% 1 ml was then injected through the needle whilst maintaining this position within the ilio-fascial space, under ultrasound vision. Appropriate spread of the local anaesthetic solution was defined as a clear separation between the ilio-psoas muscle and the iliac fascia, without femoral nerve swelling. If this was not obtained, the needle tip position was gently adjusted until appropriate spread of local anaesthetic solution was seen. When the spread was appropriate, 15 ml 0.2% ropivacaine were then injected incrementally over 3 to 5 min, under direct vision. The distribution pattern of the local anaesthetic solution around the femoral nerve observed at the end of injection was recorded.
All vascular punctures or intraneural femoral nerve injections were noted. At the end of the procedure, general anaesthesia was induced, and surgery was started. As the femoral nerve block was used for postoperative analgesia, no attempt was made to assess the quality of the block clinically. Complete recovery of function in the operated limb, and the occurrence of untoward events including paraesthesia or neurological deficit, was recorded on postoperative day 1, and after 1 month.
Descriptive analysis was performed. Results are given as mean (SD) or median (interquartile) [range], or n (%) as appropriate.
We included 100 patients with a mean age of 57 (23) years and BMI 25.6 (5.6) kg m−2. Thirteen patients had a BMI of more than 30 kg m−2. The sex ratio M/F (%) was 60/40.
When the probe was first positioned at the inguinal region in the axis of the inguinal crease, the femoral artery was already bifurcated in 53% of cases. After dynamic scanning with the ultrasound probe to obtain the best visualisation of the iliac fascia and GIPM, the probe was placed at the level of the common femoral artery, proximally to the femoral artery bifurcation in 98% of the patients. In two patients, femoral artery was already bifurcated in the inguinal region.
An image compatible with the lateral part of the femoral nerve was observed in 91 patients. The GIPM and iliac fascia were clearly seen in 85 and 68 patients, respectively. Distribution of visualisation of the GIPM, iliac fascia and the lateral part of the femoral nerve on ultrasound imaging is given in Table 1. Among the 91 patients in whom the lateral part of the femoral nerve was clearly identified on ultrasound imaging, GIPM was visualised in 80 (88%) of the cases. In the remaining nine (10%) cases in which the lateral part of the femoral nerve was not seen, the GIPM could be visualised in five (55%) patients. In two patients, only iliac fascia was identified, and in the two patients, none of these structures was correctly visualised. In the 85 cases in which GIPM was seen, the median distance between the lateral segment of GIPM, and either the skin or lateral aspect of the femoral artery was 15 (interquartile 8) [range 9 to 31], and 19 (7) [9 to 35] mm, respectively. In all these patients, the needle could be placed at the lateral segment of the GIPM. With neurostimulation, a clear motor response was obtained in all 85 cases with a median stimulating threshold of 0.46 (0.10) [0.3 to 1.2] mA. The motor responses elicited were in the rectus femoris muscle, the vastus lateralis muscle and the vastus medialis muscle in 92.5 and 3% of the cases, respectively. Two motor responses in the sartorius muscle were associated with a contraction of the rectus femoris muscle.
In 15 cases, the lateral segment of GIPM was not visualised. An image compatible with the lateral part of the femoral nerve was seen on ultrasound imaging in 11 patients (Table 1), and the needle tip was directed towards the posterolateral part of the femoral nerve. In two patients, only the iliac fascia was seen, and the needle was directed towards the lateral aspect of the femoral artery and behind the iliac fascia. In two patients, none of these three structures was seen. For these two, the needle was inserted lateral to medial in the direction of the lateral aspect of the femoral artery or its branches under neurostimulation, and the final location of the needle tip was at the point of an appropriate motor response. In all these 15 patients, a motor response in the quadriceps femoris was elicited by neurostimulation, with a median stimulating threshold of 0.54 (0.12) [0.2 to 0.9] mA. The motor responses elicited in the 15 patients were in the rectus femoris muscle, the vastus lateralis muscle, and the vastus medialis muscle in 81, 14 and 5% of the cases, respectively.
In the 85 cases when the GIPM was seen on ultrasound imaging, the local anaesthetic solution was injected through the needle placed at its lateral segment. Local anaesthetic injection resulted in better femoral nerve visualisation due to sonographic contrast enhancement in all cases. Distribution patterns of local anaesthetic below the iliac fascia are represented in Fig. 4a to d and described in Table 2. A distribution around the femoral nerve (‘doughnut sign’) was observed in 5 and 7% of cases whether GIPM was visualised or not, respectively (Fig. 4d). There were no intraneural injections or inadvertent vascular punctures.
Complete recovery of sensory and motor functions was observed in all individuals on postoperative day 1 and at follow up at 1 month.
We have shown that the groove formed by the two iliac and psoas muscles behind the femoral nerve is correctly visualised using ultrasound in 85% of patients. This is more frequent than the visualisation of the iliac fascia in front of the femoral nerve (68%).
The lateral part of the femoral nerve was not seen on ultrasound imaging in 9% of the cases despite the use of dynamic scanning to find the best image. In cases in which the lateral part of femoral nerve was not correctly seen, the GIPM could be visualised in 55% of cases. The GIPM has already been described as an appropriate position to place a catheter.11 Indirect location of nerves via their fascial space has already been described for both the sciatic (subgluteal space) and saphenous nerve (adductor canal).12,13
Although femoral nerve visualisation is somewhat operator dependent,14 this is consistent with previous reports. In some patients, the femoral nerve at the inguinal level was thin and flat, already bifurcated or branched, along with femoral artery division into superficialis and profunda femoris arteries,3,4 and was difficult to visualise using ultrasound imaging.2,7,15 Our results confirmed that ultrasound visibility of the lateral part of the femoral nerve, the GIPM and the iliac fascia was better proximal to femoral artery bifurcation.
Locating the GIPM may help to position the tip of the needle at the appropriate place for femoral nerve blockade, and positioning the needle tip under the iliac fascia has been advocated before.5,8 In our series, however, the femoral nerve and its GIPM were better visualised than the iliac fascia (80 vs. 62%), perhaps because a lateral area, just below the inguinal ligament, was easier to identify. Also, the echogenicity of the fibroconnective structure of the iliac fascia is enhanced, as the surrounding tissues including fascia lata and iliopsoas muscle increase the number of reflective interfaces, resulting in beam attenuation.2,16,17 In addition, the iliac fascia frequently adheres tightly to the anterior surface of the femoral nerve and its visibility is inconstant,18 making the needle insertion between these two structures difficult.11 Finally, a reduced anisotropic effect has been observed on iliopsoas muscle compared with the iliac fascia.2,18
When the needle tip was placed at the lateral segment of the GIPM, electrostimulation elicited a contraction of the quadriceps femoris muscle, and a therapeutic spread of local anaesthetics was observed in all cases. The high rate of quadriceps femoris muscle contractions (100%) associated with the low stimulating threshold observed was consistent with a location of the needle tip close to the lateral part of the femoral nerve (Fig. 3c).3,6,15 Local anaesthetic solution should be seen spreading behind the iliac fascia when performing femoral nerve block with ultrasound guidance.5,8 The different distribution patterns of the local anaesthetic observed in this study at the end of injection behind the iliac fascia were in keeping with this ultimate aim.
The distance between the skin and the lateral segment of GIPM that indirectly reflects femoral nerve depth was in accordance with a previous report of a mean depth of the femoral nerve at the inguinal ligament and the inguinal crease at 2.2 and 1.1 cm, respectively.19
We used electrical stimulation to assess the location of the needle tip close to the lateral part of the femoral nerve. Electrostimulation has a relatively low sensitivity for needle to nerve contact, but it remains useful for identifying the nervous structure close to the needle tip.20,21 Only femoral nerve blocks for analgesia were performed in this present study, and we could not assess either delay of onset or efficacy of femoral nerve blockade.
In conclusion, this sono-anatomic study showed that the GIPM was seen in the majority of patients undergoing ultrasound-guided femoral nerve blockade, even in some cases in which the lateral part of the femoral nerve was not visualised. The lateral segment of GIPM could be a target for location of the needle tip in an in-plane lateral medial approach of femoral nerve blockade, but further studies are required in order to evaluate the efficacy of femoral nerve block with this needle tip position.
Acknowledgements relating to this article
Assistance with the study: the authors would like to acknowledge Mrs E Laurent for the anatomical figures and Mrs E Brennan for her contribution in reading the manuscript.
Financial support and sponsorship: none.
Conflicts of interest: none.
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