Age is a predictive factor in the femoral nerve positioning:
an anatomical ultrasound study
Manabu Yoshimura1 · Toshiyuki Nakanishi1 · Seishi Sakamoto1 · Takashi Toriumi
Precise identification of the femoral nerve (FN) is essential for an ultrasound -guided femoral nerve block. We hypothesized that the distance between the FN and the femoral artery (FA) is correlated with patient age. In this prospective observa-tional study, we evaluated the FN–FA (from the lateral edge of the FA to the medial edge of the FN) distance [0.42 ± 0.42 (mean ± standard deviation) cm] in 102 patients using ultrasound. In addition, we calculated the cross-sectional area of the iliopsoas muscle using computed tomography or magnetic resonance imaging. Multiple regression analyses revealed that age was significantly and positively correlated with the FN–FA distance (R2 = 0.72, p < 0.001) and that this correlation was greater than that between height, weight, or gender and the FN–FA distance. Further, the cross-sectional area of the iliopsoas muscle per weight was significantly correlated with age (R 2 = 0.54, p < 0.001) and the FN–FA distance (R2 = 0.50, p < 0.001). These findings may help refine the ultrasound techniques used for the femoral nerve block.
Keywords Femoral nerve block · Sonoanatomy · Ultrasound
Femoral nerve block (FNB) is a useful nerve block technique with a significant clinical application for surgical anesthesia . Precise identification of the femoral nerve (FN) is par-ticularly important for an ultrasound-guided FNB.
Based on our experience, the FN is often misinterpreted as the femoral branch of the genitofemoral nerve on the ultrasound image of elderly patients undergoing the ultra-sound-guided FNB. The preconceived notion that nerves are positioned immediately lateral to the arteries can be attribut-able for such misinterpretations. Chin et al.  have reported
- case of a 25-year-old female whose ultrasound-guided FNB failed because her FN was located adjacent to the pos-terolateral aspect of the femoral artery (FA; i.e., her FN and FA were excessively close to each other). On the basis of this report suggesting the proximity between the FN and the FA in young people, we hypothesized a correlation between age and the FN–FA distance, an association between the FN–FA distance and age-related physiological change in the iliopsoas muscle. Although no report has demonstrated that
1\ Department of Anesthesiology, Japan Community Healthcare Organization, Tokuyama Central Hospital, 1‑1 Koda‑cho, Shunan, Yamaguchi 745‑8522, Japan
the thickness of the iliopsoas muscle alters the correlation between the FN–NA distance, the thickness of the skeletal muscle mass has been reported to decrease with increasing age [3, 4].
In this study, we investigated the FN–FA distance at the inguinal crease level using ultrasonography, and evaluated the cross-sectional area of the iliopsoas muscle using pelvic computed tomography (CT) or magnetic resonance imaging (MRI).
This prospective, single-arm, observational study was reported according to the STrengthening the Reporting of OBservational Studies in Epidemiology (STROBE) consen-sus. This study was approved by the Institutional Review Board (K175, K235-20160203: Japan Community Health-care Organization, Tokuyama Central Hospital, Japan) and was registered in the University Hospital Medical Informa-tion Network Clinical Trials Registry (UMIN 000020695). Oral informed consent was obtained from each patients. The institutional ethics committee granted oral informed consent.
We recruited surgical patients undergoing pelvic imag-ing tests, regardless of the type of surgery between Febru-ary and March 2016. Participants who withheld consent, were preoperatively bedridden, or were obese (body mass index > 30 kg/m2) were excluded from the study. Finally, 102 patients who underwent preoperative diagnostic pelvic CT
Fig. 1 a Correlation between patient age and the femoral nerve– artery distance. b Correlation between patient age and the cross-sec-tional area of the iliopsoas muscle per weight. c Correlation between
the cross-sectional area of the iliopsoas muscle per weight and the femoral nerve–artery distance. The line represents regression line with 95% confidence interval
or MRI were included in this study. In the operating room, the ultrasound examination was performed on patients in the supine position, and their lower extremity was rotated 45°laterally . The high-frequency (5–10 MHz range) linear ultrasound probe (SonoSite Edge™, FUJIFILM Son-oSite, Inc., Bothell, WA) was placed at the femoral crease in the transverse position to obtain a short-axis view of the FN. The distance between the lateral side of the FA and the medial side of the FN was measured on ultrasonographic image. Another examiner, who was blinded to the measure-ment of the FN–FA distance, measured the cross-sectional area of the iliopsoas muscle from the anterior superior iliac spine to the pubic symphysis using pelvic CT or MRI by picture archiving and communication system (ShadeQuest/ ViewR, Yokogawa Medical Solutions Corporation, Tokyo, Japan).
Patient demographic factors, such as height, weight, age, and gender, were analyzed, and their positive correlations with the FN–FA distance were assessed using multiple regression analyses. In addition, the associations among the FN–FA distance, the cross-sectional area of the iliopsoas muscle, and age were analyzed using Pearson’s correlation coefficient. Data are expressed as the mean ± standard devia-tion for continuous variables.
A power analysis of the Pearson’s correlation coefficient suggested the availability of 80% power with 84 individuals to detect differences in the mean with a 0.05 significance level (two tailed); the effect size was 0.3.
Statistical analyses were performed using the Statistical Package for Social Sciences (SPSS for Windows, version 21; IBM, Chicago, IL). A p value of < 0.05 was considered statistically significant.
Using ultrasound, the FN–FA distance was measured in 102 patients (62 females and 40 males). There was no patient
|Fascia lata||Fascia lata|
|Iliopsoas muscle||Iliopsoas muscle|
|Femoral sheath||Femoral sheath|
|a younger patient||an older patient|
Fig. 2 Ultrasonographic measurement of the distance between the edge of the femoral nerve and artery and the structures around the femoral nerve . The distance in the left image (0.05 cm; a 31-year-old patient) is less than that in the right panel (0.88 cm; a 73-year-old patient). The femoral nerve is enveloped by two layers of fascia iliaca,
whereas femoral vessels are contained in the femoral sheath made up of the fascia lata. White arrows indicates the distance between the femoral nerve and artery. Black arrows indicates shrinking with age. FV femoral vein, FA femoral artery, FN femoral nerve, IM iliopsoas muscle
who refused an agreement. The values of patients’ demo-graphic factors were as follows: height 159 ± 9 cm; weight 56 ± 10 kg; and age 63 ± 16 years. All patients were inde-pendent in their daily activities. No enrolled patient was a professional athlete, and no co-existing diseases prevented them from walking. The FN–FA distance was 0.42 ± 0.42 (range − 0.46–1.43) cm, and the mean cross-sectional area of the iliopsoas muscle was 631 ± 246 (range 175–1271) mm2. Multiple regression analyses revealed that age (β = 0.82, p < 0.001) was significantly correlated with the FN–FA dis-tance (R2 = 0.72, p < 0.001) (Fig. 1a) and that this correlation was greater than that between height (β = − 0.06, p = 0.54), weight (β = 0.02, p = 0.83), or gender (β = − 0.59, p = 0.53) and the FN–FA distance. Moreover, the cross-sectional area of the iliopsoas muscle per weight was significantly cor-related with age (R2 = 0.54, p < 0.001) (Fig. 1b), and the FN–FA distance (R2 = 0.50, p < 0.001) (Fig. 1c).
This is the first report that has shown the correlation between age and the FN–FA distance at the inguinal crease.
In our opinion, age-related physiological changes in the iliopsoas muscle on which the FN is located may be responsible for increase in the FN–FA distance with increase in age. Precisely, the FN is covered with the fascia iliaca (Fig. 2). The distance may become larger as the ili-opsoas muscle ages, because the muscular lacuna, a com-partment through which the FN and the iliopsoas muscle
pass, is separated from the femoral vessels. There are no specific reports of iliopsoas muscle age- related physi-ological change; however, increasing the age decreases the thickness of skeletal muscle mass [3, 4 ]. Therefore, we think aging of the iliopsoas muscle may correlate the FN–FA distance due to changes in anatomical structure.
Farag et al.  have suggested that ultrasound guidance alone (without using either a stimulating needle or needle/ catheter combination) is the best approach for the femoral perineural catheter placement. Thus, precise identification of the FN is imperative for ultrasound-guided nerve block, particularly catheter insertion.
Several studies have investigated the location of FN [5, 7, 8]. Vloka et al.  have reported that the FN was more superficial at the level of the inguinal crease than at that of the inguinal ligament. Muhly et al.  have reported an ultrasound evaluation of the anatomy of the vessels with respect to the FN where it intersects the femoral crease. They have reported the FN–FA distance to be 11.1 ± 2.9 mm. However, they have not examined the rela-tionship between the FN–FA distance and the age.
Hsu et al.  have reported that the distance between the center of the FA and the medial edge of the FN at 45° lateral rotation of both lower extremities was 1.05 ± 0.17 cm. In addition, they have reported positive correlation between the body weight and the FN–FA
distance. However, we excluded obese patients to avoid the effect of body weight in this study because the ingui-nal crease in obese patients is located more distally than that in normal-weight patients, and the FN–FA distance is commonly closed in more proximal regions. Thus, their measurement is different from what we measured in this study. Besides, they did not suggest an age correlation, and did not consider any potential confounding factors.
This study has several limitations. The study is not appli-cable for obese patients. It is also not applicable for underage patients and physically disabled ones. The observed patients did not receive lower extremity surgeries and FNB, and they had a pelvic or abdominal illness, which might affect FN location and iliopsoas muscle mass. The patient’s popula-tion apparently consist of two groups, aged (> 60 years) and younger (< 56 years) ones.
In conclusion, this study highlights a positive correla-tion between age and the FN–FA distance. Overall, these findings may help refine and improve ultrasound techniques used for the FNB.
Compliance with ethical standards
Conflict of interest Authors declare that they have no conflict of inter-est.
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