Age is a predictive factor in the femoral nerve positioning: an anatomical ultrasound study.

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 observational 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 (R2  = 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.