Relationship between ultrasound imaging and eliciting motor response during femoral nerve stimulation

Objective. Nerve stimulator-assisted localization of the femoral nerve is well described; however, direct ultrasound imaging of the femoral nerve branches may be challenging. The purpose of this study was to correlate the evoked motor responses obtained by femoral nerve stimulation and the topographic orientation of the femoral nerve branches during ultrasound examinations of the infrainguinal region. Methods. Eighty-two patients undergoing total knee replacement were enrolled in this study. A 25-mm, 5- to 10-MHz broadband linear array transducer was used to identify the femoral nerve at the inguinal crease. The medial and lateral aspects of the femoral nerve were stimulated under ultrasound imaging. Twenty cadavers were dissected to support our clinical findings. Results. A quadriceps contraction was elicited in 1.2% and 96% of the patients when stimulating the medial and lateral aspects of the femoral nerve, respectively. In contrast, a sartorius muscle contraction was elicited in 94% and 0% when stimulating the medial and lateral aspects of the femoral nerve. Our findings during anatomic dissection revealed that the femoral nerve branch to the quadriceps muscle, when compared with the branch to the sartorius muscle, originated laterally in 95% and medially in 5% of the specimens. Conclusions. When using out-of-plane ultrasound imaging at the inguinal crease, directing the stimulating needle to the lateral half of the femoral nerve may be associated with a higher probability of encountering the motor branch to the quadriceps muscle.