Technical communication: anatomy and clinical implications of ultrasound-guided selective femoral nerve block.

In this study we evaluated the anatomic basis and clinical findings of ultrasound-guided femoral nerve block performed close to the distal apex of the femoral triangle. Cadaver studies were conducted in 9 thighs of fresh bodies within 24 hours postmortem. In all cases, during injection of 10 mL of blue dye, the skin proximal to the injection site was compressed to prevent the proximal flow. In the first thigh, from the area just distal to the inguinal ligament, an epidural catheter was advanced distally beneath the fascia iliaca over the femoral nerve. In the remaining cases, 10 mL of blue dye was injected into the femoral nerve at the level of the proximal adductor canal and dye spread was evaluated after local dissection. The clinical study was conducted in 20 patients with severe varus deformities. Ten milliliters of 0.75% ropivacaine was injected as in the cadaveric series. The femoral nerve was successfully dyed in all cases of the cadaver study, whereas the muscular branch to the sartorius muscle and quadriceps muscle, with the exception of the vastus medialis muscle, evaded dyeing. All 20 patients with varus knee deformities reported analgesia; none of them experienced motor block. We conclude that local anesthetic injection at the site where the superficial femoral artery has passed beneath the medial border of the sartorius muscle (8 to 12 cm distal to the inguinal crease), combined with efforts taken to prevent proximal flow may anesthetize the sensation of the anterior-to medial aspect of the knee and motor branch of the vastus medialis muscle, without blocking the sartorius or quadriceps muscles.