Femoral nerve block: Needle insertion at the inguinal crease results in more consistent nerve localization

Introduction: A commonly suggested approach to femoral nerve block (FNB) involves insertion of the block needle 1-2 cm lateral to the femoral artery (FA) just below the inguinal ligament (IL), regardless of the technique (nerve stimulator, paresthesia, double click, arterial pulsation, infiltration, etc.).[l] It usually requires multiple attempts at localization of the femoral nerve, yielding inconsistent success rate.[2] Since performing the block at the level of the inguinal skin crease (1C) has given us more consistent results in our practice, we undertook an anatomic study of the inguinal fossa in order to explain these discrepancies. Methods: After the skin and fat of the femoral triangle were removed to the level of the fascia lata, 18 G needles were used to simulate performance of the FNB at two levels: IL and 1C. The needles were inserted: (A) just lateral to the FA and (B) 2 cm lateral to the FA at both levels. Upon completion of the anatomical dissections, the number of needle-nerve contacts, and measurements of anatomical relationships among the relevant structures were recorded. Table 1. The incidence of needle-nerve contacts after simulation of FNB at various inguinal levels and distances from the FA. Level of insertion Needle insertion Inguinal Inguinal Crease At the lateral border of FA 11.8% 70.6% 20 mm from the FA 23.5% 0 Legend: FNB=Femoral Nerve Block, FA=Femoral Artery Table 2. Relationship of theFN and FA to surrounding structures at the IL and 1C levels.______________________ Inguinal level Distance (mm) Crease Ligament p-value mean (±sD)mean (±SD) FA - fascia lata 3.2±1.5 19.7± 6.4 <0.01 FN-fascia lata 6.8± 2.7 26.4± 7.0 <0.01 FN width___________14.0± 2.0 9.8± 2.0_____O.01 Legend: FN=Femoral Nerve, FA=Femoral Artery, IL=Inguinal Ligament, IC=Inguinal Crease Results: Total of 17 inguinal triangles in 9 cadavers (4 female and 5 male) of average body frame and nutrition were dissected. The incidence of successful needle-nerve contacts during simulation of the FNB through each of the four combinations of inguinal level and needle distance from the FA is demonstrated in Table 1. Measurements of the anatomical relationship among structures of relevance to FNB are presented in Table 2. Discussion: The results of these experiments demonstrate that the optimal site of needle insertion when performing the FNB, is the inguinal crease level, and not the frequently advised 1-2 cm below the inguinal ligament. Additional advantages of performing the block at the inguinal skin crease level are: more superficial position of the femoral artery and nerve, greater width of the femoral nerve, and more consistent femoral nerve-artery relationship.