10 Inguinal Paravascular Lumbar Plexus Anesthesia (Femoral Nerve Block)
10.1 Anatomical Overview
The femoral nerve arises within the psoas muscle, usually from the anterior divisions of the four large roots L1–L4 but sometimes only from L2–L4, and is the largest nerve of the lumbar plexus (Fig. 10.1). It passes in the fascial space between psoas major and iliacus through the muscular lacuna to the thigh (Fig. 10.2). The iliopectineal fascia separates the muscular lacuna and thus the femoral nerve from the vascular lacuna through which the lymphatic vessels and the femoral artery and vein run. After giving off a few superficial cutaneous branches (anterior cutaneous branches), it lies under the fascia lata and the iliac fascia in the femoral trigone (Woodburne 1983, Hahn et al 1996, Platzer 2014; Fig. 10.3 and Fig. 10.4).
The femoral nerve provides the sensory innervation of the anterior thigh and is involved in the innervation of the hip and knee joints and of the femur. It provides the motor supply to the knee extensors and hip flexors.
10.2 Femoral Nerve Block
The inguinal paravascular lumbar plexus block was described in 1973 by Winnie (Winnie et al 1973) as a so-called 3-in-1 block. However anesthesia of the obturator nerve was not proven and is also not likely given the anatomical situation. The technique is therefore called femoral nerve block.
10.2.1 Needle Approach
Anterior superior iliac spine, pubic tubercle.
The anterior superior iliac spine and the pubic tubercle are marked and joined by a line. This connecting line corresponds to the inguinal ligament. The classical puncture site described is 1 cm below the inguinal ligament and about 1.5 cm lateral to the femoral artery (note: IVAN = from Inside: Vein, Artery, Nerve; Fig. 10.5 and Fig. 10.6).
The patient lies supine and the leg is slightly abducted and externally rotated. In difficult anatomical situations, a flat pad can be placed under the patient′s buttocks in order to show the topography of the inguinal region better.
The landmarks are marked and the femoral artery is palpated. If the artery cannot be palpated, a vascular Doppler probe can be used for orientation.
After skin disinfection and intracutaneous or superficial subcutaneous local anesthesia about 3 cm (Härtel 1916, Moore 1969) below the inguinal ligament (or 1 cm below the inguinal crease) and about 1.5 cm lateral to the artery, the skin is incised with a small lancet. The needle is then advanced cranially and dorsally at an angle of 30° to the skin and parallel to the artery until the tough resistance of the fascia lata is felt. If a short beveled needle is used, the bevel direction should be parallel to the artery. The resistance is overcome by slightly increasing pressure (Fig. 10.9 and Fig. 10.10).
While cautiously advancing the needle further, there is often a second “loss of resistance” when the tip of the needle passes through the iliac fascia (so-called “double-click”). The end of the needle should then be lowered before advancing the needle further proximally parallel to the artery under peripheral nerve stimulation (PNS).
Contractions in the quadriceps femoris muscle and “dancing” of the patella indicate that the tip of the needle is in the correct position in the immediate vicinity of the femoral nerve.
Following negative aspiration, 20 to 40 mL of a medium-acting or long-acting local anesthetic is injected. Digital pressure distal to the needle can promote distribution of the local anesthetic in cranial direction (Fig. 10.11, Fig. 10.12, Fig. 10.13).
If a continuous technique is planned, a flexible 20G catheter is advanced approximately 3 to 5 cm beyond the tip of the needle after injection of the local anesthetic. Before connecting the catheter to a bacterial filter, the catheter should be aspirated again to exclude an intravascular position. Checking the position using ultrasound makes it possible to correct the position by withdrawing the catheter if spread is insufficient in the target region (Fig. 10.14).
Local Anesthetic, Dosages
Initially: 30 mL of a 1% (10 mg/mL) medium-acting local anesthetic (e.g., mepivacaine, prilocaine) or a long-acting local anesthetic (e.g., ropivacaine 0.75% [7.5 mg/mL])
Continuous block: 8 to 10 mL/h ropivacaine 0.2–0.375% (2–3.75 mg/mL)
Combination block with the sciatic nerve: see below
10.2.2 Needle Approach with Ultrasound
Linear transducer: 10 to 12 MHz
Needle: 6 cm
Finding the femoral nerve appears to be difficult for beginners. It is always located in the inguinal region close (lateral) to the femoral artery in a triangle formed by the femoral artery (medially), the iliopsoas muscle (laterally), and the iliac fascia/fascia lata (anteriorly). The femoral nerve has stronger anisotropic behavior than other nerves, making it important to have the correct angle of the transducer (Soong et al 2005). Because they have similar reflective properties, it often cannot be clearly distinguished from the surrounding fatty tissue until after the local anesthetic is injected (Fig. 10.18).
The femoral nerve can be best visualized somewhat proximal to the origin of the deep femoral artery from the femoral artery at the level of the inguinal ligament in the short axis.
It is usually an oval hyperechoic structure lying on the iliopsoas muscle lateral to the femoral artery (Fig. 10.15). Further distally in the region of the origin of the deep femoral artery, the nerve soon begins to divide into its branches, so that even after injection of local anesthetic, the compact structure of a single nerve is no longer seen, but individual branches (Fig. 10.16). The nerve is covered by the fascia lata and iliac fascia. Lymph nodes are visible above the fascia in some patients (Fig. 10.17).