CHAPTER 26 Femoral nerve block
Muscle biopsy; skin-graft donor site; patellar fracture fixation or wiring; combined with other techniques for saphenofemoral vein ligation; hip fracture repair and hip and knee replacement; above- and below-knee amputation; knee arthroscopy; repair of fractured shaft of femur; ankle and foot surgery.
Hemorrhagic diathesis; anticoagulation treatment; local neural injury; situations where a dense sensory block could mask the onset of lower extremity compartment syndrome (e.g. fresh fractures of the tibia and fibula, or especially traumatic and extensive elective orthopedic procedures of the tibia and fibula); and distorted anatomy due to previous surgery or trauma (e.g. prosthetic femoral artery graft).
The femoral nerve arises from the ventral rami of the second, third, and fourth lumbar nerves. It descends through the substance of the psoas major, emerging from the muscle at the lower part of its lateral border, and passes down between it and the iliacus muscle, deep to the fascia iliaca (Fig. 26.1). It then passes behind the inguinal ligament to enter the thigh. The nerve lies deep to the fascia lata and fascia iliaca. The fascia iliacus separates it from the vascular bundle containing the femoral artery and vein (Fig. 26.2). It divides into two major branches (anterior and posterior) early in the proximal anterior thigh (Fig. 26.1).
Figure 26.1 Cadaver structures illustrating anatomy relevant to femoral nerve block technique. 1: Anterior superior iliac spine; 2: pubic tubercle; 3: inguinal ligament with sectioned abdominal muscles; 4: sartorius muscle; 5: iliopsoas muscle; 6: femoral nerve; 7: femoral artery; 8: femoral vein.
The anterior branch provides cutaneous innervation to the skin overlying the anterior surface of the thigh and provides motor innervation to the sartorius muscle. The posterior branch provides innervation to the quadriceps muscle and the knee joint and gives rise to the saphenous nerve, which innervates the medial side of the leg below the knee.
The main landmarks for femoral nerve block are the anterior superior iliac spine, the pubic tubercle, inguinal ligament, inguinal crease, and femoral artery (Fig. 26.3). The pubic tubercle can be palpated three fingers’ breadth from the midline, along the upper border of the pubis. The inguinal ligament is outlined by a line connecting the anterior superior iliac spine and the pubic tubercle.
Figure 26.3 Landmarks for the femoral nerve block. The anterior superior iliac spine, pubic tubercle, and inguinal ligament are outlined. The femoral artery is identified at the level of the inguinal crease.
The femoral artery lies approximately at the intersection of the medial third and lateral two-thirds of the inguinal ligament (the midinguinal point). The femoral nerve is found lateral to the femoral artery (NAVL: nerve, artery, vein, and ligament as you go toward the midline). The inguinal crease is a skin fold 3–6 cm below and parallel to the inguinal ligament. Here the artery lies at its most superficial and where relations are usually constant. Below this point, the nerve begins to disappear behind the artery.
The femoral nerve is found lateral to the femoral artery in the groin, lying outside the femoral sheath and beneath the fascia lata and iliaca. These fascial layers are seen as hyperechoic lines. Its appearance on ultrasound (Fig. 26.4) is similar to many peripheral nerves, and described as multiple round or oval hypoechoic (dark) areas encircled by hyperechoic (bright) rims. The hypoechoic structures are the nerve fascicles. The femoral nerve may be identified on ultrasound as an oval (95% oval beneath the inguinal ligament) or triangular-shaped structure measuring approximately 3 mm in anteroposterior diameter and 10 mm in mediolateral diameter. Correct transducer angulation is essential for adequate nerve visualization. Femoral nerve anisotropy occurs with transducer angulation of as little as 10° from the vertical. Anisotropy is a property of muscles, nerves, and tendons which relates to the change in ultrasound appearance of the target structure with the scanning angle used. Nerves are best visualized when the ultrasound beam hits them at 90°. In this case, the femoral nerve becomes isoechoic with surrounding structures, and disappears from view, when the transducer is angled 10–13° from the vertical.
Figure 26.4 Transverse ultrasound image of the femoral nerve lateral to the femoral artery and femoral vein, using an 8-14 MHz linear ultrasound transducer. FA: femoral artery; FV: femoral vein; FN: femoral nerve.
To identify the femoral nerve, the transducer is placed over the femoral vessels on the anterior thigh at 90° to the expected orientation of the nerve and vessels (Fig. 26.5). The vessels may be identified as large round hypoechoic structures, and blood flow may be demonstrated using colorflow Doppler (Fig. 26.6). The femoral nerve is then visualized a variable distance lateral to the femoral artery.
As for all regional anesthetic procedures, after checking that emergency equipment is complete and in working order, intravenous access, ECG, pulse oximetry, and blood pressure monitoring are established. Asepsis is observed.
The patient is placed in the supine position with the operator standing on the side to be blocked, at the level of the patient’s thigh. The needle insertion point is infiltrated with local anesthetic using a 25-G needle. A 50-mm 21-G insulated needle is oriented in a 45° cephalad and posterior orientation lateral to the palpated femoral artery at the inguinal crease (Fig. 26.7). The stimulating current is set at 1.0 mA, 2 Hz, and 0.1 ms.