With the use of ultrasound, marking surface landmarks has become less important. However, understanding relevant anatomy remains key to interpreting the sonographic anatomy which is crucial to procedural success.

 

Surface Landmarks

First, palpate the pubic tubercle (PT) in the midline and the anterior superior iliac spine (ASIS) which is found laterally over the hip and anterior to the iliac crest. A straight line drawn between the PT and the ASIS represents the inguinal ligament (IL) which serves as the upper border to the area of interest: the femoral triangle. Immediately inferior to the inguinal ligament is the fold of the inguinal crease (IC) where the femoral artery (FA) can be palpated. Medial to the FA lies the femoral vein (FV), and immediately lateral to the FA lies the femoral nerve (FN) (figure 1).

 

Figure 1: Surface Landmarks

PT = Pubic tubercle, ASIS = anterior superior iliac spine., IC = Iliac crest, FA = Femoral artery, FV = Femoral vein, FN = Femoral nerve, IC = inguinal crease

 

The Femoral Triangle

The femoral triangle holds the relevant structures needed to perform a successful femoral nerve block (figure 2). The superior boundary of the femoral triangle is the inguinal ligament, the lateral border is the sartorius muscle; and the medial border is the adductor longus muscle. The floor of the femoral triangle is made up of following muscles: Iliacus, psoas major, pectineus and adductor longus (lateral to medial). The key structures of interest which will need to be identified sonographically (from lateral to medial) include the femoral nerve, femoral artery, and femoral vein.

 

Figure 2: The femoral triangle

 

The Lumbar Plexus & Femoral Nerve

Four major nerves supply the lower extremity; the sciatic, the femoral, the obturator, and the lateral femoral cutaneous nerves. From the regional anesthesia perspective, this is important, in that an isolated femoral nerve block will not provide complete anesthesia for the lower leg. The femoral nerve supplies sensory innervation to the anterior thigh, anteromedial knee, medial lower leg and the medial aspect of the foot and ankle. Importantly, it also innervates the periosteum of the femur. In addition, it supplies motor innervation to the knee extensor muscles, including the quadriceps and sartorius (table 2).

 

Table 2: Distribution of effect of femoral nerve block

After its origin at the lumbar plexus the femoral nerve descends through the posterior third of the psoas major and iliacus muscle. It enters the thigh within the femoral triangle below the inguinal ligament and superior to the iliopsoas muscle (figure 3). Within a few centimeters of the inguinal ligament, the femoral nerve divides into anterior and posterior branches. It is important to place your blockade around the nerve prior to when it diverges, otherwise your block may be incomplete. If you are too distal, you may miss the posterior branch of the femoral nerve which provides motor innervation to the quadriceps muscles–an important consideration in patients with acute femur fractures who are experiencing quadriceps spasm (6).

 

Figure 3: The lumbar plexus