Anatomy of the Fascia Iliaca Block
The Iliacus muscle is a large, flat, triangular muscle that lines and fills the ilium. It originates from all along the upper portions of the ilium and iliac crest, sacrum and iliolumbar ligaments. The iliacus muscle joins with the lateral side of the psoas major muscle. Together they are referred to as the iliopsoas. The iliopsoas exits the pelvis from beneath the inguinal ligament, wraps around the proximal neck, and inserts into the lesser trochanter, acting as a powerful hip flexor.
The fascial covering of the iliopsoas is thin superiorly, becoming significantly thicker as it reaches the level of the inguinal ligament. This thickness provides a great deal of resistance and a large “pop” as a needle tip is passed through the fascia.
The lumbar plexus is made up of the nerve roots from the T12 through L5 vertebrae. The largest branch of the lumbar plexus is the Femoral nerve is, arising from the L2, L3, & L4 roots. The femoral nerve descends through the fibers of the psoas major and exits at the lower portion of the psoas' lateral border, passing downward between the psoas and iliacus muscle, deep to the iliacus fascia. The femoral nerve exits the pelvis into the upper thigh, lateral to the common femoral artery and vein.
The Lateral Femoral Cutaneous nerve is a purely sensory nerve arising from the L2 & L3 nerve roots that provides sensation from the iliac crest down the lateral portion of the thigh to the area of the lateral femoral condyle. The lateral femoral cutaneous nerve emerges from the lumbar plexus and travels downward lateral to the psoas muscle and crosses the iliacus muscle deep to the iliacus fascia.
The anterior and posterior Obturator nerves innervate a portion of the distal, medial thigh. They arise from the L2, L3, & L4 nerve roots and cross the iliacus muscle, deep to the fascia, to the medial thigh. The obturator nerves are sometimes involved in the FICB but probably plays little role in post-operative pain relief for most surgeries of the hip and proximal femur.
Key Points of the Fascia Iliaca Block
Make sure you are looking at iliacus fascia. While this seems easy, the sartorius muscle crosses the iliopsoas just after it passes over the edge of the ilium and passes under the inguinal ligament. The simplest way to find the correct fascial layer is to clearly identify the ilium (bone) on ultrasound. The muscle lying in contact with the bone and directly overlying it, is the iliacus muscle and so the fascial layer covering it is the iliacus fascia.
Watch for the local solution to move superiorly as you inject. We introduce the needle at the rim of the ilium and since the nerves we are after arise from the lumbar plexus, they are coming from the superomedial edge of the ilium. So, the local solution needs to travel superiorly to encounter them at the earliest opportunity. Make sure you know which side of the ultrasound picture is superior and which is inferior. To ensure that the solution travels superiorly, after inserting the needle through the iliacus fascia and injecting a small amount of solution, advance the needle tip superiorly, under ultrasound, into the space created by the injected local solution. The needle tip must remain beneath the fascia and above most of the iliacus muscle as it is advanced. You should see the injected local solution expanding or “running off” towards the superior edge of the iliacus muscle on the ultrasound image. It is alright if your local solution is injected within the body of the iliacus muscle, just try to keep it in the superficial (anterior) portion if possible.
If you are having trouble getting the injected solution to move superiorly toward the lumbar plexus consider placing manual pressure below the injection site to discourage retrograde flow.
This is a compartment block, volume is the key. The goal is not to place the local solution next to nerve, instead, we place the local anesthetic into an anatomical compartment containing nerves, and let the distribution of the local solution within the compartment take the local to the nerves. The most efficient way to take advantage of this is to make sure you are using adequate volume for the block.
We routinely use a total of 50 ml of local anesthetic mixture injected incrementally, 10 – 15 ml after needle placement, advance the needle into the space created by the volume, then inject the remainder of the local anesthetic mix.
Some centers advocate injecting a bolus of normal saline after the initial needle placement, to initiate hydro-dissection of the sub-fascial plane, followed by the local anesthetic solution. While this technique seems reasonable, since the saline and the local will eventually occupy the same space, it makes more sense to simply start and end with the solution of the final concentration.