• anatomy
• lower extremity
• postoperative pain

We read with interest the study by Vermeylen et al1 on the fascia iliaca compartment (FIC) block (FICB) in ten volunteers, especially the technique using the supra-inguinal (SI) approach described by Hebbard et al supra-inguinal fascia iliaca compartment block (SI-FICB).2

The authors injected 40 mL of lidocaine 0.5% with contrast using the SI-FICB and the pattern of spread in the FIC was beautifully presented with MRI. Unfortunately, the location of the iliac artery is misinterpreted in all images in the figure—except figure 6E–F. In figure 6A–D, the authors seem to mistake the inferior mesenteric vein for the iliac artery.

One of the outcome measures was the spread of the injectate to the obturator nerve (ON). The authors concluded that the ON was covered by the injectate between the levels of intervertebral disc (IVD) L4–L5 and L5–S1 with a success rate of 90% (9/10). However, neither the article itself nor the reference3 presents a method of locating the ON in the axial plane.

We have constructed a figure with five cross-sectional levels from IVD L4–L5 to the head of femur (figure 1) from detailed cadaveric sections (A1–E1) and standard CT scans (A2–E2). It is easy to track the ON in a standard CT scan as well as in the digitized cadaveric sections. In addition, we have meticulously plotted the spread of injectate with green color on each picture in our figure exactly simulating the spread as presented in figure 6 in their article.1

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Figure 1

Axial cross-sections at five levels from intervertebral disc L4–L5 (A1) to the head of femur (FH, E1–E2). In all images, the simulated local anesthetic spread is indicated with green color. (A1)–(E1) are cadaveric cross-sections and (A2)–(E2) are standard CT scan cross-sections. Obturator nerve (ON, white arrowhead); lumbar plexus (LP); roots of lumbar plexus (RLP); anterior ramus of L4 (L4); lumbosacral trunk (LST); psoas major muscle (PM); quadratus lumborum muscle (QL); iliacus muscle (IM); iliopsoas muscle (IPM); sartorius muscle (SM); internal obturator muscle (IOM); fascia transversalis (cyan blue line); parietal peritoneum (pink line); vertebral bodies L4, S1 (L4, S1); abdominal aorta (AA); inferior vena cava (IVC); common iliac artery (red arrows B1–C2); common iliac vein (blue arrows B1–C2); external iliac artery (red arrows D1–E2); external iliac vein (D1–E2); internal iliac artery (IIA); internal iliac vein (IIV); inferior mesenteric vein (white arrows). (A1)–(E1) is a modified excerpt from VH Dissector with permission from Touch of Life Technologies (www.toltech.net). Built on real anatomy from the National Library of Medicine’s Visible Human Project.

Obviously, the spread of injectate is nowhere near the ON at any level. From vertebral body (VB) L4 to L5, the ON is sandwiched between the anterior and posterior lamina of the psoas major muscle (PM) as part of the lumbar plexus (LP). The anterior lamina of the PM is adherent to the neuraxis which would seal off any medial spread of injectate to the LP, but the injectate does not even reach the medial aspect of the PM (figure A1–B2). The lateral spread of injectate at this level does not reach the lateral part of the LP. At the level of VB S1, the ON exits the PM on the posteromedial aspect of the muscle and enters the retropsoas compartment (RPC) which also contains the lumbosacral trunk anterior to the sacral ala (figure C1–C2). The spread of injectate does not reach the RPC as it does not extend posterior to the external iliac artery and vein and it does not enter the RPC from the lateral side.

After descending anterior to the sacral ala, the ON enters the lesser pelvis. The transversalis fascia is tightly adherent to the linea terminalis, obstructing spread from the FIC into the lesser pelvis. In the lesser pelvis, the ON is located outside the FIC in the retroperitoneal fat between the PM, the external iliac vein, the internal iliac artery, and the urinary bladder anterior to the sacrum (figure D1–E2).

Assuming that figure 6 in the article represents the entire sample of SI-FICBs, the spread of injectate with the SI-FICB apparently does not reach the ON at any level. As discussed by the authors, the spread of the local anesthetic to the ON cannot be reliably assessed by sensory and motor block; the basis of their conclusion in this article relied on the spread pattern in MRI.

The conclusion of the authors about spread of injectate to the ON seems to be effectively contradicted by their own evidence. In fact, the presented evidence beautifully illustrated that the SI-FICB with 40 mL did not spread to the ON.