Suprainguinal fascia iliaca block: does it block the obturator nerve?

To the Editor, We read with interest the dosefinding study by Kantakam et al. The authors examined the spread of injectate to the obturator nerve (ON) with a suprainguinal fascia iliaca compartment block (SIFIB). They observed that the minimum effective volume of dye in 90% of cadavers required to reach the ON was 62.5 mL in deceased subjects with an average body weight of 56 (SD 12) kg. This is more than 1 mL/kg. Although we commend the authors for their scholastic pursuit, we have concerns regarding the anatomical basis of the possible spread from the fascia iliaca compartment (FIC) to the ON. The FIC is a ‘fascia plane’, a welldefined, native, potential compartment anterior to the iliopsoas muscle limited by the firm adherence of the fascia iliaca (FI) to the FIC borders. The FIC can be opened by an injectate. However, an injectate needs to spread outside the FIC to reach the ON. This mandates creation of artificial passageway(s). Our experience with dissecting cadavers is that increasing the volume of an injectate eventually disrupts the anatomical integrity of the FI due to the combination of the volumedependent high pressure in the FIC and tearing of the FI during removal of intestines: (1) cracks in the FI allow the spread of injectate from the FIC to the ON in the retroperitoneal compartment; (2) disruption of the adhesion of the FI to the epimysium of the iliopsoas muscle along the common and external iliac arteries creates an artificial passageway to the ON in the retropsoas compartment. We know from two MRI studies of SIFIB in live patients and volunteers using 30 mL and 40 mL, respectively, that the FI remained intact and the injectate only spread in the FIC anterior to the iliopsoas muscle. 3 The injectate did not spread medial to the linea terminalis or the common and external iliac arteries and thus did not reach the ON, in either the retroperitoneal compartment or in the retropsoas compartment. Finally, even if the results by Kantakam et al were assumed not to be artifacts due to the combination of a large injected volume and tissue dissection, we are concerned that injecting such a volume would potentially expose patients to local anesthetic toxicity without a commensurate clinical benefit. We would like to thank the authors for working on this resilient idea, but we believe there is no sound anatomical basis to support its clinical development.