Frequency of Anatomical Variation and Relationship of the Femoral Nerve to the Iliopsoas Muscle in the Femoral Triangle
Glenn Woodworth, M.D., Tim H. Lee, M.D., Brooke Beckett, M.D., and Ryan Ivie, M.D.
Ultrasound-guided femoral nerve block (FNB) in the femoral triangle is a common regional anesthesia technique (1). Although anatomical variation of the femoral nerve has been described at its origin from the lumbar plexus (2) and in the iliac fossa (3), little data on anomalies of the femoral nerve in the femoral triangle is available. Variation at this location is thought to be rare (4). Chatterjee et al. reported a single case of an aberrant femoral nerve buried in the belly of the iliacus muscle (5). Descriptions of anatomical variations of the femoral nerve in the femoral triangle and their incidence could lead to improved nerve localization with ultrasound and success when performing FNBs. The aim of this study was to estimate the incidence of aberrant femoral nerves buried within the iliopsoas muscle in the femoral triangle.
MATERIALS AND METHODS
Approval was obtained from the OHSU Institutional Review Board (STUDY00017613). Sequential patients with hip MRI scans were included in the study. Exclusion criteria were patients less than 18 or greater than 80 years old; patients with femoral triangle disrupted by tumor, injection, or injury; and MRI studies where we were unable to identify the femoral nerve, iliacus muscle, psoas muscle, or femoral artery. MRI scans were obtained with a Philips 1.5 or 3T MRI scanner with axial cuts obtained using axial T1W without fat suppression. All MRI scans were reviewed by an investigator trained in radiology (BB) and measurements obtained using an IMPAX 18.104.22.16806 image archiving system. Axial cuts through the pelvis at the level of the femoral head and superior aspect of the greater trochanter of the femur were examined. This location corresponded to 2-4 cm below the mid-inguinal ligament. The location of the femoral nerve was scored as A) Superficial to the iliacus and psoas muscles, B) Partially within the iliacus muscle, or C) Completely within the iliacus muscle (Figure 1). Based on an observed incidence at our institution, a power analysis was performed to include 500 studies to yield a precision of the estimated incidence with a confidence interval that would not exceed two percent.
We are reporting preliminary data on the first 138 patients (274 MRI studies) of an intended sample size of 500 studies. 1/274 (0.36%) studies was classified as B (Figure 2) and 0/274 as C. Based on our observed incidence, we estimate the incidence of Type B between 0 to 2.29% (95% CI).
In our study, 0.36% of MRI studies were identified to have anatomical variation with the femoral nerve partially embedded within the iliacus muscle in the femoral triangle (type B). While no C types were identified, we have anecdotally identified with ultrasound during femoral nerve blocks four type C variants in the last five years at our institution with verification by nerve stimulation. Lack of knowledge of this anatomical variant could lead to failure to properly identify the femoral nerve with ultrasound and subsequently failed block.