Computed tomography scanning of the sciatic nerve posterior to the femur: Practical implications for the lateral midfemoral block

Background and Objectives: Using computed tomography (CT) scans of the thighs, this study addresses sciatic nerve anatomy at the injection site for the lateral midfemoral sciatic nerve block. It addresses the recommendation of neutral leg rotation to facilitate block placement. Method: This prospective and descriptive study involves 21 patients scheduled for CT scan imaging of the lower limbs. Transverse CT scans were analyzed at 20, 25, and 30 cm distal to the upper border of the greater trochanter (GT) of the femur with the knee externally rotated by 30degrees. The angle a formed by the broad axis of the sciatic nerve and the coronal plane, skin-to-nerve distance, great vessel-to-nerve distance, division of the sciatic nerve, and widths of the perineural space were assessed. Values are expressed as mean +/- SD. Results: At 20, 25, and 30 cm distal to the GT, the a angle was 50degrees +/- 140, 55degrees +/- 13degrees, and 56degrees +/- 26degrees, respectively. This angle increased to nearly 90degrees when the knee was rotated to a neutral position. The skin-to-nerve distance was 5.9 +/- 1.1 cm, 5.4 +/- 0.9 cm, and 5.7 +/- 1.1 cm. The section width of the perineural space was 1.8 +/- 0.9 cm(2), 3.9 +/- 2 cm(2), and 5.6 +/- 2.4 cm(2). The sciatic nerve was divided in 27% of subjects at 20 cm and in 90% at 30 cm distal. Conclusion: The characteristics of sciatic nerve anatomy described in this study support observations and clinical recommendations regarding lateral midfemoral sciatic nerve block.