Revisiting anatomic landmarks: lateral popliteal approach for sciatic nerve block based on magnetic resonance imaging

When the conventional lateral popliteal sciatic nerve (SN) block is performed, the needle angle required to localize and the level of the SN bifurcation are highly variable. The aim of our magnetic resonance imaging (MRI) study was to determine the most common range of needle-insertion angles and the relationship between skin-to-femur distance and angle. We also evaluated the variability of the SN bifurcation level and the relationship between patient height and nerve bifurcation level.

Using 289 thigh MRIs to simulate a lateral approach in the supine position, we measured and analyzed with simple linear regressions the level of SN bifurcation, the skin-to-femur and SN-to-femur distances, and the angle at which the needle must be directed posteriorly to intersect the SN at 9 cm proximal to the lateral joint line.

The mean insertion angle was 30 (SD, 8) degrees. In 95% of cases, angles ranged from 15 to 45 degrees, and the larger the thigh, the smaller the angle. The SN divided at a mean distance of 7.5 (SD, 1.6) cm (range, 1.5-12.8 cm) proximal to the lateral joint line. In 93% of cases, the bifurcation level was 10 cm or less.

Our simulated lateral popliteal SN block on MRIs shows a 15- to 45-degree range of needle-insertion angles. As the skin-to-femur distance was greater than 4.5 cm, the angles were progressively smaller than 30 degrees. Although this was an MRI study, it does provide some evidence that indicates the conventional clinically recommended 25- to 30-degree-angle ranges may need to be reevaluated. Needle insertion of 10 cm or greater proximal to the popliteal crease may increase the chance of placement at or proximal to the SN bifurcation.