Ultrasound Imaging of the Sciatic Nerve Division in the Popliteal Fossa: A Volunteer Study

The Background and Objectives: A sciatic nerve block at the level of the popliteal fossa is frequently administered for post-operative analgesia for surgery below the knee. While ultrasound continues to gain popularity as the technique of choice for guiding needle positioning during peripheral nerve blocks, practitioners can begin to utilize ultrasound to look for patterns of anatomical significance. Recognizing anatomical variations among different demographic populations can help practitioners improve in performing nerve blocks. We aim to determine if predictable variability exists in sciatic nerve bifurcation location and depth at the level of the popliteal fossa. Methods: After IRB approval, eligible subjects were screened for ASA I or II status and demographic data was collected. Fifty subjects were enrolled. The SonoSite MicroMaxx with 38-mm broadband linear array, 13 6 MHz probe with color Doppler and image capturing capabilities was used for ultrasound measurements. With subject lying prone, the location of the sciatic nerve in relation to the popliteal crease and skin-to-nerve distance were assessed via ultrasound. Two independent investigators confirmed nerve location for measurements. Analyses were performed with SAS version 9.1 using Pearson Correlation Coefficients and regression analysis. Results: Gender stratification revealed that, while males were both taller and heavier, skin-nerve measurements for depth were consistently deeper in females (p-value 0.02). Independent of the right or left leg, male gender and increased height decreases the skin-nerve distance, while increased weight increases the distance. There was no correlation between patient characteristics and crease-nerve distance. In some subjects, variability of crease-nerve distance even existed between their right and left leg. Conclusion: We show that significant variability exists for actual sciatic nerve bifurcation location, or target injection site, with consistently deeper skin depth values for female patients when compared to male patients, accounting for height and weight. These findings suggest visualization techniques such as ultrasound may lead to better localization of ideal injection sites.