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

Abstract

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.

Ultrasonographic Assessment of the Distance of Sciatic Nerve Bifurcation from the Popliteal Crease and its Depth from Skin in Volunteers

“…Our finding correlates with the study by Chiang et al where the mean distance of SN bifurcation was 5.77 cm and 6.02 cm for the females and males respectively. 11 Similar was the finding by Vloka et al and this cadaveric study showed that SN divided at 6.05 ± 2.70 cm from the popliteal crease. 9 The lesson to take away is that these distances varied greatly, from 0 to 11.5 cm, and that too in a small number of cadavers.…”

“…Chiang et al have stated in their study that 28% of their volunteers had more than 2 cm difference in the crease to nerve distance in their right and left legs and have expressed concern this difference could be significant if an extra-epineural injection is performed and there could be an incomplete block. 11 Our study has shown that this variation of more than 2 cm between limbs of the same individual is only 8.25%, less than the study by Chiang et al 11 The same study also found a predictable variability in the depth of bifurcation of sciatic nerve at the popliteal fossa and found consistent deeper skin depth values for female patients than males, 0.4 cm to be exact. Our study strengthens this view because our data shows that women have greater mean skin-nerve distances than men.…”

“…The formula n=z 2* pq/e 2 was used, where z is the z score at 95% confidence interval (1.96), p is the proportion of patients with variation in sciatic nerve bifurcation (0.21), q is 1-p (0.79) and e is the allowable error at 8%. 11 The sample size was calculated to be 99 and considering a dropout of 10%, a sample size of 110 was taken.…”

Siyatik sinirin varyasyon anatomisi ve klinik yansımaları

“…The sciatic nerve block in the popliteal fossa is frequently administered for anesthesia and postoperative pain blockage for lower extremity surgery below the knee. The sciatic nerve division in the popliteal fossa and its depth have significant implications for popliteal block (7,(15)(16)(17). Vloka et al (15) suggested that insertion of the needle at 100 mm above the popliteal crease was most suitable placement, as a different opinion according to the classical teaching.…”

“…The division of the sciatic nerve may occur anywhere from the sacral plexus to the popliteal fossa. Some anatomical variations concerning its topography and division such as the division of the sciatic nerve in the popliteal fossa may be also responsible for failure of sciatic nerve popliteal blocks (10,15,16,17).…”