Can different positions facilitate block application in ultrasound-guided obturator nerve block? A prospective comparative study

Objective: We aimed to compare the distances of the landmarks to the skin, image quality, and ease of application in the ultrasound-guided obturator nerve block (ONB) in different positions.
Materials and Methods: 40 volunteers aged between 20-65 years were included in the study. The distances of the landmarks (anterior and posterior branches of the obturator nerve; junction of the abductor longus and abductor brevis muscles with the pectineus muscle) to the skin, which were taken as a reference for the ultrasound-guided obturator block, were measured and compared in 3 different positions (P1=Neutral position; P2=45o Abduction; and P3=Flexed knee) given to the leg. We also evaluated the quality of the ultrasound image and the ease of application in each measurement by assigning a subjective observer score and comparisons were made for three positions.
Results: While the mean of the distances of the landmarks to the skin were the shortest in P3 and the longest in P1 position, there was no significant difference between the groups (p>0.05). A statistically significant difference was observed between P1 and P3 in the distance of the junction of the muscles to the skin surface (p<0.05). The highest score for the clarity of ultrasound images and ease of accessing the measurement points was the P3 position (p=0.00). In addition, in our correlation analysis, we found that as the distance of the landmarks to the skin surface decreased, the image clarity and the ease of access to the obturator nerve (score) increased, where p<0.05.
Conclusions: In ultrasound guided ONB, in P3 position landmarks get closer to the skin, and image clarity and ease of detection for landmarks increases in parallel with this position. As a result, the ultrasound guided ONB can be best done by giving flexed knee position.