Ultrasound-Guided Interscalene Block: Reevaluation of the "Stoplight" Sign and Clinical Implications.

Background and Objectives: The "stoplight" sign is a frequently described image during ultrasound-guided interscalene block, referring to 3 hypoechoic structures found between the anterior and middle scalene muscles. This study was designed to establish the ultrasound-anatomy correlation of this sign and to find any other anatomical features within the roots that could help with the interpretation of the ultrasound images obtained at the interscalene level. Methods: We performed 20 dissections of the brachial plexus in 10 embalmed human cadavers and systematically analyzed and measured the roots of C5 to C7 and then correlated these findings with ultrasonographic images on file. Results: We found that the C5 root is significantly smaller than either C6 or C7 (P

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