Objectives: The interscalene block is utilized for regional anesthesia of the upper limb, targeting the roots and trunks of the brachial plexus in the interscalene groove. The prevalence of variation, which may affect the success of this block, has not been documented in detail with respect to side and sex, nor has a classification system been proposed. Methods: Seventy-nine embalmed bodies were dissected bilaterally. The position of the roots and the subclavian artery relative to the anterior scalene muscle was documented and variations were classified according to prevalence. Differences in the prevalence of variation between left and right sides and between males and females were investigated. Results: The standard position of the nerves and subclavian artery in the interscalene groove (Type 1) was present in 31.6%. Variant positions included the following passing through the belly of the anterior scalene: C5 and C6 roots (Type 2) (46.8%), C5 root (Type 3) (15.2%), C5 and C6 roots as well as the subclavian artery (Type 4) (3.8%), and lastly, C5, C6 and C7 roots (Type 5) (2.5%). Variant anatomy was statistically more prevalent in females on the right side only. Conclusion: Variant locations of the roots and subclavian artery external to the interscalene groove were common, suggesting that ultrasound should be used to visualize variations prior to performing interscalene blocks. A classification type of variant positions has been developed for standardization.
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