Comparative study of the ramification patterns of the subclavian branches as the subclavian artery passes in front or behind the scalenus anterior muscle

We investigated the ramification patterns of four subclavian branches (i.e., vertebral artery, internal thoracic artery, thyrocervical trunk, and costocervical trunk) as the subclavian artery passes in front or behind the scalenus anterior muscle. The investigation was carried out on 56 cadavers (112 cases) during student dissection practice sessions at Osaka Dental University. In 110 of the 112 cases, the subclavian artery passed behind the scalenus anterior muscle. The pattern of ramification of the subclavian branches in these cases was classified into six types (types A–F). In the remaining two cases (two cadavers), the subclavian artery passed in front of the scalenus anterior muscle. In both of these latter cases, the pattern of ramification of the subclavian branches differed from the six pattern types observed as the subclavian artery passed behind the scalenus anterior muscle: the first branch was the vertebral artery; the second, the costocervical trunk; the third, the thyrocervical trunk; the fourth, the internal thoracic artery. This same pattern of ramification was observed in three previously reported cases (two cadavers) in which the subclavian artery passed in front of the scalenus anterior muscle. Taken together, these observations indicate that the ramification pattern reported here and in a previous investigation for the subclavian artery passing in front of the scalenus anterior muscle is characteristic of this anatomical condition.