The scalenus anterior is the most important of the three scalene muscles clinically, dividing the posterior triangle into the prescalene compartment anteriorly (subclavian vein) and the interscalene compartment posteriorly (subclavian artery and brachial plexus). These relationships make it the anatomical key to thoracic outlet anatomy — the anterior scalene is between the vessels, making its removal (scalenectomy) the primary surgical decompression for thoracic outlet syndrome.
| Origin | Anterior tubercles of the transverse processes of C3 through C6 |
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| Insertion | Scalene tubercle on the superior surface of the first rib |
| Nerve Supply | Ventral rami of C4, C5, C6 |
| Blood Supply | Ascending cervical artery |
| Actions | Bilateral: flexion of the cervical spine; Unilateral: ipsilateral lateral flexion and contralateral rotation; Elevates the first rib during deep inspiration (accessory respiratory muscle) |
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The interscalene triangle (between the anterior and middle scalenes) contains the roots and trunks of the brachial plexus and the subclavian artery. This space is the most common site of thoracic outlet compression and the target for interscalene brachial plexus block, where local anaesthetic is injected to bathe the plexus at this level.
Scalenectomy for thoracic outlet syndrome divides the anterior scalene from its first rib insertion via a supraclavicular approach, decompressing the interscalene triangle. The phrenic nerve running on its anterior surface must be identified and protected throughout the dissection. The interscalene brachial plexus block targets the brachial plexus trunks in the interscalene groove between the anterior and middle scalenes.
The anterior scalene is palpable in the posterior triangle between the sternocleidomastoid and the posterior cervical muscles, becoming firm during deep inspiration and ipsilateral head tilting.
Anterior scalene compression of the brachial plexus and subclavian vessels in the interscalene triangle producing upper limb symptoms managed with scalenectomy.
