Supernumerary subclavius muscle in Thais: predisposing cause of thoracic outlet syndrome.

OBJECTIVE To demonstrate and classify the variation of the subclavius muscle according to its insertion in the Thais. MATERIAL AND METHOD One hundred and twenty eight upper limbs were dissected out to expose the scapular region. The attachments of subclavius muscles were examined and recorded. RESULTS The subclavius muscle was categorized into 4 types according to its insertion. There are 64.06% of type I, 17.96% of type II, 15.62% of type III and 2.34% of type IV. The insertion of subclavius muscle is gradually extended from the shallow groove on the inferior surface of the clavicle towards the conoid ligament and corocoid process, to the superior transverse scapular ligament and to the superior border of the scapula adjacent to the insertion of inferior belly of omohyoid muscle. CONCLUSION The present study prevailed 64% normal subclavius muscle and other 36% of varied supernumerary subclavius muscle. The presence of supernumerary subclavius muscle could be a predisposing causative factor of thoracic outlet syndrome.