Fibromatosis colli (also known as sternomastoid tumor of infancy) is a benign proliferation of fibrous tissue infiltrating the lower third of the sternocleidomastoid, (SCM) and is the most common cause of neonatal torticollis.The mass, also known as a hematoma of the sternocleidomastoid, is firm and hard on palpation, but is neither tender nor inflamed.
The mass is easily diagnosed using ultrasound, where it is found within the SCM and enlarges the muscle. [Source: Wikipedia 
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