Fibromatosis colli
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.