Os parietale divisum.

Divisions of the parietal bone are extraordinarily rare, and have been described previously only in skull specimens in the anatomical literature. We report a case of os parietale divisum (divided parietal bone) in an infant with plagiocephaly. A 36-weeks gestation male with multiple congenital anomalies presented to the deformational plagiocephaly clinic at 11 weeks of age. CT imaging demonstrated a suture dividing the left parietal bone into two segments. He had a plagiocephalic appearance, with the greater occipital and parietal protuberances ipsilateral to the abnormal left parietal suture, and a correspondingly flatter right occiput. Head circumference was in the normal range, and the anterior fontanelle was patent and of normal size. He had mild right-sided right frontal and malar protuberance. Mandibular asymmetry was noted with a chin-point to the right of midline. He had a left-sided head tilt. He was treated with cranial molding helmet therapy for his head shape and physical therapy for his head tilt. After 16 months of helmet therapy, he had little improvement of his head shape. This case is an in vivo presentation of a rare variant, os parietale divisum.