[A case of congenital skull defect complicated with arachnoid cyst].
A 3-month-old boy was referred to our hospital with left temporal bone defect and bulging skin. The skull defect had been recognized since birth. There was no family history of any congenital anomaly and his physical development was normal for his age. His bulged left temporal region was covered by normal skin and a skull defect measuring 2 x 2 cm in size was palpated at the center. CT scan revealed skull defect at the pterion accompanied with widening of the middle cranial fossa. Arachnoid cyst of the middle cranial fossa was also suspected. The skull defect gradually enlarged and especially on crying the left temporal skin bulging became more marked. The bone edge of the defect was thin and was deflected outward. The operation was planned and performed at the age of 11 months. After excision and fenestration of the arachnoid cyst, duraplasty and cranioplasty were performed using split-thickness calvarial bone graft. Splitting the frontal calvarial bone suitable for the defect, the outer table was returned to the donor site, and the inner table was fixed into the defect. Follow-up CT one year after surgery revealed a satisfactory cosmetic result and no bone resorption. Split calvarium cranioplasty is considered to be even more useful in an infant.