Rare anatomical term; in everyday language 頭蓋骨 (ずがいこつ) is the standard word for 'skull'.
In medical books, you may sometimes see the notation '顱骨'.
ろこつは頭蓋骨の別称として使われることがあるが、一般的ではない。
ろこつ is sometimes used as another name for the skull, but it is not common.