A threaded pin or post normally used as an attachment. A screw is used as part of an orthodontic expansion plate to provide controlled separation of the plate; it is opened or closed by means of a key. The hyrax screw is an orthodontic screw designed for rapid palatal expansion. An implant screw is used as a means of primary retention of an implant or to cover an implant during the healing phase ( cover screw). A self-tapping screw is a screw that cuts its own thread in bone, dentine, or enamel.