Abstract: An arrangement usable as a core for a dental prosthesis or as a transfer jig in forming a model of an aveolar arch with one or more implants, comprising a nut which is for fitting in a hexagonal portion of a socket in the implant, a ring for fitting over the nut and seating around the mouth of the socket, and a post for screwing through the nut and the ring to the implant. The ring also has a hexagonal bore portion which receives part of the nut. Thus, the ring is located spatially and angularly relative to the implant without being socketed therein. When a dental impression has been formed with a plurality of the rings embedded therein, or when a sub-frame for a bridge has been made up on two or more such rings on a model of the aveolar arch made from that impression, it is removed by unscrewing the post and withdrawing it and the nut. Thus, the impression or bridge sub-frame is easily removed despite the probability that the axes of the implants in the aveolar arch are not parallel.