A prosthesis mounting device and assembly for securing a prosthesis on an implant supported in bone tissue. An appendage or “preppable abutment” is carried by a dental implant and is milled or “prepped” into a shape suitable to accept a dental prosthesis such as a crown or bridge. Alternatively the appendage may be cast using a lost-wax investment molding process in which an appendage model is shaped by adding and sculpting wax onto portions of a calcinable appendage and/or removing material from portions of the appendage before using the model to form a mold. In each case the appendage is formed to include a recess that extends axially upwardly from a bottom end of the appendage and is shaped to fit over an O-ring receiver abutment or “O-ball” of a dental implant.