Abstract: An improved breast prosthesis or mammary implant (20), is provided of a type known per se, comprising a single or multiple lumen elastic shell or envelope (23), for example of molded silicone, in which an aperture formed in the shell for removal from a mold is subsequently sealed with a silicone patch (24), thus forming internal lumen(s) which can then be filled by either the manufacturer or the surgeon with a fluid or gel. A substantially seamless join between the patch and the shell is formed by means of an aperture with a chamfered edge (22), and applying and bonding a patch to one side, preferably the inside, of the shell such that a seamless and ridgeless join between the patch and the shell is formed to the exterior. A method and apparatus for performing a chamfer trimming operation and applying the patch are also disclosed. A double chamfer aperture edge construction filled in with an overlay patch and an underlay patch is also disclosed.