An implantable mammary prosthesis includes an urchin buttress fill port structure. The fill port structure preferably comprises a shell, the shell including an opening, a compartment interior to the shell adapted to contain a filler. The urchin buttress is generally cup-shaped, and is disposed within the shell. The urchin buttress is preferably attached to the shell adjacent the opening to form a fluidic seal, optionally via a sealing flange. The urchin buttress may include one or more ribs, such as to provide structural support to the urchin buttress. Preferably, a reservoir is disposed external to the shell.