If you've ever done any carpentry work, you'd find that screws work best on hard materials that hold the 'bite' of the screw whereas squishy materials like silicone don't. Other than that, silicone implants are sewn (tightly) into a subperiosteal pocket to hold them down.So, they don't need screws. Although screws for an implant in (such as porex which is harder material) don't 'cause' an infection, in the event you got an infection, both the implants and the screws would have to be removed and it's easier to remove silicone implants than the screws.
