That kneel to brace himself for the pain of removing that prosthetic....fuck that must be the most painful looking thing ever in film...next to seeing the wine dripping into Two-Face's wounds in the Dark Knight.
@oozyslatt2529 guys it's a prosthetic. Basically the damage from the cyanide they gave him destroyed his mouth and resulted in him needing a prosthetic for his teeth and part of his jaw, hence why his eye sunk in and his jaw was off.