Nowadays, we virtually never replace one of these decompensated transplants with a new penetrating graft because the results of endothelial keratoplasty, particularly DMEK, are so good.
When performing a Descemet's membrane endothelial keratoplasty for an eye with a failed previous penetrating keratoplasty, numerous studies have shown that the donor DMEK graft should be sized smaller than (or equal to) the old penetrating graft. Normally, this sizing obligation is a nonissue because the typical PK averages 8 mm in diameter and because most DMEK grafts are 8 mm themselves. Occasionally,