How long do I have to lie face-up after DMEK or DSAEK?

This restriction is specific to endothelial grafts. The Academy notes that depending on your transplant you may have to lie on your back for a while after surgery. There are no stitches holding a DMEK or DSAEK graft in place, so the bubble does that job, and the bubble only presses where gravity puts it. Lying face up aims it at your cornea.

StatPearls describes the mechanics: in DSAEK an air bubble is used to adhere the graft to the cornea and after 10 to 30 minutes balanced salt solution partly replaces the air, and some surgeons use a long acting 20% sulfur hexafluoride gas bubble instead. In DMEK a larger amount of air is used and it is removed roughly 30 minutes to 2 hours later, or a small opening is made in the iris beforehand, to keep the bubble from blocking fluid flow and spiking eye pressure.

Detachment is the thing positioning is protecting against. StatPearls notes DSEK graft detachments often resolve on their own and rebubbling is reserved for complete detachments, while DMEK detachment rates in some reports run as high as 74% because the thin graft tends to roll, and central detachments or those involving more than a third of the graft may need a rebubble. That is why surgeons take positioning seriously. Follow the exact hours and break schedule you were given, and do not shorten it because you feel fine. A full thickness (PK) graft normally has no positioning requirement at all.