Inclusion Criteria:

1. Post-PK cornea at least 5 years after keratoplasty (rationale: ECD plateaus 5 years post-PK; including corneas that are in the phase of rapid cell loss prior to 5 years will confound the relationships we are studying)
2. Primary or repeat keratoplasty
3. May have had cataract extraction and IOL, but may not have history of recent cataract surgery (within 6 months)
4. Known habitual contact lens parameters if a contact lens wearer

Exclusion Criteria:

1. Active graft rejection
2. Graft on the path to failure
3. Presence of uncontrolled glaucoma (IOP>25 corrected for corneal thickness using OHTS nomogram; because high IOP causes further cell loss)
4. Active ocular surface disease (infectious conjunctivitis, infectious keratitis, atopic keratoconjunctivitis)
5. Any optical opacities or ocular disease that decreases best corrected visual acuity (examples: macular degeneration, posterior capsular opacity, corneal opacities on the visual axis)
6. Indication for PK was pseudophakic or aphakic bullous keratopathy (because this influences graft failure and cell loss)