As compared to LASIK patients, why do Epi-LASIK patients tend to have residual degree instead of perfect eyesight after surgery?
Actually, the basis of your question ie that epi-LASIK patients tend to have residual degree as compared to LASIK patients is not necessarily true. For example, in this Cochrane Database Review meta-analysis comparing LASIK with PRK (which is the same as epi-LASIK) https://www.ncbi.nlm.nih.gov/pubmed/23440799, the authors concluded that 'The two techniques appear to give similar outcomes one year after surgery.'
What's more important, I believe, is to understand why certain patients may do better with a particular technique, and also to understand the different pros and cons of LASIK versus epi-LASIK/PRK.
LASIK gives fast visual recovery and much better post-operative comfort, though with the rare possibility of flap problems. PRK/epi-LASIK/LASEK eliminates the possibility of flap problems but besides slower visual recovery and increased post-operative pain, also has a higher risk of post-operative corneal haze/scarring (which is virtually unknown with LASIK).
As such, it is my routine practice to use an anti-scarring medicine called Mitomycin C (MMC) for PRK/epi-LASIK patients, to markedly reduce the risk of corneal haze. This haze, if bad enough and thick enough, may indeed cause some amount of residual degree or regression, and may be a cause for the notion some people have that epi-LASIK patients tend to have residual degree as compared to LASIK patients. When MMC is used, though, it is extremely rare to see such haze, except in some patients with high spectacle degrees.
With the proper use of medication, and the proper selection of procedure, the 2 techniques generally do give similar outcomes after full recovery has occurred.
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