Posters

Masking effect of the epithelium on corneal wavefront aberrations in the virgin cornea and after myopic LASIK

Poster Details

First Author: M.Gobbe UK

Co Author(s):    D. Reinstein   T. Archer           

Abstract Details

Purpose:

Comparison of the corneal anterior surface wavefront (AW) and stromal surface wavefront (SW) in virgin eyes and after myopic LASIK to characterize the masking effect of the epithelium.

Setting:

London Vision Clinic, London UK

Methods:

Included were 20 virgin eyes, 20 post-myopic LASIK eyes treated with precompensating Z4,0 optimised myopic profiles (Laser Blended Vision) with the MEL80 and the VisuMax femtosecond laser (Carl Zeiss Meditec) for myopia from -4.00D to -8.00D, one post-myopic PRK eye treated in 1994 by Summit Apex Plus in a 4.5mm zone, and one post-myopic LASIK eye with a decentred 6mm optical zone treated in 1999. Elevation of the stromal surface was calculated for each eye by subtracting the epithelial thickness profile (from Artemis VHF digital ultrasound scanning) from the anterior corneal surface elevation data (from Orbscan II). Elevation data were imported into VolPro (Sarver & Assoc.) to calculate the AW and SW aberrations for a 6mm analysis zone.

Results:

In virgin eyes, there was no statistically significant difference (p=0.210) in spherical aberration (AW 0.231µm, SW 0.196µm) or coma (AW 0.233µm, SW 0.235µm), but HORMS was statistically significantly lower (p=0.003) for the AW (0.446µm) than the SW (0.515µm). After myopic LASIK, spherical aberration was much greater (p<0.001) for the AW (0.530µm) than the SW (0.056µm), coma was greater (p=0.031) for the AW (0.664µm) than the SW (0.556µm), but there was no statistically significant difference (p=0.715) in HORMS between the AW (1.039µm) and SW (1.020µm). In the 4.5mm myopic PRK eye, aberrations were much greater on the SW (HORMS=3.407µm, SA=2.989µm, coma=1.230µm) than on the AW (HORMS=1.740µm, SA=1.673µm, coma=0.392µm). Similarly for the eye with a 6mm decentred optical zone, HORMS and SA were considerably greater in the SW (HORMS=1.744µm, SA=1.407µm, coma=0.764µm) than in the AW (HORMS=1.584µm SA=1.062µm, coma=1.110µm).

Conclusions:

In virgin eyes, spherical aberration is similar for the SW and AW, although there is still an overall HORMS reduction produced by the epithelium over the SW. The epithelium contributes a large amount of masking of the stromal surface wavefront error after myopic ablations; pre-compensation of Z4,0 to reduce the induction of SA in myopic ablations appears very effective on the SW however this effect is greatly reduced after epithelial compensation sets in. FINANCIAL INTEREST: One of more of the authors... receives consulting fees, retainer, or contract payments from a company producing, developing or supplying the product or procedure presented