Topographic Changes Induced During Flap Production by Epi-LASIK surgery

1
Topographic Changes Induced During Flap
Production by Epi-LASIK surgery

• Department of Ophthalmology, University of Ulsan
  College of Medicine,
• Asan Medical Center, Seoul, Korea.
• Kyung Hoon Kim, Jooeun Lee, Chuljin Shin,
• Hee Kyung Lee, Sung Yong Kang, Jae Yong Kim,
  Hungwon Tchah

Authors have no financial interest in this study.
2
Introduction

• Changes in corneal topography and ocular
  aberration are highly evaluated as a measurement
  of visual quality after refractive surgery.
• Currently, customized surface ablation techniques
  are more frequently used to improve patients
  quality of vision.
• However, these refractive procedures are
  conducted under the assumption that removed
  corneal epithelium does not significantly affect
  outcomes of refractive surface ablation and that
  they will be regenerated into their original
  distribution.
• Thus, the purpose of this study was to
  investigate induced changes of corneal
  topographic data before and after epithelial flap
  removal during Epi-LASIK.

3
Patients and Methods

• Patients
• Eighteen eyes of ten subjects undergoing
  Epi-LASIK for correction of myopia or myopic
  astigmatism were enrolled.
• Surgical Technique
• Microkeratome (Amadeus II, AMO, Irvine, CA) for
  epithelial flap production.
• Flap diameter 9.0mm
• Flap thickness Approximately 5560µm
• Topography
• Corneal topography with ray tracing aberrometry,
  i-Trace (Technologies, Houston, Texas), was used
  to measure corneal topography before and after
  epithelium removal.
• Corneal aberrations are calculated using the
  program integrated in the i-Trace system based on
  the topographic data.

Fig.1 i-Trace
Fig.2 Amadeus microkeratome
4
Results (1)

• Simulated K showed statistically meaningful
  change between the epithelium on and off state.
• Tendency of Increasing refractive power after
  epithelium removal.
• However, other topographic parameters (central
  power, astigmatism on simulated K, asphericity
  and best fit sphere) did not significantly change.

Fig.3 Topography with epithelium
Fig.4 Topography without epithelium
5
Results (2)

• Total Corneal higher order aberrations (HOAs)
  significantly increased at 3.0, 5.0, and 7.0mm
  diameters after epithelial removal.

6
i-Trace Data With Epithelium Without Epithelium P value (Wilcoxon Test)
Simulated K (D) lt0.001
MeanSD 42.521.67 43.211.75
Range 39.5945.61 39.9546.57
Astigmatism on simulated K (D) Astigmatism on simulated K (D) 1.000
MeanSD 1.470.52 1.560.72
Range 0.592.59 0.613.12
Best-Fit Sphere, central 3.0mm(mm) Best-Fit Sphere, central 3.0mm(mm) 0.913
MeanSD 7.950.32 7.930.33
Range 7.43 8.52 7.228.63
Central Power (D) 0.913
MeanSD 42.531.69 42.631.76
Range 39.6045.44 39.0946.73
Asphericity (Q value) 0.248
MeanSD -0.100.14 0.080.84
Range -0.330.15 -1.821.52
Total Corneal Higher Order Aberration, 3.0mm(µm) Total Corneal Higher Order Aberration, 3.0mm(µm) lt0.001
MeanSD 0.0570.034 0.1700.066
Range 0.0210.161 0.0950.342
Total Corneal Higher Order Aberration, 5.0mm(µm) lt0.001
MeanSD 0.2460.097 0.6390.265
Range 0.1220.482 0.3571.352
Total Corneal Higher Order Aberration, 7.0mm(µm) lt0.001
MeanSD 0.6690.190 1.5630.763
Range 0.4371.139 0.6263.411
plt0.05
7
Discussion (1)

• According to Damien et al, epithelial
  distribution on the corneal surface before and
  after its removal during PRK is closely related
  to astigmatism and changes in corneal
  irregularity. They reported a trend for increased
  anterior prolateness after epithelial removal,
  which agrees with results of previous reports.
• In this study, however, neither astigmatism nor
  anterior prolateness were significantly altered.
  Differing methods of epithelial removal may
  underlie such a result.
• Increases of simulated K may be due to thicker
  epithelium in the periphery than in the center.

8
Discussion (2)

• Increases of total HOAs after the removal of
  epithelium may indicate its role to smoothe the
  corneal surface over the Bowmans layer and to
  decrease the corneal total HOAs.
• However, current customized surface ablation is
  conducted in the epithelium off state whereas
  pre-operative wavefront data are measured from
  epithelium on normal cornea. This assumption that
  the epithelium does not affect significantly on
  refractive surface ablation may be erroneous.
• For this reason, customized techniques in surface
  ablation may cause unpredictable results.
  Therefore, more concern should be given to
  changes of HOAs attributable to the corneal
  epithelium, and further study is needed to
  analyze causes in increase of HOAs.

9
Conclusions

• Astigmatism of simulated K in corneal topography
  showed a tendency to increase after removing the
  epithelial flap during Epi-LASIK.
• Total corneal HOAs at 3.0, 5.0, and 7.0mm
  increased after epithelial removal.
• This suggests that the corneal epithelium could
  have a significant effect on corneal topography
  and the results of corneal surface ablation.
• Future surface ablation refractive surgery may
  require more concern and investigation into the
  role of the corneal epithelium.

10
References

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