Nomogram to Predict WavefrontGuided LASIK Ablation Depth

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Nomogram to Predict Wavefront-Guided LASIK
Ablation Depth

• Christopher D. Gelston, MD1
• Michael J. Taravella, MD1
• Richard S. Davidson, MD1
• 1Rocky Mountain Lions Eye Institute, University
  of Colorado, Aurora, CO, USA
• The authors have no financial interest in any
  aspect of this poster presentation.

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Abstract

• Purpose To devise a nomogram that will predict
  wavefront-guided LASIK ablation depth based on
  pre-operative total optical aberrations.
• Methods A retrospective review of one hundred
  twenty eyes (81 patients) was performed following
  wavefront-guided LASIK myopia treatment. A
  pre-operative wavescan was obtained in all
  patients with CustomCornea software. A
  regression analysis of the absolute value of the
  total optical aberrations from the wavescan was
  then compared to the post-operative ablation
  depth following treatment with the LADARVision
  4000. Based on the pre-operative absolute value
  of the total wavefront optical aberrations a
  nomogram was calculated to predict ablation
  depth.
• Results The nomogram developed to predict
  ablation depth based on the absolute value of the
  total wavefront optical aberrations is Ablation
  Depth 3.03 X Absolute Value of Total Optical
  Aberrations 1.33. The standard of error with
  this method is 6.24 microns.
• Conclusion A nomogram based on the total
  pre-operative wavefront optical aberrations may
  help predict an accurate ablation depth and
  residual corneal thickness, in turn helping
  determine suitable candidates for
  wavefront-guided LASIK myopia treatment.

Nomogram to Predict Wavefront-Guided LASIK
Ablation Depth
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Purpose

• Laser in situ keratomileusis (LASIK) is a common
  technique to correct myopia and myopic
  astigmatism. LASIK requires the use of a
  microkeratome to create a corneal flap after
  which an excimer laser ablates the stromal bed
  and reshapes the cornea. With treatment the
  stromal bed is thinned and there is some
  controversy as to the safety of residual bed
  thickness. The minimum residual bed thickness
  has been recommended to be at least 250 microns
  to prevent post-operative complications such as
  corneal ectasia.1
• The Munnerlyn formula has previously been
  described to help determine the depth of ablation
  as a function of treatment diameter and diopter
  power correction.2 However this formula is based
  on spherical geometry and may not be useful to
  predict ablation depth in wavefront-guided
  treatments as greater ablation depths are
  required to treat higher the order aberrations.3
• The purpose of this poster is to devise a
  nomogram that will help predict wavefront-guided
  LASIK ablation depth based on pre-operative total
  optical aberrations.

Nomogram to Predict Wavefront-Guided LASIK
Ablation Depth
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Methods

• Design
• A retrospective analysis of all patients
  undergoing wavefront-guided LASIK at a single
  institution over a two year period performed by
  two physicians.
• Inclusion Criteria
• All patients had a wavefront-guided myopic or
  myopic astigmatism correction with a
  pre-operative wavescan measured with the
  CustomCornea software and treatment using the
  LADARVision 4000 excimer laser (Alcon
  Laboratories, Inc., Fort Worth, Texas, USA) with
  a 6.5mm optical zone and 1.25 mm blend zone.
• Exclusion Criteria
• Patients undergoing a refractive enhancement or
  hyperopic treatment, as well those with any
  offset used in the treatment profile or any
  pre-existing corneal pathology were excluded from
  analysis.
• Data Analysis
• The absolute value of the pre-operative total
  optical aberrations from the wavescan were
  compared to the operative ablation depth. The
  results were graphed using a scatter plot and a
  regression analysis was performed.

Nomogram to Predict Wavefront-Guided LASIK
Ablation Depth
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Results

• A total of 120 eyes (81 patients) fit the
  inclusion and exclusion criteria. A scatter plot
  of the absolute value of pre-operative total
  optical aberrations vs. operative ablation depth
  was graphed in Figure 1.
• The nomogram developed to predict ablation depth
  based on the absolute value of pre-operative
  total optical aberrations was generated with a
  linear regression
• Ablation Depth (microns) 3.03 X Absolute Value
  of Total Optical Aberrations - 1.33 with a
  standard error of 6.24 microns.

y 3.03x - 1.33 R2 0.9626
Figure 1 Scatter plot of the absolute value of
pre-operative total optical aberrations vs.
operative ablation depth.
Nomogram to Predict Wavefront-Guided LASIK
Ablation Depth
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Results

• Predicted ablation Depth 3.03 X Absolute Value
  of Total Optical Aberrations 1.33
• To determine the formulas accuracy it was
  utilized on 41 subsequent eyes (26 patients) who
  met the original inclusion and exclusion
  criteria.
• A total of 39 eyes (95) had predicted
  measurements within the standard of error of 6.24
  microns. The two outliers ranged from 7.4 to
  9.8 microns off the predicted ablation depth.

Nomogram to Predict Wavefront-Guided LASIK
Ablation Depth
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Results

• Figure 2 demonstrates an example of a patient
  who had a pre-operative absolute value of total
  optical aberrations equal to 34.2 and a
  calculated ablation depth of 102.3 microns using
  the derived formula.
• The actual ablation depth following
  wavefront-guided myopic LASIK measured 107.1
  microns which was 4.8 microns off the predicted
  value but within the standard of error of 6.24
  microns.

Figure 2 Pre-operative LASIK wavescan showing
total pre-operative aberrations.
Nomogram to Predict Wavefront-Guided LASIK
Ablation Depth
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Conclusion

• The importance of being able to predict ablation
  depth and residual corneal bed thickness is
  invaluable in determining pre-operatively who is
  a good candidate for wavefront-guided LASIK and
  those at risk for developing corneal ectasia.
  Therefore, we have developed a formula that makes
  it possible to predict ablation depth on
  wavefront-guided LASIK myopia treatments based on
  the pre-operative total optical aberrations.
  After its development we were able to show that
  95 of subsequent patients had a predicted
  ablation depth within 6.24 microns of actual
  treatment depth. This formula should be performed
  in all patients to help selectively choose the
  appropriate candidate for wavefront-guided LASIK
  and reduce the risk of cornea ectasia by
  predicting their residual bed thickness.
• While we developed the formula using the
  CustomCornea software and LADARVision 4000
  excimer laser there may be limitations in its
  accuracy if the formula is applied to other
  excimer lasers that use a different size
  treatment zone or laser delivery system such as
  the VISX S4 (AMO, Santa Ana, California, USA) or
  the Technolas 217 (Bausch Lomb, Rochester, NY,
  USA). These variables may make the formula from
  our study less accurate, and further analysis on
  the different excimer laser systems is needed to
  determine how pre-operative total optic
  aberrations correlate with ablation depth with
  the various ablation zones and laser delivery
  systems.

Nomogram to Predict Wavefront-Guided LASIK
Ablation Depth
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References

• 1. Wang Z, Chen J, Yang B. Posterior corneal
  surface topograhic changes after laser in situ
  keratomileusis are related to residual corneal
  bed thickness. Ophthalmology 1999106406-10.
• 2. Munnerlyn CR, Koons SJ, Marshall J.
  Photorefractive keratectomy a technique for
  laser refractive surgery. J Cataract Refract
  Surg 19881446-52.
• 3. Gatinel D, Malet J, Hoang-Xuan T, Azar DT.
  Analysis of customized corneal ablations
  theoretical limitations of increasing negative
  asphericity. Invest Ophthalmol Vis Sci
  200243941-8.

Nomogram to Predict Wavefront-Guided LASIK
Ablation Depth