Corneal Biomechanical Changes Following Surface Keratorefractive Surgery

1
Corneal Biomechanical Changes Following Surface
Keratorefractive Surgery

• Teeravee Hongyok, MD, Christopher J. Rapuano, MD,
  
• Ajoy Virdi, MBBS, Elisabeth J. Cohen, MD,
• Kristin M. Hammersmith, MD
• Cornea Service, Wills Eye Institute
• Jefferson Medical College, Thomas Jefferson
  University
• Philadelphia, PA
• The authors have no financial interest in the
  subject matter for this poster.
• World Cornea Congress VI, Boston, MA, USA, April
  7-9, 2010

2
Ocular Response Analyzer(ORA)

• The ORA is the only machine that can perform
  direct
• clinical assessment of corneal biomechanical
  properties.
• 4 main parameters are measured by ORA
• Corneal Hysteresis (CH) is the difference in
  inward and outward applanation pressures (P1 and
  P2). CH may reflect viscoelasticity of cornea.
• Corneal Resistance Factor (CRF) may reflect
  elasticity of cornea
• Goldmann Correlated IOP (IOPG)
• Corneal Compensated IOP (IOPCC)
• The signal peak height and multiple oscillations
  are changed in post-laser in situ keratomileusis
  (LASIK) keratectasia eyes compared to the normal
  post-LASIK eye (Glass DH, et al., ARVO 2008
  E-abstract 646).
• Using the new ORA software (version 2.04)
  launched in 2009, the device can mathematically
  describe the ORA waveform characteristics with 38
  new parameters including height, slope,
  area-under-the curve and waveform score.

Pictures from http//www.ocularresponseanalyzer.co
m/
3
Corneal Biomechanics in Keratoconus and after
Refractive Surgery

• CH and CRF have been found to be significantly
  reduced in patients with keratoconus1,2 and
  post-LASIK corneal ectasia3
• LASIK was also found to decrease CH and CRF.4-8
• The significant alteration of corneal
  biomechanics may play a critical role in
  development of this serious ectatic complication
  after refractive surgery.
• We hypothesized that surface ablation, which has
  no stromal flap and leaves a thicker residual
  stromal bed, would result in less change in
  corneal biomechanics than LASIK.

1Luce JCRS 2005, 2 Kirwan Ophthalmologica 2008,
3Kerautret JCRS 2008, 4 Pepose AJO 2006, 5Ortiz
JCRS 2007, 6Chen JCRS 2008, 7Hamilton JCRS 2008,
8 Kirwan JCRS 2008
4
Purpose

• To evaluate pre- and post-operative changes in
  corneal biomechanical properties
• using the Reichert Ocular Response Analyzer (ORA)
  
• in patients who undergo surface keratorefractive
  surgery.

5
Methods

• Prospective, non-randomized clinical trial
• Inclusion patients over the age of 18 who
  planned to undergo surface keratorefractive
  surgery on the Cornea Service at Wills Eye
  Institute from November 2008 to February 2009
• IRB approved
• Data collection before surgery, 1 and 3 months
  after surgery
• Corneal thickness U/S (Accupach V) and optical
  (Galilei)
• Corneal topography Atlas 995 (Carl-Zeiss) and
  Galilei (Ziemer)
• Reichert ORA hardware version 3.0, software
  version 2.04
• Basic biomechanics parameters and new waveform
  parameters were analyzed.

6
Results

• 9 eyes of 5 male patients underwent custom
  off-flap myopic epi-LASIK and completed 3 month
  F/U (age 36 7.8 years).

Preoperative Postoperative Postoperative
1 month 3 months
UCVA logMAR SD (Snellen) 1.20 0.17 (20/316) -0.01 0.1 (20/20) -0.06 0.05 (20/17)
MRSE (D) -4.4 1.25 0.18 0.31 -0.21 0.36
Mean keratometry (mean SD, D) Mean keratometry (mean SD, D)
Atlas 43.80 0.44 40.07 1.03 40.50 0.99
Galilei 43.39 0.44 39.64 1.16 39.96 1.08
Laser ablation depth (µm) 60.67 17.2 60.67 17.2 60.67 17.2
Pachymetry (mean SD, µm) Pachymetry (mean SD, µm)
U/S (Accupach V) 549 31 467 37 495 27
Optical (Galilei) 560 29 495 32 503 34
7
Basic Corneal Biomechanics Parameters Changes
after Epi-LASIK




P lt 0.05 (Wilcoxon Signed Rank Test)
8
Corneal Hysteresis Changes after Laser Refractive
Surgery compared to other studies
9
Corneal Resistance Factors Changes after Laser
Refractive Surgery compared to other studies
10
New Waveform Parameters Significant Changes in 9
of 38 (plt0.05)

• Decreased overall Waveform Score

Sample of Signal Overlay of Preoperative ORA and
Postoperative ORA
11
Conclusions

• Epi-LASIK, a surface ablation procedure,
  significantly reduced corneal hysteresis and
  corneal resistance factor.
• Unexpectedly, the magnitude of reduction seems to
  be similar to LASIK from previous publications.
• It appears that corneal thickness is more
  important than the flap creation on the corneal
  biomechanics.
• Waveform analysis also found significant changes
  in both peak 1 and peak 2 and a decrease in
  overall waveform score after surface ablation.

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Conclusions

• A marked decline in these parameters may reflect
  changes in the viscous and elastic qualities of
  the cornea after surface ablation.
• Future work is needed to determine whether
  differences in these corneal biomechanical
  parameters are useful in detecting patients who
  are at risk to develop post-refractive surgery
  ectasia, even after surface ablation.