Corneal Hysteresis

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Corneal Hysteresis

• Role of Corneal Biomechanics in Refractive
  Surgery

Luis Rodriguez, MD Anny Villegas, MD Clínica de
Córnea Centro Médico Docente La Trinidad Caracas,
Venezuela
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• Corneal Biomechanics is the science that studies
  the equilibrium and deformation of tissues
  submitted to any force. This science includes the
  following extrinsecal factors Intraocular
  Pressure, Cilliar Muscle, Extraocular Muscle,
  Atmospheric Pressure and Eyelids.
• Intrinsecal factors of Corneal Biomechanics
  include Central Pachymetry, Viscosity,
  Elasticity, Hydration and Regional Pachymetry.

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• The Ocular Response Analyzer (ORA) is a new
  instrument that measures the corneal
  biomechanical response (corneal hysteresis, CH).
  It measures the effect of viscous and elastic
  qualities with a calibrated air pulse in vivo.
• The ORA measures Intraocular Pressure (Goldmann).
  
• Corneal hysteresis is an indication of viscous
  damping in the cornea, re?ecting the capacity of
  corneal tissue to absorb and dissipate energy.
• It is the capacity of tissue to recover its
  original shape after external force is applied.

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• The Corneal Resistance Factor, also derived from
  corneal hysteresis, is an indicator of the
  overall resistance of the cornea.
• Corneal-compensated IOP is a pressure measurement
  that utilizes the new information provided by the
  corneal hysteresis measurement to provide an IOP
  that is less affected by corneal properties.
• The objective of this study is to discuss the
  Ocular Response Analyzer readings of corneal
  biomechanics in refractive surgery and corneal
  pathology.
• This study began in July 2006 and ended in
  January 2007.

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• Methods
• ORA was used to obtain four measurements in each
  patient, and the mean of these four readings was
  used in the analysis according to the
  manufacturers instructions.
• The study included 244 eyes from the Corneal
  Department of the Centro Medico Docente La
  Trinidad divided into the following nine
  categories
• Normal cornea, Radial Keratotomy, PostLasik
  Myopia, PostLasik Ectasia, Keratoconus, Post
  Intracorneal Segments (Intacs), Penetrating
  Keratoplasty, Fuchs Dystrophy and Graft
  rejection.

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Results
Ranges of Corneal Hysteresis (mmHg)
244 Eyes
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Frequency of Corneal Hysteresis
Frequency of Corneal Resistance Factor (CRF)
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Frequency of IOPg and IOPcc values
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16,36
15,05
16
14,86
14,73
14,49
14,23
14
11,64
11,22
11,54
12
10
9.02
8
6
4
2
0
Normal Cornea
Keratoconus
Post-LASIK
Post- LASIK Ectasia
Post-INTACS
IOPg
IOP cc
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(No Transcript)
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Keratoconus
Lasik
Ectasia Post Lasik
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• Discussion
• Patients with -6.00D (equal to or less),
  Hysteresis of more than 11mmHg, and normal
  topography are good candidates for LASIK.
• Patients with -6.00D or less, suspicious
  topography and Hysteresis between 8mmHg and
  10mmHg are good candidates for PRK.
• Patients with -6.00D or more, suspicious
  topography and Hysteresis between 7mmHg and
  10mmHg are good candidates for Phakic IOL.
• Patients, with Keratoconus and Hysteresis of
  more than 7mmHg are good candidates for
  intracorneal segments.
• Patients with Keratoconus and Hysteresis of equal
  to or less than 5mmHg are candidates for corneal
  transplant.
• When Hysteresis is less than 7mmHg PostLasik,
  patients have Ectasia.
• Hysteresis is normal in Corneal Transplants, but
  if the patient has a transplant rejection
  Hysteresis decreases significantly.

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• Biomechanical studies must be analyzed
  individually
• Ectasia is a known risk of LASIK and PRK
• Hysteresis measures corneal biomechanical
  properties
• Patients who are at risk of developing post-LASIK
  ectasia should be identified by analyzing
  Hysteresis
• Corneal Hysteresis can guide the physician in the
  choice of treatment.
• ORA helps us understand the IOP in patients with
  corneal pathologies and surgeries.