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Corneal Hysteresis

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When Hysteresis is less than 7mmHg PostLasik, patients have Ectasia. ... Ectasia is a known risk of LASIK and PRK. Hysteresis measures corneal biomechanical properties ... – PowerPoint PPT presentation

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Title: Corneal Hysteresis


1
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
2
  • 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.

3
  • 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.

4
  • 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.

5
  • 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.

6
Results
Ranges of Corneal Hysteresis (mmHg)
244 Eyes
7
Frequency of Corneal Hysteresis
Frequency of Corneal Resistance Factor (CRF)
8
Frequency of IOPg and IOPcc values
18
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
9
(No Transcript)
10
Keratoconus
Lasik
Ectasia Post Lasik
11
  • 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.

12
  • 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.
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