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Corneal Hysterisis

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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
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.
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, reflecting 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.
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.
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 manufacturer’s
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.
Results
                                         Ranges of Corneal Hysteresis
                                                   (mmHg)

           20 Normal cornea                    7,96 – 11,57

           10 Radial Keratotomy                 3,50 – 5,75

           10 Post-LASIK Ectasia                3,42 – 7,43

           34 Post-LASIK Myopia                 7,24 – 9,82

           70 Keratoconus                       5,74 – 7,71
244 Eyes
           70 Post-Intracorneal
                                                7,09 – 9,97
           Segments (INTACS®)

           20 Penetrating Keratoplasty          7,74 – 11,71

           5 Fuch’s Distrophy                   6,25 – 7,71

           5 Graft Rejection                    5,35 – 7,40
      Frequency of Corneal Hysteresis
        10
                8.93
            9                          8.62
                                                              8.24
            8                                                                      6.95
                                                                                                       6.47




                Normal Cornea
            7




                                       Post - LASIK




                                                             Post -INTACS
            6




                                                                                   Keratoconus




                                                                                                       Post -LASIK
            5




                                                                                                         Ectasia
            4
            3
            2
            1
            0




Frequency of Corneal Resistance Factor (CRF)
        9                       8.24
                                                      7.73                  7.71
        8
        7                                                                                        6.3
        6                                                                                                       5.42
        5
        4
        3
        2
        1
        0

                                       Normal Cornea
                                       Post-LASIK
                                       Post-INTACS
                                       Keratoconus
                                       Post-LASIK Ectasia
     Frequency of IOPg and IOPcc values

18                            16,36
16    14,86   15,05
                                              14,49
                                                                      14,23
                                                                                      14,73

14
                      11,64                                                   11,22
12                                    11,54


10                                                             9.02


 8
 6
 4
 2
 0
     Normal Cornea Keratoconus         Post-LASIK              Post- LASIK    Post-INTACS®
                                                                 Ectasia


                                      IOPg            IOP cc
      QuickTime™ an d a
    Cinepak decompressor
are need ed to see this p icture .
Keratoconus




Lasik




Ectasia
Post Lasik
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.
• 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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posted:5/21/2011
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