Corneal Hysteresis. Role of Corneal Biomechanics in Refractive Surgery. Luis Rodriguez, MD Anny Villegas, MD Cl ínica de Córnea Centro Médico Docente La Trinidad Caracas, Venezuela.
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Role of Corneal Biomechanics in Refractive Surgery
Luis Rodriguez, MD
Anny Villegas, MD
Clínica de Córnea
Centro Médico Docente La Trinidad
Intrinsecal factors of Corneal Biomechanics include: Central Pachymetry, Viscosity, Elasticity, Hydration and Regional Pachymetry.
The ORA measures Intraocular Pressure (Goldmann).
Corneal hysteresis is an indication of viscous damping in thecornea, reﬂecting the capacity of corneal tissue to absorb anddissipate energy.
It is the capacity of tissue to recover its original shape after external force is applied.
Corneal-compensated IOP is a pressure measurement thatutilizes the new information provided by the corneal hysteresismeasurement to provide an IOP that is less affected by cornealproperties.
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.
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.
20 Normal cornea
10 Radial Keratotomy
3,50 – 5,75
3,42 – 7,43
10 Post-LASIK Ectasia
7,24 – 9,82
34 Post-LASIK Myopia
5,74 – 7,71
70 Post-Intracorneal Segments (INTACS®)
7,09 – 9,97
7,74 – 11,71
20 Penetrating Keratoplasty
6,25 – 7,71
5 Fuch’s Distrophy
5,35 – 7,40
5 Graft RejectionResults
Ranges of Corneal Hysteresis (mmHg)
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.