Transcript Corneal Hysterisis - ASCRS/ASOA 2008

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)
244 Eyes
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
70 Post-Intracorneal
Segments (INTACS®)
7,09 – 9,97
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
8.93
8.62
9
8.24
6.95
4
3
2
6.47
Keratoconus
5
Post -INTACS
6
Post - LASIK
7
Normal Cornea
8
Post -LASIK
Ectasia
10
1
0
Frequency of Corneal Resistance Factor (CRF)
9
8
7
6
5
4
3
2
1
0
8.24
7.73
7.71
6.3
5.42
Normal Cornea
Post-LASIK
Post-INTACS
Keratoconus
Post-LASIK Ectasia
Frequency of IOPg and IOPcc values
18
16
14
12
10
8
6
4
2
0
16,36
14,86
15,05
14,49
11,64
14,73
14,23
11,22
11,54
9.02
Normal Cornea Keratoconus
Post-LASIK
IOPg
IOP cc
Post- LASIK
Ectasia
Post-INTACS®
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
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.