Analysis of Corneal Biomechanical Properties in Keratoconus Using

Ocular Response Analyzer

Hyuck Jin Choi, Joo Youn Oh, Won Ryang Wee, Mee Kum Kim,

Ji Won Kwon, Sang Mok Lee, MD, Jin Seok Choi, MD

Department of Ophthalmology, Seoul National University College of Medicine, Seoul, KoreaSeoul Artificial Eye Center, Seoul National Univerisity Hospital Clinical Research Institute, Seoul, Korea

Financial Disclosure

The authors of this poster have no financial

interest in the subject matters.

Introduction

Biomechanical Properties of Cornea•Hysteresis

•A property of physical systems that do not instantly follow the forces applied to them, but react slowly, or do not return completely to theior original state•Etymology: ‘ late, fall short’ in ancient Greek

•Corneal Hysteresis (CH)•The the difference between the inward (P1) and outward (P2) pressure values obtained during the dynamic bi-directional applanation process employed in the Ocular Response Analyzer, as a result of viscous damping in the cornea.•P1 – P2

•Corneal Resistance Factor (CRF)•the overall resistance of the cornea, including both the viscous and elastic properties (total visco-elasticity)•P1 – 0.7 x P2

Ocular Response Analyzer

Introduction

Biomechanical Properties of Cornea•Corneal-compensated intraocular pressure (IOPcc)

•Less affected by corneal properties than other tonometry•P2 – 0.43 x P1

•Goldmann-correlated intraocular pressure (IOPg)•the overall resistance of the cornea, including both the viscous and elastic properties (total visco-elasticity)•P1 – 0.7 x P2

Previous Studies•Normal Value of CH, CRF

•CH = 10.8 ±1.5 mmHg, CRF = 11.0 ±1.6 mmHg (n=165) (Ortiz et al. IOVS 2007)•CH = 10.9 ±1.6 mmHg, CRF = 11.0 ±1.6 (n=70) (Kirwan et al. Ophthalmologica 2008)

•Keratoconus, Post-LASIK•CH and CRF ⇒ low (Shah et al. IOVS 2007, Pepose et al. AJO 2007, Susan et al. BJO 2008, Kerautret et al. JCRS 2008)

Purpose

To assess the correlation between corneal

biomechanical properties such as corneal

hysteresis(CH) or corneal resistance factor(CRF)

measured by the Ocular Response Analyzer(ORA)

and topographic parameters measured with ORB

scan

Methods

Subjects•Period of data collection

•June 1, 2008 – September 31, 2008

•Outpatient clinic, Seoul National University Hospital•Inclusion criteria

•Subjects who met keratoconus index

•Control group•Age and sex-matched normal eyes

Items Value

Central Curvature

K readingSuspect

47.2 < < 48.7 D

Positive > 48.7 D

I-S index 3mmSuspect

1.4 < < 1.9 D

Positive > 1.9 D

6mm Positive > 3 D

Corneal Asymmetry

Difference Positive > 0.92 D

Parameters•Ocular Response Analyzer

•Corneal hysteresis(CH), corneal resistance factor(CRF), corneal-compensated IOP(IOPcc), Goldmann-correlated IOP(IOPg)

•ORB scan•Central curvature, I-S 3mm, I-S 6mm, SimK Max, SimK min, Simk astig, 3mm irregular astig, 5mm irregular astig, CCT

Modified Rabinowits Keratoconus Index

Results

KC Control P value

Subjects 16 14

Eyes 27 28

Gender (M/F)

11/5 9 / 5 0.630

Age (yrs) 30.0 ±7.4 27.0 ±1.7 0.142

Eye (R/L) 12 / 15 14 / 14 0.789

(mmHg) CH CRF CH-CRF CH/CRF

KC 7.8 ±1.5 7.1 ±1.7 0.8 ±0.9 1.1 ±0.1

Control 10.0 ±1.6 9.8 ±1.4 0.2 ±0.9 1.0 ±0.1

P value <0.001 <0.001 0.020 0.003

(mmHg) IOPcc IOPg IOPcc-IOPg IOPcc/IOPg

KC 15.4 ±2.5 11.6 ±3.1 3.8. ±1.7 1.4±0.2

Control 15.6 ±3.0 14.6 ±2.6 1.1±1.6 1.1±0.1

P value 0.966 <0.001 <0.001 <0.001

CH, CRF

IOP

Fisher’s exact test; Mann-Whitney U test

1. Subject Characteristics

2. ORA between KC and Control

Results

2. ORA between KC and Control

CH, CRF

CH-CRF vs. r P value

Central Curvature 0.357 0.068

I-S 3mm 0.257 0.196

I-S 6mm 0.255 0.198

Simk Max 0.450 0.019

Simk min 0.433 0.024

Simk Astig 0.280 0.157

3mm Irregular astig 0.415 0.031

5mm Irregular astig 0.552 0.003

CCT -0.394 0.042

CH/CRF vs. r P value

Central Curvature 0.347 0.076

I-S 3mm 0.259 0.191

I-S 6mm 0.272 0.169

Simk Max 0.435 0.023

Simk min 0.429 0.025

Simk Astig 0.260 0.191

3mm Irregular astig 0.421 0.029

5mm Irregular astig 0.553 0.003

CCT -0.420 0.029

Spearman’s correlation coefficient

CH – CRF CH / CRF

3. . Correlation Analysis in KC

Results

Spearman’s correlation coefficient

IOPcc-IOPg IOPcc/IOPg

IOPcc-g vs. r P value

Central Curvature 0.010 0.615

I-S 3mm 0.011 0.955

I-S 6mm 0.222 0.265

Simk Max 0.040 0.843

Simk min 0.212 0.289

Simk Astig -0.185 0.356

3mm Irregular astig 0.077 0.704

5mm Irregular astig 0.157 0.435

CCT -0.509 0.007

IOPcc/g vs. r P value

Central Curvature 0.283 0.186

I-S 3mm 0.181 0.365

I-S 6mm 0.267 0.179

Simk Max 0.263 0.185

Simk min 0.378 0.052

Simk Astig -0.004 0.986

3mm Irregular astig 0.258 0.194

5mm Irregular astig 0.337 0.085

CCT -0.510 0.007

3. . Correlation Analysis in KC

DiscussionCorneal biomechanical changes in keratoconus

• B oth CH and CRF in keratoconus ⇒ lo w er than normal control• CRF decreased more than CH.• As corneal thicknessdecreases, biomechanical stability decreases

Combined parameters in keratoconus•IOPcc-g and IOPcc/g

• IOPcc = IOPg + Corneal factor ⇒ Corneal factor = IOPcc - IOPg

• If IOPcc-g or IOPcc/g is high, there is corneal biomechanical change more than normal condition.• GAT can be underestimated in keratoconus.

Correlation Analysis with ORB Scan• CH-CRF, CH/CRF

• CCT w as negatively correlated• As w ell as CCT, simK value and irregular astigmatism of 3mm and 5mm were also positively correlated.

• IOPcc-g, IOPcc/g• Only CCT w as negatively correlated

DiscussionAbout keratoconus suspect

• Possibility to detect early keratoconus change or progression.

Limitations• Cross-sectional study

• Difficult to generalize into the change in subject

Suggestions for future studies• Study to find the risk factors of post-LASIK ectasia with various ORA parameters.

ConclusionsThe corneal biomechanical parameters and their combined parameters measured using Ocular Response Analyzer were correlated with topographic parameters. So there is a possibility to put the biomechanical parameters to practical use in detecting and monitor keratoconus.