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

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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, Korea Seoul 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 Previous Studies 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 Parameters 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 reading Suspect 47.2 < < 48.7 D Positive > 48.7 D I-S index 3mm 1.4 < < 1.9 D > 1.9 D 6mm > 3 D Corneal Asymmetry Difference > 0.92 D Modified Rabinowits Keratoconus Index 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

Results 1. Subject Characteristics Fisher’s exact test; 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 Fisher’s exact test; Mann-Whitney U test 2. ORA between KC and Control (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 0.020 0.003 CH, CRF (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 IOP

Results 3. . Correlation Analysis in KC CH – CRF 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 3mm Irregular astig 0.421 0.029 5mm Irregular astig 0.553 0.003 CCT -0.420 2. ORA between KC and Control CH, CRF Spearman’s correlation coefficient

Results 3. . Correlation Analysis in KC 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 Spearman’s correlation coefficient

Discussion Corneal 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

Discussion Conclusions About keratoconus suspect Limitations 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. Conclusions The 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.