1. 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

2. Financial Disclosure
The authors of this poster have no financial interest in the subject matters.

3. 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

4. Ocular Response Analyzer

5. 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)

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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.