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J. Liu, X. He, J. Tang, A. E. Knellinger, K. F. Baston, X. Pan; Corneal Stiffness May Influence IOP/CCT Relationship. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2841.
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The accuracy of Goldmann applanation tonometry is known to be affected by corneal properties. Theoretical analysis suggested that the variance in corneal stiffness could potentially introduce large errors in tonometric measurement of intraocular pressure (IOP); moreover, different corneal stiffness would result in different "slopes" necessary for correcting the effect of central corneal thickness (CCT).  The goal of this study is to examine in vivo how corneal stiffness influences the IOP/CCT relationship.
The study was approved by the Institutional Review Board at The Ohio State University. Eighteen subjects over 18 years old were recruited. The mean age was 55.0 ± 19.9 years (range: 25-85 years). Corneal stiffness, CCT, and IOP were measured in one eye per subject during one visit. A quantitative ultrasound method was used to determine corneal aggregate modulus, which provides a measure of corneal stiffness.  During the ultrasonic measurements, the subject was in a supine position and an eye cup filled with buffered saline was placed on the eye after the application of corneal anesthetics. CCT was measured by using an ultrasound pachymeter. IOP was measured by using a Goldmann Applanation Tonometer (AT 900, Haag-Streit). Three readings were obtained for each measurement, and the average was used for analysis. The corneas were categorized into two groups based on the measured corneal stiffness: the "soft" group had a corneal stiffness below the mean; and the "stiff" group had a corneal stiffness above the mean. The relationship between measured IOP and CCT was analyzed in all subjects and in the sub-groups.
The CCT, IOP and corneal stiffness in the study participants were 535.4 ± 38.4 µm, 12.8 ± 2.5 mmHg, and 3.02 ± 0.21 GPa (as aggregate modulus). The mean CCT and IOP were not significantly different comparing the "soft" and the "stiff" groups. The Pearson correlation between IOP and CCT in all subjects was 0.46 (P=0.06). The slope between IOP and CCT in the "soft" group was 0.02 mmHg/µm (R=0.28, P=0.42), while the slope was significantly higher in the "stiff" group: 0.05 mmHg/µm (R=0.67, P=0.07).
Both the correlation between IOP and CCT and the IOP/CCT slope were higher in the "stiff" group. This result suggests that corneal stiffness may influence the IOP/CCT relationship. Specifically, tonometric measurement of IOP may have a stronger dependence on CCT in stiff eyes than in soft eyes. Future larger sample size study is needed to validate this preliminary finding. Liu J, Roberts CJ. JCRS 2005;31:146-155. Liu J, et al. J Biomech 2007;40:1177-1182.
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