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F. Oddone, A. Kotecha, J. Shewry, E.T. White, D.F. Garway–Heath; Corneal Thickness– and Age–related Biomechanical Properties of the Cornea Measured With the Ocular Response Analyzer . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4841.
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Purpose: The purpose of this study was i) to calibrate the Ocular Response Analyzer (ORA) and ii) to establish the effects of central corneal thickness (CCT) and age on ORA and Goldmann applanation tonometry (GAT) IOP measurements. The ORA, an air–puff tonometer, measures two applanations (as the pressure rises and falls) – the difference representing a viscoelastic property called corneal hysteresis (CH). Methods: i) baseline GAT IOP and ORA 1st applanation (P1) and CH were measured in both eyes of 45 untreated ocular hypertensive (OHT) and 60 normal subjects (average age 60, range 26 – 82 years). Iopidine drops were administered to one randomly selected eye. After 3 hours GAT and ORA measurements were repeated. The relationship between change in P1 and change in CH was used to normalise CH (nCH) for P1. CH was further normalised for age. ORA IOP was defined as P1 minus nCH and calibrated against GAT and CCT by multiple linear regression. ii) GAT and ORA measurements (5 minutes between readings) were made in a randomised order in one eye of 130 normal, OHT and glaucomatous subjects (average age 61, range 22 – 83 years). The main outcome measures were the association of CH with CCT and age; agreement between GAT and ORA; the effect of CCT on measured IOP, and the effect of CCT and age on tonometer differences (power 93.5% at p = 0.05 (two–sided) for r = 0.30). Results: i) change in CH was significantly associated with change in P1 (R2 =0.14, p = 0.001). nCH was significantly associated with CCT (slope 0.227, R2 =0.30, p<0.0001) and age (slope –0.269, R2 =0.04, p=0.04). ii) the slope (per 100–µm increase in CCT) of the association of IOP with CCT was 3.2mmHg (p=0.003) for GAT and 2.4mmHg (p=0.004) for ORA. The GAT/ORA agreement (mean difference ± 95% limits of agreement) was –2.1 ± 4.5mmHg. Conclusions: CCT has a greater effect on IOP measurements by GAT than by ORA. CH is significantly greater in thicker corneas, and reduces with age. These findings suggest that the cornea’s viscoelastic property declines with age.
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