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C Riley, P Kollbaum, G Lowther; Repeatability of ORBSCAN Pachymetry Measures in Keratoconus . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3230.
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Purpose: Keratoconus is a non-inflammatory disease characterized by progressive thinning of the central cornea. Reliable and accurate measurements of corneal thickness over time are important factors in monitoring keratoconus progression. The purpose of this study is to evaluate the repeatability of ORBSCAN central pachymetry measurements in keratoconus. Methods: Three morning measurements (at least 2 hours after waking) and 3 evening measurements separated by 5-7 hours were taken on 3 days from 42 eyes of 21 diagnosed keratoconic subjects. Day-to-day, morning-afternoon, and trial-to-trial repeatability were assessed by a repeated measure ANOVA analysis and a 95% limits of agreement (LoA), taking (1) the average of central 3mm diameter ring and (2) measurement of the thinnest corneal point. Results: Fourteen male and 7 female subjects with a mean age of 40 (range: 21-63), and mean steep keratometry readings of 49 (range: 41-72) were used. Twenty-eight eyes wore RGP's and 2 eyes wore SCL's. Five eyes had central corneal scarring. The mean ±SD at the apex and the thinnest point were 479.8±61.81µm and 447.6±70.53µm, respectively. The thinnest point was more centrally located than the cone apex, which was usually located inferiorly. The instrument demonstrated good repeatability between trials (mean SD=±10.34µm). In the design of the current study, there were no significant diurnal variations (p=0.14), or day-to-day variations (p=0.20). The 95% LoA values ranged from -21.0 to +19.9µm.Conclusion: Mean thickness values obtained were similar to earlier studies using ORBSCAN and other pachymetry devices in keratoconus patients. No significant trial-to-trial, diurnal or day-to-day variations were observed. The repeatability of the ORBSCAN system in the current study suggest it could be a repeatable method for measuring corneal thickness in a multiple session or longitudinal study of the keratoconus population.
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