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L. E. Kimmel, W. Miller, N. Leach, J. Horne, T. C. Prager, S. Quintero, J. P. G. Bergmanson; Multiple Orbscan IIz Readings to Determine Measurement Variation in Normal Subjects. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3538.
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© ARVO (1962-2015); The Authors (2016-present)
The slit-scan corneal topographer/pachymeter, Orbscan IIz, is utilized for both clinical and research purposes. This study evaluates the repeatability of measures taken from a sample of normal individuals.
A randomized single eye of 10 subjects (mean age:26.6; range:23-35) with normal corneas was measured with the Orbscan IIz by a single examiner. Five scans were completed on each of two consecutive days, at the same time (+/-1.5 hrs), for a total of 10 scans/subject. Data collected included the pachymetry from the thinnest(TT) and the 12o’clock peripheral points(PT) as identified by the OrbcanIIz, the steepest(Ks) and flattest(Kf) meridians in diopters, and the maximal anterior and posterior positive(AF+,PF+) and negative(AF-,PF-) floats. In addition, corneal thickness(CT) was calculated as (PT)-(TT), anterior float(AF) as (AF+) - (AF-), and posterior float(PF) as (PF+) - (PF-). A coefficient of variation(CV) was calculated for each variable for each subject.
The mean CVs and standard errors(SE) for each variable were calculated. Low variability (CV<0.15) was found for TT=0.010,SE=0.00088; PT=0.025,SE=0.0047; K1=0.0041,SE=0.00059; and K2=0.0041,SE=0.00055. Moderate variability measures (CV=0.15-0.25) were observed for AF+= 0.20,SE=0.026; AF-= -0.23,SE =0.027; AF=0.16,SE=0.019; PF+= 0.18,SE=0.016; PF-= -0.21,SE=0.019; PF=0.16,SE=0.018; and CT= 0.20,SE=0.038.
The mean CV was low for TT, PT, K1, and K2, indicating that the OrbcsanIIz pachymetry and K-readings are highly repeatable for a population of normal subjects. The mean CVs for AF+,AF-,PF+, and PF- fell into a medium range and are therefore more variable for normal subjects, but probably still of clinical value. The variables that were calculated from 2 collected data points(CT,AF,and PF) also fell within the medium range, probably due to accumulated variability in their component pieces. Across all scans(n=100), the CV for PF was 0.323, falling into the high range(>0.25). The sensitivity of the instrument for the posterior float map in normal corneas may not be clinically helpful.
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