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J. Schwiegerling, B. Au, R. W. Snyder; Accuracy and Repeatability of Pachymetry Measurements With the Oculus Pentacam. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3539.
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© ARVO (1962-2015); The Authors (2016-present)
To test the accuracy and repeatability of measuring corneal thickness with the Scheimpflug imaging based Oculus Pentacam.
Three PMMA contact lenses with known thicknesses and curvatures were measured with the Pentacam. The lens material was doped with titanium oxide to introduce intralens scatter comparable to that of the cornea. The base side of the lens was filled with Hyprosol gel to provide a comparable surface reflection to that of the cornea/aqueous interface. Each lens was measured four times with the Pentacam and the central pachymetry was recorded. An exact raytracing model was created to determine the correction required for corneal thickness measurement since the index of refraction of PMMA is higher than that of the cornea. The center thickness of each lens was measure with a micrometer to determine its true thickness.
The raytracing model showed that a correction factor of 1.118x needs to be applied to the pachymetry value displayed by the Pentacam to account for the index difference between PMMA and corneal tissue. The micrometer measured thicknesses of the lenses were 515, 560 and 584 microns. The pachymetries of the first two lenses measured with the Pentacam were 522 +/- 2 and 555 +/- 3 microns respectively. The third lens had an abnormally high reflection from its posterior surface. It appears this elevation affects the measurement of pachymetry. Our measured value for this lens was 632 +/-3 microns. However, outside of this region of high reflection, the pachymetries are comparable to the micrometer measurement.
To create test surfaces for Scheimpflug based systems, models that exhibit scatter and reflection effects close to the that human cornea are needed. In addition, a correction factor is needed to adjust for differences in the indices of refraction of the cornea and the model material. Based on these limited results, it appears that the Pentacam gives accurate (to within 7 microns) and repeatable (+/- 3 microns) results for pachymetry. The discrepancies in thcknesses seen in the third lens are likely due to problems with the lens and not with the device.
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