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M.K. Smolek, S.D. Klyce; Standard Absolute Scales for Corneal Wavefront Error Maps . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2565.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Correct clinical interpretation of corneal topography requires an absolute standard scale based on statistical sampling of a pool of all potential conditions and a graphical mapping system designed with optimized color contrast. This study investigates the population statistics for total and high-order corneal wavefront error (CWE) and the effects of optical zone, range, and contour size on color map design. Methods: CWE was assessed using 10 corneal topography TMS maps from each of 12 conditions: normals; astigmatism; mild, moderate, and severe keratoconus (KC); KC suspects; contact lens-induced corneal warpage; myopic refractive procedures (RK, PRK, and LASIK); pellucid marginal degeneration; and penetrating keratoplasty. Zernike fitting orders 2 to 8 defined total CWE and orders 3 to 8 defined high-order CWE. CTView 5.01 extracted CWE data from each exam using optical zone diameters of 3, 5, and 7 mm. Mean, SD, median, mode, range, and frequency histograms were found for each condition and all conditions combined. The total and high-order CWE SD values of normals were used as the significance threshold for the optimum contour size when mapping across all conditions. Results: A 3 mm zone had insufficient pattern information to interpret maps. Interpretation with a 7 mm zone was complicated by the variance of 2nd order terms in normals. The 5 mm zone was acceptable for interpreting maps. Two standard maps were needed for clinical analysis. The total CWE map required a 0.75 µm contour step and a range of 19.5 µm to map 96% of all mappable points across all 12 conditions, while the high-order CWE map required a 0.25 µm contour step and a range of 6.5 µm. Both maps use 27 discrete color contours. Conclusions: The peak frequencies of total and high-order CWE histograms are centered near zero for every condition, making map interpretation more difficult than with dioptric maps. The aberration range and pattern are the key criteria for interpretation, and hence the need for an absolute scale. Adjustable scales cause aberration maps to be easily misinterpreted. Using a single 5 mm zone is advantageous for rapid clinical screening. The CWE of any corneal condition can be interpreted using 2 absolute standard scales: a high resolution scale for mapping high-order errors and a low resolution scale for mapping total error. The contour intervals and range limits allow all of the wavefront error surface to be mapped, except for some localized regions on extremely aberrated corneas, such as keratoplasty. Increasing the mappable points to 97% requires 41 distinct colors and does not make the map easier to read. The methods used here may be helpful in establishing whole eye wavefront error mapping standards.
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