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J. D. Marsack, R. A. Applegate, E. J. Sarver, M. D. Twa, J. Schwiegerling, E. DeHoog; Accuracy of Lathe-Based Manufacture of Wavefront Guided Surfaces. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4870. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Interest exists in the implementation wavefront-guided corrections in the form of a non-surgical, replaceable contact lens, specifically in regards to the highly aberrated keratoconic eye. The purpose of this work is to quantify the accuracy associated with lathe-based manufacture of wavefront-guided surfaces containing astigmatism, coma and trefoil, three common aberrations present in keratoconic eyes.
A 10th order Zernike polynomial expansion was used to describe corneal shape data over 7mm for 99 eyes comprised of keratoconus and keratoconus suspects. RMS magnitude for total astigmatism, coma and trefoil were calculated for each eye. The 1st quartile, 2nd quartile and 3rd quartile RMS values for each aberration were identified, for a total of 9 test conditions. Each test condition was combined with a 7.5mm base curve sphere in custom software written in Matlab (The MathWorks, Natick MA) and carved into PMMA rod on a 2X-ALM ophthalmic lens lathe (DAC Intl, Carpinteria CA) to form 9 models. Each model was measured 5 times on a Keratron (Optikon, Rome Italy) corneal topographer. Exam data was exported from the topographer and decomposed into a 10th order Zernike polynomial describing corneal shape with Visual Optics Laboratory (Sarver and Associates, Carbondale IL). The average and standard deviation for the term of interest in each model was calculated across the 5 measurements for each model.
Of the 9 models, 4 were within 2.5% of the designed value, 2 were within 5% and the remaining 3 were within 10%. The three largest magnitude manufacturing errors were encountered in designs for the 3rd quartile coma (0.74µm), 3rd quartile trefoil (0.15µm) and 3rd quartile astigmatism (0.11µm). The standard deviation across the 5 RMS values for all 9 models studied was < 0.06µm.
The system for lathe-based manufacture of custom surfaces is capable of implementing the aberration shapes of astigmatism, coma and trefoil. For each aberration of interest, the largest magnitude error was seen at the 3rd quartile level. Future studies will examine manufacturing precision across multiple samples of identical design to identify systematic errors. Interferometry will also be examined as a method to quantify accuracy of the process.
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