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K. S. Elder, W. J. Benjamin, BC Focus Group; What Will Replace the Vanishing Radiuscope?. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4844.
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To design and test a novel attachment for evaluation of base curves of gas-permeable (GP) contact lenses using a corneal topographer. The base curve radii measured by the topographer using the attachment were compared to those measured with a Reichert Radiuscope and a Neitz Auto Contact Gauge (Auto CG).
We devised an attachment for a topographer with the intent of measuring the surface radii of GP lenses. A front-surface mirror was fixed at a 45° angle above a hemispherical GP lens mount taken from a Radiuscope. The device was set in the chin rest of a Humphrey Atlas topographer such that the back GP surface image was focused. The base curve (BC) radii of 9 rigid polymer buttons ranging from 5.00mm to 9.00mm in 0.5mm steps were measured 3 times with the Radiuscope, Auto CG, and Atlas topographer using the BC attachment. Each measurement with the topographer included an axial radius, a tangential radius, and a radius derived from the "simulated K" reading (SimK). Mean radii (n = 3) were computed for every combination of nominal radius and measurement method. The mean radii derived with the BC attachment were compared to those derived with the Radiuscope and Auto CG.
Paired T comparisons revealed no statistically significant differences between the 5 sets of 9 mean radii (T ≤ 0.978, df = 8, p ≥ 0.357). The corresponding pairs of Reichert and Neitz mean radii differed by at most 0.013 mm. The Axial mean radii from the topographer differed by at most 0.042 mm from the corresponding radii of the Reichert and Neitz instruments, while the SimK radii differed by at most 0.037 mm. The Tangential mean radii varied by as much as 0.066 mm from those of the Reichert and Neitz instruments.
The Axial algorithm for computation of surface radius was more accurate than the Tangential algorithm, and radii determined with the SimK algorithm were closest to those of the Radiuscope and Auto CG. The mean SimK and Axial radii were within the tolerance for base curve radius, 0.05 mm, cited in ANSI Z80.20-2004 and ISO 18369-2:2006. With minor alteration we feel that the accuracy can be improved. Feasibility of this low-cost BC attachment to enable measurement of base curve radii for GP lenses using a corneal topographer was shown.
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