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T.R. Karkkainen, C. Gautreau, K. Kasovich; A novel method for determining hydrogel lens thickness in–vivo . Invest. Ophthalmol. Vis. Sci. 2004;45(13):1563.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose:Changes in hydrogel lens thickness can have significant effects on lens performance, especially oxygen transmission. The thickness of a hydrogel is difficult to verify, because it cannot be measured directly with calipers and optical methods are often complicated or inaccurate. With ultrasound pachymetry becoming commonplace in ophthalmic practice, we investigated if it could be used to determine the thickness of hydrogel lenses in–vivo. These values will be compared to a standard radiuscope technique for accuracy and repeatability.Methods:A total of 15 subjects, 10 female and 5 male, participated in the study. Each subject was an adapted contact lens wearer and free of anterior segment disease. The right eye of the subjects were fit with a hilafilcon–A, –1.00 DS hydrogel and the lens was checked to ensure proper centration. Pachymetry measurements were performed by placing the ultrasound tip on the either the central cornea or corneal/lens combination. These measurements were randomized with regards to their presentation. The lens thickness was calculated by subtracting the corneal thickness from the lens/corneal combination. The measurements were then repeated as previously described. The same lens had its center thickness determined by a popular optical method using a radiuscope. These measurements were again repeated. The mean center thicknesses were calculated and a t–test applied to determine significant differences between the means. Results:The overall mean lens thickness and standard deviation was 183.83+8.68 microns and 143.33+74.31 microns for the ultrasound and radiuscope methods respectively. A t–test revealed that the means were significantly different (p=0.01). Over half (53%) of the repeated measures with the ultrasound technique were within 5 microns and 73% were within 10 microns. Comparatively, only 13% and 40% of the repeat radiuscope measurements were within 5 and 10 microns respectively. The differences in percentages were statistically significant (p<0.05).Conclusions:Ultrasound pachymetry can be used to determine in–vivo hydrogel lens thickness. The repeatability of the technique is significantly better than values obtained using a standard radiuscope method. This method will allow for verification of lens thickness and more accurate calculations of oxygen transmission. In addition, the technique can be used by research investigators to evaluate the environmental effects (i.e. humidity, osmolarity, pH, temperature, medications etc) on lens thickness during wear.
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