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M. L. Read, C. Maldonado-Codina, P. B. Morgan; Measurement Errors Related to Contact Angle Analysis of Hydrogel Materials. Invest. Ophthalmol. Vis. Sci. 2009;50(13):6344.
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To undertake a comprehensive investigation of the measurement error associated with contact angle assessment of hydrogel contact lens surfaces.
Three silicone hydrogel contact lenses (senofilcon A, balafilcon A and lotrafilcon A) and one hydrogel contact lens (etafilcon A) were investigated. Contact angles using the sessile drop and captive bubble methods were measured using an OCA-20 contact angle analyser (DataPhysics Instruments, Germany). Ten lenses were evaluated per brand and contact angles were measured seven times for each lens type using each of the two methods. Contact angles were analysed using semi-automated analysis software and reanalysed after 24 hours (observer masked). The contact angle coefficient of repeatability (COR) as well as the 95% confidence intervals associated with three measurement conditions (image analysis COR, intra-lens COR and inter-lens COR) were determined as advocated by Bland and Altman.
There was no statistically significant difference between test and re-test image analysis measures of contact angle (all p values ≥ 0.244) and image analysis COR values were less than three degrees in all cases. Intra-lens COR values (95% confidence intervals) for the sessile drop method ranged from 10.2° (8.4°, 12.1°) (balafilcon A) to 5.2° (4.3°, 6.1°) (etafilcon A). Intra-lens COR values for the captive bubble method ranged from 5.6° (4.6°, 6.6°) (etafilcon A) to 4.0° (3.3°, 4.7°) (lotrafilcon A). Inter-lens COR values for the sessile drop method ranged from 16.5° (13.6°, 19.4°) (senofilcon) to 5.4° (4.5°, 6.4°) (etafilcon A). Inter-lens COR values for the captive bubble method ranged from 7.6° (6.3°, 9.0°) (etafilcon A) to 4.5° (3.7°, 5.2°) (lotrafilcon A).
Measurement error associated with semi-automated image analysis software was shown to be small as an absolute measure, although more significant for materials with low contact angle. Contact angles measured using the sessile drop method were typically less repeatable than those measured using the captive bubble method, likely due to lens blotting variability. Intra-lens repeatability differed between lens types suggesting varying surface wetting heterogeneity. COR values were similar for intra- and inter-lens repeatability, for all lenses except senofilcon, suggesting that pre-analysis surface blotting and changes in location of testing are the main factors influencing the repeatability of contact angle measurements.
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