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Hala M Ali, Sumith Perera; The effect of measurement variation in keratometric readings for the calculation of surgically induced astigmatism using online calculators. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4188.
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Knowledge of surgically induced astigmatism (SIA) is important in planning corneal incision parameters and calculating the power and positioning of toric lenses in lens implant surgery. The doctor-hill.com SIA Calculator uses vector analysis software to calculate SIA, using pre- and post-operative keratometric readings (Ks) and is available online for clinician usage. Keratometric measurements can give variable readings as they depend on the tear film reflections. This study aims to calculate the mean error caused by variations in repeated Ks using doctor-hill.com SIA calculator.
Three successive keratometric readings were taken by the same trained and experienced technician using the IOLMaster partial coherence interferometry device (Version S.2, Carl Zeiss Meditech Dublin California) for 36 virgin eyes of successive patients due to undergo lens extraction surgery. The IOLMaster was calibrated before each examination and only Ks that were within the recommended range were included for averaging. By inputting the Ks from the first scan as the pre-operative figure and the Ks from the second scan as the post-operative K value we obtained a value for ‘SIA’ that would arise purely due to the variability in measurement. This was repeated for scans one and three, and scans two and three. Therefore three ‘SIA’ values for each eye were obtained giving a total of 102 ‘SIA’ values.
A mean ‘SIA’ of 0.3D, (SD 0.24, range 0-1.09D, 95% CI 0.25 to 0.35D) was established purely from differences in repeated K readings.
Previous studies have shown a mean SIA of 0.5-0.7D following small incision cataract surgery. These studies have not taken into account the effect of measurement variation using biometric devices when interpreting the results. This study shows that measurement variability can introduce a significant error when calculating SIA using the doctor-hill.com online SIA calculator and should be considered when drawing conclusions from SIA associated studies.
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