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Cindy Law, Alice Zhu, Carmen Canovas, Patricia A Piers, Joseph J.K. Ma; Impact of Ocular Internal Astigmatism on Toric IOL Power Determination. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4199.
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Ocular internal astigmatism (OIA) has been studied in pseudophakic patients implanted with monofocal IOLs (Joseph J. Ma, ASCRS 2013). On average, this parameter was about +0.39D against the rule (ATR). The purpose of this study was to determine whether the incorporation of OIA improves the predictability of toric IOL power calculations.
The study retrospectively analyzed the data of 46 eyes that underwent cataract surgery with toric IOL implantation between October and December 2012. The Holladay 1 formula combined with vector analysis was used to calculate the residual refraction for each patient from preoperative biometry, implanted IOL power and measured orientation. Three devices were used to measure anterior corneal power and astigmatism: IOL Master, OPD-Scan III and an autorefractometer. For each measuring device, predicted refraction was calculated with and without adding the average OIA to the anterior corneal astigmatism. The prediction error, defined as the vectorial difference between the actual refraction and the result of the prediction, was calculated in all cases.
On average, actual postoperative refractive astigmatism was ATR (0.18±0.36D at 165 degrees). For all corneal measuring devices, mean predicted residual astigmatism was with the rule (WTR) when OIA was not included in the calculation. When the OIA is considered, the mean predicted residual astigmatism was against the rule (ATR) for all devices, in agreement with the mean actual postoperative refractive astigmatism. For each device, the incorporation of the OIA reduced the average prediction error (Table).
The prediction of residual refraction in pseudophakic patients implanted with toric IOLs was improved when adding +0.39D ATR to the anterior corneal astigmatism, irrespective of the measuring device. Therefore, results of the current study validate the existence of an ATR OIA.
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