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John Merriam, Lei Zheng; Is the A constant for the SRK/T formula the same for the right and left eyes of patients having bilateral surgery?. Invest. Ophthalmol. Vis. Sci. 2013;54(15):824.
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© ARVO (1962-2015); The Authors (2016-present)
The SRK/T formula is a popular method to select IOL power for cataract surgery. This retrospective study examines the correlation of the calculated A constant for the right and left eyes of patients having bilateral surgery with a small temporal incision.
The study includes 229 patients who had both eyes implanted with the Acrysof SN60WF IOL (Alcon). Prior to surgery the axial length and corneal curvature were measured with the IOLMaster (Zeiss). The IOL power was calculated with SRK/T formula. After surgery the manifest refraction was entered into the database in the IOLMaster to optimize the A constant for the IOL. For this study, personnel from Carl Zeiss, Inc. in Germany transferred the information from the database in the IOLMaster to a spreadsheet in Excel (Microsoft). With these data in the spreadsheet, the authors calculated the precise A constant for the measured postoperative refraction for each eye, using a program provided by one of the authors (Dr. Sanders) of the SRK/T formula. The absolute difference in the A constant, axial length, and mean corneal keratometry were calculated for each patient by subtracting the value of OD from OS.
For patients whose eyes differ by less than 0.5 mm in axial length, the mean A-constant for the right and left eyes has no significant difference (P>0.05); but within this group there are exceptions. For patients whose mean keratometry values for the right and left eyes are less than 0.5 diopter, the mean A-constant for the right and left eyes also has no significant difference (p>0.05), but there are exceptions. The reasons for the difference in A constant for the right and left eyes may relate to measurement error, an asymmetry in axial length and/or keratometry, or other factors.
The A constants for the right and left eyes are correlated, but the clinician will encounter exceptions and must be aware of the effect of both axial length and keratometry when selecting the IOL. Rather than using a single A constant for an IOL for all eyes, surgeons should consider optimizing the A constant for axial length groups.
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