Evaluation of Intraocular Lens Power Calculation Formulae Using the Ratio of Axial Length to Corneal Radius

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  • 眼軸長／角膜曲率半径比を用いた眼内レンズ度数計算式の評価

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[Purpose] The ratio of axial length (AL) to corneal radius (CR) is a good index of ocular refraction. We evaluated the predictability of three intraocular lens (IOL) calculation formulae using parameters including the AL/CR ratio.<br>[Subjects and Methods] We included 386 eyes of 271 patients with visual acuity ≥ 0.8 at 1 month after implantation of a 1-piece IOL (NY60, HOYA, Japan) during cataract surgery between January 2010 to October 2012. Preoperative parameters for the analyses including AL, CR, the AL/CR ratio, and anterior chamber depth (ACD) were measured using the IOLMaster™. The subjects were divided into three groups according to the parameter. For the AL/CR ratio, they were classified to groups with the ratio < 3.0, 3.0-3.3, and ≥ 3.3. The postoperative refractive errors were compared among groups for each parameter. Using the optimized constants for the subjects, we evaluated the predictability of the SRK/T, Haigis, and T2 formulae in this study. Predictability of the formula is considered to depend on postoperative ACD prediction. Therefore, we also examined the correlations between the postoperative ACD predicted by the formulae and the parameters.<br>[Results] With the SRK/T and Haigis formulae, significant differences in the AL/CR ratio were seen among the groups (P < 0.01, Kruskal-Wallis test). Specifically, hyperopic postoperative refractive errors were found for the SRK/T formula in the low AL/CR ratio group and for the Haigis formula in the high AL/CR ratio group. On the other hand, with the T2 formula, no significant difference was seen among the groups for any parameter. Furthermore, the AL/CR ratio had the strongest correlation with the predicted postoperative ACD among the parameters (SRK/T: r = 0.97, Haigis: r = 0.76, T2: r = 0.99; Pearson product-moment correlation coefficient).<br>[Conclusion] The AL/CR ratio is a useful parameter as a risk index of postoperative refractive error. It is desirable that an IOL calculation formula does not result in a biased prediction in terms of any parameter including the AL/CR ratio. For that, the T2 formula appeared to be ideal among the three formulae.

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