Dear Editor,
With great excitement, we read the article entitled “Accuracy of the Barrett Universal II formula integrated into a commercially available optical biometer when using a preloaded single-piece intraocular lens” by Mieno et al.[1] We congratulate the authors on their detailed analysis of the accuracy of Barrett Universal II, a newer generation formula, and comparison with commonly used formulas for Vivinex™iSert® XY1 IOL. This lens has been in use for quite some time now but not much study has been published on it. Since the protocols on clinical studies on Intraocular lens (IOL) power calculation and biometry are continually getting updated, it would be very helpful for the readers if the authors could clarify the following points.
Enrollment of one eye for each patient is recommended as it simplifies analysis and prevents the compounding of data due to the fact that ocular measurements between fellow eyes are more or less similar. Nevertheless, the selection bias and data wastage are some of its shortcomings. Hence, in the circumstances where bilateral eyes could be included, a statistically valid procedure is to select the eye randomly or selection of all right eyes or left eyes arbitrarily.[2] As the authors have selected the eye with better visual acuity, we believe that this could lead to a bias of results.
The authors have mentioned that the optimized A constant of the Sanders, Retzlaff, Kraff / Theoretical (SRK/T)formula (119.2) was used for the Barrett Universal II formula also. However, in study limitations, they have mentioned that the manufacturer’s IOL constants were used and optimization was not done. So, what is evident is that the authors have compared Barrett Universal II and SRK/T as optimized while Haigis and Holladay 1 as unoptimized, which will definitely affect the study results. As per the protocols laid down by Hoffer et al., the optimization of the lens constants has to be done mandatorily for doing any study on the IOL formula accuracy.[3] Alternatively, the authors could have used the User Group for Laser Interference Biometry (ULIB) constant for Vivinex™MiSert® XY1 IOL, which is freely available online (http://iolcon.org) instead of using the manufacturer’s constants.
The authors have used Friedman’s test for analyzing the differences in absolute error between the formulas. However, the statistical analysis used for comparison of the percentage of eyes within ± 0.50 D of prediction error has not been mentioned anywhere in the study. Ideally, the Cochran Q test should be used for analyzing the differences in the prediction error between the formulas.[2]
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References
- 1.Mieno H, Hieda O, Ikeda T, Hayashi S, Hashida M, Urabe K, et al. Accuracy of the Barrett universal II formula integrated into a commercially available optical biometer when using a preloaded single-piece intraocular lens. Indian J Ophthalmol. 2021;69:2298–302. doi: 10.4103/ijo.IJO_3455_20. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Wang L, Koch DD, Hill W, Abulafia A. Pursuing perfection in intraocular lens calculations:III. Criteria for analyzing outcomes. J Cataract Refract Surg. 2017;43:999–1002. doi: 10.1016/j.jcrs.2017.08.003. [DOI] [PubMed] [Google Scholar]
- 3.Hoffer KJ, Savini G. Update on Intraocular Lens Power Calculation Study Protocols:The Better Way to Design and Report Clinical Trials. Ophthalmology. 2021;128(11):e115–e120. doi: 10.1016/j.ophtha.2020.07.005. [DOI] [PubMed] [Google Scholar]