Comment on: Comparison of different methods to calculate the axial length measured by optical biometry

We read with great interest the paper by Savini et al. comparing different methods to calculate the axial length (AL) measured by optical biometry.1

In this paper, the authors found that the AL measured with a device using segmented AL (Argos) is shorter compared with the AL measured with a swept-source optical coherence tomography optical biometer based on the group refractive index technology (IOLMaster 700). These results were explained by different methods used by the 2 machines, mainly in long eyes.

In our opinion, these results could help to explain in part the findings of a previously published paper by De Bernardo et al. in which longer AL was measured in eyes before cataract surgery compared with the same eyes measured after surgery using the pseudophakic option obtained with an IOLMaster 500.2 The results by Savini et al. (AL obtained by Argos vs AL obtained by IOLMaster 700) are similar to those obtained in the paper by De Bernardo et al. because a similar AL difference (−0.10 mm) was found between preoperative AL measurements with the phakic option vs postoperative AL with the pseudophakic option, both obtained by IOLMaster 500.1 The reason why we wrote in part is because in the paper by De Bernardo et al., the AL shortening was obtained not only in long but also in short and normal eyes.2 For this reason, we wonder whether the AL shortening after cataract surgery could be related not only to the group refractive index, but other reasons could be involved.

The authors also adjusted AL measurements provided by the IOLMaster 700 using 4 methods, but 3 of these were designed for Holladay 1 or Holladay 2 formulas; meanwhile, the authors checked the clinical impact of different AL measurements through the SRK/T formula because the authors said that this formula does not have internal AL adjustment. This is not entirely true because this study analyzes eyes with AL >24.0 mm, and as per the SRK/T formula, when AL >24.2 mm, this parameter was adjusted according to the following formula: corrected axial length (LCOR) = −3.446 + 1.715 × AL − 0.237 × (AL × AL).3 In addition, evaluating AL adjustments designed for other formulas than the SRK/T formula could lead IOL power calculation bias: we wonder why the authors did not use the modified Wang-Koch adjustment for the SRK/T formula.4

In conclusion, although the same authors correctly affirmed that measurements from fellow eyes cannot be treated as if they were independent, they still reported data from both eyes. We fear that this could generate ambiguity in the reading and interpretation of data, for example, in the Abstract section, where the authors declared that in 102 eyes of 55 subjects, a statistically significant difference was found among different AL evaluations. It is well known that, when evaluating bilateral eyes, it should be advisable to apply specific statistical methods such as the bootstrap or generalized estimating equations to have valid results.5 Reporting data of bilateral eyes without these statistical evaluations, even if not crucial for the results of the study, could be confounding.

1. Savini G, Hoffer KJ, Carballo L, Taroni L, Schiano-Lomoriello D. Comparison of different methods to calculate the axial length measured by optical biometry. J Cataract Refract Surg 2022;48:685–689 2. De Bernardo M, Salerno G, Cornetta P, Rosa N. Axial length shortening after cataract surgery: new approach to solve the question. Transl Vis Sci Technol 2018;7:34 3. Retzlaff JA, Sanders DR, Kraff MC. Development of the SRK/T intraocular lens implant power calculation formula. J Cataract Refract Surg 1990;16:333–340. Erratum in: J Cataract Refract Surg 1990;16:528 4. Wang L, Koch DD. Modified axial length adjustment formulas in long eyes. J Cataract Refract Surg 2018;44:1396–1397 5. De Bernardo M, Cione F, Rosa N. Re: Turnbull et al.: Methods for intraocular lens power calculation in cataract surgery after radial keratotomy (Ophthalmology. 2020;127:45–51). Ophthalmology 2020;127:e87

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