In this study, we investigated the accuracy of IOL calculation formulas in high myopia using the measurements provided by IOLMaster700. Barrett Universal II, Hill-RBF3.0, Kane, EVO, SRK/T, and Haigis formulas were evaluated.
The benchmark standards for refractive outcomes after cataract surgery was established in the UK National Health Service in 2009, that using optimized A constants and partial coherence interferometry, 55% of patients achieving a refractive error within ±0.50D, and 85% of patients achieving a refractive error within ±1.00D. The outcomes of our study outperformed the benchmark standards, with a refractive error within ±0.50D and ±1.00D achieved by at least 60% and 92.3% of patients. For newer generation formulas such as Barrett Universal II, Hill-RBF3.0, Kane, and EVO formulas, the refractive error within ±0.50D and ±1.00D were achieved by at least 75.4% and 95.4% of patients, indicating an improved outcome.
Accurate refractive prediction remains challenging in cataract patients with high myopia. The largest studies analyzed 2060 high myopic eyes. In 2018, Melles et al. compared 1548 eyes with SN60WF (Alcon) and 512 eyes with SA60AT (Alcon) of AL longer than 25.50mm with the Lenstar LS900, the accuracy of formulas including Barrett Universal II, SRK/T, and Haigis. The Barrett Universal II formula had the best outcome and was less influenced by long axial length. The SRK/T and Haigis performed better than other traditional third and fourth generation formulas. In 2019, Melles et al. evaluated newer formulas including Kane, EVO, and Hill-RBF2.0 using the same dataset, and the Kane formula achieved the lowest MAE, MedAE, and SD in long and extremely long axial length eyes. The Hill-RBF2.0 and EVO formulas were less accurate than Barrett Universal II. Darcy et al. compared in 637 eyes of AL longer than 26.00mm with the IOLMaster500, the accuracy of formulas including Kane, Barrett Universal II, Hill-RBF2.0, Haigis, and SRK/T. The best formula was Kane with a MAE of 0.329D, followed by Barrett Universal II (0.338D), Hill-RBF2.0 (0.352D), Haigis (0.359D), and SRK/T (0.363D). Similarly, Cheng et al. compared the use of formulas including Kane, Barrett Universal II, Hill-RBF2.0, EVO, and Haigis in 370 eyes of AL longer than 26.00mm with IOLMaster500, and found that the Kane formula yielded the lowest MAE of 0.34D, followed by Barrett Universal II (0.37D), Hill-RBF2.0 (0.38D), EVO (0.40D) and Haigis (0.40D). The Kane formula was comparable to Hill-RBF2.0 and Barrett Universal II in the whole group and was better than Hill-RBF2.0 and Barrett Universal II in extremely myopic eyes with an AL ≥30.00mm. In a study of 79 eyes of AL longer than 26.00mm with OA2000, Rong et al. noted that the MedAE of Barrett Universal II (0.37D) was lower than Haigis (0.46D), and the percentage of eyes within ±0.50D of Barrett Universal II (70%) was higher than Haigis (54%). The refractive errors of the two formulas were lowest in eyes of AL between 28.00mm and 30.00mm. In 2020, several studies with measurements taken by IOLMaster700 were published. In a study of 164 eyes of AL longer than 26.00mm by Zhang et al., EVO was found to have a MAE of 0.35D and 79.27% of patients achieving a refractive error within ±0.50D, better than Barrett Universal II (0.38D, 73.17%). Carmona-González et al. compared in 115 eyes of AL longer than 25.00mm, the accuracy of formulas including all six formulas analyzed in our study. The lowest MAE was achieved by Barrett Universal II (0.26D), followed by Kane (0.27D), Haigis (0.27D), EVO (0.28D), Hill-RBF2.0 (0.29D) and SRK/T (0.33D).
The Kane formula is based on theoretical optics and incorporates both regression and artificial intelligence components to refine its predictions. Overall, Kane was the most accurate, with the lowest MAE of 0.323D, 80.0% and 98.5% of patients achieving a refractive error within ±0.50D and ±1.00D respectively. The MedAE of Kane was 0.298D, lower than Hill-RBF3.0 (0.240D) and EVO (0.285D). The excellent outcomes with the Kane formula in short, medium, and long eyes have been reported in several studies.[15-16, 21]
The EVO formula is a thick lens formula based on the theory of emmetropization. In our study, the EVO formula achieved the second-lowest MAE of 0.335D, a MedAE of 0.285D, 78.4% and 96.9% of eyes within ±0.50D and ±1.00D respectively, which was comparable to the results of previous studies.[16, 19] In eyes of AL within 26.00mm and 28.00mm, EVO was ranked first with an MAE of 0.328D, while in eyes of AL longer than 28.00mm, EVO had a higher MAE than Kane and Hill-RBF3.0 formulas. This is consistent with the viewpoint of Melles et al. that the emmetropization concept may break down at the extremes of the axial lengths.
The Hill-RBF is the first IOL calculation formula based purely on artificial intelligence and has been updated to version 3.0 in December 2020. To our knowledge, no published study on version 3.0 of the Hill-RBF formula exists to compare our results. Previous studies reported that Hill-RBF2.0 was less accurate than Kane and Barrett Universal II formulas in high myopic eyes.[15-17] In our study, Hill-RBF3.0 had a MAE of 0.346D, the lowest MedAE of 0.240D, 78.5% and 95.4% of eyes within ±0.50D and ±1.00D respectively. Hill-RBF3.0 finished behind the Kane formula, but ahead of Barrett Universal II, indicating that it has improved compared with the previous version. The Hill-RBF3.0 allows users to enter target refraction between -2.5D to +1.0D. For high myopic eyes with target refraction less than -2.5D, refractive predictions were obtained by artificial extrapolation, which led to inconvenience in clinical use. Nevertheless, we still included the dataset obtained by extrapolation in our analysis and achieved a good outcome.
Barrett Universal II is a paraxial ray-tracing thick-lens formula, which accounts for the varying principal planes among different-powered IOLs. It considers the effective lens position (ELP) to be a result of the ACD and a lens factor related to the physical position and the location of the principal planes of the IOL. Barrett Universal II was ranked among the most accurate formulas for high myopic eyes in many studies.[1, 22] In our study, Barrett Universal II, with a MAE of 0.361D, a MedAE of 0.313D, 75.4% and 96.9% of eyes within ±0.50D and ±1.00D, was ranked fourth behind newer formulas including Kane, EVO, and Hill-RBF3.0.
SRK/T and Haigis formulas are traditional vergence formulas, which use different numbers of variables to estimate ELP. SRK/T and Haigis formulas were reported superior accuracy over Holladay1, and HofferQ formulas for high myopic eyes. In our study, Haigis achieved a MAE of 0.415D, a MedAE of 0.360D, 66.2% and 92.3% of eyes within ±0.50D and ±1.00D, while SRK/T had a MAE of 0.450D, a MedAE of 0.370D, 60% and 95.4% of eyes within ±0.50D and ±1.00D. The refractive outcomes of the two formulas were comparable to previous studies.[23-24] SRK/T and Haigis formulas were less accurate than other new formulas as we expected. The advantage is that IOL constants of two formulas could be optimized independently, which is convenient for clinicians.
Based on AL subgroups, Kane was comparable to Hill-RBF3.0, EVO, Barrett Universal II formulas in eyes with an AL between 26.00mm and 30.00mm, and had a superior behavior over other formulas in eyes with an AL longer than 30.00mm. The excellent outcome of the Kane formula in extremely high myopic eyes was also detected in a previous study by Cheng et al.. Furthermore, the MAEs and MedAEs were lowest in eyes with an AL within 28.00mm to 30.00mm, as in two previous studies by Rong et al. and Cheng et al.. Rong et al. explained that it is the point at which refractive errors change between myopic and hyperopic. In our study, the refractive errors of Hill-RBF3.0 and Kane formulas changed from hyperopia to myopia, and the refractive errors of the other four formulas changed from myopia to hyperopia as axial length got longer, which supported the viewpoint mentioned before. To avoid hyperopic shift, EVO and Barrett Universal II formulas were preferred in eyes with an AL between 26.00mm and 28.00mm, while Kane and Hill-RBF3.0 formulas were the first choices in eyes with an AL longer than 30.00mm.
In our study, preoperative measurements were taken by a swept-source optical coherence tomography optical biometer. IOLMaster700, with a longer wavelength, is more successful at measuring AL through dense cataracts and extremely long eyes. The device provides cross retinal OCT images to detect fixation status and is expected to produce more precise AL measurement in myopic eyes with posterior staphyloma. Moreover, the device measures LT and WTW, which are optional variables in Kane (LT), Barrett Universal II (LT, WTW), Hill-RBF3.0 (LT, WTW), and EVO (LT), and is expected to further refine the accuracy of IOL formulas.
Nevertheless, the refractive outcomes achieved in our study are consistent with results based on other biometers.[17, 21, 24] The reason could be that any clinical differences, such as the range of AL included, the type of IOLs, and the selection of IOL constants, affect the refractive prediction of IOL calculation formulas. When comparing the differences in refractive prediction of different devices, biometric measurements should be carried out on the same group of patients and reduce the influence of any other clinical factors.
Our study had several limitations. 1.Lens constant optimization for new generation formulas was limited because they were not published; instead, the mean refractive errors were simply zeroed out to eliminate systematic errors. Although different from traditional lens constant optimization, that was suggested by protocols of IOL formula accuracy provided by Wang et al.. 2. Data from multiple IOL models were included, which might introduce bias due to different IOL models. However, in modern surgery, multiple IOLs are used, and the results may have greater generalizability. 3. A small number of eyes were evaluated in our study compared to other studies, and further studies with a larger sample size are needed to investigate the difference in each subgroup.