In this study, we evaluated 449 eyes and showed that the prediction accuracy of all TK formulas was similar to standard K for monofocal IOL implantation cataract surgery. Meanwhile, the analysis of the refractive prediction outcomes in different optic dimensions suggests that TK did not come up with better refractive outcomes than using K.

The application of TK has advantages over standard K in cataract surgery after laser vision correction (LVC) surgery. One of the possible reasons for this may be that the LVC procedure changes the topographic status of the cornea, causing a deviation in the estimation of the corneal refractive rate from the anterior keratometry. The superiority of utilizing TK in cataract surgery following laser refractive surgery is well-established in previous studies. Yeo et al.[14] compared K versus TK data from IOLMaster 700 in eyes with laser in situ keratomileusis or photorefractive keratectomy history and found the mean TK (38.15 D, ranging from 33.71 to 42.18 D) was lower than the mean K (39.25 D, ranging from34.99 to 42.83 D). Wang et al.[15] analyzed eyes that had undergone corneal refractive surgery previously; They found that the Barrett True-K and Haigis-TK performed better in the myopic LASIK/PRK group, which may be due to the fact that the average TK value was significantly different from that of the K. Consistently, according to the study of Lawless et al. [16], average TK values were shown to be flatter than K values for individuals with a history of myopia and steeper for patients with a history of hyperopia. However, without a history of laser surgery, the superiority of TK in the IOL calculation remains unproven in the real world. In this study, we aim to explore whether TK could achieve a better predictive outcome in conventional cataract surgery and the possibility of replacing K with TK in IOL power calculation formulas.

Contrary to eyes that have experienced corneal laser procedure, in routine eyes, TK is compatible with keratometry data[17]. Our data showed no significant difference between the average value of K (44.26D) and TK (44.28D). This evidence supports the notion that the measurement method, standard keratometry or total keratometry, has no significant effect on the prediction accuracy of each formula. As reported by Sirvannaboon et al.[9], no statistically significant differences in MAEs and MedAEs were seen between the K and TK groups. Tessler et al.[11] compared the prediction accuracy of eleven different current formulae employing the K and TK; When comparing the predictive efficacy of the two formula variations, there was no discernible difference. A recent study[18] reported that TK downgraded the formula's performance, especially in medium AL, medium anterior K, and flat K subgroups. We, along with previous reports, showed that TK did not increase the postoperative refractive outcomes in all investigated formulas.

Assessing the proportion of eyes inside ± 0.25, ± 0.50, and ± 0.75D adjusted PE, we noticed that the K and TK groups had comparable percentages in the majority of distributions. The highest proportion of eyes that achieved the refractive outcomes in all the brackets was the ones that used the BUII formula. This result is consistent with earlier discoveries. Chung et al.[19] reported 89.9%, 82.5%, 85.5%, and 85.5% of eyes achieved the refractive outcomes when applying K to the BUII, Haigis, SRK/T, and Holladay 2 formulas for multifocal IOLs, and 86.6%, 82.1%, 82.3%, and 83.4% when applying TK to the same formulas. The BUII formula is specifically designed for TK, so it is not surprising that TK can achieve the same accuracy as K. Since other formulas are developed on standard K, using TK in these formulas yield unfavorable outcomes in our study. This non-intuitive downgradation may be due to the inadaptability of the previous generation formulas or the deviation of IOL power calculation in some cases of abnormal eyes.

Furthermore, the analysis of different optical parameters showed that the difference in prediction error between K and TK formulas was insignificant regardless of AL, ACD, keratometry, and LT. Our results indicated that the extreme eye parameters would lead to a decreased prediction accuracy for all formulas. This means the formula per se brought up the lower PE when encountering the outlier data rather than the different measurement inputs. In a large multicenter retrospective study, Melles et al.[20] reported that the new generation of formulas, including BUII, is more reliable for predicting the IOP power for abnormal eyes. Our study is in agreement with this conclusion. We also found that both BUIIK and BUIITK showed the best formula performance, as reflected by a better FPI, lower absolute PE, and higher proportions of eyes achieving low PE.

Corneal diopter measured by standard K is a calculated value by measuring the anterior corneal surface information. In reality, the corneal diopter is affected by the curvature of the anterior surface, the curvature of the posterior surface, and the thickness of the cornea[3]. In theory, TK should be more helpful for determining the IOL power when the cataract surgeon designs the surgical strategy because it is more likely to reflect the actual diopter of the cornea than anterior K. But our real-world study demonstrates that TK is not better than standard K for calculating IOL power in monofocal IOL implantation cataract surgery. One possible reason is that most of the current formulas are not specifically developed for TK. The BUII formula is an exception and explains precisely why the BUII and the BUIITK have homogeneous degree of accuracy. Despite the fact that TK is currently unable to substitute for Kin IOL power calculation formulas, if the SS-OCT device becomes more prevalent in the future, TK's application potential will increase because of its broader applicability.

The limitations of our study are a retrospective case series design, differences in IOL, and no constant IOL optimization to balance ME to zero to eliminate system error. Our investigation adhered to all of the protocols that were provided by Hoffer et al.[21] for research into the precision of IOL formulas. Whether the TK formulas have a better postoperative refractive error in patients with super-long axial length needs further investigation.