Intraocular Lens Power Calculations in Keratoconus Eyes Comparing Keratometry, Total Keratometry, and Newer Formulae

Repeatability and agreement of two ocular biometers with single and dual Scheimpflug cameras in keratoconus eyes

BACKGROUND

To evaluate the repeatability and agreement of two different ocular biometers and Scheimpflug devices in keratoconus eyes.

METHODS

This prospective, comparative trial took place at the University hospital, Goethe University, Frankfurt am Main, Germany. We included eyes with keratoconus, one eye per patient, randomly selected. Measurements were taken with Galilei G6 (Ziemer, Switzerland) and Pentacam AXL (Oculus, Germany), three consecutive measurements each. Repeatability and agreement were evaluated for simulated keratometry (simK), astigmatism (simA), maximum keratometry (KMax) and its axis, total keratometry (TCP), axial length (AL), anterior chamber depth (ACD), and thinnest pachymetry (TCT).

RESULTS

Both devices showed an excellent repeatability with intra class correlation (ICC) of > 0.97 for all parameters. The 95% limits of repeatability (LoR95%) and agreement (LoA95%) were narrow for all parameters. The Galilei G6 had a narrower LoAR95% for TCT (2.1 μm vs. 4.6 μm), but a wider LoR95% for KMax (0.52D vs. 1.18D). No relevant difference was found for the other parameters. Agreement between the devices was good to moderate, especially for simK and TCP.

CONCLUSIONS

Both devices show excellent repeatability with narrow LoR95% and high ICC for all parameters. The only relevant difference was found for KMax and TCT in favor of Pentacam AXL and Galilei G6, respectively. Agreement was good to moderate, and most parameters should not be considered interchangeable.

Repeatability and agreement of total corneal astigmatism measured in keratoconic eyes using four current devices

BACKGROUND

To evaluate repeatability and agreement in measurements of total corneal astigmatism (TCA) in keratoconic eyes, using four optical coherence tomography (OCT)-based devices: Anterion, Casia SS-1000, IOLMaster 700, and MS-39.

METHODS

Three consecutive measurements were taken with each device in 136 eyes. TCA values were converted into components J0 and J45. The Anterion and the IOLMaster 700 also provided axial length (AL) measurements. The repeatability was calculated using pooled within-subject standard deviation (Sw). The agreement among the four devices was assessed by pairwise comparisons and Bland-Altman plots.

RESULTS

For all devices, the repeatability of TCA measurements showed Sw ≤0.23 D for TCA magnitude, ≤0.14 D for J0, and ≤0.12 D for J45. There were statistically significant differences in TCA magnitude for each pair, except for IOLMaster 700 with MS-39, and Anterion with MS-39. The repeatability (Sw) of axis measurements had a statistically significant negative correlation with the TCA magnitude (p < 0.001 for all devices). Both Anterion and IOLMaster 700 had high repeatability in AL measurements (Sw: 0.007 mm for Anterion and 0.009 mm for IOLMaster 700). The difference in AL between the two was 0.015 ± 0.033 mm (p < 0.001).

CONCLUSIONS

All four devices showed good repeatability in TCA measurements in keratoconic eyes, the agreement for TCA measurements between the tested devices was generally low. Anterion and IOLMaster 700 showed good repeatability and agreement in AL measurements.

Differences Between Simulated Keratometry and Total Corneal Power in Eyes With Keratoconus and a Formula to Improve IOL Power Calculation Results

PURPOSE

To compare simulated keratometry (SimK) and total corneal power (TCP) in keratoconic eyes, to determine whether the differences are systematic and predictable and to evaluate an adjusted TCP-based formula for intraocular lens (IOL) power calculation.

METHODS

In a consecutive series of keratoconic eyes, measurements of SimK, TCP, posterior keratometry, and anterior and posterior corneal asphericities (Q-values) were retrospectively collected. The difference between SimK and TCP was linearly correlated to the biometric parameters. In a separate sample of keratoconic eyes that had undergone cataract surgery, IOL power was calculated with the Barrett Universal II, Hoffer QST, Holladay 1, Kane, and SRK/T formulas using the SimK and an adjusted TCP power. The respective prediction errors were calculated.

RESULTS

A total of 382 keratoconic eyes (271 patients) were enrolled. An increasing overestimation of SimK by TCP was detected from stage I to III, with a significant correlation between the SimK and TCP difference and SimK in the whole sample (P < .0001, r2 = 0.1322). Approximately 7% of cases presented an underestimation of SimK by TCP. IOL power calculation with the adjusted TCP improved outcomes, achieving a maximum of 80% of eyes with a prediction error within ±0.50 diopters with the Hoffer QST, Holladay 1, and Kane formulas.

CONCLUSIONS

Overall, SimK overestimated TCP. Such a difference could not be predicted by any variable. The proposed TCP-adjustment formula (TCPadj = TCP + 0.56 diopters) in keratoconic eyes for IOL power calculation might be valuable for improving refractive outcomes. [J Refract Surg. 2024;40(4):e253-e259.].

Differences Between Keratometry and Total Keratometry Measurements in a Large Dataset Obtained With a Modern Swept Source Optical Coherence Tomography Biometer

PURPOSE

This study aimed to explore the concept of total keratometry (TK) by analyzing extensive international datasets representing diverse ethnic backgrounds. The primary objective was to quantify the disparities between traditional keratometry (K) and TK values in normal eyes and assess their impact on intraocular lens (IOL) power calculations using various formulas.

DESIGN

Retrospective multicenter intra-instrument reliability analysis.

METHODS

The study involved the analysis of biometry data collected from ten international centers across Europe, the United States, and Asia. Corneal power was expressed as equivalent power and astigmatic vector components for both K and TK values. The study assessed the influence of these differences on IOL power calculations using different formulas. The results were analyzed and plotted using Bland-Altman and double angle plots.

RESULTS

The study encompassed a total of 116,982 measurements from 57,862 right eyes and 59,120 left eyes. The analysis revealed a high level of agreement between K and TK values, with 93.98% of eyes exhibiting an absolute difference of 0.25 D or less. Astigmatism vector differences exceeding 0.25 D and 0.50 D were observed in 39.43% and 1.08% of eyes, respectively.

CONCLUSIONS

This large-scale study underscores the similarity between mean K and TK values in healthy eyes, with rare clinical implications for IOL power calculation. Noteworthy differences were observed in astigmatism values between K and TK. Future investigations should delve into the practicality of TK values for astigmatism correction and their implications for surgical outcomes.

Comparison of the accuracy of 9 intraocular lens power calculation formulas after SMILE in Chinese myopic eyes

As of 2021, over 2.8 million small-incision lenticule extraction (SMILE) procedures have been performed in China. However, knowledge regarding the selection of intraocular lens (IOL) power calculation formula for post-SMILE cataract patients remains limited. This study included 52 eyes of 26 myopic patients from northern China who underwent SMILE at Tianjin Eye Hospital from September 2022 to February 2023 to investigate the suitability of multiple IOL calculation formulas in post-SMILE patients using a theoretical surgical model. We compared the postoperative results obtained from three artificial intelligence (AI)-based formulas and six conventional formulas provided by the American Society of Cataract and Refractive Surgery (ASCRS). These formulas were applied to calculate IOL power using both total keratometry (TK) and keratometry (K) values, and the results were compared to the preoperative results obtained from the Barrett Universal II (BUII) formula for the SMILE patients. Among the evaluated formulas, the results obtained from the Emmetropia Verifying Optical 2.0 Formula with TK (EVO-TK) (0.40 ± 0.29 D, range 0-1.23 D), Barrett True K with K formula (BTK-K, 0.41 ± 0.26 D, range 0.01-1.19 D), and Masket with K formula (Masket-K, 0.44 ± 0.33 D, range 0.02-1.39 D) demonstrated the closest proximity to BUII. Notably, the highest proportion of prediction errors within 0.5 D was observed with the BTK-K (71.15%), EVO-TK (69.23%), and Masket-K (67.31%), with the BTK-K showing a significantly higher proportion than the Masket-K (p < 0.001). Our research indicates that in post-SMILE patients, the EVO-TK, BTK-K, and Masket-K may yield more accurate calculation results. At their current stage in development, AI-based formulas do not demonstrate significant advantages over conventional formulas. However, the application of historical data can enhance the performance of these formulas.