Keratometry and total keratometry values are similar in healthy eyes

A large international study that included data from patients in Europe, North America, and Asia reported that keratometry (K) and total keratometry (TK) values do not significantly differ in healthy eyes, and any impact on the intraocular lens (IOL) power calculations were rare.¹

A notable exception, however, was the difference between the astigmatic values associated with the 2 parameters,¹ which, according to Jascha A. Wendelstein, MD, should be investigated further. He is from the Department for Ophthalmology and Optometry, Kepler University Hospital GmbH, in Linz, Austria, and from the Institute for Refractive and Ophthalmic Surgery (IROC) in Zürich, Switzerland.

The investigators from 10 centres across the globe conducted a retrospective analysis of biometry data to explore the TK by analysing extensive international datasets representing diverse ethnic backgrounds. The primary goals were to quantify any disparities between traditional K and TK in normal eyes and assess the impact on IOL powers using the various formulas available to calculate the IOL powers.

Study rationale

The posterior cornea has garnered increased attention in recent years, Dr Wendelstein pointed out, which has led to a shift from using anterior corneal power to the total corneal power in IOL calculations.

Traditionally, instruments for measuring the curvature of the cornea focused solely on the anterior surface, a practice that persisted for over a century. Today, however, modern devices also assess the posterior corneal curvature. These advanced instruments employ technologies such as Scheimpflug imaging and (swept-source) optical coherence tomography (SS-OCT). The integration of tomography, keratometry, and biometry in current devices provides a comprehensive analysis of the cornea. The IOLMaster 700 (IOLM700, Carl Zeiss Meditec AG), based on SS-OCT, uses the anterior and posterior corneal curvatures and corneal thickness to generate TK values.

The keratometric index used for this analysis is 1.3375. Max, maximum; min, minimum.

“The IOLM700 anterior to posterior corneal curvature ratio closely resembles the Gullstrand ratio [the corneal posterior/anterior ratio in the classical Gullstrand model eye],^2-4 thus enabling surgeons to utilise established IOL formula constants in combination with IOLM700 TK values without further adjustments. However, the IOLM700 posterior curvature radii differ from those of other devices, bringing into question the utility and interchangeability of these measurements,”^2-4 the investigators explained.

Further, they noted that the idea behind the IOLM700 TK value is maintenance of the corneal power of normal eyes but to meaningfully change the corneal power in eyes with pathologies, such as may be present after refractive surgery and in eyes with keratoconus and corneal scarring, with the goal of producing more accurate results with established formula constants. In the study under discussion, the goal was to better quantify the relative and absolute difference between the IOLM700 K and TK values and how those differences affect IOL power calculations.

High level of agreement

The study evaluated 116,982 measurements from 57,862 right eyes and 59,120 left eyes.

The analysis revealed a high level of agreement between the K and TK values.On average, they reported, 93.98% of eyes were within an absolute difference of 0.25 diopter (D) between the K and TK values. Less than 0.40% of eyes showed an absolute difference of at least 0.50 D between the K and TK measurements.

The differences in the mean keratometric power K/TK values with the IOLM700 between were 43.75 ± 1.60 D and 43.81 ± 1.62 D in the right eyes, and 43.80 ± 1.60 D and 43.86 ± 1.62 D in the left eyes.

Astigmatism: An outlier

One parameter for which discrepancies were seen was astigmatism, with vector differences exceeding 0.25 D and 0.50 D observed in 39.43% and 1.08% of eyes, respectively, the authors pointed out. The data analysis showed that the anterior/posterior corneal curvature ratio seen with the IOLM700 was 1.12 ± 0.02 (95% CI, 1.08- 1.17).

The mean Cooke-Riaz-Wendelstein Index 1 (Table 1), an index used to detect eyes that had undergone myopic laser vision correction, was 3.37 ± 2.70, with a 95% CI from 0.01 to 9.96.5 The magnitude of the difference vector (DV) between astigmatism calculated with K and TK values was 0.23 ± 0.10 D (Figure). “Similar to an earlier study, we observed significant differences in K and TK derived astigmatism.²

Measurements were performed with the Pentacam (Oculus), Casia SS-1000 (Tomey), Sirius (CSO), Anterion (Heidelberg Engineering), and IOLMaster 700 (Carl Zeiss Meditec AG). In the study by Wendelstein et al, long and short eyes were defined as longer and shorter than the 98th and 2nd percentile of the dataset (27.73 and 21.75 mm).
N/a, not available; IOLM700, IOLMaster 700; OD, right eye. (Tables and figure courtesy of Jascha A. Wendelstein, MD)

In nearly 40% of all cases, the DV between both modalities was more than 0.25 D. Considering that DVs not only display differences in astigmatism magnitude but also in astigmatism orientation, the results of surgical astigmatism correction can be heavily influenced by the right modality on which to base the correction. We have observed that the higher the DV, the more frequently an against-the-rule astigmatism was present,” they said.

The investigators cited a study⁶ whose results showed that the best results were obtained when the IOL axis for the IOL rotation procedure was planned using the postoperative TK values instead of relying on postoperative measurements with the Barrett Toric Calculator or the Berdahl-Hardten AstigmatismFix tool. This finding indicates that considering TK values for IOL rotation can lead to improved outcomes in eyes with misaligned toric IOLs.⁶

Dr Wendelstein and colleagues believe that further studies are warranted to shed light on therapplicability of TK values in astigmatism correction. They concluded, “Comparisons between the K and TK values have shown generally comparable results in healthy eyes, with only a minority exhibiting significant differences between the values (Table 2). While favourable outcomes have been observed in eyes with abnormal corneas, findings in eyes with normal corneas are somewhat equivocal. Nevertheless, this large-scale study affirms that K and TK values do not significantly differ in healthy eyes, and clinically relevant differences impacting IOL power calculation are rare. Further research is needed to explore the applicability of TK values in astigmatism correction and to assess their impact on surgical outcomes.”

References

1. Wendelstein JA, Hoffmann PC, Hoffer KJ, et al. Differences Between Keratometry and Total Keratometry Measurements in a Large Dataset Obtained With a Modern Swept Source Optical Coherence Tomography Biometer. Am J Ophthalmol. 2024;260:102-114. doi:10.1016/j.ajo.2023.12.003

2. Wendelstein JA, Reifeltshammer SA, Cooke DL, et al. The 10,000 Eyes Study: Analysis of Keratometry, Abulafia-Koch regression transformation, and Biometric Eye Parameters Obtained With Swept-Source Optical Coherence Tomography. Am J Ophthalmol. 2023;245:44-60. doi:10.1016/j.ajo.2022.08.024

3. Langenbucher A, Szentmáry N, Cayless A, Wendelstein J, Hoffmann P. Comparison of 2 modern swept-source optical biometers-IOLMaster 700 and Anterion. Graefes Arch Clin Exp Ophthalmol. 2023;261(4):999-1010. doi:10.1007/s00417-022-05870-9

4. Debellemanière G, Dubois M, Gauvin M, et al. The PEARL-DGS Formula: The Development of an Open-source Machine Learning-based Thick IOL Calculation Formula. Am J Ophthalmol. 2021;232:58-69. doi:10.1016/j.ajo.2021.05.004

5. Cooke DL, Riaz KM, Murphy DA, et al. The CRW1 Index: Identification of Eyes with Previous Myopic Laser Vision Correction Using Only a Swept-Source OCT Biometer. Am J Ophthalmol. 2023;247:79-87. doi:10.1016/j.ajo.2022.10.024

6. Sharma AC, Khetan A. Comparing IOLM700 TK, Berdahl and Hardten astigmatism fix calculator and Barrett Rx formula in managing residual astigmatism due to toric intraocular lens misalignment. Indian J Ophthalmol. 2022;70(2):413-419. doi:10.4103/ijo.IJO_1760_21

Jascha A. Wendelstein, MD |E: wendelsteinjascha@gmail.com

Dr Wendelstein is from the Institut für Refraktive und Ophthalmo-Chirurgie (IROC), Zurich, Switzerland; the Department for Ophthalmology and Optometry, Kepler University Hospital GmbH, and the Johannes Kepler University Linz, both in Linz, Austria; and the Institute of Experimental Ophthalmology, Saarland University, Homburg, Germany. Dr. Wendelstein reports research support from Carl Zeiss Meditec AG and personal fees from Alcon Surgical, Carl Zeiss Meditec AG, Johnson & Johnson Vision, and Rayner Surgical outside of the submitted work.