Corresponding Author: Joon Young Hyon, MD, PhD. Department of Ophthalmology, Seoul National University Bundang Hospital, 82 Gumi-ro 173beon-gil, Bundang-gu, Seongnam 13620, Korea. Since toric intraocular lens (IOL) was first introduced in 1992, toric IOL implantation has become a widely used procedure in cataract patients with high corneal astigmatism [1,2]. Toric IOLs have been proven to be a stable, effective and predictable method for correcting corneal astigmatism in several studies [3-6]. Additionally, correcting residual astigmatism has been shown to significantly improve visual acuity at all contrast levels, for both distance and near vision [7].
It is well established that the effectiveness of astigmatism correction decreases as the toric IOL deviates from the intended axis [8]. However, discrepancies in steep axis measurements between different devices can make it difficult to determine which measurement is more reliable. Posterior corneal astigmatism can further affect total corneal astigmatism, resulting in differences between the astigmatism axis of total and standard keratometry [9,10]. These discrepancies in measurements may lead to errors in toric IOL axis calculation and consequently, in astigmatism correction.
Various devices are available to determine the toric IOL axis, including auto kerato-refractometers, optical biometry devices, and corneal topography devices. Among these, IOLMaster 500 (Carl Zeiss Meditec) and IOLMaster 700 (Carl Zeiss Meditec), based on partial coherence interferometry and swept-source optical coherence tomography, respectively, are widely used optical biometry devices. Galilei G4 (Ziemer), a dual Scheimpflug corneal topography device, is also commonly used to evaluate corneal astigmatism.
However, no previous studies have directly compared the accuracy of toric IOL axis determination between IOLMaster and Galilei G4. In this study, we aimed to evaluate the accuracy of toric IOL axis determination by comparing the postoperative gold-standard axis, calculated by an online Toric Results Analyzer (Ocular Surgical Data LLC; https://www.astigmatismfix.com/), with the preoperatively calculated axis from optical biometry (IOLMaster 500 or IOLMaster 700) and the dual Scheimpf lug topography (Galilei G4).
Materials and Methods
Ethics statement
This study was approved by the Institutional Review Board of Seoul National University Bundang Hospital (No. B-2206-761-110). The requirement for informed consent was waived due to the retrospective nature of the study. The study adhered to the tenets of the Declaration of Helsinki.
Study design and setting
We retrospectively reviewed medical records of patients who underwent phacoemulsification and posterior chamber toric IOL (Zeiss AT TORBI 709M, Carl Zeiss Meditec) implantation between July 2017 and January 2022. A total of 44 patients (64 eyes) were included: 31 eyes of 19 patients were measured using the IOLMaster 500 between July 2017 and February 2019, and 33 eyes of 25 patients using the IOLMaster 700 between March 2019 and January 2022. Corneal topography was measured using the Galilei G4 in all patients. Exclusion criteria were as follows: corneal abnormalities including corneal opacity, degeneration or dystrophy, previous refractive surgery history, postoperative corrected distance visual acuity less than 20 / 30, and age under 20 years.
Preoperative toric IOL axis calculation
IOL cylinder power and axis of Zeiss AT TORBI 709M were calculated using Z CALC Online IOL Calculator (Zeiss; https://zcalc.meditec.zeiss.com). The ocular parameters (axial length, anterior chamber depth, keratometry [K1, K2, and their meridians]) measured by IOLMaster 500 or IOLMaster 700, 0.25 D of surgically induced astigmatism, and temporal incision orientation (0° or 180° depending on the laterality of the eye) were entered into the calculator. Additionally, the preoperative toric IOL axis was calculated using the Z CALC Online IOL Calculator, with flat and steep simulated keratometry values from the Galilei G4 along with other ocular parameters (axial length, anterior chamber depth) from the IOLMaster. The intended axis was determined based on the axis calculated by the IOLMaster.
Toric IOL axis marking
Patients were advised to maintain a sitting, upright head position. Preoperative reference markings at 0° and 180° on the cornea were made using a toric reference marker (AE-2793S, Asico) prior to surgery.
Surgical technique
The surgery was performed by a single experienced surgeon (JYH). Under topical anesthesia, a 2.2-mm temporal incision was made. Continuous curvilinear capsulorhexis and phaco-chop technique phacoemulsification were performed, followed by toric IOL implantation. Toric IOL was then rotated to align with the intended axis.
Calculation of gold-standard axis of toric IOL
The implanted axis of the toric IOL was measured using the internal optical path difference (OPD) map of wavefront aberrometer (OPD-Scan III, Nidek) 1 month after surgery. Refractive outcomes were measured by autorefraction. Based on the refractive data, cylinder power, and the implanted axis of the toric IOL, the online Toric Results Analyzer was used to determine the gold-standard axis of the toric IOL [11,12].
Postoperative outcome measures
The IOLMaster and Galilei G4 were compared based on the error between the preoperatively calculated axis and the gold-standard axis. Axis error (AE) was defined as follows: “AE = (postoperative gold-standard axis) - (preoperatively calculated axis).” Absolute AE (AAE) was defined as the absolute value of the AE. AE ranges between −90° and 90°, and AAE ranges from 0° to 90°. The AE and AAE were compared between the IOLMaster and Galilei G4. Additionally, subgroup analyses were performed on patients with AAE over 10° between the IOLMaster and the Galilei G4.
Statistical analysis
Statistical analysis was performed using IBM SPSS ver. 29.0 (IBM Corp) and Prism 10 (GraphPad Software Inc). Independent t-test, paired t-test, one-sample t-test, and Mann-Whitney test were used to compare the data. The data are presented as mean ± standard deviation, and statistical significance was considered at p < 0.05.
Results
Baseline demographic data are shown in Table 1. T he mean age was 62.8 ± 13.9 years. Of the total 64 eyes, 31 (48.4%) were from male patients and 33 (51.6%) were right. The a verage spherical equivalent and a verage cylinder power of the implanted IOLs were 17.57 ± 4.73 and 3.13 ± 1.16 D, respectively. The mean postoperative refractive cylinder measured by autorefraction was 0.77 ± 0.50 D (range, 0-2.25 D). The mean difference of postoperative implanted IOL axis and gold-standard axis was 6.90° ± 6.62°.
Keratometric values and keratometric astigmatism (KA) measured by IOLMaster and Galilei G4 are shown in Table 2. Flat keratometry and steep keratometry did not show any significant difference between IOLMaster and Galilei G4. The mean KA measured by IOLMaster was 2.62 ± 1.16 D, which did not show any significant difference compared with the mean KA of 2.46 ± 0.98 D measured by Galilei G4.
Every KA axis measured by IOLMaster and Galilei G4 is represented by dot in Fig. 1. To determine whether there is a statistically significant difference in KA axis measured between IOLMaster and Galilei G4, the KA axis measured by IOLMaster was subtracted from that measured by Galilei G4, and one-sample t-test was conducted to verify if the difference is statistically significant from 0. The mean difference was −0.98° ± 8.94°, which was not significantly different from 0.
The AE was 0.19° ± 9.77° in IOLMaster, and −0.80° ± 10.45° in Galilei G4 (Fig. 2). AAE was 6.84° ± 6.92° by IOLMaster, and 7.98° ± 6.72° by Galilei G4 (Fig. 3). The AE and AAE between IOLMaster and Galilei G4 did not show any statistical difference (AE, p = 0.583; AAE, p = 0.346).
Subgroup analysis was performed on patients with KA axis difference greater than 10° between IOLMaster and Galilei G4 (Fig. 4A, 4B). Six patients met the above criteria. Among the six patients, the AE and AAE were 5.50° ± 9.48° and 6.83° ± 8.38°, respectively, for the IOLMaster, and 6.50° ± 10.50° and 10.50° ± 5.36°, respectively, for the Galilei G4. The AE and AAE did not show any statistically significant difference between the IOLMaster and Galilei G4 (AE, p = 0.667; AAE, p = 0.331).
Discussion
This is the first study to compare the preoperatively measured axis of optical biometry and Scheimpflug-based topography in toric IOL implantation. In this study, we calculated the gold-standard axis of the toric IOL after cataract surgery and compared it with the preoperatively calculated axis with IOLMaster and Galilei G4. Both AE and AAE did not show any significant difference between IOLMaster and Galilei G4.
Accurate axis determination of toric IOLs is critical for maximizing the corrective effect of astigmatism. Misalignment of the toric IOL by 10° and 20° reduces the effectiveness of astigmatism correction to two-third and one-third of the total correction, respectively [13]. A 30° misalignment produces astigmatism of the same magnitude but in a different axis, rendering the correction ineffective [1,13,14].
There are various ophthalmic devices to measure corneal astigmatism, with the IOLMaster and Galilei G4 being widely used. Discrepancies between the measurements from different devices may occur, making it challenging to determine which value to rely on. To our knowledge, no previous study has compared the postoperative gold-standard axis with preoperatively calculated axes from optical biometry and Scheimpflug-based topography. However, our study did not find any significant difference between these devices.
The IOLMaster 500 uses partial coherence interferometry and captures keratometric data from six points in a 2.4-mm zone [15,16]. On the other hand, the IOLMaster 700 gathers data from 18 spots in hexagonal patterns across three corneal zones (1.5, 2.5, and 3.5 mm) [17]. The Galilei G4 combines dual rotating Scheimpflug cameras with a Placido disc [18,19]. Dong et al. [20] suggested that differences in corneal power measurement between the IOLMaster and corneal Scheimpflug topography might be due to differences in the analytic zone size. This may explain the differences in corneal astigmatism axis measurements between optical biometry and corneal topography.
The online Toric Results Analyzer can provide information on residual astigmatism changes with toric IOL rotation, and has been used in previous studies to estimate the IOL rotation magnitude or residual cylinder reduction after toric IOL reorientation [21-24]. In this study, we used the Toric Results Analyzer to evaluate the gold-standard axis of the implanted toric IOL and compared it with the preoperative calculated axis based on optical biometry devices and Scheimpflug-based corneal topography.
AE and AAE were evaluated to compare the accuracy of corneal astigmatism measurements between the IOLMaster and Galilei G4. The AE considers the directionality of measurement error either in a clockwise or counter-clockwise direction, while AAE is a value without directionality. No significant differences were found in AE and AAE between the IOLMaster and Galilei G4.
We also performed a subgroup analysis on six patients with a KA axis difference greater than 10° between the IOLMaster and Galilei G4 to determine which device was more accurate when there was a large discrepancy between devices. The mean AE was 5.50° for the IOLMaster and 6.50° for the Galilei G4, and the mean AAE was 6.83° for the IOLMaster and 10.50° for the Galilei G4, but no significant difference was found.
There are some limitations to this study. One limitation of this study is the inclusion of both the IOLMaster 500 and IOLMaster 700 into the analysis of optical biometry. Although Jiang et al. [25] reported a high level of agreement between IOLMaster 500 and IOLMaster 700, there may be slight differences in keratometric values between IOLMaster 500 and 700. Another limitation is that we did not obtain anterior segment images with pupil dilation, which could have provided more accurate evaluations of the implanted toric IOL axis when used alongside the internal OPD map of the OPD-Scan III. Additionally, the low baseline astigmatism axis difference between the IOLMaster and Galilei G4 may lead to the lack of statistical significance. Although we performed subgroup analysis in patients with an astigmatism axis difference greater than 10° between the IOLMaster and Galilei G4, no significant difference was observed between the devices. Further investigation with larger sample sizes in patients with discrepancies between IOLMaster and Galilei G4 is needed to enhance statistical reliability.
In conclusion, this study suggests that the Galilei G4 demonstrated a similar level of accuracy to the IOLMaster in calculating the toric IOL axis, based on the gold-standard axis provided by the Toric Results Analyzer.
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