Comparison of two digital alignment systems for toric intraocular lens implantation

Introduction

Approximately 20%–30% of patients who undergo cataract surgery have a pre-existing corneal astigmatism of 1.25 diopters (D) and more.1–5 Moreover, the prevalence of corneal astigmatism higher than 2 D is 8% and higher than 3 D is 2.6%.1 This corneal astigmatism, if not corrected during surgery, leads to postoperative spectacle dependence for the target refraction. The use of toric intraocular lenses (TIOL) has resulted in less spectacle dependence and higher patient satisfaction.6

The most important factors to achieve successful astigmatism correction are the preoperative corneal measurements and the correct alignment of the TIOL itself.7 Each degree of deviation from the target axis is expected to result in a 3% reduction in astigmatism correction.8

Since the eye may undergo cyclotorsion when going from the sitting to the supine position, marking the cornea before surgery has been advised when using astigmatic correcting surgical techniques. Manual marking techniques, where the ocular surface is marked by needle, or ink staining with bubble, tonometer or pendulum markers, have shown some variability and deviations of up to 4.7°.9 10 Furthermore, the total error from the calculated target axis, starting from positioning the head at the slit lamp, manual preoperative marking and finally intraoperative marking using a Mendez or Whitman gauge, was previously described with a total of up to 5°.9

The introduction of digital alignment systems has been intended to minimise the disadvantages of manual marking, such as tilting of the head during marking, incorrect and smudged markings, as well as the total time needed.11 There is a range of digital systems that have been proven to be equal to or even better than the most manual marking methods. These include, among others, the Callisto Eye system (Carl Zeiss Meditec AG, Jena, Germany) and the Alcon VERION Digital system (Alcon Laboratories, Ft. Worth, Texas, USA).9 10 12 13 While there are several publications regarding the accuracy and comparison of these to manual marking techniques, there is only one retrospective study by Hura et al comparing these two most widely used digital alignment systems.14

The aim of this study was to evaluate and directly compare the accuracy of the two most common digital alignment systems for TIOL.

Materials and methods

Participants older than 21 years with cataracts and corneal astigmatism of less than 1.5 D were included in this prospective study. Participants were excluded from the study in the case of dense cataract or corneal pathology that would significantly influence fixation, previous ocular surgery, ocular inflammation or trauma, as well as nystagmus or other pathologies that might affect patient fixation. In advance of the study, the ethics committee (EC) of the city of Vienna approved all study documents (EC number: 21-130-0621). The study procedures followed the guidelines of Good Clinical Practice and adhered to the tenets of the Declaration of Helsinki. Written informed consent was obtained from all patients after the nature of the study had been explained. The study was registered at clinicaltrials.gov (https://clinicaltrials.gov; NCT05220683). Data handling was done in a pseudonymised way. Patients were not involved in the design, conduct, reporting or dissemination plans of this research.

Sample size calculation was done using the G*Power software V.3.1.9.2. The effect size was calculated using the mean difference of 4.16° and the SD of difference of 3.41° between the Verion and the Callisto device described in the paper of Hura and Osher.14 This resulted in an effect size of 1.22. When using a two-tailed matched paired analysis with an alpha error probability of 0.05 and an assumed power of 95%, a total sample size of 11 eyes of 11 patients was needed. To compensate for patient dropout, 15 eyes of 15 patients were included.

Surgical microscope and camera

The surgical microscope used in this study is the OPMI Lumera 700 (Carl Zeiss Meditec AG). A beam splitter was attached to the microscope to record video segments of the surgery and then extract single images after the implantation of the intraocular lens using a digital camera (Canon 5D Mark III). Using this setup, it was possible to compare the two alignment systems simultaneously (figure 1).

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Figure 1

The intraoperative setup consisted of the Zeiss OPMI Lumera 700 Microscope (1), the Alcon Mini Display (2), a Beam Splitter (3) and the attached digital camera (4) in ascending order.

Study measurements

A routine ophthalmic examination including slit lamp biomicroscopy, retinal examinations, biometry and creation of the reference images using both biometry systems took place immediately before cataract surgery. One eye of each patient with corneal astigmatism of less than 1.5 D was selected as the study eye and implanted with a non-TIOL. As the study objective was to compare the mean deviation of the projected axis, neither the implantation of a TIOL nor a specific amount of astigmatism was necessary for achieving the study aim. The reference images were imported to both alignment systems in the operating room prior to surgery. The target axis was defined by rounding up to the nearest 10 of the steep meridian, based on the IOL Master 700 biometry, taking a value between 0 and 180. Intraoperatively, both reference and alignment axes were activated and captured with a digital camera mounted on a beam splitter (figure 1). The images were postoperatively examined for the difference from the projected to the target axis. The preoperative reference images were aligned (figure 2) and five intraoperative images for each eye, in 200-ms intervals, were analysed for precision and repeatability (figure 3). Loss of tracking (LOT) was analysed during the rotation of the eye in degrees per millisecond. Additionally, the intraoperative microscope light brightness that was necessary to get matching of each of the systems was noted. There were no follow-up visits.

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Figure 2

In the first step, the preoperative reference images from both systems were aligned to make sure that the measured target axis was the same and to neutralise potential differences in head tilt during the biometric measurements. The inner, superlayed image is from the Zeiss IOL Master, while the underlying reference image is from the Alcon Argos Biometer.

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Figure 3

Five images, captured in intervals of 200 ms, of each eye were analysed regarding the projected target axis (in this example, 10°). The three blue lines represent the Zeiss system (10°), while the green dotted lines represent the Alcon system (11.1°).

Results

15 eyes of 15 patients were included in this study, of which one patient had to be excluded because of an intraoperative technical problem with the memory card of the external camera. Hence, a total of 14 patients were included in the final analysis.

There was a significant difference in precision and repeatability between the two digital alignment systems. The deviation from the actual target axis was significantly different, with a mean of 0.34°±0.75° for the Zeiss system and 1.60°±1.08° for the Alcon system (p=0.03, n=14).

To investigate LOT in more detail, this study could show that the within-subject SD (Sw) was significantly lower with 0.21° for the Zeiss system compared with 0.34° for the Alcon system (p=0.03, n=14).

In both systems, a deviation of the projected alignment axis into the clockwise direction became apparent in 9 of 14 eyes for the Zeiss Callisto and Alcon Verion systems (65%). The values, with a mean of 0.34°±0.75° and 1.60°±1.08°, ranged from −2.98° to 0.10° and −3.98° to 3.26°, respectively (figure 4).

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In terms of traceability, LOT was further analysed in 13 participants. The Alcon Verion system lost track at an average rotation of 8.2°±3.8° at a speed of 197±25 ms per degree. Since the Zeiss Callisto system remained constantly on track during these movements, no comparative analysis could be made.

While the Zeiss Callisto system recognised the ocular surface each time at the lowest lighting level of 5% of the maximum possible light level of the microscope, the Alcon Verion system required an increase in illumination in all cases. In a direct comparison, the Zeiss system was used with a significantly lower average luminosity of 5.78%±1.4% and the Alcon system with an average of 27%±14.5% luminosity for registration (p<0.001).

Discussion

Both systems evaluated in this study showed highly accurate intraoperative axis identification, which appears significantly better than that achieved with manual marking methods.10 15 16 In direct comparison, the Zeiss system performed better than the Alcon system. On average, both systems were well below the often-mentioned 3° limit, but the Alcon system had two deviations above 3°, while the highest for Zeiss was 2.98°. Overall, the Zeiss system seemed to be more stable, with a significantly lower within-subject deviation. Interestingly, both systems showed an equal number of deviations in the anticlockwise direction, even though these were of lower magnitude for the Zeiss system. Whether the differences in accuracy will result in clinically relevant differences in outcome is difficult to assess. This may play less of a role for low degrees of astigmatism but could be relevant with high toric correction.

There was a marked difference in microscope illumination necessary for the registration of the eye, where the Alcon system needed significantly more light. Since the Alcon MIDI was placed under the added beam splitter, this should not be the cause. Possibly having the system fully hardware integrated, as is the case with the Zeiss system, allows for better image analysis. However, it appears as if the need for more light is more due to the software algorithm that matches and tracks the vessels.

When it comes to marking of the steep meridian for the optimal placement of a toric intraocular lens, many methods are applied in clinical practice and might be suitable. Manual marking methods have been analysed and discussed in previous publications.9–11 15 17 18 In a head-to-head comparison by Popp et al, the manual marking methods per slitlamp and needle, tonometer, pendular or bubble marker showed variability of 1.8°, 2.3°, 2.9° and even 4.7°, respectively.10 Besides the rather high variability, these methods have an invasive character, a high user dependence and a need for an additional intraoperative axis marking with a Whitman or Mendez gauge. A cohort study could show that the sum of errors when using manual marking was as high as 5° in total.9

Previously, the Zeiss Callisto System was evaluated regarding the preoperatively calculated and postoperatively positioned TIOL axis with a mean misalignment of 0.52°±0.56°.19 A direct comparison of manual marking with the bubble marker to the Alcon Verion system showed a significant difference in the marking methods with a deviation of 2.8°±1.8° and 1.3°±1.6°, respectively.15 Although a significant difference between the marking methods could be shown with increasing frequency, no clinically significant difference in terms of uncorrected distance visual acuity was found.15 17 18 20

In a medical field with high-frequency surgery, the improved time factor with digital marking systems should not be underestimated. This advantage of digital marking systems was convincingly shown in an experimental longitudinal study by Barbera-Loustaunau et al, where a significant difference in total corneal and astigmatism marking time between the manual and digital marking groups became apparent.11

There is a growing body of literature on digital alignment systems, but very few on a head-to-head comparison between the two major systems. Hura et al compared Zeiss with the Alcon system in a retrospective study, where they found no significant difference between the two systems in 16 participants.14 A major difference compared to our study is the retrospective character and that the systems were compared regarding the projected target meridia and their similarities to each other based on two central angles of variation, while this study examined the deviation to a determined target axis.14

Regarding accuracy, the findings of Hura and Osher are in line with ours since a certain difference between the systems has been described. However, a much higher amount of drift, with a mean of 3.96° and 4.60°, regarding the two angles of variation has been described.14 Apart from the different measurement methods, the significantly lower drift of 0.21° and 0.34° in our study may be explained by improvements regarding the software since 2017 and the time intervals between captures of 200 ms as used in this study. Another difference to the aforementioned study was the initial adjustment of both reference images in our studies in order to have a fair and equal baseline starting point. Due to this fact, the deviation to the target axis could be measured with more precision, since potential differences in head tilt during the preop photography are neutralised.

Intraoperatively, the study eye was artificially moved and cyclotorsed at the end of surgery during the irrigation/aspiration to see if the two systems lost track of the eye. While the Zeiss system remained steady and the projected lines were always seen even at higher degrees of rotation, the Alcon system often lost track of the eye during the initial movement. Only after a short interval was the tracking back. This study could show that the deviation from the actual target axis was significantly different, with a mean of 0.34°±0.75° for the Zeiss system and 1.60°±1.08° for the Alcon system (p=0.03, n=14). While both systems showed good repeatability, with a mean Sw of 0.21°, the Zeiss Callisto System performed significantly better than the Alcon Verion system with a Sw of 0.34° (p=0.03). Furthermore, LOT for the Alcon Verion system amounted to an average of 8.2°±3.8° at a speed of 197±25 ms per degree.

There was a relatively high amount of microscope illumination necessary for the registration and matching of the Alcon system. Interestingly, this issue was not described by Hura and Osher. Alcon’s digital unit used in this study is a device that needs to be inserted into the beam path of the surgical microscope. The beam splitter for the external camera that allowed the capture of both overlays was inserted above the MIDI and should, therefore, not interfere with or cause loss of light energy on the way to the receiver. Further investigation of this issue would be of interest. It appears as if the matching and tracking algorithm used in the Alcon system needs more light, possibly to enhance contrast.

There are some limitations to this study that need to be mentioned. Although calibration was performed each time before surgery, an inserted display into the microscope, as is used for the Alcon system, may not be as accurate as an already built-in system, as is the case with the Zeiss system. Possibly a slight rotation of the Alcon interface or inaccuracies in the calibration procedure may have led to the slight anticlockwise offset seen in the study. However, the Alcon system also showed more variability, which must be due to another reason.

Concluding, the Zeiss system showed a significantly lower deviation from the target axis, higher stability with eye movements and less need for microscope illumination than the Alcon system.