Comparison of tendency and accuracy in predicted post-operative refraction and recommended IOL power between IOL Master and VERION before and after optimizing IOL-constant in the VERION

Background : We assessed the accuracy and tendency of the VERION image-guided system (Alcon) and the intra-ocular lens (IOL) Master 700 (Zeiss), by comparing mean refractive shift (MRS) of predicted post-operative refraction (PPR), mean absolute error (MAE) of PPR, recommended IOL power (RIP) and K-value before and after optimizing the IOL-constant in VERION, to show the importance of optimization. Methods : This retrospective study involved 72 eyes. K-value was measured with both biometers. Axial length (AL) and anterior chamber depth (ACD) measured by the IOL Master were applied to the VERION because it cannot measure these variables. The User group for Laser Interference Biometry (ULIB) IOL-constant for the IOL Master was applied to the VERION before optimizing the IOL constant, since no such official measure was established for it. MRS of PPR, MAE of PPR, RIP and K-value as measured by both biometers were compared before and after optimizing the IOL-constant in the VERION. Finally, correlations between the MRS, MAE, RIP, and K-value were analyzed in the VERION. The Wilcoxon signed-rank test was used for analysis. Results : Compared to the IOL Master, K-value was significantly higher in the VERION. Prior to optimization, MRS of PPR showed a significant myopic shift in the VERION, and MAE of PPR was significantly higher. Additionally, RIP in the VERION was significantly lower. After optimization, there were no significant differences in the MRS of PPR and RIP between the VERION and IOL Master. MAE of PPR in the IOL Master was significantly higher than in the VERION. No significant correlations were found between MRS and MAE of PPR and RIP with K-value in the VERION. Conclusions : Before optimization, the VERION was less reliable in MRS, MAE and RIP than the IOL Master. However, after optimization, the difference in MRS and RIP between the two devices became insignificant. This study indicates that optimization of IOL-constant in

the VERION is vital. After optimization, the VERION is more accurate in PPR than the IOL Master.

Introduction
A good refractive outcome following cataract surgery, has been regarded as a vital part of refractive surgery following increased patients expectations. Therefore, more accurate prediction of post-operative refraction is required [1][2][3], as surgery aims for the independence of an individual from spectacle correction and improved quality of life [4]. It is thus understandable that the accuracy of these calculations is increasingly critical [5], as post-operative visual acuity has been associated with the accuracy of measurements, prior to cataract surgery [6]. Most intra-ocular lens (IOL) calculation formulas include Kvalue as one of several biometric variables [7][8][9]. Therefore, a precise measurement of Kvalue is crucial [10].
In general, the more peripheral the cornea, the flatter it is [11,12]. However, currently available optic biometers measure the curvature of the cornea at different points [13]. devices, including the IOL Master 700 [13]. However, there is also research that has demonstrated that K-value in the VERION did not show a significant difference when compared to other optic biometers [15,16]. To add to the debate among studies, although previous research has indicated that several outcome measures with the VERION are yet to be fully evaluated [17], others support that it can be used interchangeably with the IOL Master when it comes to keratometry measurements [18]. At the same time, others suggest it cannot and that the superiority of one system over the other cannot be established [19].
The aims of the current study were to investigate the differences in K-value between the VERION and the IOL Master 700. In addition, mean refractive shift (MRS), mean absolute error (MAE) of PPR, and RIP as acquired in the IOL Master 700 and the VERION, were compared before and after IOL constant optimization for the VERION. Finally, the potential correlations between K-value in the IOL Master 700 and the VERION, both before and after IOL constant optimization and other variables were assessed.

Study population
This retrospective study included 72 eyes from 42 patients with cataracts, who underwent uneventful phacoemulsification with IOL implantation (SN60WF, Alcon Laboratories, Inc., Fort Worth, TX, USA), at Yokohama Tsurumi Chuoh Eye Clinic or Yokosuka Chuoh Eye Clinic in Japan. Additional inclusion criteria included, patients with good quality IOL Master 700 and VERION Reference Unit measurements, as indicated by both devices and postoperative best-corrected visual acuities better than 20/40. Patients with an anterior segment eye problem, high corneal irregular astigmatism, a history of traumatic or uveitis cataracts, intraocular or corneal operation, or intra-or post-operative complications were excluded. The average age was 73.8 years ± 7.7 SD (range: 50-86 years), and 40.48% of patients were men (Table 1).          Edmund et al. showed that the radius of corneal curvature changed between 1 mm and 5 mm from the corneal apex and the more central the cornea, the steeper it is [11]. Various optic biometers are currently available to measure K-value, and each device takes measurements at a different part of the cornea. For instance, the VERION, Lenstar (Haag-Streit, Switzerland) and IOL master measure corneal curvature at 0.8-1.2 mm, 1.6 mm and 2.3 mm, and 2.5 mm from the apex of cornea, respectively. Holladay illustrated the example of the theoretical differences in K-value among these devices [13]. When K-value in the VERION is 44.7 D, theoretically speaking, the same values in the Lenstar and IOL master should be 44.5 D and 44.3 D respectively. In this study, the mean K-value in VERION was 0.36 D, higher than the one in the IOL master and the difference was significant. This result comes in agreement with the aforementioned report [13].
The VERION image-guided system is a surgical-assisted system, used both pre-and intraoperatively. The image taken pre-operatively can be applied for auto-registration using the intra-operative digital marker. This system is useful to insert a toric IOL and create limbal relaxing incisions (LRI) in place [14]. The VERION Reference Unit is the biometer to measure K-value and capture the image of the anterior segment preoperatively. The image is integrated with the VERION image-guided system to be used for intra-operative registration and provides real-time visual image marking through a microscope. The image-guided system is also combined with the femtosecond laser cataract surgery platform. The Reference Unit functions for corneal incision design and LRI [14]. This subsequent process has the potential to improve the refractive outcome.
Although this state-of-the-art system sounds very sophisticated, there are also some concerns about this process. The VERION Reference Unit measures K-value at a different distance from the corneal apex compared to other major biometers. Additionally, it currently cannot measure essential biometric variables, such as ACD and AL to calculate post-operative refraction and RIP. Therefore, the VERION needs to use ACD and AL measured by different optic biometers. Furthermore, although the IOL constant is also a crucial factor for IOL calculation formulas, a published optimized IOL constant for the VERION, such as the one provided by the ULIB, does not currently exist. Since the IOL Master 700 and Lenstar are currently the gold standard optical biometers [20,21]

Conclusion
The VERION was less reliable than the IOL Master 700 before IOL constant optimization.
That being said IOL constant optimization significantly improved its predictability.
Moreover, after IOL constant optimization, the VERION became significantly more accurate than IOL Master 700.
The results of this research indicate the importance of IOL constant optimization for the VERION to improve post-operative refraction, and to make the most of the strengths of both the VERION image-guidance system and its Reference Unit. Finally, the release of a published IOL constant for the VERION from ULIB is also expected.

Ethical Approval/Informed Consent
The study was approved by ethical committees at Yokosuka Chuoh and Tsurumi Chuoh Eye clinics. Written informed consent for publication of their clinical details and/or clinical images was obtained from all patients. A copy of the consent form is available for review by the Editor of this journal.

Availability of data and materials
The data used and analyzed for the current study are available upon reasonable request from the corresponding author

Competing interests
None of the researchers involved in this study have any affiliations with, or involvement, in any organization or entity with any financial or non-financial interest.