This is the first research, which showed the importance of IOL constant optimization for the VERION to improve the accuracy of its post-operative refractive prediction. K-value in the IOL Master 700 was significantly different from the one acquired from the VERION. Regarding predicted post-operative refraction, before IOL constant optimization, the VERION showed a significant myopic shift compared to the IOL Master 700. Additionally, the VERION was significantly less accurate in PPR than the IOL Master 700. Furthermore, RIP in the VERION was significantly lower than the IOL Master 700. However, after IOL constant optimization, the performance of the VERION dramatically improved and the myopic shift of PPR became insignificant compared to the IOL Master 700. Interestingly enough, the VERION became significantly more accurate than the IOL Master 700. Furthermore, RIP in the VERION came close to that of the IOL Master 700. Concerning the correlation between K-value and other variables, K-value and refractive shift (RS) of post-operative refraction was the only significant correlation found.
K-value is one of the most important variables for the IOL calculation formulas [7-9]. 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 pre-operatively. 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], most research on the importance of IOL constant optimization is conducted with the IOL Master or Lenstar. Hsieh et. al. demonstrated that optimization of IOL constants for the IOL Master, significantly improved the accuracy of IOL calculation [22]. Aristodemou et al. [23], also reported that the IOL constant optimization could significantly improve the refractive outcome [23]. While they supported the importance of personalization of the IOL constant, Charalampidou [24] mentioned that the published IOL constant from the ULIB is as accurate as the personalized IOL constant, unless surgeons perform an extremely high number of operations per year and only use specific models for a relatively long time to apply the data for the optimization [24]. The result indicated that although personalization of IOL constant is still ideal, for the majority of surgeons, the published IOL constant from the ULIB is very useful. However, as of now, the optimized IOL constant for the VERION has not been provided by the ULIB [25]. Therefore, IOL manufacturers advise the first user of the VERION to apply the published IOL constant provided by the ULIB.
Again, K-value is one of the most important factors for the calculation of post-operative refraction and RIP, but K-value is measured at different areas from the apex of the cornea, when using a VERION or IOL Master. Therefore, to utilize the best of the functions of the VERION, it is worth investigating how the application of the published IOL constant affects the tendency and accuracy of PPR and RIP calculated using the VERION, and how the IOL optimization for it, improves outcomes.
In this study, we compared K-value, MRS of PPR, MAE of PPR, and RIP between the IOL Master 700 and the VERION, before and after IOL constant optimization. The correlations between K-value and all variables, were also analyzed. K-value in the VERION was significantly higher than in the IOL Master. This result was in line with past research [12, 13]. Prior to optimization, the VERION showed a significantly more myopic shift in MRS of PPR, higher MAE of PPR, and lower RIP than the IOL Master 700. These results indicate that before IOL constant optimization for the VERION, the device is significantly less accurate than the IOL Master 700, and the shift of PPR and RIP, has a particular tendency. However, after IOL-constant optimization for the VERION, the myopic refractive shift of PPR disappeared. MRE of PPR significantly improved, and interestingly enough, MRE of PPR in the VERION was significantly lower than the IOL Master 700. Additionally, concerning the RIP, it significantly increased in the VERION and came close to that of the IOL Master. These results indicate that after IOL optimization for the VERION, it turns out to be significantly more accurate than the IOL Mater 700. This finding has increased clinical significance as it can result in an improved quality of life and more independence from spectacles [26] for a patient who has undergone cataract surgery. The aforementioned particular tendencies disappeared. Regarding the correlation of K-value with RS of PPR, absolute error of PPR, and RIP in the VERION, K-value showed no influence on these three variables, regardless of IOL constant optimization. Whereas, K-value significantly affected RS of PPR, where the higher K-value the more myopic shift the PPR showed. This result comes in agreement to previous research [27]. Thus, we do not need to take the influence of K-value on PPR and RIP into consideration when we use the VERION, but we do need to take the influence of K-value on only PPR when we use the IOL Master.
A potential limitation of the current study is that a vital variable (AL) needed for the calculation of PPR and RIP, was not provided from the VERION and thus, the ones obtained from the IOL Master 700 were employed. Although, this was expected to have biased our results in favor of the IOL Master 700, that was not the case. It would be useful to have a published IOL constant for the VERION to assess in the future whether the pattern of the aforementioned results will change. Moreover, it can be inferred that the VERION has been found to have an accuracy close to that of gold standard devices, such as the IOL Master 700. Finally, future research could focus on the quality of vision and quality of life of those in whom post-operative outcome measurements are taken with the VERION.