Accuracy of intraocular lens power calculation using Scheimpflug tomography and OKULIX ray tracing software in corneal scarring

Background: To evaluate the accuracy of intraocular lens power (IOL) calculation using Scheimpflug tomography and OKULIX ray tracing software in corneal scarring. Methods: This study was conducted on 40 consecutive eyes, 20 cases with corneal scarring and coexisting cataract, and 20 controls with clear cornea, which underwent uneventful phacoemulsification and IOL implantation following Scheimpflug tomography and OKULIX ray tracing software and third generation IOL power calculation formulas for IOL power calculation. Accuracy of IOL power calculation was evaluated by subtracting expected and achieved spherical refraction 3 months postoperatively and was recorded as mean absolute error (MAE). Distance uncorrected visual acuity (UCVA) for each eye was measured using Snellen chart preoperatively and 3 months postoperatively; visual acuity was scored and converted to the logarithm of the minimum angle of resolution (LogMar).Values were recorded as mean ±SD (standard deviation). Student t-test (t) and Mann Whitney test (U) were used for parametric comparison of the means. Intra class Correlation (ICC) coefficient and Pearson correlation Coefficient (r) were used to assess agreement. A P value less than 0.05 was considered statistically significant. Results: In cases of corneal scarring, 20 eyes (100 %) yielded a postoperative spherical refraction which differed less than 1 diopter (D) from predicted, in 16 eyes (80 %) the postoperative spherical refraction was within 0.50 D from expected. The MAE was 0.2 D in cases, which did not differ significantly compared to controls (MAE 0.1 D). In corneal scarring cases, distance UCVA showed significant improvement from 1.3 Log Mar (Snellen equivalent 20/400) preoperatively to 0.5 Log Mar (Snellen equivalent 20/60) 3 months postoperatively. Conclusion: Scheimpflug tomography combined with OKULIX ray tracing software for calculation of IOL power provides accurate results in cases of corneal scarring.


Background
Patients with corneal scarring pose additional challenges when undergoing cataract surgery. Although in some cases, these patients may benefit from a combined keratoplasty and cataract surgery in the same sitting, sometimes corneal stromal scars have less effect on vision than expected. Undergoing cataract surgery as a first step is sometimes all that is needed to achieve better vision. In these cases, accurate IOL calculation is crucial for visual recovery. [1] Corneal topography and keratometry are the most commonly applied tools to measure the refractive power of the central cornea for IOL power calculation in cataract surgery.
Despite both methods provided satisfactory accuracy in measuring the refractive power of the corneal centre in eyes with regular cornea, such accuracy is limited in cases with scarred cornea and irregular astigmatism. [2][3][4][5] The Tomey Topographic Modeling System couples Scheimpflug and Placido disk technologies to determine corneal power and curvature. Reconstruction of both posterior and anterior surfaces of the cornea can be achieved from the video-captured slit images, allowing calculation of the total anteroposterior corneal power. In scarred corneas, these maps would provide superior accuracy than maps analyzing the anterior surface alone. [6] Lately, a novel IOL power computation software became available relying on numerical ray tracing called OKULIX (Ingenieurbu¨ ro der Leu, Hillerse, Germany). [7][8][9][10] This software is able to determine the monochromatic optical properties of the pseudophakic eye, assessing optical rays confined to the pupillary zone from cornea to fovea. Unlike conventional lens power calculation formulas, comelling with paraxial rays solely, relying on Gaussian optical principles. The OKULIX computation erratum is similar for all distance points in relation to the optical axis (≤ 0.001D). [7][8][9][10][11][12] While numerous reports evaluated the accuracy of calculation of the refractive power of the cornea following laser refractive procedures, studies evaluating the efficiency of intraocular lens power calculation in scarred corneas and irregular astigmatism are lacking. In our study, we evaluated the accuracy of IOL power calculation using Scheimpflug tomography and OKULIX ray tracing software in corneal scarring.

Methods
Subjects were enrolled from the Ophthalmology department. Informed consent was signed by participants. This research was accepted by the Research Ethics Committee, adhering to the tenets of declaration of Helsinki.
In our prospective study, we enrolled 40 consecutive eyes , 20 cases with corneal scarring and coexisting cataract, and 20 controls with clear cornea, which underwent uneventful phacoemulsification and intraocular lens implantation following Scheimpflug tomography using Topographic Modeling System TMS-5 (Tomey Corporation; Nagoya, Japan) and axial length measurement using the OA-1000 optical biometer (Tomey Corporation; Nagoya, Japan). In the next step, these data were exported to OKULIX ray tracing software for IOL power calculation. IOL power was also calculated using Holladay, SRK II, Hoffer Q and SRK-T formulas with Lenstar LS 900 biometer (Haag-Streit AG, Koeniz, Switzerland). Target refraction was set to zero.

Discussion
In the process of IOL power calculation, the corneal power is presented in a single figure: the keratometry (K) value, which is generally computed from the measurement of the central corneal mean anterior radius of curvature. Nonetheless, in cases with irregular anterior corneal surface, a solitary figure is not sufficient to accurately present the optics of the cornea and, hence, to precisely calculate the correct IOL power. [13] Since the earliest IOL power calculation formulas by Fyodorov and Gernet to the most modern by Holladay and Olsen, the corneal power was represented only by the Keratometry value, presenting the paraxial power of the cornea, computed by an approximate refractive index of the cornea to assume the non measured posterior corneal surface refractive power. The Keratometry value hypothesizes a spherical corneal shape and speculates a fixed proportion between the anteroposterior corneal curvature. This assumption yields accurate values in normal regular corneas with minimal variations in anteroposterior corneal curvature, which was proven by ray-tracing studies. [14] Nonetheless, scarred corneas with irregular surface exhibit abnormal anteroposterior corneal curvature relation, violating the hypotheses allowing intraocular lens power calculation relying on Keratometry. Furthermore, the accuracy of measurements is frequently hindered in such irregular corneas, usually encountered after corneal refractive procedures, corneal ectasia, scarred corneas, or xerophthalmia. [13] The application of keratometry in the measurement of the refractive power of the cornea relies on two assumptions, the first is that the four measured paracentral points represent the corneal central region, the second is that the corneal centre is comparable to a spherical shape and that the anterior corneal radius is 1.2 mm flatter than the posterior radius of curvature. [2][3][4] Although this proves true in regular corneas, it is not applicable in scarred and irregular corneas. [2][3][4][5] Computerized videokeratography (CVK) measures > 5000 points over the corneal surface; therefore providing superior accuracy than manual keratometry in scarred corneas with irregular astigmatism. [3,15] However, topographic corneal power measurement multiplies the anterior corneal curvature by a refractive index, assuming a fixed anteroposterior corneal curvature ratio, to calculate corneal power. [16,17] In cases with marked change in the anterior and posterior corneal surfaces relationship, the default refractive index applied by most topography systems is inaccurate. [16,17] The Tomey Topographic Modeling System couples Scheimpflug and Placido disk technologies to determine corneal power and curvature. Anteroposterior corneal suface reconstruction can be achieved from the video-captured slit images, allowing calculation of the total anteroposterior corneal power.
In scarred corneas, these maps would provide superior accuracy than maps analyzing the anterior surface alone. [18] The cornea in cases of herpetic keratitis shows scarring involving the anterior stroma, causing central flattening of the anterior corneal surface with little effect on the posterior curvature. These changes resemble alterations by refractive laser procedures and hence, CVK that relies solely on anterior corneal surface analysis would be inaccurate. [18] In Irwin et al study, computerized scanning-slit videokeratography, which analyzes the anterior and posterior surfaces of the cornea, and the contact lens overrefraction method gave good estimations of corneal power in patients with irregular corneal astigmatism, improving the accuracy of IOL calculation in patients with corneal pathology and irregular astigmatism. [18] The contact lens overrefraction method is reliable in estimating corneal power in patients with irregular corneal astigmatism, however, in cases in which the visual acuity is 20/70 or worse, contact lens overrefraction may not be accurate. [18] Lately, OKULIX IOL power computation software became available relying on numerical ray tracing, assessing optical rays confined to the pupillary zone from cornea to fovea, unlike conventional lens power calculation formulas, compelling with paraxial rays solely, relying on Gaussian optical principles. The principles of ray tracing have existed since the 17 th century, but only recently have they been applied to calculations for optical devices in ophthalmology. Although many surgeons rely on the use of third-generation IOL power calculation formulas such as the Haigis-L, Hoffer Q, Holladay 2, Olsen, and SRK/T, ray tracing is a modern technique, based on a different set of principles, that should be considered a potentially useful strategy. [19] For a collective of 153 eyes undergoing cataract surgery and applying OKULIX software for IOL power calculation, the mean prediction error was -0.05±0.67D. The slope of the regression line (0.009D/mm) was not significantly different from zero. [20] In a recent study by M. Ghoreyshi et al., the performance of OKULIX software ray-tracing IOL power calculation was not significantly different compared with SRK-T and Hoffer Q formulas. The MAE by OKULIX, SRK-T and Hoffer Q formulas, respectively, were 0.42 (±0.03), 0.36 (±0.02) and 0.37 (±0.02). [21] In the present study, The MAE was 0.2 D in corneal scarring cases, which did not differ significantly compared to controls (MAE 0.1 D, p 0.142). In another study by Karim M Nabil, OKULIX ray-tracing software accuracy was assessed in myopic cataract patients. In 83.33% of myopic patients, a prediction within ±1.00 D was obtained, whereas 70% showed a prediction within ±0.5 D. The MAE was 0.45±0.40 D. [22] In a third study, OKULIX ray tracing software yielded more accurate minus power intraocular lens calculation in extreme myopia, compared to SRK-T formula.
SRK-T calculated IOL power (-6.3 ± 2.8 D) showed statistically significant difference compared to OKULIX calculated IOL power (-4.7 ± 2.6 D), rs 0.994 p < 0.001. [23] In the present study, 3rd generation formulas which are based on anterior curvature only, had comparable results with ray tracing software, even in cases with cornel scarring. This could be explained by the fact that 75% of the studied cases involved stromal scarring, with more significant effect on the anterior, rather than posterior, corneal curvature.
One of the limitations of our study is the relatively small sample size, which could be justified by the rarity of cases of coexisting cataract and corneal scarring justified to undergo solely phacoemulsification without keratoplasty.
In summary, although corneal topography and keratometry are the most commonly applied tools to measure the refractive power of the central cornea for IOL power calculation in cataract surgery, both methods suffer limitations in cases with corneal scarring and irregular astigmatism.
In these cases, CVK that analyzes the anterior and posterior corneal surfaces provide a more accurate estimation of corneal power than CVK that analyzes the anterior surface only Conclusion In conclusion, Scheimpflug tomography combined with OKULIX ray tracing software for calculation of IOL power provides good results in cases of corneal scarring.

Consent for publication
Written informed consent to publish the results of the study and identifying images or other personal or clinical details of participants that compromise anonymity was obtained from all individual participants included in the study.
Availability of data and materials Datasets are available in additional supporting files.

Competing interests
The author declares no conflict of interest.