

ORIGINAL ARTICLE 

Year : 2022  Volume
: 23
 Issue : 3  Page : 168176 

Accuracy of intraocular lens power calculation using Scheimpflug tomography and OKULIX raytracing software in paracentral corneal scarring not interfering with postoperative refraction
Karim M Nabil
Department of Ophthalmology, Faculty of Medicine, University of Alexandria, Alexandria, Egypt
Date of Submission  31Dec2021 
Date of Decision  26Jan2022 
Date of Acceptance  01Mar2022 
Date of Web Publication  30Jul2022 
Correspondence Address: Karim M Nabil Department of Ophthalmology, Faculty of Medicine, University of Alexandria, 19 Amin Fekry Street, Raml Station, Alexandria 21523 Egypt
Source of Support: None, Conflict of Interest: None  Check 
DOI: 10.4103/djo_79_21
Purpose The aim of this study was to evaluate the accuracy of intraocular lens (IOL) power calculation using Scheimpflug tomography and OKULIX raytracing software in cases of paracentral corneal scarring. Patients and methods The study was conducted on 40 consecutive eyes: 20 with corneal scarring and coexisting cataract and 20 controls with clear cornea, for whom uneventful phacoemulsification and IOL implantation was performed. Preoperatively, Scheimpflug tomography and OKULIX raytracing software and thirdgeneration IOL power calculation formulas were used for IOL power calculation. Accuracy of IOL power calculation was evaluated by subtracting the expected and achieved spherical refraction, 3 months postoperatively and was recorded as the mean absolute error (MAE). The distance uncorrected visual acuity for each eye was measured preoperatively and 3 months postoperatively; the visual acuity was scored and converted to the logarithm of the minimum angle of resolution (LogMar) for statistical analysis. Results In cases of corneal scarring, all eyes (100%) yielded a postoperative spherical refraction which differed by less than 1 D from the predicted. In 16 (80%) eyes, the postoperative spherical refraction was within 0.50 D from the expected. The MAE was 0.2 D in cases of corneal scarring, which did not differ significantly from the controls (MAE=0.1 D), P=0.142. In corneal scarring cases, the distance uncorrected visual acuity showed significant improvement from 1.3 LogMar (Snellen’s equivalent 20/400), preoperatively, to 0.5 LogMar (Snellen’s equivalent 20/60), 3 months postoperatively. Conclusion Scheimpflug tomography combined with OKULIX raytracing software for the calculation of IOL power provided accurate results in cases of corneal scarring.
Keywords: corneal scarring, OKULIX raytracing software, Scheimpflug tomography, third generation intraocular lens power calculation formulas
How to cite this article: Nabil KM. Accuracy of intraocular lens power calculation using Scheimpflug tomography and OKULIX raytracing software in paracentral corneal scarring not interfering with postoperative refraction. Delta J Ophthalmol 2022;23:16876 
How to cite this URL: Nabil KM. Accuracy of intraocular lens power calculation using Scheimpflug tomography and OKULIX raytracing software in paracentral corneal scarring not interfering with postoperative refraction. Delta J Ophthalmol [serial online] 2022 [cited 2022 Dec 3];23:16876. Available from: http://www.djo.eg.net/text.asp?2022/23/3/168/353039 
Introduction   
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 intraocular lens (IOL) power 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 providing satisfactory accuracy in measuring the refractive power of the corneal center 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 the corneal power and curvature. Reconstruction of both posterior and anterior surfaces of the cornea can be achieved from the videocaptured 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 (Ingenieurburo der Leu, Hillerse, Germany) [7],[8],[9],[10]. This software can determine the monochromatic optical properties of the pseudophakic eye, by assessing the optical rays confined to the pupillary zone from the cornea to the fovea, unlike the conventional IOL power calculation formulas, compelling with paraxial rays solely, relying on the Gaussian optical principles. The OKULIX computation erratum is similar for all distance points in relation to the optical axis (≤0.001 D) [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 IOL power calculation in scarred corneas and irregular astigmatism are lacking. Therefore, in this study the accuracy of IOL power calculation was evaluated using Scheimpflug tomography and OKULIX raytracing software in cases of corneal scarring.
Patients and methods   
All patients were recruited from the Department of Ophthalmology, Faculty of Medicine, Alexandria University, Alexandria, Egypt, from January 2019 till July 2021. The study was approved by the Research Ethics Committee of the Faculty of Medicine, University of Alexandria, Egypt (Ethics committee approval number 0303656, date: 20/7/2017). All patients signed a written informed consent to participate in the study and for publication of data before enrollment in the study.
In this prospective study, 40 consecutive eyes, 20 with paracentral corneal scarring and coexisting cataract, and 20 controls with clear cornea were enrolled. Uneventful phacoemulsification and IOL implantation was performed in all cases. Preoperatively, Scheimpflug tomography using the Topographic Modeling System TMS5 (Tomey Corporation, Nagoya, Japan) and axial length measurement using the OA1000 optical biometer (Tomey Corporation) were performed. Subsequently, these data were exported to the OKULIX raytracing software for IOL power calculation. The IOL power was also calculated using Holladay, SRK II, Hoffer Q, and SRKT formulas with Lenstar LS 900 biometer (HaagStreit AG, Koeniz, Switzerland). The target refraction was set to zero.
Phacoemulsification was performed by Alcon Infiniti System (Alcon Laboratories, Fort Worth, Texas, USA) with intracapsular implantation of an acrylic posterior chamber IOL. The surgical procedures were performed by the same ophthalmic surgeon (K.M.N.; the author). The implanted IOLs were hydrophilic acrylic IOLs (AquaSense; Ophthalmic Innovations International, OII, Ontario, USA). The included cases were limited to paracentral corneal scarring, not interfering with postoperative refraction. Cases with corneal scarring interfering with postoperative refraction were excluded from the study.
Topcon RM8900 autorefractometer (Topcon Medical Systems, Tokyo, Japan) was used for postoperative refraction assessment followed by subjective refraction. Snellen’s chart was used for the estimation of uncorrected visual acuity (UCVA) and best spectacle corrected visual acuity, preoperatively and 3 months postoperatively. The visual acuity was scored and converted to the logarithm of the minimum angle of resolution (LogMar) for statistical analysis. The accuracy of IOL power calculation was evaluated by subtracting the expected and achieved spherical refraction 3 months postoperatively and was recorded as the mean absolute error (MAE).
Statistical analysis
All data were analyzed with the IBM Statistical Package for the Social Sciences (SPSS) software package, version 20.0. (SPSS Inc., Chicago, Illinois, USA). Values were recorded as mean±SD. Student’s t test and Mann–Whitney test (U) were used for parametric comparison of means. Intraclass correlation coefficient and Pearson correlation coefficient (r) were used to assess agreement. A P value less than 0.05 was considered statistically significant.
Results   
The data of both groups are illustrated in [Table 1] and [Table 2]. The posterior corneal curvature was significantly flatter, and the cornea was significantly thinner in the paracentral corneal scarring cases compared with the controls (P=0.038 and 0.002, respectively, [Table 1]).  Table 1 Comparison between cases and controls according to different parameters
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 Table 2 Comparison between cases and controls according to intraocular lens power, refraction, and visual acuity
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The causes of corneal scarring were trachomatous trichiasis in nine (45%) cases (superior limbal superficial stromal corneal scarring), herpes simplex keratitis in six (30%) cases (paracentral stromal corneal scarring), and trauma in five (25%) cases (paracentral fullthickness corneal scarring) ([Table 1] and [Figure 1][Figure 2][Figure 3][Figure 4].  Figure 1 Scheimpflug tomography of a patient with corneal scarring secondary to herpes simplex keratitis (original figure).
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 Figure 2 Scheimpflug tomography of a patient with corneal scarring secondary to trachomatous trichiasis (original figure).
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 Figure 3 Left: Scheimpflug tomography of a patient with corneal scarring secondary to trauma. Right: preoperative picture showing paracentral traumatic corneal scarring (original figure).
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 Figure 4 Top: Scheimpflug tomography of a patient with corneal scarring secondary to trauma. Bottom left: preoperative picture showing paracentral traumatic corneal scarring. Bottom right: postoperative picture (original figure).
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The MAE was 0.2 D in paracentral corneal scarring cases compared with 0.1 D in the controls, with no statistically significant difference (P=0.142). In corneal scarring cases, all eyes (100%) showed a postoperative spherical refraction which differed by 1.00 D or less from the predicted value, while in 16 (80%) eyes the postoperative spherical refraction was within 0.50 D from expected. The distance UCVA showed significant improvement in cases of corneal scarring from 1.3 LogMar (Snellen’s equivalent 20/400), preoperatively, to 0.5 LogMar (Snellen’s equivalent 20/60), 3 months postoperatively. The preoperative and 3 months postoperative UCVA were significantly better in the controls compared with the corneal scarring cases (P<0.001) ([Table 2]).
The intraclass correlation coefficient showed statistically significant agreement between the OKULIX raytracing software and other IOL power calculation formulas both in cases and controls (P<0.001, [Table 3] and [Table 4]).  Table 3 Intraclass correlation coefficient for OKULIX versus other intraocular lens power calculation formulas in cases
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 Table 4 Intraclass correlation coefficient for OKULIX versus other intraocular lens power calculation formulas in controls
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Pearson correlation coefficient revealed statistically significant agreement between the OKULIX expected refraction and the 3month subjective refraction both in cases and controls (P<0.001, [Table 5]).  Table 5 Agreement between the OKULIX expected refraction and the 3month subjective refraction in cases and controls
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Discussion   
In the process of IOL power calculation, the corneal power is presented in a single figure, the keratometry 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 nonmeasured 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 raytracing studies [14]. Nonetheless, scarred corneas with irregular surface exhibit abnormal anteroposterior corneal curvature relation, violating the hypotheses that allows IOL 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, while the second is that the corneal center 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 more than 5000 points over the corneal surface. Therefore, it provides 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 the corneal power [16],[17]. In cases with marked change in the relationship between anterior and posterior corneal surfaces, the default refractive index applied by most topography systems is inaccurate [16],[17].
The Tomey Topographic Modeling System couples the Scheimpflug and Placido disk technologies to determine the corneal power and curvature. Anteroposterior corneal surface reconstruction can be achieved from the videocaptured 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 the alterations by refractive laser procedures and, hence, CVK that relies solely on the anterior corneal surface analysis would be inaccurate [18].
In the Irwin and colleagues study, computerized scanningslit videokeratography was used, which analyzes the anterior and posterior surfaces of the cornea, and the contact lens overrefraction method gave good estimations of the corneal power in patients with irregular corneal astigmatism, improving the accuracy of IOL calculation in patients with corneal pathology and irregular astigmatism. The contact lens overrefraction method is reliable in estimating the corneal power in patients with irregular corneal astigmatism. However, in cases in which the visual acuity is 20/70 or worse, the contact lens overrefraction may not be accurate [18].
Lately, the OKULIX IOL power computation software became available. It relies on numerical ray tracing, assessing the optical rays confined to the pupillary zone from the cornea to the fovea, unlike the conventional lens power calculation formulas, compelling with paraxial rays solely, relying on Gaussian optical principles. The principles of ray tracing have existed since the 17th century, but only recently have they been applied to calculations for optical devices in ophthalmology. Although many surgeons rely on the use of the thirdgeneration IOL power calculation formulas such as HaigisL, Hoffer Q, Holladay 2, and SRK/T, the ray tracing is a modern technique, based on a different set of principles that should be considered a potentially useful strategy [19].
The study of Preussner et al. [20] enrolled a collective of 153 eyes undergoing cataract surgery and applying the OKULIX software for IOL power calculation. The mean prediction error was −0.05±0.67 D. The slope of the regression line (0.009 D/mm) was not significantly different from zero.
In a recent study by Ghoreyshi et al. [21], the performance of the OKULIX software raytracing IOL power calculation was not significantly different compared with SRKT and Hoffer Q formulas. The MAE by the OKULIX, SRKT, and Hoffer Q formulas were 0.42±0.03, 0.36±0.02, and 0.37±0.02, respectively. In the present study, the MAE was 0.2 D in corneal scarring cases, compared with 0.1 D in the controls, with no statistically significant difference. In another study by Nabil, the OKULIX raytracing software accuracy was assessed in myopic cataract patients. In 83.33% of myopic patients, a prediction within ±1.00 D was obtained, whereas 70% of the cases showed a prediction within ±0.5 D. The MAE was 0.45±0.40 D [22]. In a third study by Nabil, the OKULIX raytracing software yielded a more accurate minus power IOL calculation in extreme myopia, compared with the SRKT formula. The SRKT calculated IOL power (−6.3±2.8 D) showed a statistically significant difference compared with the OKULIX calculated IOL power (−4.7±2.6 D, r_{s} 0.994, P<0.001) [23].
In this study, the thirdgeneration formulas which are based on the anterior curvature only had comparable results with the raytracing 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 on the posterior corneal curvature.
One of the limitations of the present 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, the raytracing software that analyzes the anterior and posterior corneal surfaces provided a more accurate estimation of the corneal power than CVK that analyzes the anterior surface only.
Conclusion   
Scheimpflug tomography combined with OKULIX raytracing software for IOL power calculation provided good results in cases of corneal scarring.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]
