**1. Introduction**

Small incision lenticule extraction (SMILE), with over 5 million procedures performed globally, has evolved to one of the most popular and established keratorefractive procedures for the correction of myopia and myopic astigmatism. In the foreseeable future, the number of patients with prior SMILE treatment requiring cataract surgery is expected to increase accordingly in an ageing population. Inevitably, ophthalmologists will be challenged by accurate intraocular lens (IOL) power calculations for these patients.

There are three major problems in calculating IOL power after any kind of keratorefractive surgery. The first and most significant pitfall lies in the so-called keratometric index error [1]. In traditional keratometry, corneal radii are only measured for the anterior corneal curvature with the posterior corneal curvature radii being empirically extrapolated based on the assumption that the ratio between the anterior and posterior corneal curvature (A/P ratio) is constant, which is not the case after keratorefractive surgery. Secondly, some standard IOL calculation formulae tend to predict a more anterior effective lens position (formula error). Thirdly, the central zone of effective corneal power that had been artificially treated by keratorefractive surgery is estimated from traditional (paracentral) keratometry

**Citation:** Lischke, R.; Sekundo, W.; Wiltfang, R.; Bechmann, M.; Kreutzer, T.C.; Priglinger, S.G.; Dirisamer, M.; Luft, N. IOL Power Calculations and Cataract Surgery in Eyes with Previous Small Incision Lenticule Extraction. *J. Clin. Med.* **2022**, *11*, 4418. https://doi.org/10.3390/ jcm11154418

Academic Editors: Francisco Javier Ascaso and Nobuyuki Shoji

Received: 21 May 2022 Accepted: 27 July 2022 Published: 29 July 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

measurements. Therefore, the corneal power tends to be overestimated (instrument error). All these factors concordantly predispose to an underestimation in required IOL power and therefore lead to dissatisfying hyperopic residual refractive error after IOL implantation in eyes with prior myopic keratorefractive surgery [1].

Several methods have been introduced to address these sources of error and to reduce refractive surprises after keratorefractive surgery [2–9]. New technologies for corneal power measurements that incorporate measurements of the anterior and posterior corneal radii (e.g., total keratometry [10]) were established to enable more accurate predictions. Moreover, sophisticated IOL power calculation formulae have been developed by means of empirical optimization; some of which consider pre-keratorefractive surgery data (Masket [11], Modfied-Masket or Barrett True-K formula), and some do not incorporate any preoperative values (Shammas [12], Barrett True-K no history, Potvin-Hill [13] or Haigis-L formula [14]). Conveniently, a range of these formulae is readily accessible in the American Society of Cataract and Refractive Surgeons (ASCRS) post-keratorefractive surgery IOL power online calculator.

In addition to these empirical formulae, the purely physical ray-tracing approach has demonstrated very good IOL power calculation outcomes in eyes with prior laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK) [15–18]. The ray-tracing method measures the true shape of the cornea after corneal refractive surgery by using the anterior and posterior curvature radii and asphericity of these surfaces. Moreover, the central IOL thickness, the index of refraction and the true geometrical position, as defined by the ACD (distance between the posterior corneal apex and the anterior IOL apex), are used to describe the IOL and calculate its required power accurately. The ray-tracing method also obviates the need for any further historical or clinical data [5,15–17,19–21]. Ray tracing has been proven to provide reliable and satisfactory results in IOL calculations not only in treatment-naïve eyes but also in eyes after LASIK and PRK [15,16,18,21,22].

After more than one decade since the introduction of SMILE, only one case report of IOL calculation and implantation after SMILE is present in the peer-reviewed literature [23]. However, no formula comparison was reported. Hence, there is a deficiency in postoperative refractive data to optimize existing IOL power calculation formulae for post-SMILE eyes. In addition, corneal aberrometric changes after SMILE are significantly different when compared to the corneal shape changes after femtosecond laser-assisted LASIK (fs-LASIK) [24–26], which questions the validity of formulae optimized for Excimer-based photoablative procedures (e.g., the Masket formula) in post-SMILE eyes.

Consequently, the scope of the present multicenter study was to gather the first cohort of post-SMILE patients undergoing cataract extraction with IOL implantation. In this cohort, we set out to compare the refractive prediction error of IOL power calculations between ray tracing and various empirically optimized formulae available in the ASCRS post-keratorefractive surgery IOL power online calculator.

#### **2. Materials and Methods**

This multicenter cross-sectional study included patients that had previously undergone small incision lenticule extraction (SMILE) for the treatment of myopia and/or myopic astigmatism and later underwent cataract surgery with IOL implantation. The study was conducted at the University Eye Hospital of the Ludwig–Maximilians University (Munich, Germany), the SMILE Eyes Clinic Munich Airport (Munich, Germany) the SMILE Eyes Clinic Trier (Trier, Germany), the SMILE Eyes Center at the Department of Ophthalmology of the University of Marburg (Marburg, Germany) and the SMILE Eyes Clinic Linz (Linz, Austria).

Institutional review board approval of the Ludwig–Maximilians University Munich, was obtained for all aspects of this study; informed consent to use their data for analysis and publication was obtained from all subjects and all study-related procedures adhered to the tenets outlined in the The authors declare no conflict of interest.aration of Helsinki.

#### *2.1. SMILE Surgery*

All SMILE procedures were performed by highly experienced corneal surgeons utilizing the VisuMax 500-kHz femtosecond laser platform (Carl Zeiss Meditec AG, Jena, Germany) according to the local standards of the participating centers. The intended cap thickness was programmed at 120–130 μm with an intended optical zone size of 6.3 to 6.7 mm in diameter. At the superotemporal position, a single side cut of 50 degrees with a circumferential length of 3.0–4.0 mm was created. No intraoperative or postoperative complications were encountered. The surgical principles of the SMILE technique have been previously described in detail [27].

#### *2.2. Cataract Surgery and IOL Implantation*

Cataract surgery including IOL selection and implantation was performed by highly experienced corneal surgeons according to the local standards of the participating centers. Standard phacoemulsification with intracapsular IOL implantation was performed in all cases. A 2.5 mm clear incision at the steep corneal axis and two paracenthesis incisions 2 clock hours away towards both directions were created. In case of a toric IOL, a clear corneal incision was created at the temporal corneal aspect. Cohesive and/or dispersive viscoelastic agents were used at the individual surgeon's discretion. No intraoperative or postoperative complications were encountered. The implanted IOL models and powers are summarized in Table 1.


**Table 1.** Implanted IOL models, powers and observed prediction errors.

IOL, intraocular lens; D, diopter.

#### *2.3. Subjective Refraction*

Subjective manifest refraction was measured using the Jackson cross-cylinder method before and after each procedure. Best-corrected distance visual acuity (CDVA) was determined using standard ETDRS charts at 4 m.

#### *2.4. Post-hoc IOL Power Calculation*

Post-hoc IOL power calculation was performed utilizing dedicated-ray tracing software (Okulix; Panopsis, Mainz, Germany, Version 9.01) based on preoperative corneal tomography scans (Pentacam HR; Oculus Optikgeräte GmbH, Wetzlar, Germany), and preoperative optical biometry and anterior chamber depth measurements (IOLMaster 500 or 700; Carl Zeiss Meditec AG, Jena, Germany). Moreover, the American Society of Cataract and Refractive Surgery (ASCRS) post-keratorefractive surgery IOL power online calculator (Version 4.9; http://iolcalc.ascrs.org; last accessed on 4 October 2021) was used to calculate the predicted residual refractive error using the following formulae that consider pre-keratorefractive data: Barrett True K, Masket [11] and Modified Masket. Additionally, the following formulae available in the ASCRS calculator were used, which do not incorporate preoperative data: Barrett True K No History, Haigis-L [14], Potvin-Hill [13] and Shammas [12]. In accordance with recent recommendations for IOL power calculation studies [28], no IOL constant optimization was performed but (when appropriate) optimized IOL constants were used as published on the User Group for Laser Interference Biometry (ULIB) website (http://ocusoft.de/ulib/index.htm; last accessed on 4 October 2021).

#### *2.5. Statistical Analysis*

On the basis of established protocols for studies on IOL power calculation formula accuracy [28,29], the prediction error (PE) was defined as the difference between the actual residual refraction and the residual refraction predicted by the respective IOL power calculation method for the same IOL power and model. The arithmetic mean of the PE was referred to as the mean error (ME). Moreover, all negative errors were converted to positive to calculate the mean absolute error (MAE) as well as the median absolute error (MedAE). Furthermore, the standard deviation, minimum and maximum (range of PE) as well as the percentage of eyes within ±0.50, ±1.00, ±1.50 and ±2.00 diopter (D) are reported [28,29]. Boxplots were created to illustrate the differences in PE between different IOL power calculation formulae. Normal distribution was tested by the Shapiro–Wilk method. The Kruskal–Wallis test was employed to assess the differences in PE between formulae and ray tracing. In addition, the variance of ME was calculated—a smaller variance indicates better consistency of a IOL calculation method [30]. The Fisher's exact test with the Bonferroni correction was used to test for statistically significant differences between proportions of eyes with PEs within ±0.50 D and ±1.00 D, respectively., A *p*-value of <0.05 was defined as being indicative of statistical significance. All statistical analyses were performed using SPSS 27.0.0.0 for Windows (IBM Corp.; Armonk, NY, USA).
