Patients
This retrospective observational study was conducted in compliance with the institutional and government review board regulations and informed consent regulations. The study protocol was approved by the Ankara Research and Training Hospital Local Ethics Committee with the number E-2572, and the study was carried out in accordance with the Declaration of Helsinki. Written informed consent was obtained from all participants before performing CCL.
From the large database in the corneal unit, keratoconus patients were reviewed individually. Keratoconus diagnosis was based on the characteristic keratoconus signs in the anterior sagittal curvature maps (eg, an asymmetric bowtie pattern with or without skewed axes or inferior or central steeping) and at least one biomicroscopic sign (eg, a conical protrusion, Vogt striae, and Fleischer ring). Patients were excluded from the study if they were aged < 18 years, had anterior segment surgery history, corneal scarring, corneal haze > grade 1, ocular surface problems, or if they had undergone repeated CCL. Progression was defined as a consistent change in at least 2 of the following 3 parameters where the magnitude of the change was above the normal noise of the testing system: (1) progressive steepening of the anterior corneal surface, (2) progressive steepening of the posterior corneal surface, and (3) progressive thinning and/or an increase in the rate of corneal thickness change from the periphery to the thinnest point9.
First, 35 eyes of 35 progressive keratoconus patients who underwent CCL treatment and were followed up for at least 36 months after CCL were included as the cross-linked group. To make a comparison, the data of 30 eyes of 30 progressive keratoconus patients who showed progression in the 12th month of follow-up constituted the progressive uncross-linked group, and 30 eyes of 30 patients who did not show progression formed the non-progressive uncross-linked group.
Ccl Procedure
All CCL procedures were performed in an operating room under topical anesthesia with 0.5% proparacaine hydrochloride eye drops. The corneal epithelium was mechanically removed from the 8.0 mm treatment zone and then ultrasound pachymetry was applied (Sonomed 300P PacScan Pachymeter; Escalon Medical Corp., USA). If the corneal thickness was < 400 µm, hypoosmolar riboflavin was instilled until the cornea had swollen to a thickness exceeding 400 µm. When the corneal thickness exceeds 400 µm, iso-osmolar riboflavin solution (0.1% riboflavin in 20% dextran T500 solution, Meran Medicine, BNM Inc., Turkey) was instilled to the cornea every 2 minutes, for a total of 30 minutes. The cornea was exposed to 370 nm UVA light with a 5.4 J/cm² surface dosage using a commercially available UVA system (UV-X System, Peschke Meditrade GmbH, Huenenberg, Switzerland). The intended 9 mW/cm² surface irradiance was applied for 10 minutes while the riboflavin solution continued to be instilled. The cornea was irrigated with cold water at the end of the procedure, and a silicon-hydrogel bandage contact lens (Acuvue Oasys, Johnson&Johnson, base curve 8.4 mm) was applied. Postoperative treatment included moxifloxacin hydrochloride 0.5% four times a day for one week, fluorometholone 5% eye drops four times a day on a tapering schedule for two weeks, and artificial tears four times a day for one month. The bandage contact lens was removed on the fifth postoperative day.
Ophthalmological Examination
Patients underwent a complete ophthalmological examination, including best spectacle corrected visual acuity (BSCVA), slit-lamp biomicroscopy, fundus evaluation and tomographic analysis with a rotating Scheimpflug corneal tomography device (Pentacam HR, Oculus Optikgeräte GmbH, Wetzlar, Germany). The BSCVA was measured with a Snellen chart by a cornea specialist and converted to Logarithm of the Minimum Angle of Resolution (logMAR) for statistical analysis. The examination and measurement data of all the patients were recorded according to the study design.
The Pentacam HR is a non-invasive tomography device with a single rotating Scheimpflug camera. This device uses a 1.45-megapixel camera to maximally capture 138.000 data points of true elevation and a 475-nm ultraviolet-free blue light-emitting diode light for corneal illumination. All measurements were taken by an expert examiner under scotopic conditions with the natural pupil. Each participant was asked to fixate on an internal target after a complete blink, and the joystick was adjusted until perfect alignment was shown. Acceptable maps had at least 10 mm of corneal coverage without any extrapolated data in the central 8.0 mm zone. Scans not meeting acceptable criteria were repeated.
The most commonly used CCL efficacy indicators of maximum keratometry (Kmax) and thinnest corneal thickness (TCT) were noted. Minimum radius of curvature (Rminback) was noted as the point of maximum posterior corneal curvature. Asphericity (Qback) and the most commonly used higher order aberrations of spherical aberration, vertical and horizontal coma, vertical and oblique trefoil, which pertain to the corneal back surface, were noted.
Corneal light backscatter measurements were recorded via the densitometry software. This software automatically locates the corneal apex and analyzes a surrounding area of 12 mm in diameter. This area is divided into four concentric zones which are 0–2 mm circular zone and 2–6 mm, 6–10 mm, and 10–12 mm annular zones. The analyses also provide densitometric values at three different depths. The anterior layer is the superficial 120 µm region, the posterior layer is the innermost 60 µm region, and the central corneal layer lies between these two layers. For this study, the posterior corneal layer values in all concentric zones were noted. Corneal densitometry values are expressed as the pixel luminance per unit volume in grayscale units. Measurements range from 0 (maximum transparency) to 100 (totally opaque) with regard to the backscattering light degree.
The Belin-Ambrósio Enhanced Ectasia Display (BAD-III) software evaluates the pachymetric progression and the anterior and posterior elevation values of the cornea. The maximum posterior elevation value at the 5.0 mm central cornea (MaxPostelev) and at the thinnest point (ThinnestPostelev) were examined according to the best fit sphere (BFS) reference and to the enhanced BFS reference surface. The deviation of normality of the back elevation (Db) and overall deviation of normality (final D) were also noted from the BAD-III software.
The Belin ABCD keratoconus grading system is incorporated in the Pentacam software as part of the topometric/keratoconus grading display. This system uses the anterior and posterior radius of curvature (ARC and PRC) taken from the 3.0-mm zone centered on the thinnest point. The classification parameters are ‘A’ for ARC, ‘B’ for PRC, ‘C’ for thinnest pachymetry, and ‘D’ for distance visual acuity. The PRC and ‘B’ parameters were recorded from this grading system.
Statistical Analysis
Statistical analysis was performed using SPSS for Windows software (version 22.0, IBM Corp.). Conformity of the data to normal distribution was analyzed with the Kolmogorov-Smirnov test. Descriptive statistics were recorded as mean ± standard deviation (SD) values. Longitudinal analyses were performed with repeated-measures analysis of variance (ANOVA) and the Bonferroni post-hoc test. The assumption of sphericity was tested with Mauchly’s test of sphericity. When the significance level was greater than the a priori α level (p > 0.05), sphericity was assumed and the value from the univariate test table was used. When the significance level was less than or equal to the a priori α level (p ≤ 0.05), sphericity could not be assumed and Wilk’s λ test value from the multivariate test table was used. Paired t-tests were performed to determine whether the difference between two measurements of the same eye was significant. Between-group comparisons of the three groups were performed using the one-way ANOVA test. When the overall ANOVA model was significant, the Bonferroni post-hoc test was conducted to determine the pairwise comparison of the means that were significantly different. A value of p < 0.05 was considered statistically significant.