Patient Population

In this prospective, comparative study 43 patients (34 male and 9 female; 52 eyes) with progressive keratoconus were enrolled. The clinical diagnosis of keratoconus was based on corneal topography data (iTrace; Tracey Technologies, Houston, Texas, USA). Inclusion criteria were progressive keratoconus, patient age older than 18 years, corneal thickness more than 400 μm, no other ocular pathologic signs, and no pregnancy or lactation. Keratoconus was graded with the Amsler-Krumeich classification; only patients with grade I and II were included in this study.

All patients underwent uneventful CXL treatment with the use of a new high-intensity UV-A illuminator (CCL-365; Peschke Meditrade GmbH, Huenenberg, Switzerland). The selection of the CXL irradiation treatment protocol was made alternately; 26 eyes were treated for 30 minutes in an intended irradiance of 3.0 mW/cm ² according to the Dresden protocol (Group 1), while 26 eyes were treated for 14 minutes in an intended irradiance of 9.0 mW/cm ² (Group2).

Data obtained from the patient records included age, sex, preoperative ultrasound corneal pachymetry (Corneo-Gage Plus; Sonogage Inc, Cleveland, Ohio, USA), preoperative keratometry readings, endothelial cell density (ECD), and AS-OCT scans (Visante OCT 3.0; Carl Zeiss Meditec Inc, Jena, Germany).

Institutional Review Board approval from the University of Crete was obtained and all patients were appropriately informed before their participation in the study and gave written informed consent in accordance with institutional guidelines, according to the Declaration of Helsinki.

Surgical Technique

All procedures were performed by the same surgeon (G.D.K.) under sterile conditions. After topical anesthesia with proxymetacaine hydrochloride 0.5% eye drops (Alcaine; Alcon Laboratories, Inc, Fort Worth, Texas, USA), corneal epithelium was mechanically removed using a rotating brush within an 8.0–9.0 mm diameter. After epithelial removal, riboflavin (0.1% solution of 10 mg riboflavin-5-phosphate in 10 mL dextran-T-500 20% solution; Medicross; Medio-Haus, Behrensbrook, Neudorf, Germany) was instilled on the center of the cornea every 3 minutes for approximately 30 minutes in both groups. UV-A irradiation was performed using a new high-intensity UV-A optical system (CCL-365; Peschke Meditrade GmbH, Huenenberg, Switzerland). Before treatment, an intended irradiance of 3.0 mW/cm ² (Group 1) or 9.0 mW/cm ² (Group 2) was calibrated using the UV-A light meter YK-35UV (Lutron Electronic, Coopersburg, Pennsylvania, USA), which is supplied with the UV-X device. In Group 1, UV-A irradiance was performed for 30 minutes according to standard Dresden protocol in an intended irradiance of 3.0 mW/cm ² , while in Group 2 (modified-accelerated protocol) it was performed for 14 minutes in an intended irradiance of 9.0 mW/cm ² , corresponding to a total surface dose of 5.4 J/cm ² and 7.5 J/cm ² , respectively. During UV-A irradiation, riboflavin solution was applied every 3 minutes to maintain corneal riboflavin saturation. At the end of the procedure, a silicone-hydrogel bandage contact lens (14.0 mm in diameter, 8.6 base curvature, Dk = 140 barrers; lotrafilcon B, Air Optix; Alcon Laboratories Inc, Fort Worth, Texas, USA) was applied until full re-epithelialization.

Postoperative medication included chloramphenicol/dexamethasone drops (Dispersadron; Thea Laboratories, Inc, Clermont-Ferrand, France) 4 times daily until the removal of the bandage contact lens. After the removal of the contact lens, patients received corticosteroid drops 4 times daily (FML, fluorometholone 0.1%; Falcon Pharmaceuticals, Fort Worth, Texas, USA) with tapering for the next 3 weeks. Patients were encouraged to use artificial tears at least 6 times per day for at least 3 months postoperatively.

Anterior Segment Optical Coherence Tomography

All scans were performed under the same light conditions, 1 month postoperatively. All patients were asked to fixate on the optical target of the system. The image was captured when the corneal reflex, a vertical white line along the center of the cornea, was visible. The high-resolution corneal scan was used to produce an enhanced image of the cornea on the horizontal meridian (0–180 degrees). The stromal demarcation line was identified in the enhanced corneal image, and the demarcation line depth was measured using the flap tool, as provided by the manufacturer. The demarcation line depth was measured by 2 independent observers (K.T. and M.G.) in the center of the cornea. The visibility of the demarcation line was scored to obtain accuracy of the measurements (0 = line not visible; 1 = line visible, but measurement not very accurate; 2 = line clearly visible). Only measurements with score 2 were included in the study.

Statistical Analysis

All data were recorded into an Excel spreadsheet database (Microsoft, Redmond, Washington, USA) and statistical analysis was performed using SPSS for Windows, version 20 (SPSS, Inc, Chicago, Illinois, USA). A sample size analysis (G*Power 3.1.3; Franz Paul, Universitat Kiel, Kiel, Germany) was performed for a 2-tailed t test with a large difference effect size, statistical power of 80%, and statistical significance level of .05, resulting in 26 patients participating in each group in this prospective study. Normality of distribution of all measurements was confirmed using the Shapiro-Wilk test, appropriate for small sample sizes of fewer than 50 participants. To avoid bias and ensure the comparability of the 2 groups, the baseline characteristics between the groups were compared. The Bland-Altman method was used to determine the agreement between the 2 observers. The limits of agreement were defined as the mean difference of 2 measurements ± 1.96 standard deviation of the difference. Paired t tests were performed between the measurements of the 2 observers to determine whether they differ statistically significantly. Continuous variables are presented as mean ± SD (minimum, maximum). A P value <.05 was considered statistically significant.