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Ferrara intracorneal ring segments (ICRS; Ferrara Ophthalmics, Belo Horizonte, Brazil) provide improvement in both visual and topographic parameters when properly implanted in eyes with keratoconus.1--3 Studies focusing on the procedure mostly concentrated on lower order aberrations such as defocus and astigmatism. Topography, when assessed, was not largely analyzed.1--3
Ferrara ICRS act by flattening the central part of the cornea, using the general principle for corneal implants: adding corneal tissue to the periphery4 and lifting the tips of the segment, leading to additional flattening by the body of the segment at the meridian opposite the site of insertion.1
A Ferrara ICRS differs from Intacs (another implant with different criteria; Addition Technology Inc, Des Plaines, Illinois) in that it has a smaller radius of curvature (1.5 mm compared to 3.5 mm) and a triangular anterior surface instead of a flat one.1 The unique triangular shape with a flat posterior surface is supposed to induce a prismatic effect that reduces glare.2 However, night visual symptoms are still common for all designs.
The NIDEK OPD-Scan II (Optical Path Difference; NIDEK Co Ltd, Gamagori, Japan) is a multipurpose instrument that combines Placido-based corneal topography with wavefront aberrometry of the entire eye.5 It is based on an automatic retinoscopy principle, allowing high quality analysis of approximately 1440 points over a 6-mm pupil.6 Previous studies confirmed the precision and repeatability of its results.7 A detailed topographic and aberrometric analysis is yielded for the examined eye, especially with the processing of the data via OPD-Station software (NIDEK Co Ltd).5
In this study, the OPD-Scan II and OPD-Station software were used to assess the criteria of keratoconic eyes in comparison to normal eyes. The effect of Ferrara ICRS implantation was also analyzed.
Patients and Methods
This was a prospective, controlled, comparative study, in which 37 eyes of 26 patients reporting decreased vision and having a topographic diagnosis of keratoconus were enrolled. The control group comprised 38 eyes of 19 patients who had normal corneas that were eligible for LASIK. All patients signed a written consent in accordance to the tenets of the Declaration of Helsinki. No institutional review board approval was required for this study.
Patient Selection
Preoperatively, patients were excluded if they...