Subjects

This large retrospective case-series study was based on data obtained at Sorocaba Ophthalmological Hospital (Sorocaba, São Paulo, Brazil), was developed in accordance with the principles of the Declaration of Helsinki and was approved by the Committee of Ethics and Research of Sorocaba Ophthalmological Hospital (number 101.552) and São Paulo Federal University (number 1.309.808).

We analyzed records from 1355 intrastromal corneal ring implantations (1238 patients) performed between November 2009 and December 2012. The inclusion criteria were as follows: patients who underwent corneal ring implantations at Sorocaba Ophthalmological Hospital during the period of interest using rings that were 5 mm in diameter and for whom surgery and medical records were available. The exclusion criteria were as follows: patients who did not previously undergo a complete preoperative ophthalmic evaluation, comprising corneal topography/tomography and pachymetry. In total, 1222 surgeries (1196 patients) were included in this study.

The following preoperative information was collected for all eligible individuals: age, gender, type of health/medical assistance (in the Brazilian public health system or via a health/medical organization or private payment), the surgical technique used to create the corneal ring tunnel (manual or femtosecond laser-assisted), best-corrected visual acuity (BCVA) (Snellen acuity converted to logMAR scale) ¹⁸, and manifest sphere and cylinder refractive error in diopters (D).

Additional data were obtained, such as maximum central keratometry (K, expressed in D and obtained from the central three millimeters (mm) of the corneal radius), minimum central K (expressed in D and obtained from the central 3 mm of the corneal radius), and corneal thickness (pachymetry) at the thinnest point and at the ring channel (both in micrometers, µm). The last four measurements were acquired using an Orbscan IIz^® system (Bausch & Lomb, Berlin, Germany).

The cases were classified into the following six groups according to their topographical characteristics and in consideration of their maximum central K and characteristics related to their astigmatism: mild keratoconus (up to 48D), moderate keratoconus (≥48D to 52D), advanced keratoconus (≥52D to 58D), severe keratoconus (≥58D), pellucid marginal degeneration and post-graft irregular astigmatism. These groups were adopted because current classification systems for ectasias vary widely and because classifications based exclusively on keratometry values were originally developed for topographic mapping, not tomographic mapping. Furthermore, classification systems tend to change over time, and the system adopted here avoids potentially outdated groupings.

Three months after the operation, information regarding visual acuity and satisfaction were collected. We classified patients as dissatisfied if contact lens or spectacle fitting was not possible according to medical records and/or the patient perceived that their eyesight had worsened after the procedure. Patients with intraoperative or postoperative complications were also considered dissatisfied.