Long-term Evaluation of Corneal Biomechanical Properties After Corneal Cross-linking for Keratoconus: A 4-Year Longitudinal Study

PURPOSE: To compare the long-term changes in corneal bromechanics, topography, and tomography before and 4 years after corneal cross-linking (CXL) with the Dresden protocol and correlate these changes with visual acuity. METHODS: In this longitudinal study. 18 eyes of 18 patients with progressive keratoconus who were treated with CXL were included. All patients received a standard ophthalmological examination and were examined by Placido disc-based topography. Scheimpflug tomography, and biomechanical assessments with the Corvis ST (OCULUS Otikgerate GmbH, Wetzlar, Germany) and Ocular Response-Analyzer (ORA: Reichert Ophthalmic Instruments, Buffalo, NY) before and 4 years after CXL. The main outcome measures were dynamic corneal response (DCR) parameters obtained from the Corvis ST corneal hysteresis (CH), corneal resistance factor (CRF), visual acuity, refraction, corneal curvature, and corneal thickness. RESULTS: There were no significant differences in mean visual acuity, refraction, intraocular pressure, corneal to-pography, comeal astigmatism in both corneal surfaces, maximum keratometry, corneal thickness at apical and thinnest points, thickness profile indices, corneal volume. and specular microscopy before and 4 years after CXL (P > .05). Siglificant changes were observed in many DCR parameters, including radius at highest concavity and integrated inverse radius, both of which were consistent with stiffening. The CH and CRF values after CXL were not statistically significant. The new parameters using the Corvis ST include integrated inverse concave radius, which showed a significant decrease 1.07 +/- 0.93 mm(-1), consistent with stiffening. The corneal stiffness parameter at the first applanation, Ambrosio's Relational Thickness to the horizontal profile, deformation amplitude ratio. and Corvis Biomechanical Index as a combined biomechanical screening parameter did not show significant changes. CONCLUSIONS: CXL is a minimally invasive treatment option to prevent keratoconus progression over 4 years. Pressure-derived biomechanical parameters obtained from the ORA did not show any change following CXL at 4 years of follow-up, whereas the Corms ST DCR parameters detected changes in corneal biomechanical properties.