Assessment of Corneal Stiffness in Keratoconus After Crosslinking by Corvis ST and OCT.

Keratoconus

Objectives: * Measurement of corneal stiffness parameters { Deformation Amlitude (DAR1), Integrated radius, Stress parameter (SP A1), Stress strain index (SSI) and Corvis biomechanical index (CBI) } in keratoconic corneas before and after 3 and 6 months CXL using Corvis ST. * Measurement of demarcation line depth using anterior segment OCT and correlate between corneal stiffness following CXL and demarcation line depth.

Background and Rationale: Keratoconus is a progressive corneal degeneration resulting from noninflammatory thinning of the corneal stroma. Visual impairment typically commences in adolescence and progresses thereafter. Further increase in myopia, irregular astigmatism, and subepithelial scarring leads to visual impairment. Treatment modalities are based on refractive correction with spectacles or contact lenses to correct astigmatism and restore visual acuity. Such modalities do not stop ectatic progression and further visual deterioration, which ultimately necessitates corneal transplantation in 10% to 20% of patients. Corneal collagen cross-linking (CXL) has emerged as a promising techniq;ue to slow or halt the progression of keratoconus. CXL leads to an increase in intra and interfibrillar covalent bonds by photosensitized oxidation and causes a biomechanical stabilization of the cornea. Corneal visualization using dynamic Scheimpflug technology (Corvis ST, Oculus Optikger€ate GmbH) allows in vivo measurement of the corneal biomechanical deformation response to an applied air puff and is useful for evaluating the biomechanical response parameters of the cornea. It captures the dynamic corneal deformation caused by an air puff using an ultrahigh- speed camera that operates at a speed of greater than 4300 frames per second to capture a series of 140 sequential horizontal Scheimpflug images of the temporal-nasal cross section of the cornea. The consistency of the air puff has been shown and the spatial and temporal profiles have been characterized to allow calculation of the load on the cornea within the imaging window. The Corvis ST enables the measurement of several parameters by analyzing the timing and patterns of deformation at the highest concavity as well as applanation during inward deformation (loading) and outward recovery (unloading). The original parameters have been reported to be influenced most strongly by intraocular pressure (IOP) as well as age and central corneal thickness (CCT). Recently, new corneal biomechanical parameters were introduced, including the Deformation Amplitude ratio at 1.0 mm (DAR1), Deformation Amplitude ratio at 2.0 mm (DAR2), Integrated radius, Stiffness Parameter at first applanation (SP A1), Ambrosio's Relational Thickness (ARTH) , Corbic Biomechanical index (CBI) and biomechanically corrected IOP (bIOP). Laboratory studies have shown that the effective depth of CXL is confined to the anterior 300 μm of the cornea. Moreover, a stromal demarcation line between treated and untreated stroma was visible biomicroscopically at a depth of approximately 300 μm over the entire cornea after CXL treatment. This demarcation line was also evident on optical coherence tomography (OCT). The depth of this line has been postulated as an indirect measurement of cross-linking effectiveness.

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