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Sashia Bak-Nielsen, Iben Bach Pedersen, Anders Ivarsen, Jesper Hjortdal; The Rigidity of Corneas before and after Corneal Cross-linking - as measured by Corvis® ST. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1613.
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© ARVO (1962-2015); The Authors (2016-present)
Corneal Cross-linking (CXL) for treating keratoconus has been shown to stiffen the cornea in vitro and clinical studies have documented that progression of keratoconus is halted in most cases. The Corvis ST from Oculus dynamically measures corneal deformation by Scheimpflug imaging during an air puff. The pattern of deformation theoretically depends on the intraocular pressure, the corneal thickness and the material properties of the cornea. The purpose of this study was to measure the possible stiffening effect of CXL on patients with keratoconus, comparing groups with and without a previous CXL procedure.
Thirty-seven keratoconus patients were included - 19 with untreated keratoconus - 18 with keratoconus treated with CXL 4-50 months previously (median 27 months) Furthermore 31 healthy subjects were included as a control group. Apart from being measured with Corvis ST the subjects underwent a full ophthalmic examination including Pentacam topography. The Pentacam measurements were used to stage the patients into 4 groups based on the severity of the keratoconus. The stiffness of the cornea was evaluated by the radius of corneal concavity at the maximum deformation (DR) as calculated by the Corvis ST.
There was no significant difference in DR between the untreated keratoconus group and the CXL treated group although DR appeared marginally larger in eyes that had underwent CXL (un-paired t-test, p>0.05, Table 1). Increasing keratoconus severity (Grades 1-4) had only a small and insignificant influence on DR (ANOVA, p>0.05). DR in untreated and CXL treated keratoconus groups was significantly smaller compared with normal eyes.
Eyes with keratoconus have a smaller DR as measured by the Corvis ST, but the DR was similar in untreated and CXL treated eyes. As the effect of CXL mainly is exerted in the anterior part of the corneal stroma, it can be speculated that DR is insensible to changes in corneal material properties in the anterior stroma. Further studies of corneal deformation parameters in the same patients before and after CXL are needed to further evaluate the Corvis ST and the effect of CXL.
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