Abstract
PurposeTo evaluate the local geometric effects of a unilateral intrastromal ring segment with a combined variation of ring thickness and base width in a finite element simulation, and to compare it against the isolated effect of thickness or base width variation alone.MethodsA two-dimensional finite-element model of a transversely isotropic cornea was created assuming either axisymmetric stress or plane strain condition. The model geometry was composed of a three-layered corneal tissue (epithelium, anterior and posterior stroma) fixed at the limbus. The implantation of a triangular-shape asymmetric ring segment with varying ring thickness (150 to 300 μm) and base width (600 to 800 μm) was simulated. Also, changes induced by thickness or base width alone were studied and compared their combined effect in the asymmetric ring segment. Geometrical deformation of the simulated cornea and sagittal curvature were the main parameters of study.ResultsIncreasing ring thickness and base width along the arc of the asymmetric ring segment produced a more pronounced flattening in this part of the ring. The asymmetric design did find a good balance between maximizing corneal flattening at one end and minimizing it at the other end, compared to the isolated effect of ring thickness and width. Ring thickness was the most robust parameter in flattening both, the central and peripheral cornea.ConclusionThe finite-element model permitted a theoretical study of corneal deformation undergoing implantation of realistic and hypothetical ring geometries. Intracorneal asymmetric ring segments with varying thickness and base width can be a good alternative in corneas with asymmetric keratoconus phenotypes.
Highlights
Among keratoconus (KC) treatments, intracorneal ring segments (ICRS) have widely demonstrated their efficacy in improving clinical parameters and reshaping the cornea [1]
The finite-element model permitted a theoretical study of corneal deformation undergoing implantation of realistic and hypothetical ring geometries
Intracorneal asymmetric ring segments with varying thickness and base width can be a good alternative in corneas with asymmetric keratoconus phenotypes
Summary
Among keratoconus (KC) treatments, intracorneal ring segments (ICRS) have widely demonstrated their efficacy in improving clinical parameters and reshaping the cornea [1]. Despite all the technological advances in diagnosis, surgical techniques and nomograms, poor and disappointing results with ICRS have been reported in some cases, forcing positional changes in second surgeries and even changing or explantation [4,5,6] This could be explained because KC with asymmetric patterns (“snowman”, “duck”, and “irregular croissants”, with no concordance among topographic and comatic axes) are treated with symmetrical ring segments (same thickness and base width across all segment) leading to lack of concordance between the visual and tomographic results [7]. A new type of ICRS (AJL Pro+ from AJL Ophthalmic) has been recently designed for its use in KC patients These ring segments show variations along their arc length in terms of thickness and base width
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