Clinical studies report curvature changes of up to −13.75 diopters (D) after ICRS implantation,
10 as well as a small rotation of the ICRS.
2,11 Clinically, the ICRS height and optical zone are modified to modulate the desired refractive change, typically from empirical nomograms. To our knowledge only a few analytical
5,12–14 and one numerical study (Pinsky PM, et al.
IOVS 1995;36:ARVO Abstract S308) have been proposed to model the response of the cornea to the ICRS. The studies suggest a linear relation between refractive change and ICRS height (the higher the ICRS, the more effective) and optical zone (ICRS inner distance to the apex, the smaller the more effective). This relation is also reflected in the clinically applied nomograms.
15 Although the biomechanical mechanisms underlying ICRS implantation have been approached, modeling relies strongly on the mechanical properties assigned to the corneal tissue and assumptions within the model. Corneal biomechanical models have improved in sophistication, from a single-membrane model with linear elastic
16 or viscoelastic
17 material properties, to hyperelastic orthotropic shell models that account for the tissue's microstructure,
18 as well as solid viscoelastic (Kling S., EVER, 2011, Instituto de Óptica, CSIC) and nonlinear
19 corneal models. Recently, the finite-element model (FEM) has also been suggested to model keratoconic corneas.
20 The microstructure of corneal tissue is dominated by collagen fibers (mainly type I
21 ) that provide the necessary stability to maintain the spherical shape of the cornea. Keratoconus is generally thought to be associated with a local loss of collagen fibril orientation.
22,23 This results in a local decrease of corneal rigidity
24,25 and leads to the deformation in form of a cone.
26 The cone is typically slightly displaced interiorly and temporally from the corneal apex,
24 resulting in increased corneal aberrations
26,27 and a highly distorted vision.
28 Keratoconic corneas with central cones were reported to occur in approximately
29 25% of the patients.