Understanding corneal biomechanical properties is critical to model the biomechanical response of pathological corneal tissue (i.e., keratoconus, a progressive corneal disease that debilitates corneal tissue), and to increase the predictability of surgical outcomes or treatments (i.e., intrastromal ring segments, corneal cross-linking, or incisional surgery). Various methods have been used in the past to estimate the corneal modulus of elasticity (Young's modulus). The most widespread applied method is strip extensiometry,
1–4 followed by corneal button inflation,
5,6 and whole globe inflation.
4–10 In these techniques, a load is applied (typically along one axis in strip extensiometry, or radially by increasing IOP in inflation techniques). The strain upon the applied stress is measured from the lateral elongation, axial apex displacement,
4 shift of mercury droplets attached on the corneal surface,
5,7 or from changes in the corneal radius of curvature.
10,11 A new technique to estimate the corneal biomechanical properties, by measuring the corneal deformation upon air puff applanation, has recently been suggested.
12,13 In all in vitro biomechanical measurements, corneal hydration plays a role as it affects the tissue's mechanical response.
14 Also different medical solutions alter the corneal hydration and, thus, the biomechanical properties of the tissue
15 (Kling S, et al.
IOVS 2010;51:ARVO E-Abstract 4628). UV-riboflavin cross-linking (CXL) is an increasingly used technique for the treatment of keratoconus, which aims at stiffening the corneal tissue. The increase in corneal rigidity gained with this treatment is assumed to result from the reaction of the photosensitizer (riboflavin) with UV light, which creates radicals that induce additional cross-links between collagen fibrils, probably interhelically, intrahelically, and intermicrofibrillary.
15,16 Strip extensiometry stress-strain experiments showed an increase in corneal rigidity immediately after and at several months post CXL in human,
2 porcine,
2 and rabbit
1 corneas. Also whole-globe inflation experiments showed an immediate increase in corneal rigidity in eyes in vitro after CXL.
11 The biomechanical response estimated from the previous methods may be affected by the corneal shape (geometry), thickness, hydration state (in vitro), and IOP (in vivo). In this study, we developed a new two-dimensional (2D) stress-strain system that allows maintaining the original stress distribution along the corneal flap, while guaranteeing that corneal hydration is equal for all samples. This allows an accurate comparison between individual flaps of a certain layer, as well as a precise analysis of the treatment effects on a few corneal layers. To prove its application, we evaluated the change in corneal rigidity following CXL treatment in rabbits.