The characterization of biomechanical properties of the cornea is necessary to evaluate the effects of different CXL methods. Corneal biomechanical properties (i.e., Young's modulus) are usually measured by extensiometry tests on corneal strips, where a strip of cornea is subjected to tensile loading. However, the cornea is an anisotropic material, thus its mechanical response depends on the orientation of the collagen fibers, which may vary not only between different samples but also along the length of the same sample strip. While strip extensiometry can still be useful to compare samples of similar size and orientation, 2-dimensional (2D) mechanical testing provides a more suitable approach to characterize corneal biomechanical properties. In particular, 2D flap extensiometry and corneal/eye inflation have been used to characterize the changes in the corneal biomechanical response following CXL.
14 In general, these techniques rely on measurements of the corneal deformation, while the intraocular pressure (IOP) is increased in a chamber on which the cornea or 2D corneal flaps are mounted or in an ocular globe infused with saline solution.
15–17 Corneal deformation is assessed indirectly through aberrometry,
14 or directly from Scheimpflug imaging,
18 (Bekesi N, et al.
IOVS 2015;56:ARVO E-Abstract 1135) or OCT imaging,
19,20 and the mechanical properties typically estimated based on the thin-walled pressure vessel theory or using inverse finite element (FE) modeling. Air puff deformation imaging, while commercialized primarily as a tonometer, is also a promising technique to characterize biomechanical properties of the cornea in vivo. A short air pulse is emitted against the cornea and the deformation is monitored by an adequately fast imaging system (e.g., OCT
13 or Scheimpflug
18). The deformation response to the air puff depends on the mechanical properties of the cornea, among other factors.
21 The use of cutting-edge mechanical numerical simulations makes it possible to reconstruct the mechanical parameters of the cornea from the corneal deformation pattern. Kling et al.
18 used inverse modeling to retrieve material properties of normal and cross-linked porcine corneas. The corneas were modeled by finite elements and the pressure distribution of the air puff applied. The viscoelastic material parameters were changed in an iterative process to fit the deformations with the measured results. In this earlier study, we found a 2-fold increase in corneal stiffness following CXL, and a 6-fold increase in the relaxation time.