  Collagen crosslinking of the cornea (CXL) is commonly employed to prevent or
treat keratoconus. Although the change of corneal stiffness induced by CXL
surgery can be monitored with non-contact dynamic Optical Coherence
Elastography (OCE) by tracking mechanical wave propagation, the depth
dependence of this change is still unclear if the cornea is not crosslinked
through the whole depth. Here we propose to combine phase-decorrelation
measurement applied to OCT structural images and acoustic micro-tapping
(A$\mu$T) OCE to explore possible depth reconstruction of stiffness within
crosslinked corneas in an ex vivo human cornea sample. The analysis of
experimental OCT images is used to define the penetration depth of CXL into the
cornea, which varies from $\sim$100$\mu m$ in the periphery to $\sim$150$\mu m$
in the central area and exhibits a sharp transition between areas. This
information was used in a two-layer analytical model to quantify the stiffness
of the treated layer. We also discuss how the elastic moduli of partially
CXL-treated cornea layers reconstructed from OCE measurements reflect the
effective mechanical stiffness of the entire cornea to properly quantify
surgical outcome.
