High-irradiance CXL combined with myopic LASIK: flap and residual stroma biomechanical properties studied ex-vivo.

Background/aims: To evaluate ex vivo biomechanical and enzymatic digestion resistance differences between standard myopic laser in-situ keratomileusis (LASIK) compared with LASIK+CXL, in which high-irradiance cross-linking (CXL) is added.

Methods: Eight human donor corneas were subjected to femtosecond-assisted myopic LASIK. Group A (n=4) served as a control group (no CXL). The corneas in LASIK+CXL group B were subjected to concurrent prophylactic high-irradiance CXL (n=4). Saline-diluted (0.10%) riboflavin was instilled on the stroma, subsequently irradiated with UV-A through the repositioned flap. The cornea stroma and flap specimens were separately subjected to transverse biaxial resistance measurements; biomechanical differences were assessed via stress and Young's shear modulus. Subsequently, the specimens were subjected to enzymatic degradation.

Results: For the corneal stroma specimen, stress at 10% strain was 128±11 kPa for control group A versus 293±20 kPa for the LASIK+CXL group B (relative difference Δ=+129%, p<0.05). The stress in group B was also increased at 20% strain by +68% (p<0.05). Shear modulus in group B was increased at 10% strain by +79%, and at 20% strain by +48% (both statistically significant, p<0.05). The enzymatic degradation time to dissolution was 157.5±15.0 min in group A versus 186.25±7.5 min in group B (Δ=+18%, p=0.014). For the flaps, both biomechanical, as well as enzymatic degradation tests showed no significant differences.

Conclusions: LASIK+CXL appears to provide significant increase in underlying corneal stromal rigidity, up to +130%. Additionally, there is significant relevant enzymatic digestion resistance confirmatory to the above. LASIK flaps appear unaffected biomechanically by the LASIK+CXL procedure, suggesting effective CXL just under the flap.