Varying femtosecond laser induced crosslink density in corneal stroma to control degree of refractive error correction

Our recent work has demonstrated the application of femtosecond oscillators to crosslink corneal collagen in absence of photosensitizers to correct refractive errors and enhance corneal mechanical properties. Here we propose a new design of the femtosecond laser station enabling significantly reduce the time of the treatment procedure to clinically acceptable duration and report the results of one-dimensional parametric study on the control of the adjustment of corneal curvature. Fresh porcine eyes were subject to the laser irradiation ex vivo. The volumetric exposure to the laser has been executed by treating multiple planar areas at varying depths, measured from the ocular surface. The degree of the refractive error correction was adjusted by varying the number the laser treated planar areas. The eye topography has been monitored within 24-hour window post-treatment to assess degree and short term stability of the induced corrections. Results have shown that the correction of corneal refractive power can be controlled with resolution of approximately 0.75 diopter. Inflation tests were performed post-treatment accordingly to assess the viscoelastic response of crosslinked corneas. Rate dependent hysteresis curves obtained from the pressure-deformation response of treated corneas under physiological conditions showed statistically significant increase in stiffness in contrast to controls, in which no change has been observed.