Nonlinear optical crosslinking (NLO CXL) for correcting refractive errors.

Ultraviolet A (UVA) light-based photoactivation of riboflavin (Rf) to induce corneal crosslinking (CXL) and mechanical stiffening is now a well-known treatment for corneal ectasia and Keratoconus that is being used in a topographically guided photorefractive intrastromal CXL (PiXL) procedure to treat low degrees of refractive errors. Alternative approaches for non-invasive treatment of refractive errors have also been proposed that use femtosecond lasers (FS) that provide much faster, more precise, and safer results than UVA CXL. One such treatment, nonlinear optical crosslinking (NLO CXL), has been able to replicate the effects of UVA CXL, while producing a smaller area of cellular damage and requiring a shorter procedure time. Unlike UVA CXL, the treatment volume of NLO CXL only occurs within the focal volume of the laser, which can be placed at any depth and scanned into any pattern for true topographically guided refractive correction. This review presents our experience with using FS lasers to photoactivate Rf and perform highly controlled corneal CXL that leads to mechanical stiffening and changes in corneal shape.