By Dr Lee Mun Toong
When used in conjunction with LASIK (Lasik Xtra), the goal of CXL is to restore corneal strength without creating an additional change in refraction beyond that provided by the LASIK correction. Traditional CXL, when applied to the ectatic cornea, is known to cause a flattening of the cornea of several diopters; therefore, it is important to consider the differences between Lasik Xtra and conventional CXL.
The soak and irradiance times described above differ significantly from the conventional CXL treatment protocol for KC and ectasia, with shorter total riboflavin soaking times and lesser total UVA dose. The direct application of riboflavin to the stroma afforded by the lifted LASIK flap reduces the required time for sufficient riboflavin to diffuse into the targeted area of the corneal stroma. Similarly, while cross-linking for treatment of ectasia is intended to stiffen a pathologically weak cornea, it is plausible that less cross-linking may be required to return an otherwise healthy cornea to its native strength subsequent to LASIK.
Theoretical modeling using finite element analysis by Dupps et al. at The Cleveland Clinic has demonstrated that focal weaknesses of the cornea result in the progressive topographic abnormalities observed in keratoconus, and that CXL results in dramatic flattening of corneal curvature.
This finite element analysis model was applied by the same group to evaluate the impact of Lasik Xtra on response to deformation in the residual stromal bed when intra-ocular pressure is doubled, and the effect on refractive outcome. A myopic LASIK procedure with a −4.25 D spherical correction was simulated using a wavefront optimized ablation profile with an optical zone diameter of 6.5 mm and overall treatment diameter of 9 mm. The response to deformation and refractive correction was evaluated with and without the addition of simulated CXL, modeled as an increase in stiffness of the central 9 mm of the stromal bed with an effective depth of 200 μ and a stiffening factor of 1.5×.
The addition of CXL resulted in less displacement when IOP was increased (ie, cornea is stiffer), however, simulations demonstrated refractive equivalence between the cross-linked and uncross-linked eyes. This modeling demonstrated a theoretical basis for increasing the corneal stiffness without changing refractive outcome.