A finite element model for ultrafast laser-lamellar keratoplasty

A biomechanical model of the human cornea is employed in a finite element formulation for simulating the effects of Ultrafast Laser-Lamellar Keratoplasty. Several computer simulations were conducted to study curvature changes of the central corneal zone under various physiological and surgical factors. These factors included the combined effect of corneal flap and residual stromal bed thickness on corneal curvature; the effect of the shape of the lenticle on the surgical procedure outcomes and the effect of flap thickness on stress distribution in the cornea. The results were validated by comparing computed refractive power changes with clinical results. The effect of flap thickness on the amount of central flattening indicates that for flap thickness values 28% over the corneal thickness, central corneal flattening decreases. Moreover, the change in corneal curvature induced by subtraction of a plano-convex lenticle under a uniform flap, naturally imply a smaller change in the structure of the anterior layers of the cornea, but a bigger deformation in the structure of the posterior layers that are left behind the resection of the lenticle. In addition, the model also verified that the corneal curvature increased peripherally with simultaneous thinning centrally after subtraction of corneal tissue. This result shows that not only the treated zone is affected by the surgery, indicating the important role of the biomechanical response of the corneal tissue to refractive surgery, which is unaccounted for in current ablation algorithms. The results illustrate the potentialities of finite element modeling as an aid to the surgeon in evaluating variables.