[Three-dimensionnal representation and descriptive geometry of the pure spherical and pure cylindrical profiles of Excimer photoablation].

PURPOSE

To analyze the theoretical shape of the lenticules of corneal tissue ablated using Excimer laser for the correction of pure negative and positive spherical and cylindrical refractive errors, and to investigate the possible correlations between their spatial configuration and the clinical outcome for each procedure.

METHODS

To obtain a theoretical three-dimensional representation of each ablated lenticule, we used software that performs boolean operations on three-dimensional virtual surfaces (Bryce 3D, Metacreation, Dublin, Ireland). The representation of the theoretical shape of the refractive lenticule etched by a given profile of ablation based on a paraxial model over a circular optical zone was obtained by performing appropriate Boolean operations between different preoperative and postoperative surfaces or elements. These operations were repeated to obtain the representation of the theoretical additional lenticule corresponding to the volume of tissue ablated to blend any abrupt optical zone edges with a constant slope.

RESULTS

The lenticule corresponding to the negative spherical treatment had its maximum thickness in its center and no thickness at its edge, thus inducing a natural blend with the peripheral untreated cornea. The lenticule corresponding to the positive spherical treatment had its maximal thickness at the junction between the optical and transition zones, which are both circular. The lenticule corresponding to the negative cylindrical treatment had its maximal thickness along the flatter initial meridian and an elliptical transition zone contour. The lenticule corresponding to the positive cylindrical treatment had its maximal thickness at the edge of the optical zone along the initial flatter meridian, and no thickness at the perpendicular principal meridian. The transition zone is also elliptical.

CONCLUSION

The basic characteristics of the lenticule corresponding to the pure negative spherical treatment could explain the lesser degree of postoperative refractive regression than that one occurring after pure positive spherical treatment and pure positive and negative cylindrical treatments. The characteristics of the lenticules corresponding to the negative and positive cylindrical treatments may also account for the substantial spherocylindrical coupling and regression, respectively commonly observed after such treatments. The three-dimensional representation of the lenticules ablated for the correction of pure spherical and cylindrical refractive errors may help to better understand the outcome of these procedures aimed at reshaping the anterior surface of the cornea.