Multizone model for postsurgical corneas: analysis of standard and custom LASIK outcomes

Abstract

Abstract. A multizone model for postsurgical corneal topography is presented and applied to a comparative analysis of the outcome of standard and customized myopic LASIK. The different zones are segmented automatically by a clustering algorithm. The algorithm uses a set of three local descriptors, which correspond to normalized physical magnitudes computed for each point of the corneal topography map: Gauss curvature, root-mean-square ͑RMS͒ fit error to an ellipsoid surface model, and distance to the center of the topographic map. Both presurgical and post-LASIK corneal topographies of 31 eyes were analyzed using monozone and multizone models. The patients were classified into three groups according to the different LASIK treatments applied: Allegretto, Zyoptix, and PlanoScan. For post-LASIK corneas, the multizone model provided a lower fit error, an average of 1.2± 0.4 m versus 2.4± 0.7 m ͑monozone͒. The comparative analysis of the three different LASIK treatments showed no improvement of custom over standard treatments. The outcomes of Zyoptix and PlanoScan were basically equivalent and consistent with previous findings: The higher-order aberration ͑HOA͒ increased by a factor of two. The increase in HOA was higher, by a factor of three, after the Allegretto treatment. The mutizone model shows a higherfidelity representation and permits a deeper understanding of the postsurgical cornea.

Morphology, topography, and optics of the orthokeratology cornea

“…A similar methodology has been previously applied to the particular case of post-LASIK corneas by González el al. 19 The ortho-k treated cornea is a challenging condition as it usually includes significant asymmetries and irregularities. Lu et al 9 delimitated the different zones using the difference curvature tangential map and used the size of the treatment zone as a metric to correlate with the visual outcomes during the treatment onset.…”

“…It is worth mentioning that while the ellipsoid model provides reasonably good fits of normal corneas, the model fit gets poor in postortho-k corneas, similar to what happens in post-LASIK corneas. 19 Thus, elevation data fit to an ellipsoid by a monozone approach will not reflect the true shape of the ortho-k cornea. Instead, better fits are obtained when individual zones are fitted (see Table 1 RMS values monozone fit versus zonal fit), which implies that the increase in asphericity after ortho-k-especially in the OZ-would be largely underestimated by the monozone approach.…”

“…The aim of the segmentation algorithm is to identify each of these three newly formed zones. To achieve this, a cluster segmentation algorithm was implemented in MATLAB ® based on its native k-means function similar to the one described by González et al 19 for laser assisted in situ keratomileusis (LASIK) treated corneas. This algorithm was programmed to conduct an automatic segmentation based on the mean local curvature at each point.…”

Zernike vs. Bessel circular functions in visual optics

“…Many alternative functional representations have been proposed to describe ophthalmic surfaces, ranging from generalised conic functions to more complicated representations such as the fractional Zernike polynomials and spherical harmonics . Other techniques include combinations of modal and zonal approaches . Each of these approaches exhibits certain advantages and disadvantages with respect to the standard ZCP representation.…”

SyntEyes KTC: higher order statistical eye model for developing keratoconus

“…One last approach, proposed by Gonz alez et al 9 is to build a multi-zone model of the cornea. Such a model does not consist of singular surfaces, however, but rather of a collection of discrete regions that each have their own refraction that would complicate its implementation in ray tracing software.…”