Eye Contact Lens. 2005 Nov;31(6):257-62.


Comparison of axial and tangential topographic algorithms for contact lens fitting after LASIK.

Szczotka-Flynn L, Jani BR.

From the Department of Ophthalmology (L.S-F.), University Hospitals of Cleveland and Case Western Reserve University, Cleveland, OH; and Richmond Eye Associates, P.C. (B.R.J.), Richmond, VA.

PURPOSE.: Computerized videokeratography (CVK) has become the standard method for measuring the corneal curvature after refractive surgery. However, few resources exist on the use of CVK data for postoperative contact lens fitting, and no recommendations exist on the selection of the best topographic algorithm relevant to gas-permeable (GP) lens fitting. This study assessed the optimal use of topographic data to fit spherical GP contact lenses on patients who have undergone laser in situ keratomileusis. METHODS.: A retrospective analysis of CVK maps from eight post-LASIK eyes fitted with spherical GP lenses was performed in the Contact Lens Service of the Department of Ophthalmology at Case Western Reserve University and University Hospitals of Cleveland. Axial and tangential maps from the Dicon CT 200 (version 3.50) or Humphrey Atlas (version A8) topographer were analyzed. Averaged dioptric curvatures from different locations (inferior, superior, and temporal) at various distances (2.0 mm, 2.5 mm, 3.0 mm, 3.5 mm, and 4.0 mm) from the vertex normal of each eye were compared with the base curves of the prescribed GP contact lenses. RESULTS.: One-way analysis of variance, Pearson correlation analysis, and paired t tests showed that the best topographic predictors of a successful GP base curve were the average curvatures at the 4.0 mm distance on the axial maps (r = 0.8078, P=0.05) and at the 2.0 mm distance on the tangential maps (r = 0.9738, P = 0.0002). The mean dioptric powers of the GP base curve, axial map 4.0 mm curvature, and tangential map 2.0 mm curvature were 41.50 diopters (D), 42.65 D, and 42.67 D, respectively. CONCLUSION.: To simplify and guide GP fitting after LASIK, the average dioptric curvature 4.0 mm from the vertex normal on axial maps or 2.0 mm from the vertex normal on tangential maps are the best predictors of accurate GP base curve selection.

Huh?

Most ablations are 6.5 mm, although the actual effective optical zone can be smaller. I would question the relevance of mapping at a 2.0 mm radius from vertex, so the tangential maps would not seem as valid as axial maps.

Well-fitted RGPs require attention to the peripheral lens curvatures - the areas untouched by the laser. A 4.0 mm radius is usually outside the ablation zone but still subject to alteration due to the way the flap lays on the residual bed. How the flap lays depends on the pre-op curvature, the depth and diameter of the flap and the depth and diameter of the ablation. Good data is not always available at 4.0 mm from vertex.

RGP fitting on virgin corneas seeks to reduce excessive bearing of the lens on the cornea. Therefore, lens fitting is based on the area beyond a 3.0 to 3.5 mm radius of vertex - closer to where these researchers looked with the axial mapping.

It is too bad the authors of this study failed to include the pre-op topograghical data when looking at GP lens design. Such an analysis would show that good post-LASIK RGP design requires some sense of the mid-peripheral pre-op curvatures, those areas that remain relatively untouched by the procedure.

After re-reading this abstract, it is amazing for what it does not say. Were the maps taken BEFORE lenses were fitted and worn or after? Also, what is the point of averaging all the data? While a case could be made for deriving "average" corneal curvature data in the population as an aid for lens design, all dibs are off when talking about post-LASIK corneas since the degree of myopia or hyperopia and, therefore, the degree of ablation and subsequent corneal curve change, does not correlate with original curvatures.

The researchers appear to be making rather naive assumptions about post-LASIK corneas.