Comparison of axial and instantaneous videokeratographic data in keratoconus and utility in contact lens curvature prediction.

PURPOSE

Instantaneous radius of curvature data in corneal videokeratoscopy better represents corneal shape and local curvature changes in keratoconus than axial maps, especially in the periphery. Knowledge of instantaneous radius is especially important in keratoconus for analysis of the cone apex, which is often decentered. In this study, we quantitatively compared axial and instantaneous displays of videokeratographic data in keratoconus, with emphasis on apex curvature and position. We assessed the utility of instantaneous data for rigid gas permeable (RGP) contact lens curvature selection and compared it with data derived from the original axial displays in keratoconus.

METHODS

We performed modern placido-based videokeratography on 35 eyes with clinically diagnosed keratoconus. We derived the apex curvature, the position of the apex relative to the center of the map, and the steep simulated keratometry (SIM-K) reading from axial and instantaneous videokeratographic displays. We also collected the RGP base curves for 28 eyes that were fit with a minimal apical bearing or clearance RGP contact lens.

RESULTS

The instantaneous apex was consistently located closer to the center of the map than the axial apex. Means (and standard deviations) for the best fit RGP base curve, axial steep SIM-K, axial apex curvature, instantaneous steep SIM-K, and instantaneous apex curvature were 52.00 D +/- 5.92 D, 50.93 D +/- 5.94 D, 54.81 D +/- 7.14 D, 49.19 D +/- 5.54 D, and 56.86 D +/- 7.89 D, respectively. Statistical comparisons revealed axial steep SIM-K to best represent the RGP base curve for the entire group. Similarly, in the subset of patients with apex decentrations within 1 mm of the map center, the axial steep SIM-K best represented the RGP base curve. In the remaining decentered cone subset, the axial apex curvature best represented the base curve.

CONCLUSIONS

Axial and instantaneous maps differ significantly in apical position and apex curvature in keratoconus. Although instantaneous radius may better represent corneal shape, axial curvatures better predict the final RGP base curve in keratoconus patients.