Topographic pattern and apex location of keratoconus on elevation topography maps.

PURPOSE: To compare topography pattern and apex location in elevation and axial curvature topographic maps of keratoconic corneas.

METHODS

We prospectively evaluated 72 corneas of 42 patients who had one or more corneal findings of keratoconus with the elevation and axial curvature displays of the PAR Corneal Topography System (PAR-CTS) and 66 of these corneas with the axial curvature display of the Tomey Topographic Modeling System (TMS-1). Topography maps were evaluated for topography pattern and location of the cone apex.

RESULTS

Axial curvature displays of the PAR-CTS and the TMS-1 showed good concordance in terms of topographic patterns (96% for right, 86% for left corneas) and apex locations of cones (92% for right, 80% for left corneas). On the other hand, low concordances were noted when comparing topographic patterns (35.3% for right, 36.8% for left corneas) and apex locations (47% for right, 38% for left corneas) on curvature and elevation mode displays of PAR-CTS. Apices were found in the inferotemporal quadrant in 65% of corneas evaluated with the PAR-CTS. This distribution is significantly different from the apex location in axial curvature maps (p < 0.04). Twenty-nine percent of corneas that showed an apex on the axial curvature mode of the PAR-CTS had a normal pattern, without a detectable cone apex, on the elevation mode display.

CONCLUSIONS

The results of this study clearly show the difference between elevation and curvature-based corneal topographic evaluation of keratoconus. Unlike axial curvature maps, the majority of apices on elevation maps are clustered in the inferotemporal quadrant. This new information about apex location in keratoconic corneas provided by elevation topography may have better diagnostic specificity than regional differences of curvature on axial curvature maps. Because elevation mapping shows the physical location of the cone, it may improve results of contact lens fitting and surgical management.