topography topics

Calculated Corneal Indices and Analysis Modules

BY KENNETH A. LEBOW, OD
JANUARY 2000

Corneal indices are extremely important values that can aid in differentiating normal topography patterns from abnormal topography patterns. Evaluating changes in these key corneal indices provides dramatic information regarding the cornea's response to external pressures. The corneal shape factor indicates how rapidly or how slowly the flattening of the cornea occurs from the apex to the periphery. This value is especially important when trying to differentiate between corneal warpage and keratoconus, and in understanding when it may be necessary to fit an RGP steeper than a keratometry reading indicates.

Other corneal indices describe the regularity or smoothness of the corneal surface, as well as differences between the superior and inferior curvatures. The toric mean reference curvature is a valuable index that describes the mean curvature at the apex of the toric surface that best fits the measured corneal surface.

Many topography manufacturers offer systems that incorporate various indices that have been combined into special screens. These screens are useful for differentiating normal from abnormal topography patterns. Analysis modules are used to identify, classify and, sometimes, quantify keratoconus. While keratoconus maps typically demonstrate superior flattening with inferior steepening, many other conditions also present with similar patterns.

Differentiating Views

While all of the maps in Figure 1 demonstrate superior flattening and inferior steepening, not all of the images represent keratoconus. True keratoconus presents with a high eccentricity or shape factor because of the ectasia (Figs. 1 & 2, upper left). In addition, the corneal surface becomes highly irregular, either from scarring or contact lens wear.

Contact lens-induced corneal warpage can be severe enough to mimic keratoconus, but the primary differentiating index is the shape factor. While corneal irregularity may be high and significant apical toricity may be present, warped eyes typically present with low (prolate) to negative (oblate) shape factors (Figs. 1 & 2). The molding effect from RGPs flattens the central curvature and steepens the periphery, sphericalizing the corneal shape.

FIG. 1: Axial maps of keratoconus, subclinical keratoconus, corneal warpage and a displaced corneal apex.

FIG. 2: Pathfinder corneal analysis summary data for each of the conditions shown in Fig. 1.

Evaluating corneal shapes with mathematically derived indices helps you to understand various clinical corneal complications more completely.

Dr. Lebow is a member of the AOA and a Fellow of the AAO. He is in private practice in Virginia Beach, Virg.