D3 Corneal Topography and Keratometry

Topic

A Placido-disk topographer projects a series of illuminated concentric rings onto the cornea and photographs the reflection of these rings on the corneal surface. When the anterior corneal curvature is regular and uniform, the concentric rings are reflected in a similarly regular and uniform manner; however, if the anterior corneal curvature presents any irregularity, that irregularity distorts the reflection of the concentric rings precisely at the point where it occurs. Consequently, the Placido-disk topographer can reconstruct a complete map of the anterior corneal curvature based on how the reflection of the concentric rings is distorted at each point.

A Placido-disk topographer can only measure the anterior corneal curvature because it captures the reflection of the concentric rings solely from the outward-facing surface of the cornea; it cannot assess the curvature of the inner corneal surface (facing the interior of the eye) using this same reflection. To measure the posterior corneal curvature—as well as corneal thickness at any given point—a Scheimpflug camera (such as the Pentacam) is used; this device rotates around the cornea, capturing cross-sectional images of its entire thickness to reconstruct the anterior and posterior corneal curvatures and the corneal thickness at each measured point.

In addition to anterior and posterior corneal curvature, the Scheimpflug camera calculates corneal elevation: the extent to which each point on the corneal surface deviates from a perfectly spherical reference surface that has been fitted to closely match the cornea's overall shape. A corneal elevation that deviates more than expected from the reference surface within a small, localized area may reveal an irregularity that general curvature measurements alone fail to clearly demonstrate. Keratoconus—a progressive disease in which the cornea thins and bulges outward to form a cone—begins by altering posterior corneal curvature, corneal elevation, and corneal thickness in highly localized areas of the cornea before anterior corneal curvature (when measured in isolation) shows any clearly abnormal changes. Therefore, early diagnosis of keratoconus relies on combining data regarding posterior corneal curvature, corneal elevation, and corneal thickness—provided by a Scheimpflug camera—rather than relying solely on the anterior corneal curvature measured by Placido-disk topographers; by the time keratoconus causes evident changes in anterior corneal curvature, the disease has already been silently progressing in those other three parameters.

Corneal thickness measured at the very center of the cornea—using anterior segment optical coherence tomography or a Scheimpflug camera—is particularly important when planning refractive surgeries to correct conditions such as myopia, hyperopia, or astigmatism. This is because such surgeries remove a layer of corneal tissue, leaving behind a residual corneal bed that must retain sufficient thickness to ensure the cornea remains structurally stable. Normal central corneal thickness is approximately 545 microns; when a person's central corneal thickness falls below 480 microns, this reduced thickness is considered a relative contraindication for LASIK-type refractive surgery. This is because the residual corneal bed remaining after tissue removal might not retain the thickness necessary to prevent the cornea from bulging post-surgery—a phenomenon similar to the bulging observed in keratoconus.

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