How does a Placido disc topographer work?

A Placido disc topographer projects a series of concentric illuminated rings onto the corneal surface and captures their reflection with a camera. Software analyzes the spacing and shape of the reflected rings to calculate curvature at thousands of points. On a perfectly spherical cornea, rings appear evenly spaced and circular. Closer ring spacing indicates steeper curvature, wider spacing indicates flatter curvature, and distorted rings indicate irregular astigmatism.

What do the colors mean on a corneal topography map?

Using the standard convention, cool colors (blue and green) represent flatter, less curved areas with lower dioptric values. Warm colors (yellow, orange, and red) represent steeper, more curved areas with higher dioptric values. The specific dioptric range assigned to each color depends on whether an absolute or normalized scale is used and what step size is selected.

What topography pattern suggests keratoconus?

Key topographic signs of keratoconus include inferior steepening (warm colors concentrated below center), an asymmetric bow-tie pattern, a steep central or paracentral island, and an inferior-superior (I-S) value greater than 1.4 D. Early detection through topography screening is important because keratoconus is progressive and may require specialty contact lenses or surgical intervention.

Corneal Topographer Principles: Placido Disc, Color Maps, and Clinical Applications

Beyond Keratometry: Why Topography Matters

While the keratometer provides curvature data for only the central 3mm of the cornea, a corneal topographer maps the curvature of the entire corneal surface, typically covering the central 8-10mm. This comprehensive mapping reveals corneal irregularities, asymmetries, and patterns that keratometry cannot detect. For contact lens practitioners, topography provides invaluable information for fitting complex cases and identifying conditions that affect lens performance.

How Placido Disc Topography Works

The most common type of corneal topographer uses Placido disc technology. The device projects a series of concentric illuminated rings onto the corneal surface and captures their reflection using a camera system.

The operating principle:

The Placido disc (a cone or flat disc with alternating bright and dark concentric rings) is positioned in front of the patient's eye
The rings reflect off the corneal surface, which acts as a convex mirror
A camera captures the reflected ring pattern
Software analyzes the spacing and shape of the reflected rings
From this analysis, the software calculates the curvature at thousands of points across the corneal surface

On a perfectly spherical cornea, the reflected rings would appear as perfectly circular, evenly spaced concentric circles. Any deviation from this ideal pattern indicates areas of different curvature.

Rings closer together indicate a steeper (more curved) area
Rings farther apart indicate a flatter (less curved) area
Distorted or irregular rings indicate irregular corneal curvature

Placido disc topography measures the anterior corneal surface only. It calculates curvature from reflected ring patterns. Closer ring spacing = steeper curvature. Wider ring spacing = flatter curvature. Distorted rings = irregular astigmatism.

Reading Topography Maps

The topographer's software converts the ring data into color-coded curvature maps. Understanding the color scale is essential for clinical interpretation.

Standard Color Convention

Cool colors (blue, green): Represent flatter (less curved) areas with lower dioptric values
Warm colors (yellow, orange, red): Represent steeper (more curved) areas with higher dioptric values

The color scale typically uses an absolute or normalized scale:

Absolute scale: Uses fixed dioptric ranges for each color across all patients, making it easier to compare maps between patients or over time
Normalized (relative) scale: Adjusts the color range to fit each individual patient's data, providing maximum detail for that specific cornea but making comparisons between patients less straightforward

Common Map Patterns

Round (spherical): Uniform color distribution indicates a nearly spherical cornea with minimal astigmatism
Oval: Slightly elongated pattern, suggesting mild regular astigmatism
Symmetric bow-tie: Two lobes of warm color separated by cooler color, indicating regular astigmatism. The bow-tie axis corresponds to the steep meridian
Asymmetric bow-tie: One lobe is larger or steeper than the other. This asymmetry may suggest early keratoconus or other corneal irregularity
Inferior steepening: The lower portion of the map shows warmer colors than the upper, a hallmark pattern of keratoconus

When interpreting topography maps, always check the scale being used (absolute vs. normalized) and the step size (diopter interval per color). A map that looks dramatically abnormal on a normalized scale with 0.25 D steps might look quite normal on an absolute scale with 1.50 D steps. Consistent scale selection is important for meaningful comparisons.

Clinical Applications

Detecting Irregular Astigmatism

One of the most valuable applications of topography is identifying irregular astigmatism, where the corneal curvature is not uniformly distributed across the surface. Standard keratometry and refraction cannot fully characterize irregular astigmatism, but topography reveals the specific pattern and location of the irregularity.

Keratoconus Screening

Topography is the primary screening tool for keratoconus, a progressive condition where the cornea thins and bulges forward. Early topographic signs include:

Inferior steepening (warm colors concentrated below center)
Asymmetric bow-tie pattern
Steep central or paracentral island
I-S value (inferior-superior difference) greater than 1.4 D

Contact Lens Warpage Detection

Contact lenses, particularly rigid lenses, can temporarily mold the corneal shape. Topography reveals this warpage pattern, which may include:

Central flattening with peripheral steepening (from a flat-fitting RGP)
Central steepening with peripheral flattening (from a steep-fitting RGP)
Irregular patterns that do not correspond to normal corneal anatomy

Detecting warpage is important because it distorts refraction measurements and may lead to incorrect spectacle prescriptions. Patients must discontinue lens wear and allow the cornea to return to its natural shape before reliable measurements can be taken.

Post-Surgical Evaluation

After corneal refractive surgery (LASIK, PRK) or corneal transplant, topography maps show the surgical effect and any residual irregularity. This information guides post-surgical contact lens fitting decisions.

Relying solely on keratometry and ignoring topography when fitting specialty contact lenses. Keratometry measures only the central 3mm and misses peripheral irregularities that significantly affect lens fit. Topography should be performed whenever irregular astigmatism is suspected, when fitting RGP lenses on irregular corneas, or when unexplained visual complaints do not match refraction findings.

Key Takeaways

Corneal topography maps the entire corneal surface, not just the central 3mm
Placido disc technology projects concentric rings and analyzes their reflection pattern
Cool colors (blue/green) = flat areas; warm colors (red/orange) = steep areas
Symmetric bow-tie patterns indicate regular astigmatism; asymmetric patterns suggest irregularity
Topography is essential for keratoconus screening, warpage detection, and specialty lens fitting
Always check the color scale type and step size when interpreting maps