How to Read Corneal Topography for Contact Lens Fitting

What is Corneal Topography and How Does it Relate to Contact Lenses?

Corneal topography uses light-based scanning technology to map the surface of the cornea. The device projects thousands of points onto the corneal surface and measures how these points are reflected back. The result is a detailed, colour-coded map that shows the curvature of the cornea in great detail. This map is essential for contact lens fitting because it helps to determine which lens design and curvature are best suited for a patient’s eye.

 

How to Read the Corneal Topography Map for Contact Lens Fitting

The corneal topography map is typically displayed with different colours, each representing different curvatures of the cornea. These colour variations are important to understand when choosing the right contact lenses:

• Blue or Green: These colours represent flatter areas of the cornea. Flatter corneal areas may indicate a good fit for standard contact lenses, but they may need a larger lens to cover the flatter surface.
• Yellow, Orange, or Red: These colours show more curved areas of the cornea. In cases of steep or irregular corneas, such as in patients with astigmatism or keratoconus, specialized contact lenses may be needed to achieve a comfortable fit.

 

Key Features to Focus on for Contact Lens Fitting

When reading a corneal topography map for contact lens fitting, there are several key features that are particularly important to evaluate:

Central Curvature

The central curvature of the cornea plays a critical role in determining the fit of the contact lens. A cornea with a normal curvature should have a slight steepness in the center. If the cornea is too flat or too steep at the center, it could make it difficult to fit the correct lens. A flat cornea may require a lens with a larger diameter, while a steep cornea may require a smaller or more customized lens design.

Symmetry of the Cornea

The symmetry of the cornea is vital for ensuring that both eyes receive a consistent fit. While slight differences between the two eyes are normal, large asymmetries may suggest the need for customized lenses. For example, a person with a significant asymmetry in their corneal curvature might require different lenses for each eye, or personalized lenses that compensate for the irregular shape.

Asphericity of the Cornea

The asphericity of the cornea refers to how much the cornea deviates from a perfect spherical shape. A highly aspheric cornea (one that is more steep in the center and flatter on the periphery) can present challenges in fitting standard contact lenses. In such cases, specialty lenses like toric lenses (for astigmatism) or corneal rgp/scleral lenses (for irregular corneas) may be required to provide proper alignment and comfort.

 

Curvature Profiles

In addition to the colour map, corneal topography often includes curvature profiles, which provide detailed measurements of curvature along various radial lines from the center to the periphery. This helps eye care professionals determine the best fitting lens size and design. For example, if the curvature profile indicates a sharp change in curvature near the edges of the cornea, the practitioner may opt for a larger diameter lens to ensure it stays properly centered on the eye.

 

Analyzing Abnormalities for Contact Lens Fit

One of the most common uses of corneal topography in contact lens fitting is identifying corneal abnormalities that may require specialized lenses. Conditions like keratoconus or astigmatism (irregular curvature) may require custom contact lenses for optimal vision and comfort. In such cases:

• Keratoconus: The topography map of a person with keratoconus will typically show a steep, cone-shaped area in the center of the cornea. In these cases, scleral lenses or rigid gas-permeable lenses are often used because they can vault over the irregularly shaped cornea and provide a smooth surface for better vision.
• Astigmatism: If the map shows significant differences in curvature in different meridians of the cornea, toric lenses are often required to correct the astigmatism. These lenses are designed to match the specific shape of the cornea and provide clear vision by compensating for the uneven curvature.

 

Customizing the Contact Lens Fit Based on Topography Data

Once the corneal topography is fully analyzed, the eye care professional will use the data to select the most appropriate type of lens. Depending on the curvature, symmetry, and shape of the cornea, the professional may recommend:

• Standard soft contact lenses for relatively uniform and regular corneas.
• Toric lenses for astigmatism, as they are designed to fit the varying curvatures in different areas of the cornea.
• Rigid gas-permeable lenses for individuals with moderate corneal irregularities.
• Scleral lenses for more severe cases, such as keratoconus, where the lens vaults over the cornea and provides a smooth optical surface.

 

Conclusion

Reading corneal topography for contact lens fitting is essential for ensuring a comfortable and effective lens fit. By analyzing the map’s curvature, symmetry, and asphericity, eye care professionals can select the best contact lens type and design for each individual. Understanding these key features allows for a customized lens fitting that maximizes comfort, visual acuity, and eye health. Whether the patient has a standard cornea or requires specialty lenses for an irregular shape, corneal topography provides the necessary information to create the best solution for optimal vision correction.