Corneal Topography & OCT for Contact Lens Fitting

Corneal Topography & OCT for Contact Lens Fitting

Quick Answer: Corneal topography uses Placido disc reflections to map curvature, power, and elevation across the entire corneal surface — far more than the central 3-4 mm a keratometer samples. Color scales run warm (steep) to cool (flat), and patterns such as bow-tie, asymmetric bow-tie, or inferior steepening flag astigmatism and keratoconus. Anterior-segment OCT adds cross-sectional imaging that lets fitters measure sagittal depth, limbal clearance, and landing zone relationships — essential for scleral and ortho-K fitting.

Topography: Principle

A Placido disc projects a series of concentric light rings onto the tear film. The system analyzes the spacing and shape of the reflected rings: closely spaced rings indicate a steep cornea; widely spaced rings indicate a flat cornea. Modern systems combine Placido imaging with Scheimpflug cameras or slit-scanning to add true elevation data.

Color Scale

Standardized color scales use:

• Warm colors (red/orange) = steep (high power, short radius)
• Cool colors (blue/green) = flat (low power, long radius)
• A green central zone is typical of normal curvature (~43-44 D)

Always check the scale step size (1.0 D vs 0.5 D) — smaller steps exaggerate pathology. Absolute scales aid comparison across visits; relative scales highlight local variation.

Map Types

Patterns & Pathology

• Round / oval pattern: regular astigmatism, symmetric bow-tie.
• Symmetric bow-tie: with-the-rule or against-the-rule regular astigmatism.
• Asymmetric bow-tie: skewed axis, one lobe larger — early keratoconus suspect.
• Inferior steepening ("claw" or dropping bow-tie): classic keratoconus pattern.
• Superior steepening: contact lens-induced warpage or superior ectasia.
• Post-LASIK pattern: central flat zone (blue) surrounded by annular steepening (red donut); oblate pattern.
• Post-RK pattern: radial flat spokes with steep incision bridges.
• Post-PK pattern: irregular, often high astigmatism; topography mandatory before fitting.

Clinical Uses in Contact Lens Practice

1. Keratoconus screening — topography detects sub-clinical ectasia before slit-lamp signs. Indices such as Sim-K, I-S value (>1.4 D suspicious), and KISA% aid diagnosis.
2. RGP fitting — align the lens with the flattest meridian; tangential maps show where the lens will bear.
3. Scleral lens fitting — elevation maps guide vault; OCT confirms clearance.
4. Ortho-K fitting and monitoring — tangential maps show the treatment zone (central flat) and reverse curve; confirm centration and assess for decentration.
5. Post-PK and post-refractive fitting — irregular corneas require topography-driven designs.

Anterior Segment OCT

OCT uses low-coherence interferometry to produce cross-sectional images of the cornea, tear film, and conjunctiva/sclera. For contact lens fitting, AS-OCT provides:

• Sagittal depth (vault) of scleral lenses — central clearance target ~200-300 µm after settling.
• Landing zone relationship — confirms the lens lands on conjunctiva, not on the limbus.
• Limbal clearance — prevents limbal staining and vascular constriction.
• Tear film thickness — helps evaluate dry eye and lens dehydration.
• Post-lens tear reservoir — verifies fluid reservoir under scleral lenses.
• Pachymetry — corneal thickness for edema monitoring in extended wear.

Topography vs Keratometry

Ortho-K and Post-Surgical Considerations

For ortho-K, the tangential map is the workhorse: it shows the central flat treatment zone, the reverse curve (steep ring), and the alignment zone. A well-centered treatment zone 5-6 mm wide with a symmetric reverse curve indicates a good fit. Decentration, smiley-face patterns, or central islands signal lens design or positioning problems that need re-design before continued wear.

For post-PK (penetrating keratoplasty) corneas, topography is mandatory before any fitting attempt because the graft often has high, irregular astigmatism that keratometry cannot capture. Elevation maps guide RGP and scleral designs that vault the graft-host junction. Fitting should wait until sutures are out and curvature has stabilized — typically 3-6 months post-suture removal.

For post-LASIK / PRK patients, the oblate cornea requires a reverse-geometry RGP or a scleral lens. Topography reveals the central flat treatment zone and transition zone; the lens must vault the central flat area and land on the midperipheral steep ring. Misreading a post-LASIK map as a normal oblate pattern leads to central bearing and staining.

Limitations of Topography

• Tear film disruption degrades the reflected Placido image.
• Placido-only systems miss true elevation data (need Scheimpflug or slit-scanning).
• Color scales can mislead if not standardized.
• Reference sphere selection changes elevation maps; always compare to a normal baseline.
• Topography measures the anterior surface only; posterior ectasia (e.g., posterior keratoconus) requires Scheimpflug or OCT.