Corneal Topography and Tomography

Corneal Contours - Intro to Imaging

• Corneal Topography:
  • Analyzes anterior corneal surface curvature and shape.
  • Primarily uses Placido disc reflection principle.
  • Detects surface irregularities like astigmatism, keratoconus.
• Corneal Tomography:
  • Provides comprehensive 3D corneal structure analysis.
  • Images anterior surface, posterior surface, and full corneal thickness (pachymetry).
  • Utilizes technologies like Scheimpflug imaging or Optical Coherence Tomography (OCT).

⭐ Tomography provides a 3D reconstruction (anterior, posterior, pachymetry), unlike topography (anterior surface only). oka

Imaging Arsenal - Gadgets & Glory

• Corneal Topography: Maps anterior corneal curvature.
  • Methods: Placido disk (Keratoscopy, Videokeratoscopy).
  • Outputs: Axial, Tangential, Refractive power, Elevation maps.
  • Uses: Keratoconus screening, contact lens fitting.
• Corneal Tomography: Comprehensive 3D corneal structure.
  • Slit-scanning (e.g., Orbscan): Anterior/posterior elevation, pachymetry. ⚠️ Post-LASIK pachymetry may be overestimated.
  • Scheimpflug imaging (e.g., Pentacam): Rotating camera; detailed anterior/posterior elevation, pachymetry.

    ⭐ Scheimpflug imaging (e.g., Pentacam) is crucial for assessing posterior corneal elevation and pachymetry, vital for ectasia risk assessment.

  • AS-OCT: High-resolution cross-sectional images, precise pachymetry. )

Map Mysteries - Decoding Displays

Interpreting corneal maps is key for refractive surgery planning and ectasia screening.

• Axial Map: Overall power; astigmatism (WTR/ATR). Good for initial screening.
• Tangential Map: True local curvature; precise cone localization.
• Elevation Maps: Height vs Best Fit Sphere (BFS). Anterior/posterior float vital for ectasia risk.
• Pachymetry Map: Displays corneal thickness. Identifies thinnest point.

📌 Color Code: Hot (reds): steep/elevated/thick. Cool (blues): flat/depressed/thin.

⭐ Asymmetric bowtie with skewed radial axis (SRAX) and inferior steepening on axial maps are hallmarks of keratoconus.

Clinic Compass - Guiding Decisions

⭐ The Belin/Ambrósio Enhanced Ectasia Display (BAD-D) is a critical index for early keratoconus detection, combining anterior, posterior, and pachymetric data.

• Refractive Surgery Screening:
  • Detect ectasia risk: Kmax > 47.2 D, posterior float elevation, thin pachymetry (<500µm).
  • Identify forme fruste keratoconus (FFKC).
  • Guide procedure choice: LASIK, PRK, SMILE.
• Keratoconus:
  • Early diagnosis: Asymmetry, skewed radial axes, BAD-D > 1.6.
  • Staging (e.g., Amsler-Krumeich) & monitoring progression for CXL.
• Post-Op & Other:
  • Evaluate ablation centration/quality.
  • Detect post-surgical ectasia.
  • Accurate IOL power calculation (post-refractive).
  • Specialty contact lens fitting.

Index Insights - Quantifying Curves

• SimK (Simulated Keratometry): Steep K, Flat K, Avg K, Astigmatism.
• Pachymetry: Thinnest point (TP), Central Corneal Thickness (CCT). Crucial for ectasia risk.
• Elevation Maps: Anterior & Posterior float vs. reference sphere; detects early keratoconus.
• Key Indices:
  • Kmax: Max keratometry.
  • I-S value: Inferior-Superior asymmetry (Normal < 1.4 D).
  • SRAX: Skewed Radial Axes Index.

⭐ Posterior corneal elevation > +15 µm (8mm BFS) or thinnest pachymetry < 470 µm are key post-LASIK ectasia risks.

High‑Yield Points - ⚡ Biggest Takeaways

• Placido disk for anterior curvature; Scheimpflug for 3D analysis (anterior/posterior, pachymetry).
• Axial maps guide IOL power; Tangential maps excel at keratoconus detection.
• Keratoconus indicators: asymmetric bowtie, inferior steepening, ↑Kmax, posterior elevation.
• Central Corneal Thickness (CCT) is crucial; LASIK typically needs >500 µm.
• Elevation maps compare cornea to Best Fit Sphere (BFS) or Toric Ellipsoid.
• Belin/Ambrósio Display (BAD) integrates data for ectasia risk stratification.
• Corneal biomechanics (e.g., Hysteresis) assessed by devices like ORA (Ocular Response Analyzer).