Course Title:
CORNEAL TOPOGRAPHY
CORNEAL TOPOGRAPHY: Intro & Overview
Lecturer: M. Patrick COLEMAN, ABOC, COT DISCLOSURE STATEMENT I have no vested financial interest in anything we will be talking about.
CELL PHONE? Please silence cell phones @ this time! Thank you!
What is CORNEAL TOPOGRAPHY?
What is CORNEAL TOPOGRAPHY? (cont.)
Warm colors vs. Cold colors & Contour lines close together vs. f a r apart
Comparing Corneal Topography to Manual Keratometry
Manual KERATOMETER: Only measures FOUR points within central 3mm of cornea!
Corneal TOPOGRAPHY: • Measures 8,000 points over a 10mm area of the cornea! • Provides much more detail of the anterior corneal surface
Comparing Corneal Topography to Manual Keratometry (cont.)
Keratometer mires reflecting off 3mm area of cornea (NOTE: image is offset to get this picture!)
Topography image of ‘placido placido rings’ rings reflecting off 10mm area of cornea
Comparing Corneal Topography to Manual Keratometry (cont.)
Comparing Corneal Topography to Manual Keratometry (cont.) Suspect KCN
Corneal topography map = 10mm area measured = 10,000 points measured!
Comparing Corneal Topography to Manual Keratometry (cont.)
Comparing Corneal Topography to Manual Keratometry (cont.) K’s: 45.50 / 44.50
K’s: 45.50 / 44.50
43.9 D @ 114 43.2 D @ 24
Manual Keratometry would show these two corneas to have the same K readings (45.50 / 44.50) Keratometry Missed the Keratoconus!
Topography as a diagnostic tool – seeing the big picture: PLACIDO DISK design
But, do these two cornea’s look the same with TOPOGRAPHY?
Topography as a diagnostic tool – seeing the big picture: Cone of Focus (triangulation principle)
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• One of the Placido Disk rings is used as a ‘master cone-of-focus ring’ for calculation purposes
Topography as a diagnostic tool – seeing the big picture: Cone of Focus (using “Placido Disk” principles)
Topography as a diagnostic tool – seeing the big picture
The conecone-ofof-focus RING is spaced equidistant from adjacent “Placido Disk” rings. What does that mean? You are at the correct distance to capture accurate information! The center ring is clearly focused and aligned; this is another indicator we will get a good measurement!
Topography as a diagnostic tool – seeing the big picture
Topography Views & Maps Available The Map Printout Pupil marker
Data Values: -Sim Sim K’ K’s -Shape Factor -Pupil size -HVID (i.e. ‘white white to white’) white
Pupil center (x)
Center of Map; Vertex (X)
1mm x 1mm grid indicators
Corneal apex (Δ)
Cursor value & position
Topography Views & Maps Available (cont.) MOST COMMONLY USED MAPS
LEAST USED MAPS:
Curvature Maps
Shape Maps
Function Maps
• Axial
• Elevation
• Refractive
• Tangential
• Irregularity
• Corneal Wavefront
• Mean Map • (Keratometry Map)
Topography Views & Maps Available (cont.) Corneal Topography maps can describe corneal... ...SHAPE or corneal ...OPTICS
Shape ..........................................Optics
• Image Simulation • Point Spread Function (PSF) • Modulation Transfer Function (MTF)
Elevation Mean Tangential Axial Refractive Wavefront
Topography Views & Maps Available (cont.)
Topography Views & Maps Available (cont.)
Examples of some different VIEWS (displays) available:
Examples of some common MAPS you might use: Axial Curvature
Single View
Mean Curvature
Trend Analysis
Topography Views & Maps Available (cont.)
Purpose of the different MAPS
Axial vs. Tangential Maps
Axial Map "Looking at a mountain and its overall distortion" Great for basic screening Excellent for ‘seeing’ astigmatism (steepest & flattest meridians on the cornea) Steep radius = hot, red colors Flat radius = cool, blue colors Axial Map is the starting point, point, not the answer Measures from the center out for everything; less accurate for peripheral ‘things’ things’
Purpose of the different MAPS (cont.) Tangential Map “Looking at each radius of the mountain’s surface – no matter where you are located on the mountain" Sensitive in picking up details in curvature changes on the cornea Displays small distinct areas of curvature with great detail Good for evaluation of peripheral areas of the cornea Each radius on the surface is measured independent from the center of the surface
Purpose of the different MAPS (cont.) Axial Map
Tangential Map
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Always measures from the corneal vertex - “the center”- OUTWARD
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Each measured point is INDEPENDENT of the corneal vertex
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Kind of like - “Average Trip MPG” from the corneal vertex (center) to any point
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Kind of like - “Instantaneous MPG” @ any point
Purpose of the different MAPS (cont.) Axial Map vs. Tangential Map Axial
Tangential
“center of map is only reference”
“finds local curvature”
“MOUNTAIN = irregularity on cornea “EARTH” = cornea 8,000 miles
8,006 miles
Purpose of the different MAPS (cont.) Elevation vs. Irregularity Maps: “Orange vs. Melon” Melon”
6 miles
8,000 miles
Elevation Map
Irregularity Map
• “Best Fit Sphere” model (Orange shape) • Shows difference in height (microns) between cornea and a sphere • A normal map? Always above BFS in center, always below BFS in periphery •Less sensitive for pathology detection than Irregularity Map
• “Best Fit Ellipsoid” model (Melon shape) • Shows difference in height (in microns) between cornea & aspheric surface (melon!) • Better fits natural cornea (aspheric & prolate) • May better detect irregularities for a prolate cornea (i.e., a cornea that has w/the rule astig.)
Optical Axis
“Earth with Mountain Analogy” With Tangential Map: Size, shape, & location of corneal abnormalities, such as keratoconus, keratoconus can be seen more accurately.
Purpose of the different MAPS (cont.) Going back to Axial Map…
Elevation vs. Irregularity Maps ( or “Orange vs. Melon”) Elevation Map:
Center always above BFS Orange/Red colors Periphery always below BFS Blue colors
• Is it simply steep? • Or is cornea elevated @ this point?
Irregularity Map:
Ectasia • Uses a “Best Fit Ellipsoid” (BFE) model • Matches the natural aspheric cornea better • More sensitive @finding corneal irregularities
----------------------------Unless you have elevation… it is impossible to know!
Purpose of the different MAPS (cont.) MEAN Curvature Map: Your “water filter” for the Axial Curvature Map: Filters out the “noise noise” Removes local (normal) corneal astigmatism That “exposes exposes” underlying local changes in the curvature, revealing pathology such as keratoconus (KCN) Most accurate centrally & peripherally! peripherally “Average” curvature in all directions around one data point
Purpose of the different MAPS (cont.) MEAN Curvature Map: Suppresses nonnon-pathological features (like w/the-ruleastigmatism) while preserving pathological features, such as Keratoconus (KCN)
AXIAL MAP With-the-rule astigmatism pattern
MEAN MAP
Pattern eliminated through local curvature averaging by “Mean” map
Purpose of the different MAPS (cont.) AXIAL curvature map vs. MEAN curvature Map:
How Corneal Topography Helps in Fitting Contact Lenses MasterFit II Contact Lens Software for RGPs!
WithWith-thethe-Rule Astigmatism? (AXIAL map)
…or early Keratoconus (KCN)? (MEAN map)
Common Corneal Disorders as seen via Topography These are just some of the more COMMON DISORDERS you can find with Topography: • With-the-Rule Astigmatism • Against-the-Rule Astigmatism • Irregular Astigmatism • Keratoconus • Pellucid Marginal Degeneration • Corneal Scars • Contact lens (CL CL) induced changes • …AND LOTS MORE!
Helps doctor “visualize” effects of lens parameter changes on the lens fit Simulates fluorescein pattern and tear-film clearances Suggests most appropriate design & type of RGP lens (spherical, toric, aspheric)
Common Corneal Disorders as seen via Topography PathFinderTM II Corneal Analysis Software Checks to see if corneal shape is within normal limits…or not Provides “probabilities” for 5 different corneal conditions by comparing topography exams against an extensive clinical database Can screen “high risk” patients (e.g., keratoconus [KCN] suspects or those that wanting refractive surgery) surgery
PathFinder™ II Software: The Probability View
CORNEAL TOPOGRAPHY SUMMARY & CONCLUSION:
The OBJECTIVES we covered today were to… •Explain what corneal Topography is vs. Keratometry •Identify various “maps maps” used to display topography results •Discuss how topography can help w/clinical diagnosis •…and contact lens fitting LECTURER: M. Patrick COLEMAN, ABOC & COT