7. Corneal Topography and Tomography - Advanced Contact Lenses Fall 2026

Describe a modern topographer

Placido ring + video + computer software

What does a keratometer measure?

curvature only at center 3-4mm of the cornea

What does topography provide related to keratometry?

Topography provides tools to evaluate and measure a larger area of the cornea (8-10mm)

What is topography helpful for?

In getting the entire picture of the cornea to fit gas permeable contact lens that are 8-10mm in diameter

What does corneal topography measure?

curvature of the cornea

Does corneal topography measure elevation?

No

More curved cornea = (steeper/flatter) curve

steeper

Less curved cornea = (steeper/flatter) curve

flatter

More curved cornea corresponds to a (larger/smaller) circle

smaller

Less curved cornea corresponds to a (larger/smaller) circle

larger

Radius of curvature can be converted to ____ power

diopter

How to Find the radius power from a given Diopter power?

Radius = 337.5 / diopter power

What are the corneal curvature devices that are on the market?

-Medmont topographer

-OPD

-Pentacam tomographer

Topographers with SMALL CONES are capable of what? What are they better at?

-Capable of collecting more data points

-Better accuracy

What is the downside to topographers with SMALL CONES?

A little more difficult to capture the image

Topographers with LARGE CONES benefit?

Easier to manipulate and capture data

Describe the Medmont Topographer

-Uses placido ring

-Is tear film dependent

On the Medmont Topographer, it is recommended to get ___ scans to ensure reliability (serial topography)

3

OPD is often used for what?

Entrance testing

Can OPD readings be repeated multiple times?

Yes

Do we use OPD readings for BC selection?

No not usually

Describe Pentacam

-Tomography

-Tear film independent

True or False:

One scan on Pentacam is often enough

true

Which of the corneas in the attached image is steeper?

The smaller circle with the smaller radius of curvature will be steeper

True or False:

On corneal topography, there is a color scale to show the relative curvature of the cornea

Colors used are red, orange, yellow, green and blue

true

Red on corneal topography = (steep/flat)

steep

Blue on corneal topography = (steep/flat)

flat

Does corneal topography look at the elevation of the cornea?

NO

What does corneal topography measure?

the curvature of the cornea

What are the types of maps that are available for corneal topography?

-reliability indices

-axial map

-tangential "instantaneous map"

-refractive map

-elevation map

-difference map

Axial, Tangential, and Elevation Map on Medmont Corneal Topography (See Pic)

Axial, Tangential, and Elevation Map on Medmont Corneal Topography (See Pic)

What is the corneal topography axial map showing?

Overall shape of the cornea

What is the appearance of an axial map on corneal topography?

Average of data so it is smooth

The curvature on axial map of corneal topography is relative to what?

optic axis

On the axial map of corneal topography the ____ power is more accurate

central

There is _____ accuracy of axial map in the peripheral cornae

limited

What are corneal topography axial maps good for?

Corneal GP and SCL base curve

Tangential Maps in Corneal Topography are also known as what?

Instantaneous maps

Tangential Maps in Corneal Topography are the most ____ map

sensitive

Describe the corneal power/curvature shown in Tangential Maps of Corneal Topography

Localized corneal power/curvature

The curve of Tangential Maps in Corneal Topography are measured relative to what?

tangent to normal

What are Tangential Maps in Corneal Topography good for?

-diagnosing keratoconus

-orthoK F/Us

What maps of corneal topography are the MOST TRUE representation of corneal shape?

Elevation map

What are Elevation map of corneal topography useful for?

for determining the best lens for an irregular cornea

Based on the elevation map, what can be predicted?

The NaFl pattern

How does topography calculate the elevation map?

Complicated algorithm used to indirectly calculate elevation relative to a best fit sphere

How does tomography calculate the elevation map?

Directly measures elevation, but output is still displayed relative to a best fit sphere

Is refractive power on topography used often?

No

What does refractive power on topography give an interpretation of?

quality of vision through the pupil

When is refractive power on topography most often used?

in pre- and post-refractive surgery procedures

Difference Maps Used in Pre and Post Refractive Surgeries and Ortho K (Pic)

Difference Maps Used in Pre and Post Refractive Surgeries and Ortho K (Pic)

How to use difference maps on topography?

Subtract the most recent power from the baseline power

Shape Factor =

e^2

What is the IS Index on Corneal Topography?

The difference between the avg inferior and superior power

What is the Surface Asymmetry Index (SAI) on Corneal topography?

A measure of the asymmetry of the eye

How is Surface Asymmetry Index (SAI) measured on corneal topography?

Difference in power between corresponding points at the same chord 180º apart

A value of _____ indicates an irregular cornea according to Surface Asymmetry Index (SAI) on corneal topography

>1

What is Surface Regularity Index (SRI) on corneal topography?

A measure of surface regularity

Surface Regularity Index (SRI) on corneal topography will increase with increasing _____

asymmetry

>______ is considered an abnormal Surface Regularity Index (SRI) on corneal topography

>0.8

True or False:

IS Index, SAI, and SRI will flag as yellow or red on corneal topography if they are irregular

true

Good Tear Film on Corneal Topography (Pic)

Good Tear Film on Corneal Topography (Pic)

Poor Mires on Corneal Topography (Pic)

Poor Mires on Corneal Topography (Pic)

Poor Interpretation of the Corneal Surface on Corneal Topography (Pic)

Poor Interpretation of the Corneal Surface on Corneal Topography (Pic)

Can GP lenses be simulated on the eye with corneal topography?

Yes

What is the Scheimpflug Principle?

Removes image distortions with aerial images

What are the Scheimpflug Principle Imaging Devices that are available on the market?

-Pentacam

-Galilei

Scheimpflug Principle (Pic)

Scheimpflug Principle (Pic)

What 5 measurements does the Pentacam take?

-Pachymetry

-Topography

-Corneal Curvature

-Astigmatism Determination

-Scheimpflug Imaging of the Lens

How many images does the Pentacam take / 2s?

50 images

What are the advantages of the Pentacam Tomographer?

Gives maximal depth of focus and minimal image distortion for an obliquely tilted object

Where is the depth of focus on the Pentacam with an eye?

The depth of focus of the device extends from the cornea to the posterior lens

Pentacam Scheimpflug Image (Pic)

Pentacam Scheimpflug Image (Pic)

Anterior Segment Slice with Pentacam (Pic)

Anterior Segment Slice with Pentacam (Pic)

What are the individual maps that can be created with Pentacam?

-Instantaneous (Tangential)

-Axial

-Elevation of the Anterior and Posterior Corneal Surface

Pentacam 4 Map Refractive (Pic)

Pentacam 4 Map Refractive (Pic)

Pentacam 4 Map Refractive Values (Pic)

Pentacam 4 Map Refractive Values (Pic)

True or False:

The Pentacam 4 Map Refractive gives the elevation of the front AND back surface of the cornea

true

Belin/Ambrosio Enhanced Ectasia Printout on Pentacam (Pic)

Belin/Ambrosio Enhanced Ectasia Printout on Pentacam (Pic)

On the Belin/Ambrosio Enhanced Ectasia Map, do you want there to be a lot of difference in elevation between the front and back surface of the cornea?

No

Belin Ambrosio measures based on the ______

Best Fit Sphere

What does the regular BFS measure?

-From central 8mm of the cornea

-Includes all corneal curves in the central 8mm will result in a steeper BFS (yellow curve on the pic)

What does the enhanced BFS measure?

-Removes the 3.5mm surrounding the thinnest part of the cornea

-Removing those steep curves will result in a flatter BFS

BFS vs Enhanced BFS on Anterior and Posterior Corneal Surface (Pic)

BFS vs Enhanced BFS on Anterior and Posterior Corneal Surface (Pic)

Corneal Thickness Spatial Profile (Pic)

**want the corneal thickness to stay w/in the normal range

Corneal Thickness Spatial Profile (Pic)

What is the PIT value on Belin/Ambrosio Enhanced Ectasia Display?

(Mean corneal thickness in the ring - thinnest corneal thickness) / Thinnest corneal thickness

What are the D (differential values) on Pentacam?

-Changes from BFS vs enhanced BFS

-Reported as the standard deviation from the population mean

What are the D (differential values) reported on Pentacam?

-Df (front)

-Db (back)

-Dp (Patch)

-Dt (thinnest value)

-Da (thinnest displacement)

True or False:

D values will be in yellow or red with irregualrity and compared to normative database values

true

Pentacam Display (Pic)

Pentacam Display (Pic)

Keratoconus Grading Scales on Pentacam (Pic)

Keratoconus Grading Scales on Pentacam (Pic)

Keratoconus Grading Scales on Pentacam (Pic)

**Grade 3-4+ on this display

Keratoconus Grading Scales on Pentacam (Pic)

KISA% > ______ indicates keratoconus on Pentacam

100

Progression of keratoconus is defined as what?

a change in 2 of the following: anterior corneal surface, posterior corneal surface, thinnest patch

Can OCT be used for tomography?

Yes

What is special about using OCT for tomography?

Can get epithelial thickness w/ better resolution

OCT (Pic)

OCT (Pic)

Pentacam Subtle Diagnosis (Pic)

Pentacam Subtle Diagnosis (Pic)

What to do if no Pentacam is available?

Can use subjective VA, topography, or aberrometer

True or False:

Topography is unable to detect any surface irregularity

true