Deck #4: Corneal Topography

List the clinical uses of Corneal Topography.

• determine:

  • Refraction

  • Corneal curvature

  • Amount and direction of corneal astigmatism

• Progressive myopia

• High ametropia

• Trauma

• Detection & evaluation of irregular corneas

• Monitoring of corneal disorders

• CL fitting

• Evaluate surgical candidacy & treatment plans

What are the corneal zones?

Central - 3-4mm zone (1er refracting zone of the cornea)

Paracentral / Peripheral - 7-8mm zone

Paralimbal - 11mm zone

Limbal - 12mm zone

What is the average dioptric power of the cornea?

43.00-44.00D

What index of refraction is used for calculations involving the tear film vs cornea?

Tear film = 1.3375

Cornea = 1.376

How does corneal curvature relate to power?

More curvature (steeper) = more power

Less curvature (flatter) = less power

How does corneal curvature differ from center to periphery?

Center = more power (steeper)

Periphery = less power (flatter)

What corneal area does standard keratometry measure?

over the central 3mm

What is the central 4mm considered to be?

Spherical → gets aspheric/flatter peripherally

What the difference between Prolate and Oblate corneas?

Prolate – central steep, peripheraL flat

Oblate – central flat, peripheral steep

What is enantiomorphism in corneal topography?

concept that OD & OS are mirror images and non-superimposable

What is Meridional specific asphericity?

nasal cornea is flatter than temporal due to recti muscle insertion

What’s the difference between Regular vs Irregular astigmatism?

Regular =  Principle meridians are 90° apart (WTR, ATR, Oblique)

Irregular = Principle meridians are NOT 90° apart

What is WTR, ATR & Oblique?

WTR

• more power vertically

• spectacle Rx = near 180°±30°

.

ATR

• more power horizontally

• spectacle Rx near 90°±30°

.

Oblique

• Principle meridians are ±15° of 45° & 135°

What astigmatism is this?

WTR

What astigmatism is this?

ATR

What astigmatism is this?

Oblique

Define Topography.

→ 2D representation of elevation & depression of the surface

• only images the front surface of the cornea

Provide examples of Topography.

• Reflection based topography

  • Keratometry

  • Placido disc (Photokeratoscope, Videokeratoscope)

• Wavefront sensors

Define Tomography.

 → 3D build up of a tissue & its layers

• can create images of multiple surfaces within the eye

  • Anterior cornea

  • Posterior cornea

  • Lens

Provide examples of Tomography.

• Scanning slit

  • Orbscan

• Scheimpflug photography

  • Pentacam

Note: tomographers also produce topography images.

Placido disk based topographers provide [2D/3D] representation.

Placido disk based topographers provide 2D representation.

Slit based units (Orbcan, Pentacam) provide _______ & _______.  

Slit based units (Orbcan, Pentacam) provide topography & tomography.  

What is Reflection based Corneal Topogrpahy?

• Over ⅔ of the refractive power of the eye comes from the air-tear interface at the anterior corneal surface

• Object is reflected off an area of the cornea with known dimension & distance from a light source

• Virtual upright image is produced in the anterior chamber

• Ratio of object to image diameters is used to estimate the radius of curvature of the cornea

Examples: Keratometry & Placido disk

What is Keratometry?

• Quantitative measure

• Rotate optics to align with principle meridians

• Adjust power to adjust for distance of rings from each other

  • Distance changes as a function of the size of the rings in relation to corneal curvature

• ONLY 4 data points are generated

  • 2 on the steepest axis

  • 2 on the flattest axis that is 90° away

List the limitations to Keratometry.

• Visible iris diameter is ~11.5mm

• Only measures the central 3mm (Average pupil range 2-5mm)

  • larger pupil will get more light irregularities

What information is recorded during Keratometry?

• Power

• Toricity (corneal astigmatism)

• Horiz@meridian / Vert@meridian

• Amount, type, mires

What is Placido Disk?

• Qualitative

• Device made of concentric rings on a background of a different color that is reflected off the corneal surface

• Photokeratoscope -12 mires over 70% of corneal surface

• Videokeratoscope - 15-38 mires over 95% of corneal surface

How does a steep/cornea appear on a Videokeratoscope?

Steep cornea – narrower rings, closer together

Flatter cornea – broader rings, further apart

How does a regular vs irregular astigmatism appear on Videokeratoscope?

Regular astigmatism – elliptical mires

Irregular astigmatism – non-elliptical, distorted mires

What is the theoretical range of videokeratoscopes?

8-110 D (avg cornea is 44D)

• 110 D = steep

List the types of Videokeratoscope.

1. Atlas 9000

2. Medmont E300

3. Keratron

4. Topographic modeling systems

What is the Atlas 900?

→  Large cone (22 rings – prevents ring overlap)

• Power range: 15-95 D

What is the Medmonth E300?

• 32 Rings

• Power Range: 10 - 100 D

• can capture composite images (covering 0.25mm to 14mm)

• better visualizes the curvature out to sclera

• tear film analysis (Tear film surface quality, Tear break up time)

What is a Orbscan? What does it measure?

→ Uses both a scanning slit & Placido disk

• Measures:

  • Topography

  • CT

  • Posterior corneal elevation (float)

• Shows tomography

• uses 40 slits (20 from nasal, 20 from temporal) projected on the cornea to assess 240 points on each slit

• Triangulation between the reference slit beam surface & reflected beam captured by the camera can be used to analyze the anterior & posterior corneal curvature & pachymetry

What is a Pentacam?

• Allows maximized depth of field of an object tilted obliquely relative to the axis of the lens system

• Anterior seg of the eye in focus from cornea to lens

• 3D assessment of anterior chamber structures

  • Limbus to limbus measurement

  • Measures lens data

• Scheimpflug camera measures 25,000 data points

  • CT, anterior & posterior corneal curvature & elevation

  • Anterior chamber depth & volume

  • Angle measurement in multiple positions

    

What is the principle behind Pentacam?

Scheimpflug principle: Describes the plane of focus of an optical system when the lens plane is not parallel to the image plane

What colors represent flat, steep, average surfaces?

Cool colors (Blue, purple, black) = flat

Warm colors (Red, orange) = steep

Normal colors (green, yellow) = average

What colors represent high vs low power?

Warm colors (Deep red, Red, orange, yellow) = high power

Cool colors (Deep green, Blue, purple) = low power

What colors represent thickness?

Warm colors (Deep red, Red, orange, yellow) = ↓ CT

Cool colors (Deep green, Blue, purple) = ↑ CT

What’s the difference between Absolute scale vs Normalized scale?

Absolute scale

→ used in routine practice / screenings

• may not notice minute details

• 35-50D: each color represents 1.5D interval

• <35 or >50D: each color = >5D

.

Normalized scale

→ Scale changes based on the most appropriate colors for “that eye”

• shows more minute details

• hard to make quick comparisons

Define Axial Curvature.

→ Length of line drawn from normal to corneal surface to optical axis

• Sagittal curvature

  • Averages data between rings

  • Radius = distance from corneal surface to optical axis

  • assumes spherical cornea (Less accurate in aspheric periphery)

• doesn’t capture flat periphery

• Topography represented by 23 colors in 0.50D steps

• Corneal power map

Define Tangential Curvature. What is it best vs poor indicator of?

• Instantaneous curvature

• Radius of curvature at each data point is true local curvature (uses the “best” sphere for every aspect of the cornea)

• Tangents projected outward from center vertex

• More accurately represents corneal curvature compared to axial maps

  • Best indicator = corneal shape

  • Poor indicator = corneal power

What type of scan is this?

Axial scan

What type of scan is this?

Tangential scan

What type of scan is this?

Axial scan

What type of scan is this?

Tangential scan

What do elevation maps measure?

→ height difference corneal curvature & reference shape fit to cornea

• can be done for both anterior & posterior cornea surface Anterior or posterior elevation – float

• describes the surface figure of the cornea better than sagittal & tangential displays

• “starting point” for analyzing aberrations of the corneal surface

• displayed relatively & is signed (+/-)

  • (+) – cornea is higher (steeper) than reference curve

  • (-) – cornea is below (flatter) than reference curve

What colors represent elevation?

Warmer colors (Deep red, Red, orange, yellow) = above reference sphere

Reference plane = Green

Cooler colors (Deep green, Blue, purple) = lower than reference sphere

What map is this?

Axial Map (curvature)

Note: Orientation looks opposite depending on whether the scan is looking at curvature or height

• more curved corneal surface is lower than the less curved surface

What map is this?

Elevation (height) map

Note: Orientation looks opposite depending on whether the scan is looking at curvature or height

• more curved corneal surface is lower than the less curved surface

What is the Mean curvature?

→ Average curvature of 2 principal meridians for any given corneal point

• Average astigmatism (takes local average curvature at each point)

• Suppresses non-pathological features (Corneal astigmatism)

• Preserves pathological features (Keratoconus)

• helps detect subtle corneal-shape abnormalities

What is Ring image? List 4 things its useful for.

→ actual ring map during measurement

• Raw data

.

Useful for:

• Observing tear film quality

• Horizontal visible iris diameter

• Pupil measurement

• Geometric offset from the pupil can be displayed

What is Snell’s law map of power useful for? What index does it use?

(+) Accounts for SA (unlike axial map)

• Refractive powers may differ from axial view, especially in the periphery

(+) Helps determine optic zone in RGP lenses or refractive surgery

n = 1.3375

What is True net power map useful for? What index does it use?

→ Optical power of the total cornea (front + back) using sagittal curvature values

• Anterior = 1.376

• Posterior = 1.336

What are the different zones assessed in a keratometry view?

Central = 0-3 mm from corneal center

Mid-periphery = 3-6 mm

Periphery = 6-9 mm

What is a Sim-K value and what information does it provide?

→ similar to a standard keratometry reading, where it “assumes the principal meridians are 90° apart”

• Shows corneal curvature in the two main meridians:

  • Red = Steepest meridian

  • Blue = Flattest meridian

• Eccentricity values are displayed at meridians: 0°, 10°, 15°, 25°, 30°

What is Eccentricity (e)?

→ Rate at which the corneal curvature changes center to periphery (0-1)

• ↓ e-value = gradual curvature changes (oblate)

• ↑ e-values = abrupt curvature changes (prolate)

What is Shape factor?

Shape factor = e²

(+) SF = prolate

(-) SF = oblate

What is the eccentricity (e) value for average vs prolate corneas?

Average ≈ 0.55

Prolate ≈ 0.2 - 0.5

What are the normal, borderline and abnormal Shape factor (SF) values?

Normal = 0.13 to 0.35

Borderline = 0.02 to 0.12 & 0.36 to 0.46

Abnormal = -1.0 to 0.01 & 0.47 to 1.0

What is mean toric keratometry (TKM)?

• Uses elevation data to compare toric reference to actual cornea

• Provides the most accurate toric representation of a pts cornea

What are the normal, borderline and abnormal mean toric keratometry (TKM) values?

Normal = 43.10D to 45.90D

Borderline = 41.80D to 43.00D & 46.00D to 47.20D

Abnormal = 36.00D to 41.70D & 47.30D to 60.00D

What is the Irregularity Map?

→ Displays corneal-surface irregularities as wavefront errors (scaled in x, y, & z axes)

• uses a toric ellipsoid (non-spherical) reference shape to account for toricity & asphericity

• identifies which depressions or elevations CAN’T be fit by a more realistic surface

• quantifies areas of distortion

What is the corneal irregularity measurement (CIM)?

→ Number that represents the irregularity of the corneal surface

• predict irregular astigmatism or visual distortions

• ↑ CIM = harder to fit CLs

  • indicate ocular pathology

What are the normal, borderline and abnormal corneal irregularity measurement (CIM) values?

Normal = 0.03µ to 0.68µ

Borderline = 0.69µ to 1.0µ

Abnormal = 1.1µ to 5.0µ

What is the Pachymetry?

→ Measure of local corneal thickness

• Cooler colors = thicker cornea (thicker more periphery)

• Warmer colors = thinner cornea (thinner more centrally)

What can be measured using crystalline lens imaging?

→ can measure lens thickness & densitometry

• useful in pt with dense cataract or corneal opacities

What maps make up the 4 Map Refractive?

1. Axial map

2. Anterior float (elevation)

3. Pachymetry

4. Posterior float (elevation)

What are artifiacts?

• Shadows on the cornea from long or aberrant lashes

• Ptosis, blink or small fissure width

• Irregularities in the tear film

• Poor working distance

• Misalignment of the instrument

Note: There’s false artifacts in any imaging technique, which give us a false interpretation

What are the different Topographic patterns?

Regular patterns

• Round

• Oval

• Steepening – inferior or superior

.

Astigmatic patterns

• Symmetrical

  • With or without skewed axis

• Asymmetrical & orthogonal

  • With superior steepening

  • With inferior steepening

  • Bow-tie with skewed axis

• Irregular

  • anything thats not a circle, bowtie or steeping

How does the “normal” cornea change with age?

→ Small changes in curvature occur throughout life

Infancy - spherical

Childhood / adolescence - WTR astig

Adulthood - spherical

Late adulthood - ATR astig

What is Bow-tie/Dumb bell?

• Indicate astigmatism

  • Direction determines WTR / ATR / oblique

  • Look for symmetry

    • should be a roughly the same size in the same meridian

    • Irregular patterns look tilted or with one side larger than the other

What is Keratoconus?

→ Ectacic area of cornea, usually in inferior nasal position

• Nipple cone

• Oval cone

• Globus cone

What is Pellucid marginal degeneration?

• Steeper inferior cornea than superior cornea

• show high ATR / irregular astigmatism

• Kissing doves / lobster claw pattern

What disease does this indicate?

Kerataconus

What disease does this indicate?

Pellucid Marginal degeneration

What disease does this indicate?

Corneal warpage

What corneal error does a Pterygium induce?

• induces WTR astigmatism

• May have area not mapped by instrument due to steepening

What factors determine the effect of trauma or surgery on the cornea?

• Location

• Severity / extent & depth of injury

• Size

• Type of trauma

How does corneal trauma (ex: laceration) affect corneal curvature?

Flattening = laceration meridian

Steepening = opposite meridian

Identify this pattern.

Regular WTR astig

• symmetric bowtie

• no skew

Identify this pattern.

Regular ATR astig

• symmetric bowtie

• no skew

Identify this pattern.

Spherical cornea

Identify this pattern.

Inferior steepening (indicative of kerataconus)

Identify this pattern.

Irregular astigmatism (granular dystophy)

Identify this pattern.

Irregular ATR astig

Cause: pellucid marginal degeneration

Identify this pattern.

Axial scan - spherical center (with center flattening)

Anterior float - central depression

Posterior float - normal corneal profile

pachy - central thinning

Cause: Post-lasik (for Lasik, posterior float doesn’t get affected)

Identify this pattern.

Axial scan - center flattening

Anterior float - central depression

Posterior float - central depression

Pachy - irregular thickness distribution

Cause: post corneal transplant

Identify this pattern.

Axial map / Curvature map for Kerataconus

What does it mean if a patient reports an image as being “smudgy”?

uncorrected regular or irregular astigmatism