Optics: Ant Seg Topography Flashcards by Bella Forster

The corneas refractive power is derived from its ____________ and the difference in refractive index between _______ and ___________

Anterior curvature

Difference in refractive index between air and the cornea

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The cornea is steeper at the ________ and gradually flattens toward the ________

Steeper at the center

Gradually flattens toward the periphery

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To accurately measure total corneal power, we need to measure what 3 parameters?

Anterior corneal curvature
Posterior corneal curvature
Central corneal thickness

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What parameter does the keratometric equation take into account?

The anterior cornea

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How does radius of curvature affect curvature of the cornea?

Larger radius of curvature = smaller curvature
Smaller radius of curvature = larger curvature

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What does corneal mapping provide an assessment of? And for what reason?

Corneal thickness for the diagnosis and treatment of corneal disorders

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The cornea is thinnest at the ________ of the cornea

Apex

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CCT average range

520 - 580 um

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Does corneal thickness change with age?

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Corneal thickness is slightly thicker during what part of the day?

Upon waking up.

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How does corneal thickness change throughout the day?

Becomes slightly higher at night

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How is corneal pachymetry involved with refractive surgery?

Screening for LASIK or PRK
Post-op assessment

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In what ways is Corneal Pachymetry important to understanding ocular health?

Assess endothelial cell damage to cornea
Corneal edema
Viral infections
Diabetes
Thick or thin cornea
Indicator of Ectasia (keratoconus)

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What meridians does Topography measurements assess?

Any meridian, not limited

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Keratometry measurements are only at the center _______ mm

3 mm

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Keratometry can assess curvature and optical power for how many meridians?

2 meridians only

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What are the 3 main Topography techniques?

Placido Disc
Slit Scanning - Orbscan
Scheimpflug imaging

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Limitations to placido disc technique

Affected by tear film
Only provides anterior surface details

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Steeper cornea = __________ mires

Crowded or denser

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Flatter cornea = __________ mires

Sparse

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How does radius of curvature affect dioptric power?

Larger radius of curvature causes a decrease in dioptric power

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A ‘normal’ cornea is _______ at the center and gradually _____________

Steep at the center

Gradually flattens toward

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What is Sim-K?

Simulated Keratometry readings derived from corneal topography data

Axial Power Map: axial curvature is derived by calculating the radius of curvature at __________________________, assuming what type of model?

Each corneal point relative to the optical axis

Assuming a spherical model

Tangential Curvature Power map: radius of curvature is calculated independently at each point without assuming the center lies along the _____________________

Optical axis

Tangential Curvature Power map provides __________ spatial resolution

Higher

Tangential Curvature Power map allows precise localization of ____________ or ____________

Steepening or flattening

With Scanning Slit Technique as seen in OrbScan, what is used to assess the corneal surface?

Multiple complimentary slits

In the Orbscan, 40 slits are projected on the cornea to assess __________ points on each slit

240 points

Scanning slit technique (Orbscan) can analyze what aspects of the corneal surface?

Anterior corneal curvature Posterior corneal curvature Corneal thickness

When an object is not parallel to the image plane, it will not be possible to focus all of the image on a plane parallel to the image plane, thus this may lead to _______________

Image distortion

When a planar subject is not parallel to the image plane, an oblique tangent can be drawn from the image, object and lens planes. The point of intersection is called the _______________

Scheimpflug intersection

What does Scheimpflug imaging do? And why?

Rotates the electronic camera in order to create a sharp image at all depths in the anterior chamber of the eye. ## Footnote Increases depth of focus

Scheimpflug technique: 5 advantages

Anterior curvature Corneal thickness Anterior chamber depth Anterior and posterior float elevation maps Amount of corneal and lenticular opacity (Densitometry)

How does the Galilei imaging system differ from the Sirius and Pentacam?

The Galilei has a dual Scheimpflug

Corneal topography can be used to screen for corneal _________

Ectasia

Corneal topography can help monitor and treat __________ disorders

Corneal

Corneal topography can help manage __________ surgery

Refractive

Corneal topography can help manage ocular surface disorders like _________

Dry eye

4 topography maps

Axial maps/sagittal maps Anterior elevation maps Posterior elevation maps Pachymetry maps

Axial maps/sagittal maps provide a quick overview of ___________ and __________

Corneal power and curvature of cornea

Axial maps/sagittal maps average the data to create a smooth map. How does this affect the accuracy?

Less accurate

Axial maps are ideal for base curvature selection of a _____________________________

Corneal or soft contact lens

Apical astigmatism is a type of corneal astigmatism where the astigmatic effect is greater ______________ than ____________

Centrally than peripherally

Elevation maps - anterior and posterior, display the shape of the cornea by comparing it to a _________________

Computerized best-fit sphere

With elevation maps, how are surfaces elevated from the reference fit sphere noted?

In Plus - steeper

With elevation maps, how are surfaces depressed from the reference fit sphere noted?

Negative sign - flat

Anterior elevation > __________ um Or Posterior elevation > __________ um at the thinnest point may suggest ____________

>than 15 um anteriorly or > than 20 um posteriorly, ectasia

In corneal topography, standard best-fit sphere calculations, which are based on the central 8 mm of the cornea, can be inaccurate in cases of _______________

Ectasia like keratoconus ## Footnote Enhanced best-fit sphere method by excluding central 3 mm data

Pachymetry map is a map of?

Map of the thickness of cornea

Can Placido disc instruments provide a pachymetry map?

On a corneal thickness profile, what is the red line? And what should it align with if values are within normal limits?

The red line is the cornea being tested, it should align with the middle black line if values are within normal limits

Mean anterior Keratometry value of > _______ D indicates abnormal findings

0.3 D

Mean Posterior Keratometry value of > _______ D indicates abnormal findings

0.1 D

Thinnest Pachymetry value of > _______ microns indicates abnormal findings

12 microns

Front elevation at thinnest location value of > _______ microns indicates abnormal findings

2 microns

Back elevation at thinnest location value of > _______ microns indicates abnormal findings

5 microns