05 Cornea

How to clinically measure the corneal surfaces?

Keratometry

Corneal Topography (placido disc, mire)

Corneal Tomography (slit scan, OCT)

Corneal topography definition

Relates to the anterior surface shape only. Uses P1.

Corneal tomography definition

3D structures (anterior/posterior cornea & anterior lens surface)

5 clinical uses of corneal topography

Corneal optics

Contact lens fitting

Refractive surgery

Cataract surgery

Diagnosis & monitoring of keratoconus, pellucid marginal degeneration, pterygium & dry eye

Clinical use of corneal topography on corneal optics

Power (sphere & cyl) & irregularities (aberration)

Clinical use of corneal topography on contact lens fitting

Rigid Gas Permeable (RGP), scleral, ortho-K

Clinical use of corneal topography on refractive surgery

topography guided laser vision correction

Clinical use of corneal topography on cataract surgery

IOL power calculations & optimal selection of premium IOLs

4 research uses of corneal topography

Relative contribution of corneal aberration to optical quality of the eye

Refractive surgery induced spherical aberration

Characterizing corneal pathologies (KC, PK)

Importance of posterior corneal surface in KC

Principles of Reflection Keratometery and Corneal topography the image is formed by a….

convex mirror

Keratometric formula

Note this one measures radius of curvature & not dioptric power

Corneal first-surface topographers cannot directly measure ______ of the cornea.

dioptric

What do corneal topographers estimate? What don’t they estimate?

Estimate the radius of the corneal first surface at numerous locations using set of assumptions

Those measurements tell us nothing about the back corneal surface nor thickness

What’re the 4 assumptions of corneal topography

Small angle approximation = less reliable in peripheral mapping

Tear film thickness is uniform = dry eye?

Perfect spherical surface as reference = “prolate” human cornea

Perfect alignment & fixation = astigmatism error

What quantifies tear film breakup time (TBUT)?

Tear surface topography

The keratometer and corneal topographers to express their measurements in diopters of corneal power assumes an _________ of the cornea to account for the ________.

artificial value for the index, 1.3375 (“keratometric refractive index”)

posterior corneal surface power

Keratometer and corneal topographers equation with the artificial index of refraction

Total corneal power for “normal eye” is…

43 D

Assuming the keratometric refractive index can lead to serious errors in some clinical cases like….

Post-Refractive surgery cornea: anterior curvature different from normal

Cataract surgery: IOL power calculation (SPH & CYL)

Pathological cornea: Keratoconus (irregular astigmatism or higher order aberrations)

How to fix the clinical error arising from assuming the keratometric refractive index?

Measuring both surfaces directly & optical ray tracing to estimate true corneal power

Corneal topography measure the…

shape of the surface

mire in keratomoeters & videokeratoscopes

What is used as objects in keratometers & videokeratoscopes?

(mires) Placido or keratocopic disc is a series of illuminated concentric circles that are imaged by the cornea acting as convex mirror.

As in standard keratometers, what is measured vs. calculated?

size of corneal image is measured & the radius of curviture is calculated

Manual keratometer (3 characteristics)

Keratometry vs. Corneal Topography

Keratometry samples 2 points in each two meridians (total of 4 points) & estimates the best spherical surface through these two points

Corneal topography samples thousands of points allowing the reconstruction of the corneal surface

Keratoscopy (Placido-disk or mires) categories! (manual/automated & qualitative/quantitative)