PC - Assessment of Corneal Curvature - Week 1

Question 1

List 7 uses for measuring corneal curvature.

Answer 1

Screening tool
Contact lens fitting
Stability of the cornea/tear flim
Diagnosis/monitoring of corneal pathology
Gold standard for pre/post refractive surgery evaluation
Estimating astigmatism
IOL determination

Question 2

What is the most common instrument to measure corneal curvature, and what surface curvature does it measure?

Answer 2

Keratometer, which measures the anterior surface curvature of the lens.

Question 3

List the dioptres that classify as the following:
Flat cornea
Typical cornea
Steep cornea

Answer 3

Flat - <41.00D
Typical - 43.00-44.00D
Steep - >46.00D

Question 4

Describe the clinical procedure to using a keratometer.

Answer 4

When looking through the eyepiece, notice the circle with a plus inside it.
Have it overlap exactly with the bottom-right circle.
Align the plusses of the bottom-right circle with the plusses of the bottom left circle.
Have the minuses of the bottom-right circle overlap with the minuses of the top-right circle.

Question 5

Give the template for a keratometer recording, and an example.

Answer 5

Corneal power (D) curvature along the two principle meridia (mm) @ power axis

42. 00D (7.75mm) @ 175 H
43. 75D (7.60mm) @ 85 V

Question 6

Describe a limitation of keratometers in terms of refractive indices.

Answer 6

Modern keratometry assumes a refractive index of 1.3375

The tear flim is 1.336 and the cornea is 1.376

Question 7

Describe a limitation of keratometers in terms of corneal shape.

Answer 7

Modern keratometry assumes the cornea is spherical or toric.

Question 8

Describe a limitation of keratometers in terms of symmetry.

Answer 8

Modern keratometry assumes the cornea is symmetrical.

Question 9

Describe a limitation of keratometers in terms of where it measures curvature.

Answer 9

Modern keratometry only measures the central 3mm in two principle meridians, 6% of the cornea.
The peripheral cornea is not measured.

Question 10

Describe a limitation of keratometers in terms of irregularly shaped corneas.

Answer 10

Modern keratometry has limited capacity to measure and monitor irregular corneal surfaces and/or
decentred corneal apex.

Question 11

Describe how a keratoscope works.

Answer 11

A placido disc is used, which is a disc with concentric rings (bullseye). A hole in the centre allows a practitioner to view the patient’s eye.
The reflection of the rings can be seen in the patient’s eye. Local size of the reflected image is used to infer curvature.

Question 12

Describe videokeratoscopy.

Answer 12

A series of illuminated annular rings are projected onto the cornea.
Reflection picked up by a digital camera and analysed.
Anterior corneal slope is measured and curvature computed.
The shape (elevations) is computed indirectly.

Question 13

Describe a limitation of placido disc-based keratometry in terms of reference points.

Answer 13

Changes in the reference point or viewing angle results in changes to the curvature.

Question 14

Describe a limitation of placido disc-based keratometry in terms of what it is able to measure.

Answer 14

It can only measure the anterior surface curvature, and not posterior.

Question 15

Can placido disc-based keratomertry be used to assess corneal surface curvature for LASIK outcomes?

Answer 15

No, its not good enough.

Question 16

Consider elevation-based keratometry. What three additional points of information does it provide?

Answer 16

Front and back surfaces of the cornea
Corneal thickness
Structure of the anterior chamber

Question 17

Is there any difference between shape determined by curvature maps and elevation maps? Is it important?

Answer 17

Yes, but it is clinically important only in unusual or difficult cases.

Question 18

What keratometry method represents the true shape of the cornea?

Answer 18

Elevation based keratometry.

Question 19

Describe the Scheimpflug principle, and the system it applies to.

Answer 19

Used for pentacams.
The law states that to achieve a greater depth of focus, move the three planes, provided that the object plane, the lens plane, and the image plane cut each other in one line or one point of intersection.

Question 20

What should be done first, topography or tonometry?

Answer 20

Topography

Question 21

How many maps should be generated per eye as a baseline?

Answer 21

4 at least.

Question 22

What is needed for contact lens wearers who require topography mapping?

Answer 22

Contact lens wear must be discontinued for 2 weeks prior.