PC - Assessment of Corneal Curvature - Week 1 Flashcards
List 7 uses for measuring corneal curvature.
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
What is the most common instrument to measure corneal curvature, and what surface curvature does it measure?
Keratometer, which measures the anterior surface curvature of the lens.
List the dioptres that classify as the following:
Flat cornea
Typical cornea
Steep cornea
Flat - <41.00D
Typical - 43.00-44.00D
Steep - >46.00D
Describe the clinical procedure to using a keratometer.
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.
Give the template for a keratometer recording, and an example.
Corneal power (D) curvature along the two principle meridia (mm) @ power axis
- 00D (7.75mm) @ 175 H
- 75D (7.60mm) @ 85 V
Describe a limitation of keratometers in terms of refractive indices.
Modern keratometry assumes a refractive index of 1.3375
The tear flim is 1.336 and the cornea is 1.376
Describe a limitation of keratometers in terms of corneal shape.
Modern keratometry assumes the cornea is spherical or toric.
Describe a limitation of keratometers in terms of symmetry.
Modern keratometry assumes the cornea is symmetrical.
Describe a limitation of keratometers in terms of where it measures curvature.
Modern keratometry only measures the central 3mm in two principle meridians, 6% of the cornea.
The peripheral cornea is not measured.
Describe a limitation of keratometers in terms of irregularly shaped corneas.
Modern keratometry has limited capacity to measure and monitor irregular corneal surfaces and/or
decentred corneal apex.
Describe how a keratoscope works.
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.
Describe videokeratoscopy.
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.
Describe a limitation of placido disc-based keratometry in terms of reference points.
Changes in the reference point or viewing angle results in changes to the curvature.
Describe a limitation of placido disc-based keratometry in terms of what it is able to measure.
It can only measure the anterior surface curvature, and not posterior.
Can placido disc-based keratomertry be used to assess corneal surface curvature for LASIK outcomes?
No, its not good enough.
Consider elevation-based keratometry. What three additional points of information does it provide?
Front and back surfaces of the cornea
Corneal thickness
Structure of the anterior chamber
Is there any difference between shape determined by curvature maps and elevation maps? Is it important?
Yes, but it is clinically important only in unusual or difficult cases.
What keratometry method represents the true shape of the cornea?
Elevation based keratometry.
Describe the Scheimpflug principle, and the system it applies to.
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.
What should be done first, topography or tonometry?
Topography
How many maps should be generated per eye as a baseline?
4 at least.
What is needed for contact lens wearers who require topography mapping?
Contact lens wear must be discontinued for 2 weeks prior.
