CL Pre fit, Ocular measures and anterior imaging

Describe the shape of the cornea

The cornea is aspheric.

It thickens and flattens towards the periphery- they are described as prolate ellipses.

What is the average apical radius of a cornea?

7.90mm Horizontal

7.80mm Vertical

WTR (steeper vertically)

What is the average p value of a cornea?

0.77 Horizontal

0.82 Vertical

What can we use to measure corneal curvature?

A keratometer (most common)

Topography

Wavefront analysis

OCT

Describe the differences between a one position and 2 position keratometer

One position: measures both corneal meridians at once. The image is doubled but the mires are fixed.

2 position: measures each principle meridian separately. Fixed doubling, variable mires location

What are the advantages and disadvantages of 2 position keratometry?

Advantage: doesn’t assume Px has regular astigmatism

Disadvantage: takes longer

What is corneal topography?

Uses black and white rings that use reflections to measure corneal contour.

Describe the types of corneal topography

Absolute/ axial: uses true values and curvature to describe the scale of curvature throughout the cornea.

• Good to assess curvature compared to general population.

• Good for calculating raw curvature values

• Masks detail and subtle corneal differences

Normalised/ Tangential: Intervals are adjusted to fit equally between steepest and flattest radii values for that eye.

• Great for detail, comparison and to calculate astigmatism

• Not possible to compare maps for diff eyes/ sessions

What are the benefits and downsides of corneal topography?

Benefits:

• easier to use

• More detail

• Allows custom + specialist lens

• Detailed corneal tear film quality

• Need for any laser Tx

Downsides:

• more expensive

• Less available

• Specialist product

• Unstable tear film quality→ inaccuracy

• No posterior cornea analysis

Advantages and disadvantages of a videokeratoscope

Advantages:

• Faster and more measurements

• Anterior and posterior measurements

• More accurate

Disadvantages:

• Reduced corneal transparency can affect accuracy

• Tends to overestimate corneal thickness

• Can be fussy with images

What is an OCT?

Allows 3D rendering of cornea

Can estimate thickness of cornea, show limbus, attachments and the cornea/ CL interaction

However, it cannot assess curvature.

What is the average Vertical Palpebral Aperture?

8-10mm

Average Px measurements

• Radius of curvature: 7.86mm (flatter horizontally)

• Diameter: 11.8mm

• Centre thickness: 0.52mm

• Peripheral thickness: 1.00mm

• Pupil size: varies from 2-3mm to 7mm

• VPA: 9.3mm

• Sag: 3.6mm