W3 Corneal Topography for CL Fitting

How much of the optical power of the eye is the cornea?

2/3

What does topography depend on?

• Surface curvature

• Surface regularity

What does the radius of the cornea vary from?

7.4 to 8.8mm

How is the cornea divided?

• Central zone(4.5mm of constant curvature diameter)

• Peripheral zone (longer diameter than central)

Do the 2 eyes differ?

They’re very similar- almost a mirror image

What are measures of a-sphericity?

• e - eccentricity which is the rate of deviation of the curvature from a circle

• Shape factor - p

• Asphericity parameter - Q

What is the average e2?

0.22 +- 0.15

What is the perfect circle?

E2 is not asphericitc at all, equal to 0

What is an ellipse oblate circle?

E2 is less than 0

What is ellipse prolate?

E2 is less than 1 and more than 0

What happens when e2 becomes +1?

It becomes parabola

What happens when e2 becomes more than +1?

Becomes hyperbola

How has corneal curvature been assessed in the past?

• Placido used discs with black and white rings in 1880’s to assess irregularities on the cornea

• Gullstrand photographed and analysed keratoscope images

What is the placido disc?

• Klein keratoscope

• +6D lens in centre

• Internally illuminated

• Purely qualitative

How can we alternatively measure the shape of the cornea?

• Inspect image formed by cornea acting as a convex mirror

• Done via placido disc, Gullstrand’s and ophthalmometry

• ALTERNATIVELY, you can create a profile with a slit of light

How is correct working distance monitored?

Monitored through Schneider disc or similar system

• Double image unless exactly aligned which must focus on eyepieces

• Axis orientation achieved by rotating mirrors until horizontal limbs of mirror are coincident

What are Javal-Schiotz mires keratometers?

• Has 2 refractive points evaluated as 2 pairs to determine radius of curvature in that meridian

• 2 meridians at 90 degrees to each other determined

• Oranges and purple should only barely touch in the periphery

What do we assume about keratometry?

• Assume spherical arc- uniform

• Orthogonal axes (lines that intersect each other at right angles)

• Back surface of cornea has less than 10% power of front surface power

What are errors when operating keratometry?

• Focusing

• Proximal accommodation

• Misalignment

• Orientation of instrument

What are px errors when operating keratometry?

• Corneal distortion causes mire distortion

• Fixation steadiness- unable to focus eye

What are uses of keratometry?

• Baseline data

• Measure degree of corneal astigmatism

• Measure and select lens choice

• Lens flexure- when px blinks see if keratometry reading differs following blink- if px blinks see if lens bends

• Post wear oedema/corneal moulding

• Contact lens verification

What can we do with video keratometry?

Measures 7-9mm area, various types of display possible, repeatability must be assessed

What are all the inaccuracies with video keratometry?

• Image from tears reflects light differently

• Dry eyes need drops which affect dispense

• Corneal disease

• Less coverage and more shadows

• Astigmatism more than 1D affects alignment

Where should the eye by aligned?

Centre of entrance pupil most ideal but VK directed towards centre of curvature

What does a corneal map indicate?

• Warm colours correspond to steeper corneal radius

• Cooler colours correspond to flatter radius

• In astigmatism bow tie shape is seen- becomes narrower and more obvious

What’s a corneal map used for?

• Corneal maps show corneal curvature and power in 2D image. Useful diagnosis and monitoring corneal conditions and assessing changes in corneal power changed by conditions. Corneal maps larger and more useful for viewing central detail

• Less sensitive for measuring corneal curvature

Whats a tangential map used for?

• more accurate than corneal maps

• Easier to detect localised changes in cornea

• Smaller and better for showing true shape of cornea

• Used to select best contact kens design for irregular cornea

Why is a corneal map good for laser refractive surgery?

• Map denotes the distance of corneal surface

• Used for post operative refractive corrections

Demonstrate irregular vs asymmetrical astigmatism.

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