22. How to: Keratometry

Question 1

3 reasons why corneal curvature is measured?

Answer 1

1. Allows selection of appropriate radius of curvature for RGP CL.
2. Gives measure of the amount of corneal astigmatism.
3. Baseline measurement to compare any future changes.

Question 2

Describe the shape of the cornea?

Answer 2

Spherical at the center, aspherical on the periphery.

Question 3

What is meant by an regular cornea in terms of meridians?

Answer 3

2 meridians typically 90 degrees apart (regular astigmatism)

Question 4

How is keratometry results used to compare spherical vs Toric central cornea?

Answer 4

r1 and r2 values used.
Great difference between r1 and r2 values is noted. One value will show the flattest curve, one value will show the steepest curve.

Question 5

Regular astigmatism meaning?

Answer 5

Two principle meridians 90 degrees to each other

Question 6

Irregular astigmatism ?

Answer 6

Two principle meridians that are not 90 degrees to each other

Question 7

With-the-rule-astigmatism?

Answer 7

Ks steep in vertical meridian, the refractive error is greater in the vertical meridian (cyl axis 180 +/-30)

Question 8

Against the rule astigmatism?

Answer 8

Ks steeper in the horizontal meridian. The refractive error is greater in the horizontal meridian (Cyl axis 90 +/- 30)

Question 9

Oblique astigmatism?

Answer 9

Two principle meridians are more than 30 degrees from the horizontal or vertical. (the flatter and steeper curves are closer to 45 and 135)

Question 10

When given 2 K values, how is the flattest and steepest K values identified?

Answer 10

Flattest K = biggest number
Steepest K = smaller number

Question 11

Keratometry takes what measurement?

Answer 11

Only measuring central 3-4mm.
Gives a starting point to select radius of curvature for RGP lenses.

Question 12

Why is keratometry not used to select soft lens fitting?

Answer 12

Keratometry can not indicate how periphery of the lens will fit.

Question 13

2 main types of keratometry in the clinic?

Answer 13

One position: Measures both meridians simultaneously.
Meridians fixed at 90 degrees from one another.
Two position: Measures each meridian separately.
Meridians not fixed at 90 degrees from one another.
Hence, one step not useful in Px’s with irregular astigmatism.

Question 14

Is keratometry used in RGP fitting?

Answer 14

yes

Question 15

Is measure of corneal curvature useful for soft lenses fitting?

Answer 15

No- only useful for RGP lenses fitting

Question 16

3 causes of irregular curvature of cornea?

Answer 16

1. Keratoconus
2. Laceration- affects cornea
3. Trauma- could be penetrative

Question 17

Why is fixation important when using keratometry ?

Answer 17

Need good fixation or it affects results