3.1.3: Use instrument to measure corneal curvature & assess regularity

Question 1

Which area of cornea mesured using B&L and JS keratometer?

Answer 1

Central 3.0mm

Question 2

How much does the difference in K’s equate to?

Answer 2

0.25DC for every 0.05mm difference
0.50DC for every 0.1mm difference

Question 3

What is the main use of keratometer?

Answer 3

determine central corneal radii (mm) & power (D) and principal meridians of cornea

Question 4

What can keratometer be used for?

Answer 4

Differentiate regular & irregular astigmatism
Observe quality of reflected mires
Measure NIBUT
Measure BOZR of rigid lens
Select most appropriate lens based on how steep cornea is

Question 5

What is the disadvantage of the B&L keratometer?

Answer 5

Cannot assess irregular astigmatism on it e.g. due to keratoconus, pterygium or corneal scarring/surgery

Question 6

What is with-the-rule astigmatism and what is against-the-rule astigmatism?

Answer 6

With-the-rule: steeper meridian along 90 deg (or close to)
Against-the-rule: steeper meridian along 180 deg (or close to)

Question 7

What are the other uses of keratometer?

Answer 7

Relate corneal astigmatism to spec Rx to determine lenticular astigmatism
Help to decide on design of 1st choice lens i.e. spherical, toric, back surface toric, front surface toric, bi-toric