Lecture 1 keratometry

Question 1

What is keratometry?

Answer 1

The measurement of the principal radii of the anterior surface of the cornea (tear film)

Question 2

Why do we do keratometry?

Answer 2

*To determine the shape of the cornea

*To determine the central corneal radii (mm) (can be converted into power) of the two principal meridians of the cornea

*Used to differentiate between regular (meridians are 90 degrees from each other) and irregular astigmatism

*To observe the quality of reflected mires (light is reflected from cornea which can tell you quality of tears, shape of cornea and give indication of keratoconus

*Measure NIBUT (non-invasive break up time)- can measure how long it takes for tears to evaporate

*Measurement of BOZR (back optic zone radius) of a contact lens

Question 3

What are the principles of keratometry?

Answer 3

*Cornea (tears) acts as a mirror from which light is reflected
*We are measuring the radius of curvature of this ‘mirror’
*Uses 1st purkinje image

Size of image depends on:
-size of the object (mires)
-radius of curvature of cornea
- distance between mires and cornea

*Image is formed within the cornea

Question 4

How does variable image doubling work?

Answer 4

*One mire
*Prism in the instrument is used to double the image horizontally and vertically
*When measuring the curvature in the horizontal meridian, we are trying to make the line between actual mire and horizontally doubled mire touch
*Same can be done in vertical meridian

Question 5

What are the types of keratometers?

Answer 5

*ONE POSITION:
-Referred to as the Bausch and Lomb
*Can measure both meridians at the same time so called one position keratometer
*Mire separation is fixed but image doubling system is variable

TWO POSITION:
*Referred to as Javal-Schiotz
*Only one meridian can be measured at one time
*Can measure each meridian independently so can measure irregular corneal astigmatism
*There is a internal fixation light/ mirror for px to fixate so will help align
*Has an occluder in it as well
*You decide on the correct distance from eye (so staircase and block is single and clear

Question 6

What are the limitations of keratometry?

Answer 6

*Only curvature of central cornea is assessed
*No peripheral info so no idea about shape of cornea
*Instrument inaccuracy means the 0.05 mm changes in k aren’t clinically significant (hard to compare overtime)
*K readings may be dependent on the instrument used due to calibration differences (must use same one if comparing to previous results)
*CL fitting can only be assessed by placing lens on eye (using diagnostic lenses)