Lesson 15 - Keratometry and Corneal Topography

Question 1

Rotating grip

Answer 1

Used to determine the axis

Question 2

Horizontal and vertical measuring drums

Answer 2

Used for measuring horizontal and vertical corneal curvatures

Question 3

Contact lenses are usually fit to this curvature of the cornea.

Answer 3

Flattest

Question 4

What is a keratometer? What does it help determine?

Answer 4

Measures the front surface curvature of the cornea by providing the radii of curvature, or the flattest and the steepest curves. It is also used to determine the axis of corneal astigmatism. These measurements are used to help determine the initial trial contacts (diagnostic fitting) when fitting contact lenses, to determine if the cornea has any irregularities before refractive surgery, if there are any corneal disruptions caused by disease, and to look at the surface of the cornea of patients with dry eye. There is a manual keratometer and an automated version.

Question 5

Corneal astigmatism

Answer 5

The difference between the flattest and steepest curvatures of the cornea. Sometimes this amount agrees with the refractive astigmatism. This means that all the astigmatism in the prescription is due to corneal astigmatism.

Question 6

Residual astigmatism

Answer 6

If the corneal astigmatism does not match the prescription astigmatism, there is more astigmatism that is due to structures within the eye.

Question 7

Corneal topographer

Answer 7

Used to image and measure the entire corneal surface showing the locations, thickness, and depth as well as elevations and depressions. Thickness is measured in microns, where the thickest portions are the higher numbers. Later versions have incorporated computer imaging, using more sophisticated programs and providing more detailed images showing where the elevations and depressions are through color enhancements. The topographical map will show measurements in colors corresponding to the various elevations of the surface.

Question 8

Placido’s disc or keratoscope

Answer 8

Illuminated target made up of concentric black circles reflected onto the cornea to measure the corneal surface. If the surface of the cornea is not spherical, the image will not be spherical and the areas adjacent to each other will not be symmetrical. These aspherical surfaces will help us measure the power of that eye—specifically, the amount of corneal astigmatism of that eye.

Question 9

Intraocular lens implants

Answer 9

Lens implants used in cataract surgery

Question 10

Traditional gas permeable contact lenses

Answer 10

Usually considered “hard” contacts, but the materials are much more oxygen permeable now, so they are now called “gas permeable” contacts.

Question 11

Scleral lenses

Answer 11

Large as permeable lenses made to cover the entire cornea

Question 12

Parts of the manual keratometer

Answer 12

1. Focusing knob: Focuses the images
2. Rotating grip: Finds the axis
3. Horizontal and vertical measuring drums: Locates the horizontal and vertical axes of astigmatism.
4. Elevating knob: Raises and lowers the unit
5. Locking knob: Locks the unit in front of either eye
6. Chin rest elevation knob: Leveling sight, align the patient’s outer canthus to this.

Question 13

Autokeratometers give the corneal measurements at the push of a button once you focus on each cornea. The disadvantage of these is what? For this reason, what are now used?

Answer 13

Autokeratometers give the corneal measurements at the push of a button once you focus on each cornea. The disadvantage of these is that the clarity of mires is not seen. If the mires are distorted, it helps the doctor diagnose corneal and tear film irregularities. For this reason, topographers are now used.

Question 14

How to calibrate a keratometer

Answer 14

Before taking measurements with the keratometer, you must ensure that the measurements will be accurate by calibrating the instrument.

To do so, follow these steps:

First, turn the horizontal measuring drum until the plus signs are superimposed. Note the reading on the horizontal measuring drum.

Turn the vertical measuring drum until the minus signs of the circle are superimposed. Note the reading on the vertical measuring drum.

If both the readings on the horizontal and vertical measuring drums match the diopter value of the sphere (plus or minus an eighth of a diopter), then the keratometer is calibrated.

Refer to the following web activity: Refer to the following web activity: https://purcellexchange.com/abo-practice-area_v2/keratometry-tutorial/menu.html

Question 15

How to take measurements with a keratometer

Answer 15

1. Clean the chin rest with an alcohol swab or cloth with soap and water, dry.
2. Have patient place their chin on the chin rest. Line up the outer part of their eye (outer canthus) with the leveling sight. Use the chin rest elevating knob if needed. Make sure their forehead is snug against the forehead bar.
3. Focusing: Move the occluder in front of the eyepiece and look through the eyepiece. Turn the focusing knob all the way to the left (counterclockwise) and slowly to the right (clockwise) until the cross is in focus.
4. Move the occluder out of the way but keep the drum in front of the cornea until you can see the three mires reflected on the cornea. Look through the eyepiece.
5. Use the rotating axis knob to line up the horizontal “-” lines and the vertical “+” crosses. You must be very accurate lining up the lines.
6. Use the vertical and the horizontal measuring drums to superimpose the two “-” and “+” lines. You must be very accurate lining up the lines.
7. Read and record the keratometer readings. Remember that the left drum is the horizontal axis drum and the highest axis goes with the number on the left drum. The right drum is the vertical axis drum, and the lesser axis number corresponds with the right drum power.

Refer to the following web activities: https://purcellexchange.com/abo-practice-area_v2/keratometry-tutorial/menu.html

http://abo-ncle.org/abo-practice-area_v2/keratometer-practice/practice_1/index.html

Question 16

The keratometer readings are focused on the curvature of the central 3-3.5 mm of the corneal surface and are in units of diopters of power along a certain axis. These corneal measurements are used to know what? Usually, you’ll start your fitting using a lens with the base curvature that matches what?

Answer 16

The keratometer readings are focused on the curvature of the central 3-3.5 mm of the corneal surface and are in units of diopters of power along a certain axis. These corneal measurements are used to know what diagnostic contact lenses to start your fitting with. Usually, you’ll start your fitting using a lens with the base curvature that matches the flattest corneal curvature.

Question 17

Typically, the flattest curvature is ___ degrees away from the steepest curvature. If the flattest curve is along the horizontal axis, we call it what? If the flattest curve is along the vertical axis, we call it what?

Answer 17

Typically, the flattest curvature is 90 degrees away from the steepest curvature. If the flattest curve is along the horizontal axis, we call it “with the rule astigmatism.” If the flattest curve is along the vertical axis, we call it “against the rule astigmatism.”

Consider the following prescription:

O.D. 44.00 @ 180 / 45.00 @ 90

You would start your fitting using a diagnostic contact lens with a contact lens curvature that is the flattest, so 44.00 diopters.

The contact lenses are manufactured and distributed with the curvature designated in millimeters instead of diopters.

337.50 ÷ 44.00 diopters = 7.67 mm

You would start with a lens with a base curve of 7.67 mm.

Question 18

Sometimes the flattest and steepest curvatures are not 90 degrees apart, which can be a normal variation. The steepest curvature is what?

Answer 18

Sometimes the flattest and steepest curvatures are not 90 degrees apart, which can be a normal variation. The steepest curvature is the larger of the two numbers in diopters.

For example, consider the following prescription:

O.D. 42.25 @ 135 / 45.00 @ 045
O.S. 43.00 @ 180 / 44.50 @ 090

In this example, the right eye shows the flattest curvature is 42.25 diopters along the 135 axis, and the steepest curvature is 45.00 diopters at axis 045.

It is appropriate to write the axis with three digits in order to avoid the chance that there is a missing digit.

Question 19

The base curves of gas-permeable contact lenses are designated by the radius in millimeters rather than diopters. Therefore, you’ll need to convert diopters to millimeters to fit these contacts. What formulas are used to convert to millimeters, or millimeters to diopters?

Answer 19

337. 5 ÷ diopters = millimeters
338. 5 ÷ millimeters = diopters

Example: convert 43.00 diopters to millimeters

337.5 / 43 = 7.85 mm

Question 20

If there is a gradual increase in corneal astigmatism over time and the mires of the keratometer are irregular, aspheric, wavy, or the axes are not 90 degrees apart, this is suspicious of a condition called what? Define it. What type of contact lens best fits this type of lens?

Answer 20

Keratoconus - a progressive corneal disease where the cornea continues to thin. This type of lens is best fit with a hard gas-permeable contact lens.

Question 21

What are some important reasons for taking keratometry readings?

Answer 21

Before refractive corneal surgery, the surgeon knows if there are any corneal surface issues. Corneal surface problems would result in poor outcomes and rule the patient out as a candidate for contact lens wear. Additionally, before cataract surgery, it is important to know the amount of corneal astigmatism versus lenticular astigmatism in order to calculate the power of the intraocular lens needed.

Question 22

Given the K readings of OD: 46.00 @ 070 / 43.00 @ 160, which is the flattest curvature of this cornea?

Answer 22

43.00 diopters. It is always the lesser of the two K readings.

Question 23

Does this cornea, given the K readings of OD: 46.00 @ 070 / 43.00 @ 160, have with-the-rule astigmatism or against-the-rule astigmatism?

Answer 23

With-the-rule astigmatism. Remember that the direction of the flattest curvature determines the direction of the astigmatism.

Question 24

How to interpret a topograhical map

Answer 24

It is usually helpful to first look for the steepest areas (highest diopter powers), then look at how they flatten (lower diopter powers) as the rings move outward. The flatter surface areas have lines not as close together. Since contact lens fits and certain eye surgeries depend on where the steepest and flattest curves of the cornea are, doctors will begin looking at the steepest and flattest areas on the printout. The higher-diopter numbers are steeper, and the lower numbers are flatter.

Remember the measurements will usually be in diopters and you may need to convert the numbers you need to millimeters when you fit contact lenses.

Question 25

In relation to corneal topography, if the eye is too dry or too wet, the lines that separate the areas may be blurred, so you may need to what? If the corneal surface has been damaged, infected, or otherwise disrupted, you may need to what?

Answer 25

If the eye is too dry or too wet, the lines that separate the areas may be blurred, so you may need to redo the measurement. If the corneal surface has been damaged, infected, or otherwise disrupted, you may need to take a second measurement.

Question 26

Traditional rigid gas-permeable contact lenses are small—usually about 8 or 9 mm in diameter—so you would only need the information from what part of the map? For larger scleral lenses, discussed in a later lesson, topography becomes an essential tool that describes what?

Answer 26

Traditional rigid gas-permeable contact lenses are small—usually about 8 or 9 mm in diameter—so you would only need the information in the central portion of the map. For larger scleral lenses, discussed in a later lesson, topography becomes an essential tool that describes the entire corneal surface on which the scleral lens will sit.

Question 27

True or false? Each topographical map will have a color scale showing the diopters that correspond to certain colors on the map.

Answer 27

True. Each topographical map will have a color scale showing the diopters that correspond to certain colors on the map. This helps to read the printout, since each company makes a different color scale. Typically, the reddish colors are the steepest portion, and the darker colors such as green or blue are flatter.

Question 28

It is important to note that sometimes a topographical map can distinguish between what? The topographical map will show what?

Answer 28

It is important to note that sometimes a topographical map can distinguish between corneal dystrophies or degenerations as the cause of visual problems rather than a cataract. The topographical map will show discrete pockets of irregular curvatures in certain areas, which would be diagnostic of corneal diseases.

Question 29

Given the K reading 42.00@170/43.00@080, does this cornea have with-the-rule or against-the-rule astigmatism?

Answer 29

With-the-rule astigmatism. Remember the direction of the flattest curvature determines the direction of the astigmatism.