Chapter 9- Contact lenses I (2) Flashcards
The two meridians when it comes to astigmatism corrects what:
1- Blurry vision
2- Eye strain
3- Excessive squinting
There are two types of astigmatism on the cornea:
1- “With the rule” astigmatism- where the vertical meridian is more curved than the horizontal meridian (like a rugby ball)
2- “Against the rule” astigmatism- where the horizontal meridian is more curved than the vertical meridian
Examples of astigmatic prescriptions
How to work it out:
The most positive meridian is the spherical power
The difference between the two powers is the negative cylinder power
The axis is the one with the most positive power reading
For the last one:
Plano basically means 0
To find the angle you add 90 to 45 to get 135. This because in astigmatism, the two angles are normally 90 degrees apart
You find the difference between the two powers
Take the angle that is not part of the plano
How to convert astigmatism spectacle Rx into occular Rx
1- Find the corresponding value on the table for the power. If the corresponding value is like -6.12, round it to the nearest 0.25 which will be -6.00
2- Then add the two values in the prescription. For example, in the first one, -6.50 plus -1.00 will equal -7.50. Then find the corresponding value for -7.50.
3- Use the corresponding values from your step 1 and step 2 and find the difference to find the cyl. Make sure you round to the nearest 0.25.
4- The angle will stay the same in this case
What is keratometry?
Keratometry is the measurement of the principal radii of the anterior surface of the cornea (tear film)
Why do we use keratometry?
1- To find out the central corneal radius (mm) and power (D) in both principal meridians of cornea
2- Differentiate between regular and irregular astigmatism
3- Measurement of radius of contact lenses
With keratometry, the greater the radius…
The greater the radius, the less curved the eye is
The smaller the radius, the more curved the eye is
When rounding the radius, you would round to the nearest 0.05mm
Why do we measure the diameter of the cornea? (Horizontal visible iris diameter)
To help to determine the size of the diameter of lenses.
How do we measure the diameter of the cornea?
Measure with a PD rule
Or comparator scale- semi circles that are representative of different diameters that you would match with the person’s iris.
What is the average measurement and range of the diameter of the cornea?
The average measurement is expected to be 11.6 mm
The range is normally between 10.2- 13.0 mm
