Fundamentals of GP Lens Design and Fitting Flashcards
Describe the gradual adaptation schedule for first time wearers of RGP lenses
First Day- 4 hours
Day 2- increase by 2 hours
Day 3- increase by 2 hours
etc….. up to a full day
What is a good way to describe the irritability of wearing them before adaptation?
‘You’ll have an awareness of the lens” … better than “It’ll be painful”
How do we know what base curve lens to try as a diagnostic lens?
Use HVID (horizontal visible iris diameter)
What is the normal range of HVID?
11.4 to 12.1 mm
What is a large HVID?
> 12.2mm
What is a small HVID?
Less than 11.4 mm
If the patient has a HVID of 11.5 or greater, what diameter should you try as a diagnostic lens first?
9.5 mm
If the patient has a HVID of less than 11.4, what diameter should you try as a diagnostic lens first?
9.0 mm
When dispensing an RGP lens, should you anesthetize the eye?
No
What is the term do we use when describing how a lens clears the central cornea?
Vault
The lens should rest (land) where on the cornea ideally?
Midperipheral cornea along the horizontal meridian
Ideally the lens should have unobstructed movement along what meridian?
Vertical
In the mid-periphery, the lens will always ‘land’ at the point of greatest elevation.
Free card
There was a second type of topography that we looked at once the lens was on. What do the colors and numbers represent?
Red (+ numbers) higher than the reference sphere
Blue (- numbers) lower than the reference sphere
Beyond how many diopters of WTR will a spherical RGP begin to rock vertically?
Beyond 2 Diopters
What is corneal eccentricity?
Basically the cornea not being perfectly spherical as it flattens toward the periphery. One of the slides had 3 different examples (i.e. 0.6, 0.4, 0.2; the 0.6 cornea was flatter). This is why we use a diagnostic flat K lens, but then try steeper or flatter lenses.
When you have a K reading, how do you round to get your first diagnostic lens
They mentioned rounding up is best (i.e. K is 43.7, use a 44.0D diagnostic lens)
What does a lens drifting side to side indicate?
Flatter than K lens
What does a lens riding high indicate?
Flatter than K Lens, too flat
What does a low riding lens indicate?
Steeper than K lens, too steep
A lid can push the lens downward giving a false impression that it’s riding low. Patient’s must keep their eyes wide open as you access this.
Free Card
What are 3 GP fitting factors?
1- center of lens should clear central cornea
2- Should be midperiphery contact point along horizontal meridian
3- Lens should maintain unobstructed movement along the vertical merician
Where will a RGP lens move on the eye?
In the direction of least mechanical resistance, hopefully vertically
What does inferior clearance look like
Fluorescein will be thinner superiorly. May be caused by unequal toricity on cornea or upper lid pressure attributing to this.
When doing an overrefraction, if you under X diopters, you do not need to do a vertex calculation. How many diopters is this?
4D
How much of the corneal astigmatism will be neutralized by tear film under RGP lens?
Hopefully 100%, this is the main reason why RGP lenses offer superior optics compared to soft lenses.
If you have ATR astigmatism, where will you have touch/clearance?
Touch- 12 & 6
Clearance- 3 & 9
