ABO Contact Lens Fitting 2 — Questions and Answers
Question 1: A patient with keratoconus is best managed with which contact lens modality?
- Soft toric lens
- Rigid gas-permeable (RGP) lens (Correct answer)
- Daily disposable spherical lens
- Extended-wear soft lens
Correct answer: Rigid gas-permeable (RGP) lens
RGP lenses vault the irregular cornea and provide a smooth refracting surface, making them the preferred option for keratoconus.
Question 2: When fitting a soft toric lens, the lens consistently rotates 15° clockwise. What axis adjustment should be made?
- Add 15° to the prescribed axis (Correct answer)
- Subtract 15° from the prescribed axis
- Order the lens with the same axis
- Switch to a spherical lens
Correct answer: Add 15° to the prescribed axis
Using the LARS rule (Left Add, Right Subtract), clockwise rotation means you add degrees to the axis.
Question 3: Which base curve selection guideline is most appropriate when fitting a standard soft lens?
- Match base curve exactly to flat K reading
- Select a base curve 1.0 mm flatter than flat K
- Choose a base curve 0.5–1.0 mm flatter than flat K (Correct answer)
- Use the steepest K reading as the base curve
Correct answer: Choose a base curve 0.5–1.0 mm flatter than flat K
Soft lenses are typically fit 0.5–1.0 mm flatter than the flat K to allow appropriate drape and movement.
Question 4: A patient's over-refraction with a trial RGP lens yields −0.75 D sphere. What does this most likely indicate?
- The lens is too steep
- Residual astigmatism is present
- The lens power needs to be adjusted by −0.75 D (Correct answer)
- The lens diameter is too small
Correct answer: The lens power needs to be adjusted by −0.75 D
A spherical over-refraction result is simply added to or subtracted from the trial lens power to determine final lens power.
Question 5: Which fluorescein pattern indicates an appropriately fitted RGP lens on a normal cornea?
- Apical touch with heavy mid-peripheral pooling
- Apical pooling with heavy peripheral touch
- Feather touch or minimal apical clearance with even mid-peripheral bearing (Correct answer)
- Complete corneal clearance with no touch zone
Correct answer: Feather touch or minimal apical clearance with even mid-peripheral bearing
An alignment fit shows slight apical touch or minimal clearance, even mid-peripheral bearing, and adequate edge lift.
Question 6: Which parameter primarily determines the oxygen transmissibility (Dk/t) of a contact lens?
- Lens diameter
- Base curve radius
- Center thickness and material Dk (Correct answer)
- Water content alone
Correct answer: Center thickness and material Dk
Dk/t is calculated by dividing the material's oxygen permeability (Dk) by the lens center thickness (t).
Question 7: A presbyopic patient with low myopia wants to try monovision contact lenses. Which eye is typically corrected for distance vision?
- Non-dominant eye
- Dominant eye (Correct answer)
- Either eye, patient preference only
- The eye with less astigmatism
Correct answer: Dominant eye
In monovision, the dominant eye is corrected for distance and the non-dominant eye for near.
ABO Contact Lens Fitting 2
A patient with keratoconus is best managed with which contact lens modality?