NOCE Contact Lens Fitting and Dispensing 1 — Questions and Answers
Question 1: In RGP contact lens fitting, a fluorescein pattern showing central touch (dark area centrally) with excessive edge lift indicates:
- An optimal alignment fit
- A steep lens fit
- A flat lens fit (Correct answer)
- Adequate corneal clearance
Correct answer: A flat lens fit
A flat-fitting RGP lens results in central bearing (dark area with fluorescein) and excessive peripheral edge clearance because the lens back surface is flatter than the cornea.
Question 2: When initially selecting a base curve for a soft contact lens, which measurement is most commonly used as the starting point?
- The steeper K reading only
- The flatter K reading only
- The average of both K readings (Correct answer)
- The vertex distance measurement
Correct answer: The average of both K readings
For soft contact lens fitting, the base curve is typically selected based on the average keratometry reading because soft lenses drape over the entire cornea.
Question 3: A patient's spectacle prescription is -4.50 DS with a vertex distance of 12mm. What contact lens power would you expect to order?
- -4.50 DS
- -4.25 DS (Correct answer)
- -4.75 DS
- -5.00 DS
Correct answer: -4.25 DS
Compensating for the 12mm vertex distance using the effectivity formula, the effective power at the corneal plane is approximately -4.25 DS (less minus than the spectacle Rx).
Question 4: Which lens parameter describes the peripheral edge design and tear exchange area of a rigid gas permeable contact lens?
- Optical zone diameter
- Peripheral curve radius (Correct answer)
- Base curve radius
- Center thickness
Correct answer: Peripheral curve radius
The peripheral curve radius (and its width) defines the edge design of an RGP lens, governing tear exchange and lens-to-limbus relationship.
Question 5: What is the minimum oxygen transmissibility (Dk/t) recommended for daily wear soft contact lenses to prevent corneal hypoxia?
- 24 × 10⁻⁹ (Correct answer)
- 87 × 10⁻⁹
- 125 × 10⁻⁹
- 150 × 10⁻⁹
Correct answer: 24 × 10⁻⁹
Holden and Mertz established that a minimum Dk/t of 24 × 10⁻⁹ is required for daily wear soft lenses to prevent significant hypoxic stress to the corneal epithelium.
Question 6: When fitting a toric soft contact lens, which factor is MOST critical for achieving adequate visual correction?
- Base curve radius selection
- Overall diameter
- Axis alignment and rotational stability (Correct answer)
- Oxygen permeability of the material
Correct answer: Axis alignment and rotational stability
Proper axis alignment is critical for toric lenses because mislocation of even a few degrees from the intended axis can significantly reduce the cylinder correction.
Question 7: A patient with keratoconus needs contact lens correction. Which lens type is MOST appropriate for this condition?
- Standard spherical soft lenses
- RGP lenses or specialty designs such as scleral lenses (Correct answer)
- Daily disposable soft lenses
- Extended wear silicone hydrogel lenses
Correct answer: RGP lenses or specialty designs such as scleral lenses
Keratoconus patients typically require RGP or specialty lens designs (scleral, hybrid) because they vault over the irregular cornea, providing a smooth anterior optical surface.
NOCE Contact Lens Fitting and Dispensing 1
In RGP contact lens fitting, a fluorescein pattern showing central touch (dark area centrally) with excessive edge lift indicates: