Contact Lens Fitting and Dispensing Flashcards
7 cards from real NOCE practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Contact Lens Fitting and Dispensing flashcards as text
In RGP contact lens fitting, a fluorescein pattern showing central touch (dark area centrally) with excessive edge lift indicates:
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.
When initially selecting a base curve for a soft contact lens, which measurement is most commonly used as the starting point?
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.
A patient's spectacle prescription is -4.50 DS with a vertex distance of 12mm. What contact lens power would you expect to order?
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).
Which lens parameter describes the peripheral edge design and tear exchange area of a rigid gas permeable contact lens?
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.
What is the minimum oxygen transmissibility (Dk/t) recommended for daily wear soft contact lenses to prevent corneal hypoxia?
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.
When fitting a toric soft contact lens, which factor is MOST critical for achieving adequate visual correction?
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.
A patient with keratoconus needs contact lens correction. Which lens type is MOST appropriate for this condition?
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.