NOCE Contact Lens Fitting and Dispensing 2 — Questions and Answers
Question 1: During an RGP lens follow-up, a lens rides superiorly and shows temporal lag with each blink. What is the most likely cause?
- The lens is fitted too steeply
- The lens is fitted too flat (Correct answer)
- The lens diameter is too large
- The lens power is incorrect
Correct answer: The lens is fitted too flat
A flat-fitting RGP lens has poor corneal alignment, causing it to decenter superiorly and exhibit excessive movement and temporal lag on the blink.
Question 2: What is the purpose of performing an 'over-refraction' during a contact lens fitting?
- To determine the patient's bifocal add power
- To refine the contact lens power while the lens is on the eye (Correct answer)
- To measure corneal curvature with the lens in place
- To assess binocular vision balance
Correct answer: To refine the contact lens power while the lens is on the eye
Over-refraction is performed with the trial contact lens in situ to determine what additional power is needed to achieve best corrected acuity with that specific lens.
Question 3: A soft lens patient reports significant end-of-day dryness and discomfort. Which change would MOST likely improve comfort?
- Reducing the lens replacement frequency
- Switching to a higher water content conventional hydrogel
- Switching to a silicone hydrogel lens material (Correct answer)
- Increasing daily wearing time gradually
Correct answer: Switching to a silicone hydrogel lens material
Silicone hydrogel lenses provide higher oxygen transmissibility and tend to maintain better surface wettability throughout the day, reducing end-of-day dryness compared to conventional hydrogels.
Question 4: What does 'limbal clearance' refer to in the context of contact lens fitting?
- The space between the lens edge and the corneal limbus (Correct answer)
- The clearance between the back lens surface and the corneal apex
- The diameter of the lens optical zone
- The amount of lens movement observed on blink
Correct answer: The space between the lens edge and the corneal limbus
Limbal clearance is the space between the edge of a contact lens and the corneal limbus (corneoscleral junction), which is important for limbal vessel health and comfort.
Question 5: For a patient with a spectacle prescription of -2.50 -1.50 x 180, which contact lens type would provide BEST visual correction?
- Spherical soft lens at -2.50 D
- Spherical soft lens at spherical equivalent (-3.25 D)
- Toric soft contact lens (Correct answer)
- Monovision fitting with a spherical lens
Correct answer: Toric soft contact lens
With 1.50 D of cylinder, a toric soft lens is the most appropriate choice since the residual astigmatism with a spherical lens would likely reduce visual acuity significantly.
Question 6: What is the primary purpose of sodium fluorescein dye when fitting RGP contact lenses?
- To disinfect the lens before insertion
- To visualize the tear layer distribution beneath the lens (Correct answer)
- To measure the optical power of the lens
- To stain corneal abrasions exclusively
Correct answer: To visualize the tear layer distribution beneath the lens
Sodium fluorescein, illuminated with a cobalt blue light, reveals the tear film distribution beneath an RGP lens, allowing the practitioner to assess the fitting relationship.
Question 7: What is the ideal movement of a correctly fitted soft contact lens following a blink?
- No movement at all (0 mm)
- 0.5 to 1.0 mm of movement (Correct answer)
- 2.0 to 3.0 mm of movement
- More than 3.0 mm of movement
Correct answer: 0.5 to 1.0 mm of movement
A well-fitted soft contact lens should exhibit approximately 0.5 to 1.0 mm of movement after a blink, indicating adequate tear exchange without excessive lens instability.
NOCE Contact Lens Fitting and Dispensing 2
During an RGP lens follow-up, a lens rides superiorly and shows temporal lag with each blink.
What is the most likely cause?