ABO Contact Lenses 5 — Questions and Answers
Question 1: A patient's spectacle Rx is -4.00 -2.00 x 180. Using the spherical equivalent, what power soft contact lens would you initially trial?
- -3.00 D
- -4.00 D
- -5.00 D (Correct answer)
- -6.00 D
Correct answer: -5.00 D
Spherical equivalent = sphere + (cylinder/2) = -4.00 + (-2.00/2) = -4.00 + (-1.00) = -5.00 D.
Question 2: Which contact lens material property is described by the term 'wettability'?
- The lens's ability to transmit oxygen
- The degree to which the lens surface attracts and retains the tear film (Correct answer)
- The resistance of the lens to protein deposition
- The elasticity of the lens polymer
Correct answer: The degree to which the lens surface attracts and retains the tear film
Wettability refers to how well the tear film spreads and adheres to the lens surface; poor wettability leads to dry spots, discomfort, and visual fluctuation.
Question 3: During slit-lamp evaluation of a newly fitted soft contact lens, what finding indicates the lens may be too tight?
- Excessive lens movement (>1.5 mm) on upward gaze
- Conjunctival blanching at the lens edge (Correct answer)
- Lens edge standing off from the globe
- Rapid tear film breakup over the lens surface
Correct answer: Conjunctival blanching at the lens edge
Conjunctival blanching (vessel compression) at the lens edge indicates excessive tightness causing mechanical restriction of blood flow at the limbus.
Question 4: What is the recommended minimum central corneal clearance (vault) when fitting a scleral contact lens?
- 50–100 microns
- 100–200 microns
- 200–400 microns (Correct answer)
- 500–600 microns
Correct answer: 200–400 microns
A minimum central clearance of approximately 200 microns (some guidelines say 200–400 µm) is recommended at dispensing to account for settling over the wear period.
Question 5: A rigid contact lens has a Dk of 100 and a center thickness of 0.20 mm. What is its Dk/t value?
- 20
- 50
- 100
- 500 (Correct answer)
Correct answer: 500
Dk/t = Dk ÷ thickness in cm = 100 ÷ 0.020 cm = 500 × 10⁻⁹ (Fatt units); dividing 100 by 0.02 gives 500.
Question 6: Which condition is characterized by fine, diffuse corneal punctate staining in a patient wearing silicone hydrogel lenses with a preserved multipurpose solution?
- Solution-induced corneal staining (SICS) (Correct answer)
- Microbial keratitis
- Acanthamoeba keratitis
- Recurrent corneal erosion
Correct answer: Solution-induced corneal staining (SICS)
Solution-induced corneal staining (SICS) results from an incompatibility reaction between certain preservatives (especially PHMB or Alexidine) and silicone hydrogel lens materials.
Question 7: When converting a spectacle prescription with vertex distance 12 mm to a contact lens power for a highly myopic patient (-10.00 D spectacles), the contact lens power will be:
- More minus than the spectacle Rx
- Less minus than the spectacle Rx (Correct answer)
- Equal to the spectacle Rx
- More minus only if the cornea is steep
Correct answer: Less minus than the spectacle Rx
Moving a minus lens closer to the eye (from 12 mm vertex to contact lens position) reduces its effective power; thus, the contact lens needs less minus than the spectacle prescription.
ABO Contact Lenses 5
A patient's spectacle Rx is -4.00 -2.00 x 180.
Using the spherical equivalent, what power soft contact lens would you initially trial?