COT Contact Lens Principles 2 — Questions and Answers
Question 1: Which contact lens parameter describes the curvature of the back surface of the lens?
- Diameter
- Base curve radius (Correct answer)
- Center thickness
- Optical zone
Correct answer: Base curve radius
The base curve radius (BCR) describes the curvature of the posterior (back) surface of a contact lens.
Question 2: A patient with keratoconus would most likely be best corrected with which type of contact lens?
- Daily disposable soft lens
- Rigid gas-permeable lens (Correct answer)
- Extended-wear silicone hydrogel
- Bandage soft lens
Correct answer: Rigid gas-permeable lens
Rigid gas-permeable lenses mask the irregular corneal surface of keratoconus by creating a smooth refractive interface with the tear lens.
Question 3: What does the term 'oxygen transmissibility' (Dk/t) measure in contact lenses?
- Rate of lens dehydration
- Oxygen passing through a lens of specific thickness (Correct answer)
- Lens surface wettability
- Ion permeability of the lens material
Correct answer: Oxygen passing through a lens of specific thickness
Dk/t (transmissibility) is calculated by dividing the oxygen permeability (Dk) by the lens center thickness (t).
Question 4: Which fluorescein pattern under a rigid lens indicates excessive apical clearance?
- Dark central zone with peripheral pooling
- Bright central pooling with dark midperiphery (Correct answer)
- Even green fluorescence throughout
- Crescent-shaped bubble at 6 o'clock
Correct answer: Bright central pooling with dark midperiphery
Excessive apical clearance produces a bright central pool of fluorescein (central pooling) with a dark (bearing) midperipheral ring.
Question 5: A soft contact lens is described as '38% water content.' What does this mean?
- 38% of the lens volume is gas-permeable polymer
- 38% of the lens weight at equilibrium is water (Correct answer)
- The lens transmits 38% of available oxygen
- 38% of lens surface is coated with wetting agent
Correct answer: 38% of the lens weight at equilibrium is water
Water content percentage refers to the proportion of the fully hydrated lens weight that is water at equilibrium.
Question 6: Which lens movement pattern on blink is considered ideal for a well-fitted soft contact lens?
- No movement at all
- 0.25–0.50 mm of upward movement (Correct answer)
- 1.5–2.0 mm of lateral movement
- Downward movement followed by lag
Correct answer: 0.25–0.50 mm of upward movement
An ideal soft lens fit shows approximately 0.25–0.50 mm of smooth upward movement on blink to allow adequate tear exchange.
Question 7: What is the primary advantage of silicone hydrogel lenses over conventional hydrogel lenses?
- Lower cost and greater availability
- Significantly higher oxygen transmissibility (Correct answer)
- Greater resistance to protein deposits
- Better UV-blocking capability
Correct answer: Significantly higher oxygen transmissibility
Silicone hydrogel materials have much higher Dk/t values, allowing more oxygen to reach the cornea and reducing hypoxia-related complications.
COT Contact Lens Principles 2
Which contact lens parameter describes the curvature of the back surface of the lens?