COA Contact Lens Fundamentals 1 — Questions and Answers
Question 1: What does the base curve of a contact lens describe?
- The total diameter of the lens
- The curvature of the central posterior surface of the lens (Correct answer)
- The oxygen permeability of the lens material
- The power of the lens in diopters
Correct answer: The curvature of the central posterior surface of the lens
The base curve describes the radius of curvature of the central posterior (back) surface of the contact lens, which determines how the lens fits against the cornea.
Question 2: What does 'Dk' represent in contact lens terminology?
- The lens diameter in kilograms
- The oxygen permeability of the lens material (Correct answer)
- The dioptric power of the contact lens
- The dissolving constant of the cleaning solution
Correct answer: The oxygen permeability of the lens material
Dk is the oxygen permeability of a contact lens material, where D is the diffusion coefficient and k is the solubility coefficient of oxygen in the material.
Question 3: A patient's contact lens shows a tight-fitting pattern on fluorescein evaluation with a rigid gas-permeable lens. Which of the following best describes this fit?
- Flat fit with excessive edge lift
- Steep fit with central pooling and minimal edge clearance (Correct answer)
- Alignment fit with uniform fluorescein pattern
- Interpalpebral fit with superior bearing
Correct answer: Steep fit with central pooling and minimal edge clearance
A steep (tight) RGP fit shows central pooling of fluorescein under the center of the lens and reduced edge lift, indicating the lens curves more steeply than the cornea.
Question 4: Which type of contact lens is most appropriate for correcting irregular astigmatism caused by keratoconus?
- Soft toric lens
- Daily disposable spherical soft lens
- Rigid gas-permeable lens (Correct answer)
- Monovision soft lens
Correct answer: Rigid gas-permeable lens
Rigid gas-permeable (RGP) lenses vault over the irregular corneal surface and use the tear lens to neutralize irregular astigmatism, making them the preferred option for keratoconus.
Question 5: What is the term for the measurement of the contact lens that describes its total width from edge to edge?
- Base curve
- Optical zone diameter
- Overall diameter (OAD) (Correct answer)
- Back vertex power
Correct answer: Overall diameter (OAD)
The overall diameter (OAD) refers to the total width of the contact lens measured from one edge to the other, and it is a key parameter in contact lens fitting.
Question 6: Which contact lens design uses alternating concentric zones to provide correction for both near and distance vision?
- Monovision
- Aspheric multifocal
- Translating bifocal
- Concentric/annular multifocal (Correct answer)
Correct answer: Concentric/annular multifocal
Concentric (annular) multifocal contact lenses have alternating rings of distance and near power arranged in concentric zones, allowing simultaneous vision correction.
Question 7: What is the primary advantage of silicone hydrogel contact lenses over conventional hydrogel lenses?
- They are less expensive to manufacture
- They have significantly higher oxygen transmissibility (Dk/t) (Correct answer)
- They provide better UV protection
- They require no cleaning or disinfection
Correct answer: They have significantly higher oxygen transmissibility (Dk/t)
Silicone hydrogel lenses have much higher oxygen transmissibility (Dk/t) than conventional hydrogels, allowing more oxygen to reach the cornea and reducing hypoxia-related complications.
COA Contact Lens Fundamentals 1
What does the base curve of a contact lens describe?