ABO Contact Lens Fitting — Questions and Answers
Question 1: What is the primary measurement needed before fitting rigid gas permeable (RGP) contact lenses?
- Corneal curvature (keratometry readings) (Correct answer)
- Pupil size
- Palpebral fissure width
- Iris color
Correct answer: Corneal curvature (keratometry readings)
Keratometry readings provide the corneal curvature measurements essential for selecting the initial base curve of an RGP lens.
Question 2: A contact lens prescription of -3.00 D corresponds to a spectacle prescription of -3.25 D at a 12mm vertex distance. Why is the contact lens power less minus?
- The contact lens sits closer to the eye, requiring less minus power (Correct answer)
- The contact lens material absorbs some light
- The tear film adds plus power
- Manufacturing tolerances require rounding
Correct answer: The contact lens sits closer to the eye, requiring less minus power
When a minus lens moves closer to the eye (decreasing vertex distance), less minus power is needed because the effective power of the lens increases at shorter distances.
Question 3: What does a flat-fitting soft contact lens typically demonstrate on slit lamp evaluation?
- Excessive movement and edge standoff (Correct answer)
- No movement with bubbles trapped underneath
- Lens decentration inferiorly
- Central bearing with peripheral touch
Correct answer: Excessive movement and edge standoff
A flat-fitting soft lens has a base curve that is flatter than the cornea, causing excessive movement, poor centration, and visible edge standoff.
Question 4: Which contact lens care solution component is responsible for killing microorganisms?
- Disinfectant (such as hydrogen peroxide or polyquaternium) (Correct answer)
- Surfactant cleaner
- Wetting agent
- Chelating agent
Correct answer: Disinfectant (such as hydrogen peroxide or polyquaternium)
The disinfectant component, such as hydrogen peroxide or polyquaternium-1, is specifically designed to kill bacteria, fungi, and other microorganisms on the lens surface.
Question 5: What is the Dk/t value of a contact lens measuring?
- Oxygen transmissibility through the lens (Correct answer)
- Lens diameter to thickness ratio
- Lens curvature to power ratio
- Moisture content percentage
Correct answer: Oxygen transmissibility through the lens
Dk/t measures oxygen transmissibility, where Dk is the oxygen permeability of the material and t is the lens thickness. Higher Dk/t values indicate more oxygen reaches the cornea.
Question 6: What is the recommended minimum Dk/t for extended (overnight) contact lens wear?
- 87 (Holden-Mertz criterion) (Correct answer)
- 24
- 125
- 50
Correct answer: 87 (Holden-Mertz criterion)
The Holden-Mertz criterion established that a minimum Dk/t of 87 is needed for overnight wear to prevent corneal swelling beyond acceptable limits.
ABO Contact Lens Fitting
What is the primary measurement needed before fitting rigid gas permeable (RGP) contact lenses?