Contact Lens Fitting Flashcards

What is the primary measurement needed before fitting rigid gas permeable (RGP) contact lenses?

Answer: Corneal curvature (keratometry readings)

Keratometry readings provide the corneal curvature measurements essential for selecting the initial base curve of an RGP lens.

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?

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.

What does a flat-fitting soft contact lens typically demonstrate on slit lamp evaluation?

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.

Which contact lens care solution component is responsible for killing microorganisms?

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.

What is the Dk/t value of a contact lens measuring?

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.

What is the recommended minimum Dk/t for extended (overnight) contact lens wear?

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.