Contact Lens Principles Flashcards

Which contact lens parameter describes the curvature of the back surface of the lens?

Answer: Base curve radius

The base curve radius (BCR) describes the curvature of the posterior (back) surface of a contact lens.

A patient with keratoconus would most likely be best corrected with which type of contact lens?

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.

What does the term 'oxygen transmissibility' (Dk/t) measure in contact lenses?

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).

Which fluorescein pattern under a rigid lens indicates excessive apical clearance?

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.

A soft contact lens is described as '38% water content.' What does this mean?

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.

Which lens movement pattern on blink is considered ideal for a well-fitted soft contact lens?

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.

What is the primary advantage of silicone hydrogel lenses over conventional hydrogel lenses?

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.