Contact Lenses: Materials, Fitting, and Care
What this domain tests
Contact-lens questions test material properties, soft-lens fitting relationships, fluorescein evaluation, over-refraction, and safe care. The exam expects you to distinguish oxygen performance from water content and to recognize when a fit must be changed or referred.
Core rules
- Dk is oxygen permeability; Dk/t is transmissibility through a given lens thickness. Silicone hydrogels achieve high Dk/t with moderate water content.
- Water content affects oxygen transport in conventional hydrogels and influences modulus, dehydration, and deposit formation. Higher water content is not automatically healthier.
- A steeper base curve tightens the fit; a flatter base curve loosens it. Diameter and sagittal depth also change the fit, so base curve alone is not enough.
- Evaluate centration, coverage, movement, edge lift, and fluorescein pattern. A steep fit shows central pooling and mid-peripheral bearing; a flat fit shows central bearing and peripheral pooling.
- Care includes hand washing, approved solutions, rubbing and rinsing when indicated, proper case hygiene, and the prescribed replacement schedule. Never use tap water.
Worked example
A patient wears monthly silicone hydrogel lenses and reports 4 p.m. dryness with good vision. Evaluation shows adequate centration but tight edges and limbal redness. The lens is too steep or too large in sag. Flattening the base curve slightly or reducing diameter may improve tear exchange. Over-refraction is −0.25 DS; do not simply copy the spectacle Rx, because vertex distance and lens-to-eye relationship differ.
Traps
Equating high water content with high oxygen; changing power without checking the fit; ignoring tight-edge redness; and permitting tap-water rinsing. Pain, photophobia, or vision loss warrants lens removal and prompt referral.