Contact Lenses
Core framework
Understand lens material properties (Dk/t for oxygen transmission, water content, modulus), fitting relationships (base curve vs corneal curvature), and care systems. Know the difference between soft and rigid gas-permeable lenses and when each is appropriate.
Concepts to know cold
- Oxygen: corneal edema results from hypoxia; higher Dk/t reduces it. Extended wear demands higher Dk/t than daily wear.
- Fit assessment: for soft lenses, evaluate centration, movement, coverage, and comfort. For RGPs, look at fluorescein pattern: alignment, apical bearing, edge lift. A steep fit shows central pooling; a flat fit shows central bearing.
- Complications: microbial keratitis (especially with overnight wear), giant papillary conjunctivitis, corneal neovascularization, and solution-related hypersensitivity.
- Keratoconus: RGPs or specialty designs often improve acuity by creating a tear lens over the irregular cornea.
Traps
Questions confuse steep and flat fluorescein patterns, or ask which parameter to change for a given fit problem. Remember: steeper base curve = tighter fit; flatter = looser. Overnight wear is the strongest risk factor for microbial keratitis.
Rule of thumb
Change one parameter at a time and re-evaluate. For soft lens comfort problems, check fit before blaming the material.
Worked example
An RGP shows central pooling with mid-peripheral bearing. That is a steep fit; flatten the base curve (increase radius) to improve alignment and distribute bearing.