Contact Lens Practice
Concepts
Know lens materials (hydrogel vs silicone hydrogel Dk/t, water content, modulus), base curve and sagittal depth relationships, and the difference between rigid gas-permeable and soft lens fitting philosophies. Understand the tear film's role, oxygen transmissibility (Dk/t), and the Holden–Mertz criteria for overnight wear. Master keratometry-to-base-curve conversion (base curve ≈ average K + 0.8 mm for soft lenses), and the use of fluorescein patterns to assess RGP fit (alignment, steep, flat).
Traps
Questions often confuse Dk (permeability) with Dk/t (transmissibility) - a thicker lens has lower Dk/t. Another trap: higher water content does not always mean higher oxygen permeability in silicone hydrogels. Also watch for the difference between a steep fit (central pooling, edge lift) and a flat fit (central bearing, excessive edge lift).
Worked rule of thumb
For soft lenses, base curve = average K + 0.8 mm. If K readings are 7.8 mm and 7.6 mm, average = 7.7 mm, so base curve ≈ 8.5 mm. For RGPs, choose a base curve 0.1 mm steeper than the flattest K for a starting alignment fit.