Contact Lens Fitting and Management
This domain tests whether you can translate corneal shape into a lens parameter set, interpret the fit, and recognise when a lens is harming the eye.
Fit logic
- Rigid lenses: keratometry in millimetres; a base curve slightly steeper than the flat K is a common starting point; assess with fluorescein - alignment shows an even thin central pool, mid-peripheral clearance and adequate edge lift; steep fits pool centrally and bear peripherally; flat fits bear centrally.
- Soft lenses: total diameter, base curve, power, water content, modulus, and centre thickness all affect comfort and physiology.
- Oxygen: Dk is permeability (barrer); Dk/t is transmissibility, and it is Dk/t that predicts corneal swelling. Higher water content does not automatically mean more oxygen; silicone hydrogel chemistry matters more.
Worked example
A silicone hydrogel with Dk 100 barrer and centre thickness 0.10 mm gives Dk/t = 100/0.10 = 1,000 × 10⁻⁹ (cm/s)(mL O₂/mL·mmHg). Thinning the same material to 0.07 mm raises Dk/t to about 1,430 units. Commonly cited thresholds sit around 24 × 10⁻⁹ for daily wear without oedema and much higher for extended wear, so the calculation, not the marketing label, decides.
Traps
Confusing Dk with Dk/t; assuming a smaller base curve number means a flatter lens (it means steeper); using K readings directly as a soft lens base curve; skipping the over-refraction; and missing hypoxia signs - limbal redness, neovascularisation, microcysts - because the patient reports good comfort.