Contact Lens Fundamentals
Parameters and materials
Contact lenses are described by base curve, diameter, power, material, and center thickness. Hydrogels (low and high water content) and silicone hydrogels transmit oxygen; Dk is permeability and Dk/t is transmissibility, the clinically meaningful figure. Rigid gas permeable lenses are described by base curve, diameter, optic zone, and power, and are judged by fluorescein pattern, centration, and movement.
Key measurements
Keratometry gives corneal curvature in both millimeters and diopters (337.5 / r in mm gives corneal power). Also record horizontal visible iris diameter, lid position, tear film quality, and pupil size.
Traps
- Dispensing a spectacle refraction unchanged as a contact lens power; vertex conversion matters most above ±4.00 D.
- Forgetting that a steeper base curve tightens the fit and a larger diameter generally tightens the fit as well.
- Assuming oxygen needs are the same for daily and extended wear; extended wear demands much higher Dk/t.
Worked example: vertex conversion
A −8.00 D spectacle prescription at 12 mm vertex distance: F = −8.00 / (1 − 0.012 × −8.00) = −8.00 / 1.096 ≈ −7.30 D, so order approximately −7.25 D at the corneal plane.
Referral red flags
Pain, photophobia, reduced acuity, corneal infiltrates, or neovascularization warrant prompt referral rather than a refit.