Contact Lens Fitting and Evaluation
Prefit data and lens-to-cornea relationships
Begin with keratometry, corneal diameter (HVID), refraction, lid anatomy, and tear film quality. Convert K readings with D = 337.5 ÷ r(mm): a 7.80 mm meridian is 43.27 D. Remember that a smaller base curve radius in millimeters is a steeper lens - a frequent wording trap.
Rigid evaluation reads the fluorescein pattern: central alignment with a thin, even tear layer, mid-peripheral clearance, and a peripheral band of edge lift 0.5–1.0 mm wide that brightens on blink. Apical pooling means the lens is steep; central bearing with a dark, dry appearance means it is flat. The push-up test confirms edge clearance and movement.
Soft lens evaluation judges centration, full limbal coverage, lag of roughly 0.5–1.0 mm on upward gaze, and 0.2–0.5 mm of movement on push-up. Excessive movement suggests a flat or loose lens; a tight lens shows minimal movement, conjunctival indentation, and later limbal hyperemia.
Scleral evaluation is measured in micrometers: apical clearance typically 200–400 µm, limbal clearance without impingement, and a landing zone that does not blanch conjunctival vessels.
Over-refraction is performed over the lens. A spherical over-refraction over a rigid lens quantifies residual astigmatism; vertex-adjust corrections above about ±4.00 D.
Rule of thumb: steepen (decrease) the base curve to increase clearance; flatten to reduce it. A 0.10 mm base curve change is roughly 0.50 D.