How To Design Rigid Gas-Permeable Lenses
Why Exact RGP Parameters Matter
RGP success hinges on tight alignment between base curve, diameter, peripheral curve system, and the tear lens that forms beneath the optic zone. A 0.10 mm change in base curve or diameter shifts sagittal depth, tear exchange, centration, and oxygen performance. Capturing “gas permeable” and “hard lens calculator” intent here consolidates related searches into one tool.
From Keratometry to Initial Base Curve
Convert Ks with D = 337.5 ÷ r (r in mm). For a spherical start, choose a base curve at or slightly flatter than flat K (about 0.05-0.10 mm) to encourage alignment and tear flow. When corneal toricity is not masked (≈2.50 D or more, or significant limbal toricity), move to back surface toric or bitoric designs. Refine with fluorescein: look for central touch or clearance, mid-peripheral alignment, and adequate edge lift.
Tear Lens Effect and Final Power
Tear lens contribution ≈ (Kflat - Base Curve) in diopters: steeper than K adds plus, flatter adds minus. Ordered power = spherical over-refraction + tear lens effect, vertexed if the refraction exceeds high-power thresholds. For bitorics, calculate each principal meridian separately to deliver full cylinder while preserving centration and comfort.
Design Refinement and Follow Up
- Diameter: Larger lenses improve centration and lid interaction; smaller diameters increase translation for multifocal optics.
- Peripheral Curves: Multi-zone systems enhance tear pump and reduce seal-off risk.
- Residual Astigmatism: Use front surface cylinder or go bitoric when internal cylinder persists after corneal neutralization.
- Aftercare: Reassess for staining, impingement, bubbles, decentration, and stability before finalizing supply.
FAQ: When choose bitoric over front surface toric?
Opt for bitoric when corneal toricity is ≥2.50 D or a front surface cylinder alone cannot stabilize residual astigmatism without sacrificing centration.