Modifying Base Curve, Diameter, Sag & Power from Findings

Modifying Base Curve, Diameter, Sag & Power from Findings

Quick Answer: Once you have assessed fit, comfort, and over-refraction, the next step is translating those findings into parameter changes. For soft lenses, modify sagittal depth (via base curve and/or diameter) and lens power. For RGP lenses, modify base curve, diameter, and power (using SAM/FAP). Each finding maps to a specific parameter change — this section gives you the decision tables.

Soft Lens Fit Modifications

Steep (Tight) Soft Lens

Findings: minimal lens movement, tight on blink, conjunctival injection, patient reports "tight" sensation, corneal edema after extended wear, reduced VA with blink, fluorescein shows central pooling with peripheral touch.

Goal: reduce sagittal depth so the lens vaults less and moves more.

Flat (Loose) Soft Lens

Findings: excessive lens movement, edge standoff visible at slit lamp, lens decentered on blink, patient reports "loose" or "floating" sensation, fluctuating VA, fluorescein shows edge lift and poor corneal coverage.

Goal: increase sagittal depth so the lens vaults more and grips better.

Why Sagittal Depth Matters Most

Sagittal depth (sag ≈ D²/8r, where D is diameter and r is base-curve radius) determines how much the lens vaults the cornea. Because sag scales with the square of diameter but only linearly with base-curve radius, a small diameter change can produce a larger sag shift than a typical base-curve change:

• A 0.5 mm increase in diameter (e.g., 14.0 → 14.5 mm) on a lens with 8.4 mm base curve raises sag by roughly 0.20 mm
• A 0.2 mm flattening of base curve (8.40 → 8.60 mm) on a 14.0 mm lens reduces sag by roughly 0.06 mm

When a soft lens needs a meaningful fit change and you can change only one parameter, change diameter first.

RGP Fit Modifications

RGP fitting relies on fluorescein pattern interpretation to assess the tear lens relationship between the rigid lens and the cornea.

Steep RGP Fit

Fluorescein pattern: central pooling (bright green pool under the optic zone), peripheral touch (dark ring at edge), minimal lens movement, air bubble may be trapped centrally.

Goal: flatten the base curve to reduce vaulting.

Power adjustment: flattening the base curve creates a minus tear lens → FAP (Flat Add Plus) → add plus power to the RGP.

Flat RGP Fit

Fluorescein pattern: central touch (dark area under optic zone where the lens contacts the cornea), peripheral clearance (bright green ring at edge), excessive lens movement, edge standoff.

Goal: steepen the base curve to increase vaulting and improve centration.

Power adjustment: steepening the base curve creates a plus tear lens → SAM (Steep Add Minus) → add minus power to the RGP.

RGP Diameter and Fitting Philosophy

Diameter determines how the lens interacts with the lids:

Interpalpebral fitting is preferred when the upper lid is tight or sharply curbed (lens would be uncomfortable under the lid), or for steep corneas where lid attachment is unstable. Lid-attachment fitting is preferred for normal lid anatomy, with-the-rule astigmatism, and when translation is needed (e.g., translating bifocals).

Diameter also interacts with centration: a larger diameter centers better on a flat cornea but may irritate a tight upper lid. A smaller diameter clears a tight lid but may decenter on a flat cornea.

Power Modifications: Combining Over-Refraction with SAM/FAP

When you change only lens power (no base-curve change), add the over-refraction directly to the current lens power:

New power = current lens power + over-refraction

When you change both base curve AND power, apply SAM/FAP first, then add the over-refraction:

New power = current lens power + over-refraction + SAM/FAP adjustment

Where SAM/FAP adjustment is:

• -0.25 D per 0.05 mm steepened (SAM: steep add minus)
• +0.25 D per 0.05 mm flattened (FAP: flat add plus)

Worked Example: RGP Power Change with Base-Curve Steepening

Current RGP: base curve 7.85 mm, power -6.00 D. Over-refraction: -0.50 DS. You steepen the base curve to 7.75 mm (0.10 mm steeper) to fix central touch.

• Over-refraction adjustment: -0.50 D
• SAM adjustment: 0.10 mm steepened = 2 × 0.05 mm = -0.50 D (add minus)
• New power = -6.00 + (-0.50) + (-0.50) = -7.00 D

Worked Example: RGP Power Change with Base-Curve Flattening

Current RGP: base curve 7.70 mm, power -8.00 D. Over-refraction: +0.25 DS. You flatten the base curve to 7.80 mm (0.10 mm flatter) to fix central pooling.

• Over-refraction adjustment: +0.25 D
• FAP adjustment: 0.10 mm flattened = 2 × 0.05 mm = +0.50 D (add plus)
• New power = -8.00 + 0.25 + 0.50 = -7.25 D

Decision Table: From Finding to Parameter Change

Final Verification

After any parameter change, repeat the full evaluation:

1. Reassess comfort, centration, movement, and fluorescein pattern (RGP) after 10–15 minutes of wear
2. Repeat over-refraction — it should be closer to plano
3. Confirm VA at distance (and near for multifocals)
4. Verify toric axis stability across blink and gaze (Section 9.1)
5. Document the final parameters and schedule follow-up

Iterative fitting is normal: expect one or two parameter modifications before arriving at the final lens. Patients should always leave with a clear follow-up plan and backup spectacles.