Two Ends of the Fitting Spectrum
In GP contact lens fitting, the base curve determines whether the lens is steep, flat, or aligned relative to the corneal curvature. A steep fit and a flat fit represent opposite ends of the fitting spectrum, each with distinct fluorescein patterns, clinical signs, and consequences. Understanding these differences is fundamental for achieving an optimal fit.
Steep GP Fit
A GP lens is considered steep when its base curve is steeper (shorter radius) than the patient's corneal curvature.
Fluorescein Pattern
- Apical clearance: Bright green fluorescence centrally where the lens vaults over the corneal apex
- Mid-peripheral bearing: A dark ring where the lens contacts the mid-peripheral cornea
- Reduced edge clearance: The tight periphery shows minimal fluorescence at the lens edge
Clinical Signs
- Minimal lens movement: The mid-peripheral bearing creates a seal effect, restricting lens sliding during blinking
- Good centration: The steep fit tends to hold the lens in a central position
- Air bubbles: Trapped under the central vault area due to poor tear exchange
- Lens adherence: In extreme cases, the lens may bind to the eye and resist removal
Consequences
- Corneal edema: Restricted tear exchange reduces oxygen delivery, causing stromal swelling
- Central corneal staining: Pooled, stagnant tears beneath the vault can cause toxic effects on the epithelium
- Spectacle blur: After lens removal, the cornea may be temporarily distorted (steepened centrally), causing blurred vision with spectacles. This effect typically resolves within hours but can persist with chronic steep fitting
- Corneal warpage: Chronic steep fitting can mold the cornea to match the steep lens, altering its natural curvature
Correction
To correct a steep fit: flatten the base curve (select a longer radius, larger mm value). For example, if the current BC is 7.50mm and the fit is steep, try 7.55mm or 7.60mm. Each 0.05mm change alters the tear lens by approximately 0.25 D.
A steep fit shows central pooling (green center, dark mid-periphery, tight edge). Think: the lens is "too curved" and vaults over the cornea like a dome. The correction is to flatten (increase) the base curve radius.
Flat GP Fit
A GP lens is considered flat when its base curve is flatter (longer radius) than the patient's corneal curvature.
Fluorescein Pattern
- Central touch/bearing: Dark area centrally where the lens presses on the corneal apex
- Mid-peripheral clearance: Fluorescence pools in the mid-periphery as the lens lifts away
- Excessive edge clearance: A wide, bright fluorescent ring at the edge
Clinical Signs
- Excessive lens movement: The lens rocks on the corneal apex, creating significant movement with each blink
- Poor centration: The lens may decentrate, typically inferiorly, as it slides off the steeper central cornea
- Lens rocking: A see-saw or tilting motion is visible as the lens pivots on the corneal apex
- Lens ejection: In extreme cases, the lens may be blinked off the cornea entirely
Consequences
- Central corneal abrasion: The mechanical bearing on the apex can damage the epithelium
- Three and nine o'clock staining: Desiccation staining at the 3 and 9 o'clock positions (horizontal meridian) due to lens-induced drying. The lens edge lifts away from these areas, creating exposed zones that dry out between blinks
- Central corneal flattening: Chronic pressure on the apex can flatten the central cornea (opposite of the steepening seen with steep fits)
- Uncomfortable wear: Excessive movement and rocking cause awareness and irritation
Correction
To correct a flat fit: steepen the base curve (select a shorter radius, smaller mm value). For example, if the current BC is 7.80mm and the fit is flat, try 7.75mm or 7.70mm.
When evaluating for three and nine o'clock staining, look for punctate staining at the horizontal meridian of the cornea, just inside the lens edge. This pattern is specific to GP lenses and results from desiccation of exposed corneal tissue between the lens edge and the lid margin. It can occur even with properly fitting lenses but is more common with flat fits.
Side-by-Side Comparison
| Feature | Steep Fit | Flat Fit |
|---|---|---|
| Central fluorescein | Pooling (bright green) | Touch (dark) |
| Mid-periphery | Bearing (dark ring) | Clearance (fluorescence) |
| Edge clearance | Minimal | Excessive |
| Lens movement | Minimal | Excessive |
| Centration | Usually good | Often poor |
| Corneal effect | Central steepening | Central flattening |
| Correction | Flatten BC | Steepen BC |
Confusing the fluorescein pattern direction with the base curve change needed. If you see central pooling (steep), the reflex might be to "add more" but the correct action is to flatten. If you see central touch (flat), the reflex might be to "take away" but the correct action is to steepen. Remember: the pattern tells you the current problem, and the correction is always the opposite direction.
Key Takeaways
- Steep fit: central pooling, mid-peripheral bearing, minimal edge lift, minimal movement
- Flat fit: central touch, mid-peripheral clearance, excessive edge lift, excessive movement
- Steep fit causes corneal edema, spectacle blur, and potential corneal steepening
- Flat fit causes central abrasion, 3 and 9 staining, and corneal flattening
- Correction: steep = flatten BC; flat = steepen BC
- Each 0.05mm base curve change alters the tear lens by approximately 0.25 D