8.1 Lens Selection & Application Procedures

8.1 Lens Selection & Application Procedures

Quick Answer: Diagnostic fitting starts before any lens touches the eye. You use K readings, manifest Rx, topography, lid tension, and palpebral aperture to choose an initial trial lens. Soft trials are picked by base curve (BC), diameter, material, and modality; RGP trials are picked by BC on/near flat K, diameter 8.5–9.5 mm, and Dk-based material. After insertion, allow soft lenses 10–15 min and RGP lenses 20–30 min to settle before judging fit.

Pre-Fit Data You Must Collect

Before opening a trial lens kit, gather the following five pieces of clinical data. The CLRE expects you to know what each one contributes to the selection:

Soft Lens Trial Selection

Soft lens selection uses four variables. Adjust each based on the measurements above:

1. Base curve (BC) — Typical soft BCs are 8.3–8.7 mm. A steeper (lower) BC suits a steeper cornea; a flatter (higher) BC suits a flatter cornea. When in doubt, start mid-range (8.6 mm).
2. Diameter — 14.0–14.5 mm is standard. Diameter must be 1.0–2.0 mm larger than HVID so the lens overhangs the limbus by 0.5–1.0 mm on all sides.
3. Material — Silicone hydrogel (high Dk/t) for long wear or compromised corneas; hydrogel for daily wear with lower oxygen need; hypergel for comfort/moisture.
4. Modality — Daily disposable for occasional/low-cylinder use; monthly/biweekly for full-time wear; toric modality when cylinder ≥ 0.75 D and stable.

Power selection: vertex the manifest Rx if the spherical equivalent is ≥ ±4.00 D. Round plus lenses up to the nearest 0.25 D; round minus lenses down (more minus) by 0.25 D when vertexing matters.

RGP Trial Selection

RGP selection is more sensitive to K readings because the lens is rigid and maintains its shape:

1. Base curve — Start on or 0.05–0.10 mm flatter than flat K. For with-the-rule astigmatism ≤ 1.50 D, a spherical lens on flat K usually aligns. For higher cylinder, consider a toric or aspheric back surface.
2. Diameter — 8.5–9.5 mm is standard. Larger diameter distributes weight better and improves centration; smaller diameter reduces lid interaction in tight lids.
3. Material by Dk — Low Dk (~25–40) for daily wear with healthy corneas; high Dk (60–100+) for extended wear or compromised corneas. Material also affects surface wettability and flexure.
4. Power — Use the lacrimal lens method: RGP power ≈ (vertexed manifest sphere) − (flat K − base curve in diopters). A base curve flatter than K adds plus lacrimal lens power; steeper adds minus.

Application (Insertion) Procedure

The CLRE expects you to know the precise insertion sequence. The same sequence applies to soft and RGP trials:

1. Wash and dry hands — soap and lint-free towel; apply non-perfumed lotion only if needed.
2. Inspect and rinse the lens — remove from case, inspect for nicks/tears, rinse with appropriate solution (soft: multi-purpose or saline; RGP: wetting solution).
3. Place lens on the index finger — concave side up, at the fingertip, not the pad.
4. Fixate — patient looks straight ahead or slightly down.
5. Pull up the upper lid with the non-dominant hand, pressing the lid against the brow to prevent blinking.
6. Pull down the lower lid with the middle finger of the dominant hand.
7. Place the lens on the sclera (temporal) or directly on the cornea. Soft lenses can be placed on the sclera; RGP lenses are usually placed directly on the cornea to avoid trapping air.
8. Release the lower lid first, then the upper lid slowly.
9. Instruct the patient to blink gently and look in all directions to seat the lens.
10. Inspect with slit lamp for centration, movement, and bubbles.

Removal Techniques

Soft lens removal: Pull the lens onto the temporal sclera with the index finger, pinch gently between thumb and index finger, and lift out.

RGP removal: Two common methods:

• Two-finger ( lids ) method: Open the lids wide, press the lid margins near the lens edge, and the patient blinks — the lens pops out onto a clean towel or hand.
• Suction tool (DMV): Wet the suction cup, touch the lens, lift. Use only on RGP lenses, never soft.

Settling Time

Do not evaluate fluorescein patterns, movement, or coverage before settling is complete — early readings over-call steepness and under-call movement.

Key Takeaways

• Trial lens selection is data-driven, not trial-and-error: K + Rx + topography + lids + aperture drive the first lens.
• Soft BC is usually 0.6–0.8 mm flatter than the flat K equivalent; RGP BC is on or 0.05 mm flatter than flat K.
• RGP diameter is 8.5–9.5 mm; soft diameter is 1–2 mm larger than HVID.
• Always vertex powers ≥ ±4.00 D.
• Respect settling time — soft 10–15 min, RGP 20–30 min — before judging fit.