Diagnostic Fitting & Evaluation

A well-fitting soft lens shows:

- Movement: ~0.25-0.5 mm on blink in primary gaze (up to ~1 mm can be acceptable) — enough for tear exchange, not so much that vision fluctuates.

- Centration: covers the cornea in all gazes with ~0.5-1 mm of overlap onto the sclera and full limbal coverage.

- Coverage & comfort: stable, comfortable, with clear, consistent vision.

Push-up test: push the lens up with the lower lid and watch recovery. Smooth, moderate resistance with prompt re-centration indicates an ideal fit.

Too STEEP (tight): little or no movement, conjunctival indentation or drag on push-up, bubbles trapped centrally, initial comfort that worsens through the day, and blur that clears briefly on the blink. Restricted tear exchange risks tight-lens syndrome.

Too FLAT (loose): excessive movement and lag, edge stand-off or fluting, decentration, lens awareness, and vision that fluctuates with each blink.

Confirm the fit after ~5-10 minutes of settling, not immediately on insertion. Evaluate in primary, up-, and lateral gaze, and always check that the lens is not inside-out — a subtle flared edge causes discomfort and a poor fit.

Rigid lenses are evaluated with sodium fluorescein under cobalt-blue light (a yellow barrier filter boosts contrast):

- Apical clearance (steep): central pooling (bright green) with mid-peripheral bearing; the lens vaults the apex.

- Alignment (on K): thin, even fluorescein across the cornea with a smooth edge band — usually the goal.

- Apical touch (flat): central bearing (dark touch) with peripheral pooling; the lens rocks on the apex.

Edge clearance — the peripheral fluorescein band — should be roughly 0.5 mm wide to allow blink-driven tear exchange; too little seals the lens, too much causes edge awareness and 3-and-9 staining.

The tear (lacrimal) lens & SAM/FAP: tears trapped under a GP form a powered lens.

- Steeper Add Minus — a base curve steeper than K creates a plus tear lens, so add minus to the lens power.

- Flatter Add Plus — a flatter base curve creates a minus tear lens, so add plus.

- Rule of thumb: each 0.05 mm of base-curve change equals about 0.25 D of tear-lens power to compensate.

Because the tear lens fills the surface, a spherical GP masks most regular corneal astigmatism (roughly up to ~2-3 D) without a toric.

Soft toric lenses correct astigmatism and therefore need rotational stability so the cylinder axis stays put. Stabilization methods:

- Prism ballast — a thicker inferior lens that gravity and lid forces orient.

- Peri-ballast and dynamic stabilization (thin superior/inferior zones, or double slab-off) — the lids squeeze the thin zones to align the lens.

- Truncation — a flattened lower edge that rides the lower lid.

Toric lenses carry laser scribe marks (commonly at the 6-o'clock position) so you can read rotation at the slit lamp.

LARS — Left Add, Right Subtract: if the marking rotates to the patient's left (from your view), ADD those degrees to the prescribed axis; if it rotates right, SUBTRACT.

- Example: prescribed axis 180, mark rotates 10° left → order 180 + 10 = 190 → 010.

- Example: axis 090, mark rotates 15° right → order 075.

Let the lens settle and re-check rotation across several blinks; a lens that rotates inconsistently or more than ~20° needs a different design. Scleral lenses vault the entire cornea and limbus, landing on the sclera over a fluid reservoir — ideal for irregular corneas (keratoconus) and severe dry eye.

Because a spectacle lens sits ~12-14 mm in front of the cornea, its effective power at the corneal plane differs once the correction moves onto the eye. Compensate whenever the power exceeds ±4.00 D; below that the change is within tolerance.

Effectivity (vertex) formula:

Fc = Fs / (1 − d × Fs)

where Fs is the spectacle power, d is the vertex distance in meters, and Fc is the contact lens power.

The rules that matter for the exam:

- Minus lenses need less minus at the eye (power moves toward plano).

- Plus lenses need more plus at the eye.

Worked minus example: −8.00 D at 12 mm → Fc = −8.00 / (1 − 0.012 × −8.00) = −8.00 / 1.096 = −7.30 D.

Worked plus example: +8.00 D at 12 mm → Fc = +8.00 / (1 − 0.012 × +8.00) = +8.00 / 0.904 = +8.85 D.

For a sphero-cylinder Rx, vertex-compensate each principal meridian separately, then recombine. Always work in the minus-cylinder form and account for any residual astigmatism a soft sphere cannot correct.

A lens fits the eye by its sagittal depth (sag) — the vault from the lens back surface to the plane of its edge — not by base curve alone. Two variables set sag:

- Base curve radius: a steeper (smaller-radius) base curve increases sag.

- Overall diameter: a larger diameter at the same base curve increases sag.

This is why a flatter base curve can still fit tightly if the diameter is large — the two must be judged together.

Soft lenses come in a small range of base curves (for example 8.3-8.8 mm). Because soft material drapes over the cornea, base-curve changes have a modest effect; diameter changes are often more powerful. Refine mostly by movement and coverage.

GP lenses are selected relative to K. For with-the-rule corneal astigmatism, a common approach fits the base curve between flat and steep K (splitting the difference) so the lens aligns without excessive rock. Adjusting the base curve changes both the fluorescein pattern and the tear-lens power (SAM/FAP).

Choosing the right sag balances centration, movement, and comfort while preserving tear exchange.