17.2 Fitting Workflows: K Readings, Tear Lens, and the SAM/FAP Rule

17.2 Fitting Workflows: K Readings, Tear Lens, and the SAM/FAP Rule

A contact lens fit is a sequence: measure the cornea, pick a base curve relative to it, evaluate the physical fit, then refine power. The COA stem usually hands you a number and asks for the next correct step.

Step 1 — Keratometry (K readings)

Keratometry measures the curvature of the central anterior cornea in two principal meridians, reported in diopters or millimeters (e.g., 43.00 @ 180 / 44.00 @ 090). The difference between the two meridians is the corneal (with-the-rule or against-the-rule) astigmatism. K readings are the anchor for choosing an RGP base curve. Conversion is fixed: a steeper cornea has a higher diopter value and a smaller millimeter radius; 7.85 mm corresponds to about 43.00 D.

With-the-rule astigmatism has the steeper meridian vertical (near 90 degrees), the most common pattern in young eyes; against-the-rule has the steeper meridian horizontal and becomes more common with age. Keratometry also assesses tear-film quality — distorted or jumping mires suggest dry eye that will compromise lens comfort, a point the assistant should flag before fitting.

Step 2 — Pick the base curve relative to K

For RGP lenses the relationship of base curve to K defines the fit:

A practical starting rule for spherical corneas: begin on K for low astigmatism, fit slightly steeper as corneal cylinder increases (often 0.25 D steeper for each 0.50 D of with-the-rule astigmatism), and verify with fluorescein. The goal is a centered lens that moves 1–2 mm on a blink and exchanges tears without bubbling or sealing off.

Step 3 — The tear lens and SAM/FAP

When an RGP sits on the cornea, the tear film trapped under it acts as a lens. The exam-critical rule:

• SAM — Steeper Add Minus: if you fit the lens steeper than K, the tear lens has plus power, so you must add minus power to the contact lens to compensate.
• FAP — Flatter Add Plus: if you fit flatter than K, the tear lens has minus power, so you add plus power.

Magnitude: roughly 0.25 D of tear-lens power for every 0.05 mm (about every 0.25 D) of base-curve change from K. Worked example: spectacle Rx is −3.00 D, K is 43.00 D, and you fit 0.50 D steeper than K. Steeper Add Minus → add −0.50 D → the contact lens power becomes about −3.50 D. A spherical RGP tear layer neutralizes the great majority of regular corneal astigmatism, which is why RGPs give crisp vision on astigmatic corneas.

Step 4 — Vertex distance

Spectacles sit ~12 mm in front of the cornea; contacts sit on it. For powers above about ±4.00 D, you must convert spectacle power to the corneal plane. Minus lenses need less minus as a contact; plus lenses need more plus. Use the formula F_cl = F_spec / (1 − d·F_spec), with d in meters (0.012 m). Below ±4.00 D the change is clinically negligible and is usually ignored.

Soft-lens workflow contrast

Soft lenses are fit empirically: choose base curve and diameter from the patient's K and corneal diameter, insert, let settle 5–10 minutes, then assess centration, movement (about 0.25–0.50 mm on blink), and coverage. Too little movement = tight (flatten the lens / increase the base-curve number). Too much movement = loose (steepen the lens / lower the base-curve number). A soft toric must also show stable rotational orientation.

Reading fluorescein patterns

For RGP fits, sodium fluorescein dye is instilled and viewed with a cobalt-blue light. The dye glows green wherever the tear film is thicker. A well-aligned (on-K) fit shows a thin, even film with slightly more pooling at the edge. Central pooling (bright green center) means the lens is too steep — apical clearance — and traps tears; central touch (a dark, non-staining center surrounded by an edge ring) means it is too flat — apical bearing — and the lens rocks on the apex. Excessive edge lift lets the lens dislodge; inadequate edge lift seals the lens and starves the cornea of tear exchange.

Note that high-water soft lenses absorb fluorescein, so the standard dye is not used to assess soft-lens fit; movement and centration are judged instead.

Trial-lens overrefraction

The most reliable way to finalize power is to put a trial lens of the chosen base curve on the eye, let it settle, then overrefract — measure the residual refractive error over the lens and add it to the lens power. This automatically accounts for tear-lens and vertex effects, which is why an experienced fitter trusts the overrefraction over a paper calculation when the two disagree.

Putting the steps together

The full RGP sequence the exam expects: take K readings → select a diagnostic base curve relative to K → insert and settle → evaluate fluorescein pattern and movement → overrefract → apply SAM/FAP and vertex logic to confirm power → order final parameters. Skipping straight to ordering from spectacle numbers without a diagnostic fit is the classic wrong workflow. If two answers seem reasonable, the better one keeps a diagnostic lens on the eye and verifies the fit physically rather than relying on calculation alone.