Contact Lens Fitting
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Direct answer
The fluorescein pattern tells you everything a fitting chapter tries to say: a rigid lens fitted on the flat K should show a thin, uniform green film across the cornea; a steep (too-tight) lens pools dye centrally and barely moves; a flat lens touches the apex, floods the periphery and rides around excessively. Soft lenses are fitted by vault — a base curve roughly 0.6–0.8 mm flatter than the flat corneal radius, centred, covering the limbus, moving about half to one millimetre on a blink — while toric soft lenses are judged by their rotation marker, corrected by the LARS rule (left add, right subtract). Oxygen carries the safety case: the Holden–Mertz criterion sets Dk/t of about 87 as the threshold for overnight wear, which is why silicone hydrogel dominates extended wear.
What you must remember
- Fitting data first: keratometry, horizontal visible iris diameter, dim-light pupil diameter, palpebral aperture and tear quality (break-up time comfortably above 10 seconds).
- Rigid (RGP) base curve: begin on the flat K, typically 0.05–0.10 mm flatter for an alignment fit; diameter commonly 9.2–9.8 mm.
- Fluorescein patterns: alignment — even thin film; steep — central pooling, midperipheral touch, minimal movement; flat — central bearing with excessive edge clearance and movement.
- Soft lens fit: base curve flatter than the cornea (about 8.3–8.9 mm for average eyes), diameter 14.0–14.5 mm extending 1–1.5 mm beyond the limbus, movement 0.5–1.0 mm on blink, stable centration in all gazes.
- Toric soft lenses: the orientation mark should sit at 6 o'clock; rotation is corrected by LARS — if the mark rotates to your left, add the degrees to the cylinder axis; right, subtract.
- Oxygen: Dk/t decides safety — Holden–Mertz found roughly 87 needed for overnight wear versus about 24 daily; silicone hydrogel clears the overnight bar standard hydrogels fail.
- Trial lenses settle 20–30 minutes before over-refraction; the final power is the over-refraction added to the diagnostic lens power.
- Therapeutic (bandage) soft lenses protect the cornea in epithelial disease; paediatric and aphakic fits follow the same principles.
Fitting a rigid lens on an astigmatic cornea
A 26-year-old wants out of spectacles: K readings 44.25 at 180 and 45.75 at 90 — 1.50 D of with-the-rule corneal astigmatism. The flat K of 44.25 D converts to a radius near 7.63 mm, so the diagnostic rigid lens selected is about 7.65–7.70 mm, fractionally flatter. Sodium fluorescein under a burton lamp reads the fit: a faint vertical band of touch and horizontal pooling — the lens bridges the steep vertical meridian slightly, so a marginally steeper base curve is tried and the second pattern evens out.
On blink the lens drops a millimetre and recentres — good. The over-refraction after settling is −0.50 DS, so the order takes the diagnostic power minus half a dioptre. Had the pattern shown a dark central touch with a bright edge ring, the lens was too flat — the exact opposite reading, and the two are swapped by more candidates at viva than any other fact in fitting. Toric back-surface designs enter only if the astigmatism climbs much higher.
Where students slip
The steep/flat inversion is the classic: steep pools centrally, flat touches centrally — remember that a steep lens grips the cornea like a cap and therefore barely moves, and the pattern follows logically. LARS trips the second wave: the rule refers to the observer's view of marker rotation, and the correction applies to the axis, not the power. Third, comfort is not a fitting parameter: a tight soft lens moves nothing, blanches the edge conjunctiva and starves the cornea — push-up test and blink movement are judged at the slit lamp, not by asking the patient. And the oxygen question has a number for an answer: Dk/t near 87 (Holden–Mertz) for overnight wear — "high oxygen permeability" earns half a mark at best.
Frequently asked questions
How is a rigid lens base curve initially selected?
On the flat keratometry reading — about 0.05–0.10 mm flatter than the flat K for alignment — then refined by fluorescein pattern, movement and centration.
What fluorescein pattern indicates a steep rigid fit?
Central pooling with midperipheral bearing, reduced movement and edge seal-off — the lens vaults too much; flatten the base curve.
What is the LARS rule for toric soft lenses?
If the marker rotates to the observer's left, add the degrees of rotation to the cylinder axis; if right, subtract — applied before ordering.
How much should a well-fitted soft lens move on blink?
About 0.5–1.0 mm of lag with smooth recentring; no movement suggests a tight fit, excessive movement a flat or loose one.
What Dk/t is required for overnight wear?
Around 87 per the Holden–Mertz criterion — roughly 24 suffices for daily wear; silicone hydrogel materials meet the overnight threshold.
Why does a trial lens settle before over-refraction?
Tears and temperature equilibrate and the lens finds its final position over 20–30 minutes, so the over-refraction reflects the fit the patient will actually wear.