Lesson · NCLE
Contact lens fitting
From K readings to a finished order: base curve, spectacle-to-CL power, corneal and residual astigmatism, SAM-FAP, LARS, fluorescein patterns and over-refraction.
After this lesson you can
About 20 minutes- Convert K readings to a base curve and back
- Turn a glasses Rx into a spherical or toric soft lens power
- Work an RGP power with the four-step method and SAM-FAP
- Correct a rotating toric with LARS and read a fluorescein pattern
K readings and base curve
The keratometer measures the curvature of the front of the cornea in its two principal meridians. The flatter reading is “flat K”. The difference between the two is the corneal astigmatism.
BC (mm) = 337.5 ÷ K K (D) = 337.5 ÷ BC
K to base curve
K reading 43.50 D.
- 337.5 ÷ 43.50 = 7.759
Answer: 7.76 mm
- With-the-rule (WTR): the vertical meridian (near 90) is steeper. In minus cylinder the axis is near 180.
- Against-the-rule (ATR): the horizontal meridian (near 180) is steeper. In minus cylinder the axis is near 90.
Try it now
Glasses Rx to soft lens power
- Put the Rx in minus cylinder.
- Cylinder 1.00 D or less: use the spherical equivalent and fit a spherical lens. More than 1.00 D: fit a toric and work each principal meridian.
- Compensate for vertex distance, from the refracting distance to 0 mm on the cornea.
- Round to the nearest 0.25 D.
Spherical soft lens
Glasses Rx −4.50 −0.75 × 180, refracted at 12 mm.
- Cylinder 0.75 is 1.00 or less, so use the SE: −4.50 + (−0.75 ÷ 2) = −4.88
- DC = 12 × 4.875 × 4.875 ÷ 1000 = 0.285
- Moving in, compensate with plus: −4.88 + 0.29 = −4.59
Answer: −4.50 D
Try it now
RGP power: corneal or residual, tear lens and SAM-FAP
A rigid lens traps a tear lens between itself and the cornea. That tear lens neutralises the corneal astigmatism and adds its own power, so the RGP power is worked in four steps.
- Minus cylinder: put the Rx in minus-cylinder form.
- Corneal or residual: if the Rx cylinder equals the K difference, the astigmatism is corneal and the tear lens corrects it; drop the cylinder. If not, it is residual and a spherical RGP will leave it uncorrected.
- Vertex: compensate the sphere to the cornea and round to 0.25 D.
- SAM-FAP: fitting steeper than K adds plus tear lens, so add minus; flatter than K adds minus, so add plus.
RGP power
Rx −3.00 −1.00 × 180 at 12 mm; K 43.00 @ 180 / 44.00 @ 090; base curve 43.50 D.
- Rx already in minus cylinder.
- K difference 1.00 = Rx cylinder 1.00: corneal. Drop the cylinder.
- Vertex −3.00: −2.89, rounds to −3.00
- BC is 0.50 D steeper than flat K, so add minus: −3.00 − 0.50 = −3.50
Answer: −3.50 D
Try it now
Toric rotation: LARS
A toric soft lens has a mark that should sit at 6 o’clock. If it rotates, correct the axis you order by the amount of rotation, as seen from your side of the slit lamp.
Left Add, Right Subtract
Correcting a rotated toric
Lens ordered −2.00 −1.25 × 170; the mark rests 15° to your left.
- Left, so add: 170 + 15 = 185, which is written 005
Answer: −2.00 −1.25 × 005
Reading a fluorescein pattern
| Pattern | Fit | Change |
|---|---|---|
| Even, light green centre with a bright edge band | Aligned, on K | None |
| Bright pool in the centre, dark band in the mid-periphery | Too steep (apical clearance) | Flatten the base curve |
| Dark centre, wide bright edge | Too flat (apical touch) | Steepen the base curve |
| Dark band along 3 and 9 o’clock, pooling top and bottom | With-the-rule cornea | Confirm with the K readings |
| Dark band along 12 and 6 o’clock, pooling at the sides | Against-the-rule cornea | Confirm with the K readings |
Try it now
Over-refraction
An over-refraction is done over the trial lens and added to it. Put the over-refraction in minus cylinder first.
- Cylinder 1.00 D or less: add its spherical equivalent to the lens sphere.
- Cylinder more than 1.00 D: add sphere to sphere and cylinder to cylinder, and use the over-refraction axis.
Over-refraction on a spherical lens
Trial lens −3.00 DS; over-refraction +0.50 −0.50 × 090.
- Cylinder 0.50 is small, so use the SE: +0.50 + (−0.50 ÷ 2) = +0.25
- −3.00 + (+0.25) = −2.75
Answer: −2.75 DS
Contact lens tolerances
| Parameter | PMMA | RGP | Soft |
|---|---|---|---|
| Diameter | ± 0.05 mm | ± 0.05 mm | ± 0.25 mm |
| Base curve | ± 0.25 mm | ± 0.05 mm | ± 0.20 mm |
| Centre thickness | ± 0.02 mm | ± 0.02 mm | ± 0.03 mm |
| Power 0.00 to 5.00 D | ± 0.12 D | ± 0.12 D | ± 0.25 D |
| Power 5.12 to 10.00 D | ± 0.18 D | ± 0.18 D | ± 0.25 D |
| Power 10.12 to 15.00 D | ± 0.25 D | ± 0.25 D | ± 0.50 D |
| Power 15.12 to 20.00 D | ± 0.37 D | ± 0.37 D | ± 0.50 D |
| Power over 20 D | ± 0.50 D | ± 0.50 D | ± 0.75 D |
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