Choosing a Contact Lens Base Curve from K Readings

Radius in millimeters is 337.5 divided by K in diopters, and the reverse. Corneal astigmatism is the steep K minus the flat K. GP base curves start from the flat K and follow a fitting philosophy: on K, steeper or flatter. Soft lenses are chosen by trial fit, because K readings predict the best soft base curve poorly.

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K readings recap

A keratometer measures the central cornea along its two principal meridians and reports each as a radius, a power, or both. The flatter one is the flat K, the steeper one the steep K. How to take and record readings is on the keratometry basics guide.

The 337.5 conversion

• Radius (mm) = 337.5 ÷ K (D)
• K (D) = 337.5 ÷ radius (mm)

The constant comes from the standard keratometric index, 1.3375. Higher diopters mean a steeper cornea and a shorter radius.

Try the keratometer

Take readings on the keratometer simulator, then convert both meridians by hand with the formula above.

Corneal vs refractive astigmatism

Corneal astigmatism is steep K minus flat K. The spectacle cylinder can differ from it, since the rest of the eye, such as the crystalline lens, adds its own. That difference is one reason a fitter compares the two before choosing a lens type.

GP vs soft base-curve logic

GP lenses. The flat K is the usual starting point. Fitting philosophies set the base curve against it: on K, steeper than K, or flatter than K, often varying with the amount of corneal astigmatism. These are conventions, not rules. The lens design’s own fitting guide governs, and the fluorescein pattern on the eye decides.

Soft lenses. K readings are a poor guide to the best soft base curve, and the number on the label is not the lens’s actual curvature once on the eye. Fitters trial-fit and judge centration, coverage and movement.

Worked examples

1. Convert both meridians

K readings 43.00 @ 180 and 44.50 @ 090.

1. Flat: 337.5 ÷ 43.00 = 7.85 mm.
2. Steep: 337.5 ÷ 44.50 = 7.58 mm.
3. Corneal astigmatism: 44.50 − 43.00 = 1.50 D, steeper at 090.

2. Steeper than K

Same eye, and a fitting guide calls for 0.50 D steeper than flat K.

1. 43.00 + 0.50 = 43.50 D.
2. 337.5 ÷ 43.50 = 7.76 mm. The radius is shorter than the on-K radius of 7.85 mm.

3. Radius to diopters

337.5 ÷ 7.80 = 43.27 D.

Practice set

Original problems. Round radii to two decimals.

1. Convert a K of 42.25 D to millimeters.

   Show answer

   337.5 ÷ 42.25 = 7.99 mm

2. Convert a radius of 7.40 mm to diopters.

   Show answer

   337.5 ÷ 7.40 = 45.61 D

3. K readings 41.75 @ 180 and 43.00 @ 090. How much corneal astigmatism, and which meridian is steeper?

   Show answer

   43.00 − 41.75 = 1.25 D, steeper at 090.

4. Flat K 45.00 D. What base-curve radius is “on K”?

   Show answer

   337.5 ÷ 45.00 = 7.50 mm

5. A soft lens comes in two base curves. The Ks are 43.50 / 44.25. Which do you order?

   Show answer

   Neither from the Ks alone. Trial fit and judge centration and movement on the eye.

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