CLToday Fitting Tip of the Month

May 2002

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Let the E-Value Be Your Guide When Fitting Aspheric GP Lenses

Having fitted more than 10,000 aspheric GP lenses in Holland over the past 20 years, I’d like to offer this tip. The fitting advice is the standard rule for these types of lenses and based on the specific lens eccentricity value.

The most common aspheric GP lenses in Holland (and throughout Western Europe) have an e-value of approximately 0.4 (or 0.45). (The second most used e-value is 0.6.) First, I’ll describe the general design of these lens types.

The central part is more or less spherical up to 20 or 25 degrees, followed by the elliptical shape up to the peripheral band of 0.3 mm or 0.4 mm wide with a curve of 12.0 mm, 12.25 mm or 12.5 mm. The edge curve radius is generally 0.6 mm or 0.65 mm.

For astigmatism from 0.50D to 3.00D, here's my standard fitting advice:

For lenses with an e-value of 0.4: Mean K + 0.10 mm to 0.12 mm gives the first choice base curve radius (BCR).

For lenses with an e-value of 0.6: Mean K + 0.05 mm to 0.07mm gives the first choice BCR.

It’s not unusual for experienced practitioners to order a lens with an e-value of 0.5 or even 0.7 or 0.8.

Ideal fluorescein pattern, based on with-the-rule corneal astigmatism of 1.00D for all these lenses up to e-value 0.7:

Flattest meridian: Almost full alignment from the center up to the small second curve. You'll see a very gentle touch at the cornea periphery. The edge gives a small, medium-bright green appearance.

Steeper meridian: Full alignment in the center but starting from the mid-periphery, the fluorescein pattern becomes gradually more apparent.

When a with-the-rule cornea is fitted properly, you’ll see the famous “bow-tie” or “propeller” shape.

As for movement and positioning of the lens, after each blink, you’ll generally see smooth, downward movement, good central position, and then the lens moves downward slowly to the eyelid until the next blink. The general comfort is very good.

For higher with-the-rule astigmatism (> 3.00D), choose peripheral toric designs; in Europe we use bi-axial aspherics.

Of course, if the astigmatism is against-the-rule, choose a little bit steeper BCR, but not more than 0.05 mm steeper.

Three and nine o’clock staining is very rare.

Mid or higher Dk materials are the best.

If tear exchange is poor, you may observe some lens binding or sticking after some time. If this occurs, choose the base curve 0.05 mm flatter with the same eccentricity or choose the e-value 0.1 higher with the same base curve.

Use good, non-polishing lens cleaners.

John Schilperoort
Contact Lens Practitioner
Member of the Educational Team of the
Utrecht High School for Optometry
Section Contact Lenses

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