Section two Rigid gas-permeable lens fitting

RULE OF THUMB FOR SPHERICAL LENSES

An increase in the BOZD of 0.50 mm requires the BOZR to be flattened by 0.05 mm to maintain the same fluorescein pattern.

The principle of clinical equivalents still applies to aspheric lenses, but the differences found with tear layer theory are greater.2

Example 2 (elliptical lens notional BOZR:BOZD): 7.70:7.00 ≡ 7.75:7.70 ≡ 7.80:8.50

RULE OF THUMB FOR ASPHERIC LENSES

An increase in the BOZD of 0.70 mm requires an increase in the notional BOZR of 0.05 mm to give the same tear layer thickness.

11.7 Astigmatic fitting

A with-the-rule astigmatic cornea fitted with a spherical lens shows:

•With an alignment fitting, blue touch in the horizontal meridian and green stand off vertically.

•With a steep fitting, a vertical ellipse of fluorescein pooling with midperipheral horizontal touch.

In fitting a toric cornea with a spherical lens, the best result is achieved with a horizontal band of touch by choosing a lens on flattest ‘K’ or slightly flatter. Touch of this nature limits lens flexure and provides good vision as long as edge clearance is adequate in one meridian and not excessive in the other.

Against-the-rule astigmatism shows the same fitting patterns with the  meridians reversed.

11.8 Peripheral fitting

The ideal peripheral clearance is 60–80 µm, equivalent to an edge lift of 0.12– 0.15 mm. It depends on corneal topography and method of fitting. In practical terms, this gives an annulus of fluorescein about 0.40 mm wide at the edge of the lens. The limit of clinical significance is about 10 µm.

Tight periphery

Appearance

•Good centration.

•Periphery presses the limbus on blinking and may cause discomfort.

•Poor tears exchange under the lens and several blinks necessary for fluorescein circulation.

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Fluorescein patterns and fitting 11 Chapter

To improve a tight periphery

An increase in edge clearance of at least 10–15  m is necessary to give a discernible change in fluorescein pattern. Axial edge lift should be increased in increments of 0.03 mm (e.g. from 0.12 mm to 0.15 mm).

•Use a flatter peripheral radius (e.g. 10.75 mm instead of 10.25 mm).

•Add one or more flatter peripheral curves (e.g. 12.25 mm, 0.4 mm wide; or 15.00 mm, 0.2 mm wide).

•Increase the width of the peripheral curves.

•Use a flatter BOZR and smaller BOZD.

•Increase blending of peripheral curves.

•Change the lens design. Centration affects the peripheral interaction with the cornea.

Loose periphery

Appearance

•Lens rides high and does not drop after a blink.

•Bubbles are found at the edge and superior dimpling may occur.

•The edge lifts away from the cornea on blinking.

•The lens is unstable on excursion movements.

•Frequently gives 3 and 9 o’clock staining.

To improve a loose periphery

To improve a loose periphery requires a decrease in edge clearance of at least 10–15  m. The axial edge lift is decreased in increments of 0.03 mm (e.g. from

0.15 mm to 0.12 mm).

•Use a tighter peripheral radius (e.g. 10.00 mm instead of 10.50 mm).

•Reduce the width of the peripheral curves.

•Use a larger BOZD, possibly with a flatter BOZR to compensate.

•Change the lens design and possibly try an aspheric.

Lens position (Table 11.1)

The position the lens assumes on the cornea may indicate a fitting problem in respect of lid tension or possible decentration of the corneal apex.3

GENERAL ADVICE

•Use a negative carrier for low-riding plus lenses.

•Aspheric lenses sometimes give better centration and comfort with plus lenses.

•A displaced corneal apex causes lens decentration even if the fitting is not too flat. Use a larger TD if flare is a problem.

•Tight lids with a toric cornea also cause decentration. Consider a back surface toric lens.

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Section two Rigid gas-permeable lens fitting

Table 11.1  Lens position

References

1.Phillips AJ. Rigid gas permeable corneal lens fitting. In Contact Lenses, 5th edn, Phillips AJ, Speedwell L, editors. Oxford: Butterworth-Heinemann; 2007.  

p. 313–357.

2.Atkinson T. The development of the back surface design of rigid lenses. Contax 1987;November:5–18.

3.Morris J. RGP lenses Part 2 – Fitting procedures. Optician 2004;228(5976):28–35.

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Aspheric lens design has evolved because clinical models have shown the overall form of the cornea to be elliptical.1 The variation in the shape of an ellipse is called the eccentricity (e) and is an important factor in lens design and fitting. Mathematically, it is always less than 1 (see Section 9.3).

12.1 Advantages and disadvantages of aspherics

Advantages

•Fit more closely to the corneal topography (Figure 12.1).

•Distribute pressure more evenly over the cornea.

•Edge lift or Z value is smaller, giving less lid sensation.

•Can sometimes fit up to 4.00 D of astigmatism.

•Some designs can give improved distance vision; others can assist presbyopia (see Section 24.4.1).

•Absence of transition zones assists tear flow.

Disadvantages

•Manufacture requires sophisticated lathes.

•Reproducibility and verification more difficult.

•Aberrations with some back surface designs.

•Decentration if fitted flatter than ‘K’ to obtain movement.

•Decentration with a decentred corneal apex.

©2010 Elsevier Ltd, Inc, BV

DOI: 10.1016/B978-0-7506-7590-1.00011-X