Section TWO Rigid gas-permeable lens fitting

References

1.Stone J. The possible influence of contact lenses on myopia. British Journal of Physiological Optics 1976;31:89–114.

2.Black-Kelly TSB, Butler D. The present position of contact lenses in relation to myopia. British Journal of Physiological Optics 1971;27:33–48.

3.Perrigin J, Perrigin D, Quintero S, Grosvenor T. Silicone/acrylate contact lenses for myopia control: 3-year results. Optometry and Vision Science 1990;67:764–5.

4.Mountford J. Accelerated Orthokeratology Brisbane; 1996.

5.Cho P, Cheung SW, Edwards M. The longitudinal orthokeratology research in children LORIC in Hong Kong: a pilot study on refractive changes and myopia control. Current Eye Research 2005;30(1):71–80.

6.Jessen GN. Orthofocus techniques. Contacto 1962;6(7):200–4.

7.Orthokeratology. Vol 1. International Orthokeratology Section of N.E.R.F publication; 1972.

8.Orthokeratology. Vol 2. International Orthokeratology Section of N.E.R.F. publication; 1974.

9.Grant SC, May CH. Orthokeratology – a therapeutic approach to contact lens procedures. Contacto 1970;14(4):3–16.

10. Coon LJ. Orthokeratology Part 2: Evaluating the Tabb method. Journal of the American Optometric Association 1984;55:409–18.

11. Swarbrick, H, Alhabria, A. Overnight orthokeratology induces central corneal epithelial thinning. Poster, AAO Meeting, December 2001; 2001.

12. Mountford J. Advanced orthokeratology: Part 1: History, lens design and mode of action. Optician 2002;224(5862):20–5.

Further reading

Mountford J, Ruston D, Dave T. Orthokeratology. Oxford: Butterworth-Heinemann; 2004.

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Section

Hydrogel and silicone hydrogel fitting THREE

Soft lens fitting 15CHAPTER and design

15.1 Fitting considerations

15.1.1 Sagittal height and corneal measurement

The curvature of the central cornea has a relatively small influence on its sagittal height compared with normal variations in shape factor and diameter. Keratometry on its own is therefore a poor predictor of the optimum soft lens radius. There is also a positive correlation between corneal diameter and corneal curvature so that flatter corneas generally have larger diameters and vice versa. A similar relationship exists for sagittal height. One of the most important influences on the optimum soft lens fitting is therefore the corneal shape factor or p value, although this can generally only be obtained by using either an autokeratometer or topographer.

Atypical combinations of corneal radius and diameter are occasionally also found:

Large corneas with steep radii

•Have relatively large sagittal heights.

•Need soft lenses with large sagittal depths.

•Require steep lenses.

Small corneas with flat radii

•Have small sagittal heights.

•Need soft lenses with small sagittal depths.

•Require flat lenses.

©2010 Elsevier Ltd, Inc, BV

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

Section THREE Hydrogel and silicone hydrogel fitting

15.1.2 Dynamic assessment of fitting

Soft lens fittings of all types are assessed dynamically in relation to lens movement with:

•The ‘push-up’ test.

•Blinking.

•Upwards gaze.

•Lateral gaze.

The ‘push-up’ test

This simple test, which has become one of the main assessment techniques for soft lenses, can be used to evaluate two aspects of the dynamic performance of most soft lenses on the eye.

•The lens is pushed vertically upwards by digital manipulation of the lower lid margin and the resistance to decentration is assessed. An optimum fitting gives little or no resistance to movement.

•When the pressure of the lower lid is released, the speed of recovery is observed. Rapid movement, similar to that observed on blinking, indicates a satisfactory fitting whereas a slow recovery may be indicative of tightness.

A grading system has been suggested where 100% denotes a lens which is impossible to move and 0% one which would fall off the eye: 50% represents an optimum fitting.1 Both components of the test, however, are important so that a lens which is difficult to decentre but recovers rapidly suggests negative pressure within the tear film. This type of fitting should be particularly avoided with silicone hydrogels.

15.1.3 Design factors

The most appropriate design has to be selected from the very wide range of lens forms now available. Several factors must be taken into account:

•Size (corneal or semi-scleral).

•Material (hydrogel or silicone hydrogel).

•Water content (low, medium or high).

•Dk.

•Thickness (standard or thin) (see Section 17.4).

•Geometric and optic design (spherical or aspheric).

•Manufacturing method (lathed or moulded).

•Lens flexibility (see Section 17.2).

•Lens power (see Section 17.1).

•Disposable or conventional.

All of these factors have some influence on vision, comfort and fitting characteristics but an important consideration has always been the total diameter. The two main fitting philosophies into which soft lenses were traditionally

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divided were therefore corneal and semi-scleral. There has been a progressive convergence between these two approaches so that there is now considerable overlap, especially with single diameter lenses. With large corneas, some lenses become corneal fittings by default.

The majority of disposable lenses are available only with a single total diameter and many offer no choice of radius. This simplifies fitting but means that if a lens proves unsatisfactory on the eye it is necessary to change to a different variety. It is therefore essential to draw on a wide range of lens types to provide an optimum fitting for the greatest number of patients.

15.2 Corneal diameter lenses

True corneal diameter lenses with TDs less than about 13.50 mm are now in the minority. They are almost invariably conventional as opposed to disposable and historically were manufactured from low to medium water materials to give reproducible lenses of good durability. The thinner varieties, in particular, cause minimum interference with corneal metabolism and give excellent cosmetic appearance. Some patients reject the cosmetic appearance of a semi-scleral lens which overlaps onto the sclera.

Indications

•Small corneas.

•Where all disposable lenses are too large.

•Small palpebral apertures.

•Difficulty in handling larger lenses.

•Cosmetic reasons.

Contraindications

•Very large corneas.

•Shallow corneoscleral junction allowing decentration.

•Tight lids causing lens decentration.

•Sensitive lid margin.

•Sensitive limbus.

Fitting

Radius

•Radius selection is based on keratometry.

•Most radii are between 7.90 and 8.90 mm.

•Less flexible low water content materials may require radii 0.70 mm or more flatter than ‘K’.

•The radius for standard HEMA lenses is usually between 0.30 and 0.60 mm flatter than ‘K’.

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Section THREE Hydrogel and silicone hydrogel fitting

•High water content lenses are fitted closer to alignment.

•Fitting steps are usually between 0.20 and 0.40 mm.

•Most corneal lenses have a single curve back surface.

Total diameter

•Lenses should be just slightly larger than the horizontal visible iris diameter (HVID). They should extend beyond the limbus by up to 0.50–0.75 mm to avoid irritation.

•Most corneal lenses vary in size from 12.50 to 13.50 mm, with the possible range from 12.00 to 14.00 mm.

•High water content lenses are fitted approximately 0.50 mm larger than HEMA.

•High plus and high minus lenses are fitted approximately 0.50 mm larger than low powers in order to achieve stability on the cornea.

•Fitting steps are usually 0.50 mm.

Power

After allowing for vertex distance considerations, the lens power is usually within 0.25 D of the spectacle Rx. Thicker designs require about 0.25 D less minus than thin lenses.

Fitting appearance

Fitting characteristics are mainly as described in Chapter 16, but it is essential for a correctly fitting lens to give complete corneal coverage with proper centration to avoid the risk of epithelial dehydration and arcuate staining of any exposed area. The slit lamp should be used for careful observation of centration and movement, since these can be significantly influenced by factors such as:

•Corneal topography.

•Limbal topography.

•Lid pressure.

•Tear forces.

•Size of palpebral aperture.

•Position of cornea within palpebral aperture.

Figure 15.1 shows the four common ways in which a lens may position on the cornea with the eye in the primary position:

(a)An optimum fitting is shown. The lens is perfectly centred and there should be 0.25–0.50 mm of vertical movement on blinking.

(b)The lens is riding high, influenced perhaps by a tight upper lid. This may prove acceptable, provided that the decentration is no more than about 0.50 mm. An attempt should be made to improve the fitting by selecting a larger diameter.

(c)The lens is riding in a low position. It generally represents an unsatisfactory fitting which is either too small or too flat and the patient is likely to complain of unacceptable lid sensation. It can also occur with the downwards pressure of a relatively heavy upper lid. This

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creates a fitting which is too tight, although initially quite comfortable. There is the possibility, after several hours of wear, of arcuate staining at the superior limbus together with oedema from insufficient tears exchange.

(d)The lens is eccentrically located. This may also be due to a fitting which is too small or too flat, or because of lid pressure with a shallow corneoscleral junction. It represents an unsatisfactory fitting and a larger total diameter should be tried. However, if the decentration is limited to 0.50 mm, it may occasionally prove acceptable. An attempt should always be made to improve the fitting characteristics of a decentred lens by selecting a larger diameter. Where this fails, it may be necessary to consider a semi-scleral lens.

A B

C D

Figure 15.1  The four common positions (see text) taken up by soft lenses of corneal size on an eye in the primary position: (A) correctly centred;

(B) slightly high; (C) slightly low; (D) laterally decentred

Clinical equivalents

The principle of clinical equivalents applies where two lenses of the same design and material, with different but related parameters, give similar fitting characteristics on the eye.

RULE OF THUMB

A change in diameter of 0.50 mm ≡ a change in radius of 0.20 mm.

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To improve a loose fitting

•Select a larger total diameter.

•Select a steeper radius.

•Use a more rigid or lower water content material.

•Use a different lens thickness.

To improve a tight fitting

•Select a flatter radius.

•Select a smaller total diameter.

•Use a less rigid or higher water content material.

•Use a different lens thickness.

15.2.1 Example of a corneal diameter lens

Lunelle ES 70 (CooperVision)

A high water content mainly corneal diameter hydrogel lens, manufactured by lathing. One of the few high water content lenses available in a small diameter.

Material properties

Lens geometry

•Centre thickness is 0.20 mm at −3.00 D

•Back surface is a single curve.

•Front surface is lenticulated.

Parameters available

See Table 15.1.

Table 15.1  Parameters available for Lunelle ES 70 lenses

Fitting technique

•The 14.00 mm diameter is selected for corneas larger than 11.25 mm; the 13.00 mm diameter for corneas 11.25 mm or smaller.

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