Section THREE Hydrogel and silicone hydrogel fitting

•Excessive movement on blinking (over 0.75 mm).

•Excessive lag on lateral or upwards eye movements (over 1.5 mm).

• Poor centration.

•Poor comfort because of lid sensation.

•Visual acuity variable.

•Retinoscopy reflex clear centrally with peripheral distortion.

•Variable vision.

•Refraction variable because of lens movement.

•Keratometer mires vary with lens movement, giving peripheral distortion.

• Buckling of lens edge.

16.4 Summary of soft lens fitting characteristics

The general fitting characteristics for soft lenses are summarized in Table 16.1. Each lens type and lens make is likely to have its own individual fitting characteristics, depending upon its material, water content, total diameter, method of manufacture and the various other points discussed in Chapter 15 on ‘Soft lens fitting and design’ and Chapter 17 on ‘Other soft lens fitting considerations’. Table 16.1 is therefore intended to give an overview of the general fitting char-

acteristics which the contact lens practitioner may expect to find.

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17.1 Lens power

Power is an important consideration in deciding which type of lens to fit, particularly with new patients.

Low minus (<−2.00 D)

•Thin lenses should be avoided because of handling difficulties and the greater chance of dehydration.

•A thicker silicone hydrogel, or medium to high water content hydrogel should be selected.

High minus (>−6.00 D) and medium to high plus (>+3.00 D)

•Silicone hydrogels are the likely first choice to avoid any physiological problems.

•Low water content hydrogels should be avoided because of their greater thickness and low Dks.

•Larger diameter (semi-scleral) lenses usually give better stability   of fitting.

©2010 Elsevier Ltd, Inc, BV

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

Section THREE Hydrogel and silicone hydrogel fitting

Medium minus (−2.00 D to −6.00 D) and low plus (<+3.00 D)

•Silicone hydrogels are the likely first choice.

•High water content or thin hydrogels are also possible, except where problems arise with fitting characteristics, dehydration or visual acuity.

17.2 Lens flexibility and modulus of elasticity

An important influence on the fitting characteristics of all lenses is the flexibility of the material. This explains why two lenses of apparently the same specification but different material can behave in entirely different ways on the cornea.1 The original Permalens, for example, which is very flexible, was often required to be fitted steeper than ‘K’ compared with other more rigid materials where the more usual flatter than ‘K’ approach is correct. Similarly, moulded or spun-cast lenses with a thin overall cross-section and inherently greater flexibility than their lathed counterparts lend themselves better to a ‘one-fit’ fitting philosophy which relies on draping the cornea.

Another important influence on the fitting characteristics of all lenses is the modulus of elasticity of the material (see Table 19.1). This defines a material’s relative stiffness which has a strong influence on lens fit. Materials with a high modulus are generally much easier to handle but are more likely to cause arcuate staining (see Section 19.5.1). Materials with a low modulus, in addition to handling difficulties because of their flexibility, may well show less movement together with greater decentration on the eye. Variations in modulus of elasticity can also give a large difference in sagittal depth which, in turn, affects the fitting characteristics.

PRACTICAL ADVICE

•Because of flexibility and manufacturing considerations, the lenses from one laboratory cannot necessarily be duplicated by another merely by ordering the same nominal specification.

•Fitting should be carried out with trial lenses of the type to be ordered to ensure optimum reliability.

•For conventional lenses, this is no longer feasible with current regulations but many laboratories will now supply prescription lenses on an exchange basis.

•With disposables, lenses can be taken from a fitting bank or trial lenses ordered from the laboratory.

17.3 Additional visual considerations

Flexure and liquid lens power

Flexure occurs when a soft lens fitted flatter or steeper than ‘K’ bends to follow the corneal curvature. The refractive effect with both plus and minus lenses is to add negative power.2,3

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Other soft lens fitting considerations 17 Chapter

A liquid lens occurs if the posterior surface of the soft lens fails to conform to the front surface of the cornea.4 This is more likely to be present with semi-scleral designs, whereas there is virtually no liquid lens with ultrathin lenses which completely drape the cornea.

PRACTICAL ADVICE

•Any discrepancy between contact lens and spectacle Rx (allowing for vertex distance) is caused mainly by flexure but possibly by liquid lens power.

•This is unlikely to be greater than 0.50 D with a satisfactory fitting.

•Thin corneal lenses require more minus power than thick semi-scleral lenses.

Astigmatism

The usual limit for acceptable acuity with a thin spherical lens is about 0.75 DC, but it can be as little as 0.50 DC for critical observers or patients with large pupils. Occasionally, acceptable vision is obtained with much higher cylinders but with the wide range of conventional and disposable torics now available, these should be considered for astigmatism from 0.75 D upwards. With an amblyopic eye, however, there may be no advantage in the additional complexity and cost of a toric lens.

Soft lenses are generally fitted flatter than ‘K’ so that less minus is required compared with the best vision sphere used with rigid lenses.

Example:

Spectacle Rx −3.50/−1.00 × 180

Likely BVP of spherical soft lens −3.50 D.

Wavefront technology and ocular aberrations

Some varieties of soft lens (e.g. Purevision from Bausch & Lomb; Definitions from VeniVidi) employ a similar approach to that used in laser surgery where wavefront technology can improve the potential visual acuity with aberration control. Soft lenses are manufactured to incorporate spherical aberration to neutralize the average value of the human eye. It is claimed that such lenses can also give improved acuity with low levels of astigmatism, e.g. Frequency 55 Aspheric.

Environmental factors

The power of a soft lens depends on its basic dimensions of radius, diameter, thickness and refractive index which can all vary with environmental factors. These include ocular effects such as temperature, pH, tonicity and volume of tears. Some of these are in turn influenced by external factors such as ambient temperature, humidity, or the degree of lens hydration when placed on the eye. Generally, high water content hydrogel materials undergo larger changes and give greater variation in vision than HEMA or silicone hydrogels.

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