Section three Hydrogel and silicone hydrogel fitting

projected lifespan is now reduced, with the greater emphasis on high water content and silicone hydrogel lenses and the disappearance of intensive cleaning systems. In addition, modern multipurpose solutions are easier to use but less efficient at cleaning than peroxide systems. Many practitioners have therefore devised schemes that encourage patients to replace lenses on a frequent and planned basis before they reach the troublesome stage. The time interval is usually 3 months.

The advent of modern production techniques, particularly moulding, has reduced manufacturing costs so that what used to be regarded as conventional lenses are now produced specifically for planned disposal. The replacement interval should be controlled by the practitioner­ but depends upon the make of lens, laboratory system and individual patient requirements. Sometimes, new lenses are exchanged with the patient at three monthly aftercare visits with the additional advantage that the older lenses are removed from circulation.

18.2 Disposable lenses

The ultimate frequent replacement lens is the disposable which is discarded  and replaced on a much more frequent basis, usually monthly, two weekly  or daily.

Disposable lenses were originally recommended for weekly extended wear because they avoided most patient handling and eliminated the need for solutions and disinfection. In theory, they should have proved safer, but several studies have implicated ionic lenses and, in particular extended wear, with a significant increase in microbial keratitis.1,2,3

Disposable soft lenses have now assumed the dominant place in routine contact lens practice for daily wear. Extended wear is now almost entirely the province of silicone hydrogels which are gradually becoming the lenses of first choice for all wearing schedules. Disposables offer several clinical advantages over their conventional counterparts.

Advantages

•Lenses rarely reach the stage where they build up deposits.

•Reduced risk of allergies and infections.

•Reduced incidence of CLIPC.

•Spare lenses are always available.

•Replacement cost is significantly less if lost or damaged.

•Less time and effort required with lens cleaning.

•Eliminate the need for professional lens cleaning.

•Cost savings on solutions.

•Patients like the regular fresh feeling of new lenses.

•Easier to fit since fitting parameters are limited.

•Ideal for children requiring soft lenses.

•Theoretically better for most extended wear (see Chapter 21).

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Disposable lenses and frequent (planned) replacement 18 Chapter

Disadvantages

•Restricted range of fittings and powers.

•Restricted availablity for complex lenses such as torics or bifocals.

•Increased cost on an annual basis.

•Uncertain management of patient compliance.

•Branded lenses are available by mail order so that some element of control is removed from the practitioner.

•In the event of a clinical problem, patients may merely replace lenses rather than seek professional advice.

•No longer feasible to check lenses prior to dispensing.

•Administrative complexities because of the volume of lenses.

18.3 Types of disposable lens

Monthly and two weekly

A wide range of disposable lenses is now available in terms of design, water content, material, fitting, power range and wearing schedule. Just some of these are shown in Table 18.1. The majority of lens systems are designed for monthly use which represents a good compromise between lifespan, convenience and cost. Most are of medium to high water content and manufactured by moulding. Several have a handling tint, UV filter or both.

The most commonly used two-weekly lenses are manufactured by Johnson & Johnson and are available with two radii and a wide range of powers (+6.00 to −12.00 D). The lenses are sometimes prescribed on a weekly basis and are the basis of toric and bifocal designs.

N.B. Fitting parameters, lens names and wearing schedules may vary in different countries.

Daily disposables

Since daily disposables should never be reused, they represent the only lens form that genuinely requires no disinfection by the patient. Saline is sometimes employed for rinsing while handling. Lens costs are significantly greater when lenses are used on a regular basis. They tend to be thin, some are difficult to handle and breakage in the eye may occur with some makes. Nevertheless, they provide several important advantages:

•No risk of solutions allergy.

•They avoid patient mistakes in using an incorrect solution.

•A lens case is not required and therefore no contamination is introduced from this source.

•No build-up of surface deposits.

•Ideal for patients who wish to use lenses intermittently.

•Can often be used while patients recover from CLIPC.

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*See Table 19.1 for silicone hydrogel disposables. **Equivalent radius.

N.B. Some lenses are supplied with different names or replacement intervals in different countries.

Disposable lenses and frequent (planned) replacement 18 Chapter

There are several makes of lens available and examples are shown in  Table 18.2.

Toric lenses

Monthly toric disposables are produced by several manufacturers but generally with a simplified range of cylinders and axes, concentrating on the horizontal and vertical meridians. Lenses are fitted according to the principles of stock torics, described in Section 23.4, and most laboratories provide diagnostic lenses for trial fitting. Examples are shown in Table 18.3.

There are also daily toric disposables as shown in Table 18.4.

Multifocals and bifocals

There is also a good range of multifocal and bifocal lens designs and these are shown in Table 18.5 (see also Chapter 24).

Silicone hydrogels

For details of the increasing range of silicone hydrogel disposables, see  Chapter 19.

18.4 Fitting disposable lenses

The selection of lens type and fitting methods follow the principles described in Chapters 15, 16 and 17. Most lenses have a limited range of fitting parameters with the TD fixed for a given power range at approximately 14.0 mm or greater. The TD cannot therefore be used as a fitting variable, and disposable lenses may be too large for small corneas and vice versa.

It is important to recognize when a satisfactory fitting cannot be achieved so that it is necessary to prescribe more than one type of disposable lens. In this way, an improvement to the centration, fit or comfort can often be managed by changing from one brand to another. It is essential to avoid tight lenses if they are to be used for extended wear.

Disposables are generally fitted from a stock bank and it is a simple matter to supply patients with temporary lenses for an extended tolerance trial of a week or more. Where there is some doubt concerning the ideal fitting or water content, patients can be given either a mixed set of lenses or two different pairs and instructed to wear the more comfortable combination at the first aftercare examination.

18.5 Aftercare with disposable lenses

Lens handling

Many disposable lenses rely on a thin, flexible draping design in order to fit the widest cross-section of corneal curvatures. Handling is often more difficult than

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BS: Back surface toric.