Silicone hydrogels 19 Chapter

Indications for silicone hydrogel lenses for daily wear

•For better physiology.

•To increase wearing time.

•For occasional overnight wear.

•For better comfort.

•Vascularization with existing lenses.

•To reduce conjunctival injection with existing lenses.

•For dry eye problems.

•For easier lens handling because of increased rigidity.

•To reduce lens breakage.

•As bandage lenses.

Contraindications

•Where complex lenses are still unavailable.

•Patients predisposed to superior arcuate staining.

•Patients predisposed to CLIPC.

•Sensitive lids where patients have been able to tolerate only thin or high water content soft lenses.

•Additional costs.

19.1 Fitting disposable silicone hydrogels

General fitting points

•Fitting characteristics should follow the principles outlined in Chapter 16 and are assessed with the slit lamp according to lens movement or push-up test.

•Mobility must remain sufficient to ensure an adequate exchange of tears behind the lens to allow the removal of debris.

•With extended wear, it is essential to recognize when lenses give unsatisfactory performance and discontinue.

•A small number of patients are unable to tolerate silicone hydrogels. Those who have adapted to very thin hydrogel lenses sometimes cannot support the extra rigidity and thickness; and patients with very sensitive lids may be intolerant of the surface treatments.

Radius

•Most silicone hydrogels have only one radius and no adjustment is possible.

•Where there is a choice of two fittings, consider the steeper option for better centration but the flatter for increased mobility.

227

Section three Hydrogel and silicone hydrogel fitting

•Lenses fit a wide range of corneal curvatures and successful results are achieved from 7.20 to 8.40 mm.

•The main cause of an unsatisfactory fitting is edge buckling. The lens is unable to conform to the corneal curvature because of its greater rigidity compared with hydrogels.

•Buckling usually occurs with steeper corneas and higher powers and is very uncomfortable.

•If buckling has not settled within about fifteen minutes, success is unlikely.

Total diameter

•Most varieties have similar TDs which cannot be altered.

•It is important to recognize when lenses are too small with poor centration or too large and likely to give inadequate tears exchange.

•Because silicone hydrogels give minimal dehydration, they maintain their TD on the cornea and often appear larger than the equivalent diameter hydrogel.

Power

Because of their rigidity, it is less easy to correlate the powers of silicone hydrogels with standard ‘thin’ hydrogel lenses. This is particularly true with plus powers. Trial lenses must be allowed to settle fully in order to assess the optimum result.

PRACTICAL ADVICE

•When inserting a lens, minimize finger contact to avoid the possibility of surface greasing. If the lens has been placed initially onto the sclera, position the lens either by digital pressure through the lids or ask the patient to move the eye towards the lens.

•Patients are usually comfortable within about 3 days. If they have not adapted by 1 week it is better to refit with another variety of silicone hydrogel – probably with a lower modulus – or a traditional hydrogel lens.

19.2 Fitting custom made silicone hydrogels

Silicone hydrogels were originally introduced as disposable lenses. It was subsequently realized that, in order to fit the widest possible range of patients, it was necessary to extend the available parameters for individual fitting. Lenses by companies such as CIBAVision, Cantor + Nissel, Mark ’Ennovy and Ultravision are available for quarterly (frequent replacement) or yearly renewal.

Air Optix Individual (CIBAVision)

A silicone hydrogel lens for individual fitting and quarterly replacement, manufactured by lathing.

228

Parameters available

See (Table 19.2).

Fitting method

Lenses are selected on the basis of the initial fitting guide (Table 19.2).

Typical specification

8.40 : 14.00 −15.00

Related lenses

•Air Optix Aqua Plus, a monthly disposable silicone hydrogel.

•Air Optix Aqua for astigmatism, a monthly disposable toric.

•Air Optix Night & Day Aqua Plus, a monthly disposable for extended wear.

•Air Optix Aqua Plus Multifocal, a monthly disposable for presbyopia.

19.3 Complex lenses

Silicone hydrogels have gradually been introduced in an increasingly wide range of complex lenses to correct astigmatism and presbyopia. Some of these are highlighted in Tables 18.3 and 18.5, respectively.

229

Section three Hydrogel and silicone hydrogel fitting

19.4 Dispensing silicone hydrogels

Handling

•Handling considerations for silicone hydrogels also relate to their greater rigidity.

•Most patients find them easier to handle than ‘thin’ or high water content hydrogels.

•Breakages are less common and lenses are rarely lost from the eye.

•If a lens is first placed on the sclera it may be less easy to recentre.

•After initial insertion, however, trapped air bubbles from the solution are frequently observed. These tend not to resolve on their own and may cause dry spots or may be confused with mucin balls. They are best removed in the same way as a foreign body by sliding the lens onto the sclera and carefully recentring.

Wearing schedule

•Existing lens wearers should be able to wear silicone hydrogels for several hours from the first day.

•Patients new to lenses can start at about 6 hours with 2 hour   increments.

•Extended wear patients should first use lenses for daily wear for a few days. They should then be carefully assessed after one or two overnights.

•Most lenses for extended wear have this designated for either 6 or 30 nights. Patients can generally manage the appropriate number of nights without problems. Some, however, are happy to remove lenses on a more frequent basis with overnight disinfection.

Lens disinfection

•Lens disinfection can be carried out with most of the proprietary   products. There has been some suggestion that products containing polyhexamide increase the occurrence of corneal staining but, even where this seems to be the case, it is normally of low level and rarely exceeds Grade 1.2

•Hydrogen peroxide solutions give the lowest level of corneal staining.

•Some solutions such as Synergi have been specially formulated for silicone hydrogels.

19.5 Aftercare

Aftercare considerations follow those given in Chapters 28 and 30. Although silicone hydrogels generally give much improved ocular response for extended wear, practitioners must still observe the cautions noted in Chapters 21. The

230

Silicone hydrogels 19 Chapter

uptake of standard fluorescein into the lenses is small because of their low water content. It can therefore be used at all aftercare visits, preferably in combination with some form of grading system. Because of their particular physical characteristics, some aspects of aftercare require special attention.

19.5.1 Daily wear

Arcuate staining

Arcuate staining, close to the superior limbus (SEAL), is fairly common with higher modulus lenses because of the shearing force of a relatively rigid material against the corneal surface. The patient may or may not be symptomatic. Staining is sometimes resolved simply by stopping lens wear for a few days. If this is unsuccessful, it is generally necessary to change to a different make of lens or refit a hydrogel.

Mucin balls

A phenomenon largely connected with the introduction of first generation silicone hydrogels is the mucin ball. Other terms employed have been pre-corneal deposit or lipid plug.3,4 Mucin balls are commonly associated with high modulus silicone hydrogels because of their surface properties and lens rigidity, although they were described with rigid lens materials as long ago as 19943 and have been observed even with thin daily disposables. They tend to form in the superior half of the cornea, beneath the upper lid, and are the result of poor tears exchange beneath the lens failing to remove cell debris and other products of corneal metabolism. Mucin balls have proven quite benign and most patients are completely asymptomatic requiring no action apart from routine observation. They can be distinguished from microcysts because of:

• Their grey translucent or opalescent appearance.

• Their larger size (10–50  m).

•They do not exhibit reverse illumination.

•They leave dimples in the corneal surface from which fluorescein can be washed out by irrigation.

Papillary conjunctivitis

CLIPC is occasionally seen where it had not previously occurred with hydrogels.

Dryness

Most patients find silicone hydrogels very satisfactory in respect of dry eye symptoms and they may well be the first choice in these cases. Sometimes, lenses exacerbate symptoms and patients are unable to tolerate a particular type for either daily or overnight wear. On the other hand, silicone hydrogels are often very successful as bandage lenses for dry eye patients.

231

Section three Hydrogel and silicone hydrogel fitting

19.5.2 Extended wear

Microcysts

Microcysts in the past have been a common problem with extended wear. On refitting with silicone hydrogels, there is sometimes an initial increase in number as the cornea adapts to a much improved physiological environment, referred to as the ‘rebound effect’. Once they have migrated to the corneal surface and resolved, there is usually a marked reduction and subsequently they are rarely

a problem. Microcysts exhibit reverse illumination and are small with diameters of 15–20  m.

Acute red eye

Acute red eye reactions (CLARE, see Section 29.1.1) are seen in a minority of cases, sometimes where they were unknown with hydrogel lenses. Reactions can sometimes occur within a few days of fitting. If they recur, silicone hydrogel lenses should be discontinued.

Contact lens peripheral ulcers

Contact lens peripheral ulcers and other inflammatory reactions (see Section 29.1.1) are not uncommon and generally associated with continuous wear. They are sometimes found with a red eye reaction but are frequently asymptomatic. Some patients seem predisposed to CLPUs and the practitioner must use careful judgement as to whether silicone hydrogel wear should be continued.

Microbial keratitis (see Section 29.1.1)

Microbial keratitis still occurs with silicone hydrogels, although the severity of infection is less than with hydrogel lenses. Patients using lenses on an extended wear basis should be given careful advice in writing concerning the appropriate symptoms.

Vascularization

Vascularization is most unlikely to occur because of the extremely high oxygen permeability. In some cases, where patients have discontinued extended wear with lower Dk materials for this reason, 30 days continuous wear has proved perfectly feasible with blood vessels ‘ghosting’ within a few weeks.

Papillary conjunctivitis

CLIPC is occasionally seen. Papillae may be atypically dome shaped and close to the lid margin. With extended wear, patients may notice a reduction in vision because of deposits on the lens surface.

232