Section SIX Children and therapeutic lenses

Table 32.1  Parameters for SoftPerm

of a soft lens. The main disadvantage is the low Dk available for a large lens which covers the entire cornea and limbus.

SoftPerm (CIBAVision)

The rigid centre is Synergicon A with a Dk of 14; the hydrophilic skirt has a 25% water content and a Dk of 5.5 (Table 32.1).

Synergeyes KC (Paragon)

The rigid centre is HDS 100 with a non-ionic hydrophilic skirt of 27% water content and Dk of 9.3 (see also Section 32.8.2).

Piggy-back systems

Piggy-back systems use a rigid gas-permeable to provide good vision fitted over a soft lens to improve comfort (see Section 32.8 below). They can prove beneficial where typically sensitive keratoconic eyes cannot tolerate a rigid gas-  permeable on its own. As fluorescein is not useful, the position, movement, visual acuity and comfort are the best fitting indications.

Scleral soft lenses

A multicurve soft scleral lens with a diameter of 23.00 mm can be designed from a mould of the eye using a shadowgraph.

32.3 Aphakia

Unilateral aphakics derive considerable visual benefit from lenses because the reduced image size and lack of distortion allow some degree of both fusion and binocular vision to be established. The field of view is also considerably improved. Visual acuity, however, is often reduced by about one line because of the absence of spectacle magnification.

The key problems with aphakic contact lens wearers are:

•Different retinal image sizes.

•Handling difficulties.

•Centration problems.

•Pupil shape and flare.

Particular care is now required with aphakia as the most straightforward cases will have received intraocular implants.

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32.3.1 Rigid lenses

Rigid gas-permeable and PMMA lenses give excellent visual results and are therefore the ideal optical correction for those able to handle lenses.

Corneal lens fitting

•The BOZR is often fitted between mean and steepest ‘K’ to help stability   and centration and give the preferred fluorescein pattern of apical clearance.

•The BOZD is chosen between 7.00 mm and 8.50 mm, depending on pupil size and position.

•The TD is generally larger than with the equivalent low-powered lens to help centration; it varies between 8.80 and 10.50 mm.

•The peripheral curves are usually spherical. The axial edge lift is greater than normal, of the order of 0.15 mm.

•Lenses are lenticular in form to reduce weight and thickness. The FOZD is often 0.50 mm larger than the BOZD; 8.00 to 8.50 mm is fairly standard, depending on pupil shape and position and where the lens sits. The reduced optic varies inversely with power.

•The front peripheral curve is often in the form of a negative carrier, although parallel and even positive shapes are used.

•A typical centre thickness is 0.35–0.45 mm because of the high powers required.

•Edge thickness should be at least 0.16 mm to avoid a fragile ‘knife edge’ and make removal easier.

PRACTICAL ADVICE

•Some high plus lenses always assume a decentred, superior temporal position because the corneal apex has been drawn in this direction during surgery. Choose the TD and BOZD to give sufficient pupil coverage for adequate vision.

•If lenses decentre down because of gravity and lens mass, fit larger TDs.

•Use a negative carrier to give a ‘hitch-up’ lens.

•Avoid a back surface toric unless centration is a problem. Toric peripheries are sometimes necessary with high degrees of astigmatism.

•Tints reduce photophobia and assist handling; a different density or colour between right and left lenses helps identification.

•If exposed corneal sutures cause peripheral staining or are rubbed by the lens, they should be removed to enable comfortable wear. This may be necessary up to several months later.

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Corneoscleral lenses

Apex lens

The Apex is a very large, modified corneal lens with a scleral rim of about 2.00 mm and reduced optic to minimize mass. It is a bicurve construction, with the peripheral curve at least 0.70 mm flatter than the BOZR. The lens is used where the corneal curvature is very flat in one meridian, or grossly irregular, or where an eccentric pupil requires a very large optic. The lenses give stable acuity because of their limited movement on the cornea and are especially useful for uniocular senile aphakics and also for grafted aphakic eyes. The wearing times can be reasonably long because of reduced corneal sensitivity.

•The BOZR is chosen approximately 0.50 mm flatter than ‘K’ to give apical touch.

•The BOZD varies from 8.00 to 9.50 mm to allow corneal alignment over a fairly large central area.

•The TD varies between 11.50 and 13.00 mm, depending on the corneal diameter. The large size assists handling.

•The lens periphery usually has up to six fenestrations.

Typical specification: 8.20:8.50/10.50:12.00 BVP +17.00 Tint Grey Reduced Optic

It is important to ensure that the patient can handle the lens before ordering. Elderly aphakics often have loose lids, so removal is difficult. A suction holder is more useful if it incorporates a light source, but should only be used by patients who are aware of the lens position and can aim correctly. Bilateral aphakics can use a spectacle frame glazed on one side only. The help of a friend or relative is invaluable.

The success rate is greater with binocular cases, whereas young unilateral aphakics often find tolerance difficult with a rigid lens. The cause of the aphakia is often traumatic and they find it difficult to appreciate the potential consequences of lost binocular vision, amblyopia and divergence. A comfortable hydrophilic lens often achieves greater success.

PRACTICAL ADVICE

•Expect some degree of corneal staining because of the fitting technique.

•The wearing time may be limited, but even a few hours of good vision is appreciated if nothing else has worked.

Dyna intra-limbal

A modern large diameter corneal lens is the Dyna intra-limbal which has a typical TD of 11.2 mm chosen 0.2 mm smaller than the HVID. The lens can be made from a a variety of gas-permeable materials. For conditions such as pellucid margin degeneration, keratoconus and highly astigmatic eyes, the first BOZR is chosen 0.2 mm flatter than mean ‘K’ and light touch on the central cornea is ideal. The desired peripheral edge clearance is 0.2 mm and standard, steeper or flatter edge lifts are available. Fitting these lenses after keratoplasty,

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the first BOZR chosen is on mean ‘K’. The desired fluorescein pattern is minimum clearance over the donor cornea and alignment with the host. Should visual enhancement surgery have taken place following surgery, a reverse geometry design can be incorporated.

32.3.2 Soft lenses

Hydrogels

Some major companies and most smaller, independent manufacturers produce lenses in a range of water content, in powers of +10.00 D to +20.00 D. Fitting criteria are mainly as given in Chapter 16 for standard soft lenses.

Most hydrogel lenses for aphakia are now medium to high water content and of semi-scleral bicurve design. Lenses generally have TDs between 13.00 and 14.50 mm, but sizes as large as 16.00 mm may sometimes be necessary for proper centration.

BOZRs range from 7.00 mm (for infants) to 9.30 mm for very large lenses. The normal range for elderly aphakics is from 7.80 to 9.00 mm. Lenses are selected from about 0.3 mm flatter than ‘K’ for diameters up to 14.00 mm. The radii are steeper than with earlier lower water content lenses because the softer materials require fitting closer to ‘K’ to ensure stability. BOZDs vary from 7.00 to 8.00 mm.

Lens size must always be chosen to avoid the limbus and give good centration. Irregularities at the limbus, such as sutures or drainage blebs, influence the choice of diameter. Exposed sutures should be removed, while deformities at the limbus may need to be vaulted with a diameter as large as 16.00 mm.

Continuous wear

Elderly aphakics who are unable to handle lenses may leave them in continuously. Careful practitioner management and regular aftercare visits are extremely important, since many documented infections associated with extended wear have been with this type of patient.

Lens movement is especially important because of overnight dehydration and the need to ensure the removal of debris. A saline eyewash in the morning and evening can be recommended, ideally using minims. Deposits are a major problem and some patients need a new lens every 3–6 months.

Silicone hydrogels

Silicone hydrogels are now the preferred material because of their superior physiological performance. Lens selection follows the principles of hydrogel lenses but the radius is generally flatter because of the greater rigidity of the material. The most common TD is 14.50 mm with the range from 13.00 to 16.00 mm. Most lenses are available only to individual order so that factors such as BOZD, thickness and front surface lenticulation are determined by the laboratory. They generally fit into a system of quarterly planned replacement.

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