Section SIX Children and therapeutic lenses

32.4 Corneal grafts (keratoplasty)

32.4.1 Rigid lenses

The main considerations are:

•The size of the graft. It is better to keep the TD within the limits of the graft tissue.

•The tilt of the graft, which may cause a problem in position and stability. Lens decentration often occurs but is acceptable if the graft is not compromised.

•Stability. Sometimes a very large lens (>12.00 mm) is necessary even to stay on the cornea with blinking.

•Staining of the grafted tissue is less acceptable than with a normal cornea and requires careful observation. Any coalescent areas are unacceptable.

Reverse geometry lenses

It is sometimes beneficial to fit reverse geometry lenses consisting of:

•A spherical back optic zone.

•An aspheric intermediate curve, 1.00 D flatter than the equivalent intermediate curve for standard reverse geometry lenses.

•An aspheric peripheral curve, wider than that used for a standard reverse geometry design.

Rose K post graft keratoconus lens

•The range of BOZRs is from 6.90 to 9.00 mm.

•The standard TD is 10.40 mm, with the range from 9.00 to 12.00 mm.

•There are standard, flat and steep peripheral systems.

•The first lens is 0.30 mm steeper than mean ‘K’.

•There should be central fluorescein pooling.

•Fitting increments for radius should be in large steps of up to 0.2 mm.

•If a lens decentres towards the steepest part of the cornea, try steeper or larger lenses.

•If a toric lens is needed because of poor centration, try a lens 0.2 to 0.3 mm steeper than keratometry in both meridians.

•Fenetrations are sometimes needed to aid tear circulation.

32.4.2 Soft lenses

A soft bandage lens can compress or mould a low rigidity graft or realign one with partial eversion. A soft lens is often used as a protective membrane immediately after the sutures have been inserted and may also be used to treat graft rejection. The soft ‘splint’ is often kept in place for several weeks.

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32.5 Corneal irregularity

32.5.1 Rigid gas-permeable lenses

The initial lens is chosen on the basis of the best keratometry readings obtainable. The lens is fitted in the normal way but the fluorescein pattern nearly always shows the irregularity of the corneal surface. The fitting is decided according to visual improvement. In some cases, a small amount of apical clearance gives stability and good vision, whereas in others, alignment or touch is necessary. The TD may need to be larger than normal for lens stability.

Bubbles over an irregular area can cause long-term corneal desiccation and may ultimately require a scleral lens.

32.5.2 Soft lenses

Soft lenses usually conform too closely to the cornea to give any great optical benefit. Some improvement is occasionally achieved with a thick lens or rigid material.

32.6 Albinos

Rigid lenses do not always give visual improvement despite frequently heavy and restrictive spectacles for bilateral hypermetropic astigmatism. Magnification is lost by fitting a contact lens and the cylinder gives unstable vision. The main benefit of contact lenses is to help photophobia by means of a tint and occasionally soft lenses may also be used. Adaptation is a stressful period and any nystagmus can increase initially, although it tends to stabilize once the lenses have settled down. Nystagmus sometimes reduces where vision is improved.

PRACTICAL ADVICE

•Use a diagnostic rigid lens close to the anticipated power to give an accurate assessment of both BVP and fitting.

32.7 Radial keratotomy and photorefractive keratectomy

Radial keratotomy (RK) is a surgical technique to reduce myopia by flattening the cornea with radial incisions.

Photo-refractive keratectomy (PRK) is a laser technique to sculpt the central corneal surface producing a reduction in myopia, hypermetropia or astigmatism. Following PRK in myopes, the peripheral cornea retains its normal contour but the central, ablated zone is much flatter.

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Rigid lens fitting with conventional designs for both RK and PRK results in central fluorescein pooling, although an acceptable fit may sometimes be achieved using aspherics (e.g. Quantum, Persecon E). Reverse geometry lenses can often give much better corneal alignment and improved comfort (see Section 32.4.1). Lenses are best designed from topographical maps of the cornea (see Section 2.3) to avoid the use of numerous diagnostic lenses.

Soft lens designs can work with RK and PRK, but trial and error is often necessary to find a lens that fits adequately, achieves good centration and avoids bubble formation. Silicone hydrogel materials can also be considered.

32.8 Combination lenses

32.8.1 ‘Piggy-back’ lenses

The combination of a rigid lens on top of a soft or silicone hydrogel lens is used with keratoconus and graft cases to achieve good vision with improved comfort where all else has failed.

The foundation soft lens has a large diameter for stability and a typical front surface radius of about 7.60 mm. This is achieved by altering the power of the best fitting, so that a minus lens is necessary for ‘K’ readings between 6.00 and 7.00 mm. A low plus lens is required if the cornea is flatter than 8.00 mm.

The rigid lens has a TD of 9.50 mm or larger to give good centration. The BOZR is based on the front surface curvature of the soft lens, measured with the keratometer.

The problems with combination lenses are:

•A large steep soft lens is required to find a satisfactory fitting.

•Stabilizing the rigid lens on the soft lens takes practice.

•Different solutions are needed for each part of the combination, although this is made easier if a daily disposable can be used.

•Lenses are removed separately. In some cases, the soft part is used for extended wear with the rigid lens put in place to help vision during the day.

Using a silicone hydrogel lens gives improved rigidity and enhanced oxygen transmissibility which may improve success.6

The reverse combination of a corneal rigid lens covered by a thin soft lens can be used for sporting purposes to avoid risk of loss.

32.8.2 Rigid centre with soft periphery

This combination (e.g. Softperm, SynergEyes) is designed to give the acuity of rigid lenses with the comfort of soft.

Softperm

Softperm parameters are given in Table 32.1. The basic fittings consist of radii from 7.10 to 8.10 mm in 0.1 mm steps and the first choice of lens is near to

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Table 32.2  Parameters for SynergEyes A

flattest ‘K’. Fitting characteristics of movement and centration are based on soft lens criteria. Large molecular weight fluorescein can be used.

SynergEyes®

See Table 32.2 for SynergEyes A parameters. There are three different fitting sets:

•SynergEyes A. Used for ametropia, mild keratoconus and corneal scarring.

•SynergEyes KC. Used for keratoconus and prolate corneas.

•SynergEyes PS. Used for oblate post-surgical corneas.

32.9 Silicone rubber lenses

Silicone rubber lenses have the following therapeutic uses:

•Aphakia.

•Dry eyes (e.g. Sjögren’s syndrome).

•Exposure problems following lid reconstruction.

•Corneal perforations.

•Corneal ulceration.

Lenses are a monocurve design and the parameters available are radii from 7.50 to 8.30 mm, with TDs from 11.30 to 12.50 mm in 0.50 mm steps. The wide range of powers includes plano and aphakic (Zeiss (Pace) and Silsoft (Bausch & Lomb)). The initial lens is chosen to be between 0.2 and 0.4 mm flatter than flattest ‘K’ and 1.0 mm larger than HVID. Some movement and tear exchange is essential.

Advantages

•Very high Dk permits continuous wear.

•Good vision.

•They do not dehydrate and are ideal for dry eyes or tear film problems.

•Low risk of loss or damage.

•Good for corneal reconstruction. The radius can be refitted as the cornea reforms under the lens.

•Resistance to bacterial colonization and therefore ideal for eyes open to infection.

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