Section SIX Children and therapeutic lenses

32.1 High myopia and hypermetropia

32.1.1 Rigid gas-permeable lenses

The main problem with powers over ±10.00 D is the lens mass. Stability and position may be improved by fitting:

•A TD about 0.50 mm larger than usual.

•On mean ‘K’ to show apical clearance with the fluorescein pattern.

•Lid attachment (hitch-up) lenses (see Section 8.5).

•A reduced optic and ordering the optimum design of carrier (e.g.   parallel for high minus with tight lids; negative for high plus) (see Section 8.2.7).

32.1.2 Hydrogel lenses

The main difficulty with myopes is the mass of material at the limbus. Even with a reduced optic, long-term problems such as oedema and vascularization can occur. The size of the optic is important as it is affected by the lenticulation and flare can occur if it is too small.The highest power normally available is −30.00 D Hypermetropes have all the lens mass centrally. This can cause central oedema even with high water content materials. A normal cornea is under greater stress than an aphakic eye because it has higher oxygen demands. The

highest power normally available is +30.00 D.

Radii are chosen in the normal way, but larger total diameters are necessary to aid stability.

32.1.3 Silicone hydrogel lenses

Silicone hydrogel lenses, where the prescription is feasible, are usually the first choice. Air Optix Individual (CIBAVision) and lathed lenses from other manufacturers are available to deal with high prescriptions including astigmatism (e.g. mark ’ennovy). The physiological advantage is useful but surface problems can arise with some individuals. Most lenses are provided on a 3 months replacement schedule.

32.2 Keratoconus

Keratoconus is the classic case in which a rigid contact lens provides a new refracting surface for an irregular cornea and gives improved acuity, whereas spectacles frequently offer no significant benefit. Corneal lenses are the preferred form of correction in the early stages of the condition.

The apex of the cone is usually displaced inferior and nasal relative to the pupil. The common problem in all keratoconus fitting is to make the BOZD of the lens, which tends to centre at the apex of the cone, sufficiently large to cover

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the pupil area without the formation of either dimples or a stagnant pool of tears. Another problem is that mid-peripherally all spherical lenses are tightest where the cornea is flattest. This can result in lenses either impinging excessively onto the superior limbus or dropping to an uncomfortable low position.

Keratometry readings are typically steep, astigmatic and irregular, except in the early stages. As the cone advances, readings eventually fall outside the range of the instrument, although this can be extended with a supplementary plus lens (see Section 2.2.4). Modern autokeratometers have a much greater range and, despite corneal distortion, can sometimes gives readings as steep as 4–5 mm. Corneal topographers are ideal and can help with the selection of the first lens (see Section 2.3).

The main problems with keratoconus fitting are:

•Decentration on the irregular cornea.

•Discomfort because of increased lid and corneal sensitivity.

•Photophobia (helped by a tint).

•Lens thickness because of the high negative power.

•Dimpling, which may necessitate fenestrations or a change in peripheral design.

•Oedema, often due to lack of lens mobility and reduced tears exchange.

PRACTICAL ADVICE

•Try more than one type of fitting because different designs can give very different acuities.

•Always over-refract with the type of lens to be used, since the BVP may vary by at least 1.00 D between different designs.

•Centration also varies with design and lenticulation.

32.2.1 Rigid lenses

The three major techniques for fitting rigid lenses in keratoconus are:1

1.Flat, or two-point touch, with primary lens support on the apex of the cornea, where the central optic zone of the lens touches or bears on the corneal apex and further support at the superior limbus with, for example, a high riding lens.

2.Divided support, or three-point touch, with lens support and bearing shared between the corneal apex and the paracentral cornea – the most common.

3.Steep, with lens support and bearing directed off the apex and onto the paracentral cornea, with clearance or vaulting of the corneal apex.

Spherical lenses to give two-point touch

•Use the apex of the cone as a fulcrum.

•Allow the lens to be held by the top lid to achieve a high-riding fitting.

•Need to be fitted sufficiently flat to allow for lid attachment.

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•Can possibly change the non-affected part of the cornea by mechanical pressure.

•Are thought to be the major cause of scarring at the cone area.

Spherical lenses to give three-point touch

•The BOZR is chosen on or near the flattest ‘K’.

•The BOZD is between 5.00 and 7.50 mm.

•The TD ranges from 8.50 to 10.00 mm.

•The peripheral curves are designed to flatten off rapidly to follow the topography of the cornea.

•When the cone area is fitted, the BVP is usually much higher minus than expected because of the steep BOZR. In advanced cases, the periconal area may be used and require less power.

The optimum three-point touch fluorescein fit (Figure 32.1) gives the overall effect of a ‘bull’s-eye’ pattern. There is light touch on the cone surrounded by a ring of fluorescein which is in turn surrounded by an annulus of mid-peripheral bearing with peripheral edge clearance. If the central touch is too heavy, it may allow rocking of the lens and cause corneal abrasion. This can be improved either by changing the BOZR or by altering the degree of peripheral bearing.

Typical diagnostic lenses:

7.00:6.00/8.00:8.20/10.00:9.20 −3.00 6.50:6.00/7.50:8.00/9.50:9.00 −5.50 5.50:5.60/6.50:7.60/8.50:8.60 –11.00

Figure 32.1  Typical ‘bull’s-eye’ fluorescein pattern (dark shading represents corneal touch)

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The rule that a radius change of 0.05 mm ≡ 0.25 D breaks down because of the very steep curves.

RULE OF THUMB

A change in radius of 0.05 mm ≡ 0.50 D with lenses steeper than 6.90 mm.

PRACTICAL ADVICE

As a starting point, consider selecting the initial BOZD to be at least 0.2 mm larger than the numerical value of the BOZR (e.g. 7.40 mm with 7.17 mm radius; 6.35 mm with 6.15 mm radius).

Apical clearance lenses

•A spherical central zone is chosen to vault the apex of the cone with minimum clearance.

•A spherical or non-conic peripheral zone is designed to slide over the flatter superior corneal surface.

•In some cases, a junctionless transitional zone is used, broad enough to allow adequate edge clearance.

•Careful selection of the peripheral curves achieves best fitting results.

Spherical lenses to fit the corneal periphery

In fairly early keratoconus, especially with large corneas, lenses can be fitted on the basis of matching the relatively unchanged corneal periphery,2 but with the addition of a much steeper BOZR. The actual peripheral curves are very similar to those found in a lens designed for a normal cornea, but to allow for the steep centre a tricurve becomes a four or possibly five curve lens.

Examples:

Offset and aspheric lenses

These designs have the advantage that, if the conical zone of the cornea lies in front of the visual axis, a small-diameter lens can be fitted.

•The BOZR is chosen on flattest ‘K’ to give an area of central touch.

•The BOZD varies from 2.00 mm for a true conoid to 7.70 mm for some offsets.

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•The TD is typically 8.00 mm, but may be as small as 7.00 mm.

•Axial edge lift is the standard 0.10–0.15 mm and assessed by fluorescein.

•Fenestrations may be used to improve tears exchange and reduce frothing.

•A parabolic curve with edge flattening of 0.7 mm is sometimes useful when it is no longer feasible to fit spherical or aspheric lenses.

•Typical offset trial lens: 6.00:5.50/AEL (ff) 0.1 at 8.50.

Elliptical ‘K’ (Persecon keratoconus)

The design has a bi-elliptical back surface and works well for early keratoconus, especially where patients have large pupils.3

•The BOZR is chosen on flattest ‘K’.

•The optic zone is 8.00–8.50 mm, allowing an even pressure distribution over the lens centre.

•The TD is either 9.30 mm or 9.80 mm.

•The eccentricity of 0.39 is the same as that of the standard Persecon E.

•The peripheral zone flattening has an elliptical curve of the same eccentricity but flatter vertex radius.

The design is predetermined by the laboratory, so that if the fluorescein pattern is unsatisfactory another type of fitting should be used.

Aspheric periphery lenses

Designed with a spherical back surface and either a flatter edge curve with an aspheric edge (e.g. Conflex/KE) or a spherical back surface with an aspheric-type peripheral bevel for greater edge lift (Aspheri-KD).

Quasar K

The continuous curve technology of the original Quasar (see Section 12.2.1) is used for the Quasar K so that the design eliminates the sharp junction with the periphery. The peripheric aspheric curvature can be altered by selecting either the Sag I and Sag II options. The AEL is dependent on the BOZR and TD.

Sag I

BOZR: 5.70 to 8.00 in TD: 8.80 to 9.60 mm AEL: 0.421 to 0.196 mm 0.1 mm steps

Sag II

BOZR: 5.00 to 8.00 in TD: 8.60 to 9.60 mm AEL: 0.728 to 0.276 mm 0.1 mm steps

Acuity lenses

Fitted from diagnostic lenses with a combined aspheric and multicurve design. Each lens is categorized according to a cone radius.

The initial lens is selected on the basis of steepest ‘K’. The final fitting should give minimal central touch.

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The Acuity K design is useful for cases of advanced keratoconus more  usually found in hospital contact lens practice.4 There is also a soft lens version.

Rose K lens

A multicurve lens with spherical radii clearing the flat mid-peripheral and peripheral cornea. The radii are blended to form a controlled ‘aspheric’ peripheral lens geometry. These curves can be adjusted to give a looser or tighter peripheral fit.

•TD: 7.90 mm to 10.20 mm. Usually 8.70 mm is used; if not, then as the TD is increased or decreased the BOZD and secondary curve widths are also increased or decreased in a fixed ratio.

•BOZR: 4.75 to 8.00 mm.

•To keep pooling at the base of the cone to a minimum, the back optic zone diameter must decrease as the radius steepens and therefore varies with total diameter.

•The secondary curve width varies with diameter.

•AELs: standard (STD), increased (INC) and decreased (DEC), increasing as the base curve is steepened.

•Choose a lens 0.20 mm steeper than mean ‘K’ or with a topographer choose the BOZR based on the dioptric axial radius 3.0 mm temporal to the geometric centre of the cornea.

•Initially aim for light touch at the cone apex. A low riding lens means the radius is too steep or the optic zone too large.

•Consider the peripheral fit and order increased or decreased edge lift as needed. It is acceptable to have lower edge stand-off and it may be needed to ensure adequate superior alignment.

•Smaller TDs (8.10 to 8.30 mm) work well on steep corneas. A large TD will tend to make the lens sit higher.

•There is often a shallow pool at the cone base which is acceptable.

•The lower edge may give slight stand-off from the cornea because of inferior corneal steepening. This is acceptable and often essential to ensure adequate superior alignment or clearance.

Rose K2

Essentially the same design as the original Rose K but with spherical aberration control over the back optic zone diameter. This gives an improvement in vision for the high minus powers necessary with very steep radii but generally results in a tighter fitting in the periphery.

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Profile design

A design based on the concept of axial profile (Jack Allen). The fitting system is designed to give equal apical profile increments of 0.1 mm of the central sag across the whole range of lens radii. Two peripheral designs of ‘standard’(N)  and ‘reduced’(R) axial edge lift (by 0.1 mm) are available with three TDs of 8.70 mm(S), 9.30 mm (N) and 9.90 mm (L). Each sag has a number from 1 to 13, 1 being the smallest, and comes with a letter relating to TD and axial edge lift.

‘CLEK’ standardized fitting set

There is debate over whether there is correlation between central touch of rigid lens fitting and subsequent scarring. A wide-ranging study as part of the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) has shown that the majority of rigid lenses fitted give some degree of apical touch. In order to determine a standard fitting protocol, the study has suggested a methodology to produce apical clearance.5 All lens parameters are uniform except the BOZR and BPZR which are varied according to fluorescein assessment. The first lens is chosen on mean ‘K’ and then steeper lenses used until apical clearance is first seen.

Corneoscleral lenses

These are essentially large rigid gas-permeable lenses that equally distribute pressure over both corneal and scleral surfaces. They are sometimes referred to as mini-sclerals.

So2Clear

The total diameters vary between 13.3 mm and 15.5 mm with a relationship between the BOZR and the scleral periphery. There is a spherical 9.6 mm optic zone with a multicurve design. Lenses are made from Boston XO (Dk 130) with a plasma coating. There are three designs, Standard, Keratoconus (KC) and KC Aspheric, all fenestrated for easier removal.

The four main principles involved in the fitting of So2Clear are:

•BOZR – central alignment with slight mid-peripheral corneal clearance.

•Peripheral curve – alignment with the scleral conjunctiva.

•Total diameter – 1.00 to 1.25 mm of scleral coverage.

•Movement – maximum of 0.25 mm of movement on blinking.

Other factors include:

•Lenses can be fitted using corneal topography, although there is generally not enough information about the peripheral curves and sclera to achieve the correct scleral curve fitting in every case.

•The BOZR is labelled as the base curve in dioptres.

•The standard set is for non-keratoconus corneas, including normal eyes and post graft, post-LASIK or radial keratotomy cases.

•The keratoconus set has a spherical optic zone of 9.6 mm but the intermediate curve flattens more rapidly to match the flattening found with keratoconic corneas.

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•There is also an aspheric keratoconus set with an aspheric optic zone of 9.6 mm. This set gives better results for smaller and steeper central cones. Other fittings aspects are similar to those used with spherical keratoconus design.

Insertion

With a normal cornea, the lens is wetted with solution and inserted like a soft lens but directly onto the cornea. A drop of saline in the bowl of the lens will aid comfort and help prevent bubble formation.

For keratoconus or other corneal irregularity, it is sometimes difficult to avoid bubble formation. The bowl of the lens should be filled with solution and inserted from below with the patient’s head in a horizontal position.

Removal

Lenses are removed by using pressure with top and bottom lids to break any suction between the lens and the eye. This method may not work if the peripheral curves are too tight or if the lens is too dry. In these cases, a suction holder is advisable. It should be angled from the side to break the surface tension at the periphery rather than pulling it from the centre of the lens.

Fitting

Central fitting

The initial lens is selected to be on flattest ‘K’, although several lenses may need to be tried before achieving the optimum corneal fitting. The preferred fluorescein pattern is alignment or, in the case of high corneal astigmatism, the typical dumb-bell appearance.

Peripheral curve

The peripheral curve is fitted only after the optic radius has been decided upon. The ideal fit is in alignment with the sclera. There should be:

•A maximum of 0.25 mm of movement on blinking.

•No edge stand-off.

•No pinching or blanching of the limbal or conjunctival vessels.

•Good subjective comfort.

Progressively flatter lenses are inserted until the periphery is slightly too flat, indicated by excessive movement or minimal edge stand-off. The desired fitting is the peripheral curve of the lens one step steeper. The central radius is ignored during peripheral fitting. The periphery is specified as the same as, or the number of steps steeper or flatter than the central fitting.

Total diameter

The diameter should provide 1.00 to 1.25 mm of lens coverage beyond the HVID.

•If the HVID is <11.5 mm, TD = 13.5 mm.

•If the HVID is 11.5 to 12.00 mm, TD = 14.0 mm.

•If the HVID is 12.0 mm, TD = 14.5 mm.

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Fluorescein instillation

Fluorescein can be instilled in two ways:

•Applying to the conjunctiva in the usual way. This is recommended for normal eyes as the way fluorescein spreads under the lens edge gives a further assessment of the peripheral fit. If the time taken is more than one or two minutes, it suggests the lens is too tight.

•Adding to the liquid in the bowl of the lens prior to insertion. This saves time and is recommended for keratoconus fitting.

Keratoconus

For mild to moderate cones, fitting is begun with the standard set. The steeper lenses in the keratoconus set are used when necessary.

•The periphery is fitted in the same way as normal eyes.

•So2Clear can also be used for keratoconus patients with Intacs implants and for those with post-LASIK-induced corneal ectasia.

•In general, there are four fitting patterns seen with So2Clear lenses: Vaulting the cone

Skimming the cone Light cone touch Heavy cone touch.

In some cases with a very irregular cornea, a satisfactory fitting cannot  be achieved because it is impossible to avoid a large air bubble in the central  area of the lens. This can sometimes be helped by trying a lens without fenestration.

Ordering

The lens order should specify:

•Lens design (i.e. standard, keratoconus or aspheric).

•Radius (or lens number).

•Diameter.

•Power.

•Peripheral curve.

GENERAL ADVICE ON KERATOCONUS

•Do not fit too steep as this may result in a large air bubble.

•The steeper the radius, the higher the minus power.

•Small changes in BOZR can give larger than expected changes in refraction.

•Modifying peripheral curves can affect the lens position and the required BVP.

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•Even if the first diagnostic lens is not ideal, over-refract to get an idea of the likely acuity. It is always better to assess the vision with at least two diagnostic lenses to ensure that the refractive results correlate.

•Lens design may depend on visual improvement.

•Expect to use at least two lenses per eye.

•Sometimes a high Dk material is less comfortable than a low Dk material.

32.2.2 Soft lenses

Soft lenses can sometimes be used for keratoconus, depending upon the degree of corneal distortion. Centre thickness is deliberately increased or a conical back surface used.

Conical soft lenses

Cone curves of 5.40 mm to 7.60 mm with blended bicurve peripheral fittings of extra steep (SS), steep (S), normal (N), flat (F) and extra flat (XX) are the basis of the Acuity Shephard design. Lenses have a standard TD of 14.50 mm and BVPs range from +20.00 to −40.00 D. The peripheral and cone fit are separate entities but the result should be a lens that just traps an apical bubble that is expelled by the lightest of blinks. If it is not possible to introduce a bubble that will remain until a blink then the cone fit is probably too flat.

Spherical soft lenses

The minimum centre thickness should be 0.35 mm, but 0.60 mm is advisable. The edge is reduced to 0.18 mm by means of lenticulation. A relatively rigid  silicone hydrogel lens in plus power masks a considerable degree of corneal distortion and reasonable acuity may be obtained with the additional use of spectacles.

Rigidity can be achieved also with a combination of increased thickness and a proprietary series of front and back curves (e.g. Kerasoft, UltraVision International). Four BOZR values are available in Series A, B, C and D, where D is the flattest. Each series is available in three TDs of 14.00, 14.50 and 15.00 mm. The prescription lens usually requires a cylindrical component which can incorporate up to −11.00 DC.

Kerasoft 3 is the equivalent design in silicone hydrogel material but the fitting is relatively flatter than for hydrogel lenses. The four series have radii from 8.00 to 8.60 mm with a standard TD of 14.50 mm. The 8.60 mm radius is recommended for corneas between 6.80 and 7.20 mm and the 8.00 mm radius for corneas steeper than 6.00 mm.

Hybrid lenses

Hybrid lenses consist of a rigid gas-permeable centre surrounded by a hydrophilic soft skirt. They aim to provide the vision of a rigid lens with the comfort

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