Section four Complex lenses

D

D

Figure 24.2  Bifocal lens movement: (A) primary gaze; (B) down gaze (D, distance portion; N, near portion)

•The bottom lid should be no lower than the inferior limbus in order to support the lens.

•There should be minimal disturbance of the lens on blinking or the reading portion is drawn in front of the pupil for distance and the patient complains of variable vision.

RULE OF THUMB

There must be sufficient movement to ensure that approximately threequarters of the pupil area is covered by the correct section of the lens for both distance and near1 (Figure 24.2).

24.4 Fitting rigid multifocals and bifocals

24.4.1 Simultaneous types

Aspherics

Rigid gas-permeable aspheric multifocal lenses usually have good optical quality and are relatively easy to fit. High and low eccentricities are now available.2

High eccentricity designs

•Are typically fitted very steep, as much as 0.60 mm steeper than flattest ‘K’.

•Posterior aspheric designs can provide an effective near addition up to +2.50 D.

•The greater the near add the smaller the distance zone.

•Good centration is essential.

•A central fluorescein pool will be seen.

Back surface aspheric (e.g.Quasar Plus)

These lenses typically have a 2 mm central spherical zone with an aspheric periphery of e = 0.45 to 0.6. The front surface is totally spherical. The near add

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point is set 3 mm from the geometric centre. Lenses are usually fitted 0.4 mm steeper than flattest ‘K’ as good centration will be obtained with the help  of the steep BOZR. The fluorescein pattern exhibits apical clearance with midperipheral alignment. Permitted movement is 1.0 to 1.5 mm, with 2 mm on downwards gaze. An increase in the reading add is achieved by fitting a steeper BOZR. The distance zone, however, is reduced and this may compromise distance acuity. For high additions of up to +4.00 D, +2.50 D is generated on the back surface with the remaining power worked as an aspheric front surface. This design can also accommodate a back surface or front surface toric.

PRACTICAL ADVICE

The lenticular design means that it is possible to:

•Add negative power to the distance area.

•Add positive power to the near carrier.

•Increase to a small extent the size of the distance area.

Low eccentricity designs

•Have a lower range of posterior flattening. The BOZR is chosen only 0.05 to 0.10 mm steeper than flattest ‘K’.

•Must centre well or slightly superiorly with an alignment fluorescein pattern.

•Some translation on downwards gaze helps with near vision.

Aspheric (e.g. Aqualine MF, Cantor + Nissel; Maxim Varifocal, Bausch & Lomb)

These aspheric front or back surface lenses are designed to produce a progressive power curve which increases from the centre outwards. The lenses therefore give centre distance with an intermediate to near addition of up to +2.00 D within the pupil area. Plus power is further enhanced in upwards or downwards gaze.

Control of spherical aberration within the power gradient gives a smooth transition from distance to near vision. Some designs (e.g. Astrocon (CIBAVision)) require a fitting 0.05 to 0.10 mm steeper than flattest ‘K’ to achieve the change in power gradient over the pupil area, while others (e.g. Aqualine MF) can be fitted in alignment as a standard lens.

The reading addition cannot work effectively if the lens is fitted too steep or too flat. If a higher add is required, the non-dominant eye can be over-plussed by +0.50 D, but care should be taken not to over-minus distance. For best near vision while reading, the patient should hold the chin up to use the intermediate zone.

S-form technology

•Creates a posterior power gradient over the entire optical area.

•Ensures the desired power is constant over the whole of the specified front optic diameter.

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•Gives total power control over the whole of the desired optical area.

•Excludes spherical aberration effects.

•Gives fewer problems with pupil flare in low illumination.

•Permits increased lens movement.

•Is not dependent on lens centration.

Aspheric multifocal (Blanchard Essential, David Thomas)

Three series are available. Series I is used for adds up to +1.50 D, Series II from +1.50 D to +2.25 D; and Series III for adds greater than +2.25 D. An alignment fitting philosophy is used with upper lid attachment. The initial lens is selected on the basis of flattest ‘K’ and the degree of astigmatism. Success can sometimes be increased by adding more plus power to the distance BVP. This should be tried before using a series with a higher power, although the stronger add may well prove helpful in the non-dominant eye. The problem of corneal moulding with higher additions is reduced by using the Blanchard Reclaim HD which has the increased addition on the front surface in an aberration controlled design.3

Material implications

Choosing a high refractive index material and a low specific gravity means a thinner and lighter lens which helps with centration and acceptability of higher additions without disturbing distance vision (see Tables 7.3 and 7.4).

Concentric bifocals

Concentric bifocals are especially suitable when good near vision is required above eye level, although some patients find the superimposed­ images difficult to ignore. The diameter of the distance portion is dependent on the pupil size and the lenses must be fitted to centre well with minimum movement. Back or front surface designs are possible (Figure 24.3), although the latter are more frequently fitted.4

A B

Figure 24.3  Concentric bifocals: (A) front surface design; (B) back surface design (from Phillips and Speedwell, Contact Lenses, 4th edn, ButterworthHeinemann, Oxford, by permission)

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Front surface

These are CD lenses made as lenticulars, with a central flatter curve giving a small optic area for distance surrounded by a steeper carrier, the radius of which is selected to give the correct power for near (e.g. Quantum Bausch & Lomb, Polycon II CIBAVision) (see Figure 24.3a). The distance portion varies between 3.00 and 5.00 mm and should cover 50% of the pupil area; 4.00 mm is needed in 70% of cases. It is often useful to bias one eye for distance with a larger segment and the other for near with a smaller central area. A stable fit is achieved with large TDs between 9.60 and 9.90 mm, although some vertical movement is essential for satisfactory near vision. A low-riding lens works better than a high-riding one.

Typical specification

R and L: 7.80:7.50/8.40:8.60/10.50:9.80 −3.00 Add +2.00

Right seg. size 4.30 mm; Left seg. size 3.50 mm

Back surface

The de Carle lens is a CD design4 with a very steep central back curve which relies on the partial neutralization of the tears. The surrounding annulus for near is used as a bearing surface (see Figure 24.3b).

Rigid gas-permeable diffractive bifocals (e.g. Diffrax)

These diffractive bifocals were designed with a circular phase plate incorporated into the lens optic so that light could be made to interfere constructively at two focal points and to interfere destructively at other points on the optic axis.5 The design, which was pupil-independent, is no longer available.

24.4.2 Alternating types

Solid segmented bifocals (e.g. Scotlens, Cantor + Nissel)

The anterior and posterior surfaces are smooth and the segment is a monocentric inclusion within the anterior and posterior surfaces, i.e. the optical centre of the segment is located at the segment top to give ‘jump-free’ vision. Despite the use of prism for stabilization, the lens is thinner and lighter than the solid segment Tangent Streak design (see below). Segment size and shape are often fixed (e.g. 6 mm wide and 3 mm high) but all other parameters are custom designed. The amount of prism can be varied from 1.25 to 3.00 and depends on the BVP. It is usually greater for more negative powers.

With minimal astigmatism a lens is selected 0.05–0.15 mm flatter than ‘K’. Over 1.00 DC, a steeper radius is used. The ideal fitting gives alignment or minimal apical touch with the truncation resting on the lower lid. Vertical movement should be about 1 mm with rapid recovery on blinking. There should be at least 2 mm between the top of the lens and the superior limbus in the primary position. On downwards gaze, the lens should be lifted by at least 2 mm and translate over the superior limbus.

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The segment top should be level with or slightly below the inferior pupillary margin in normal ambient lighting. Excessive lens rotation can be controlled by changing the prism power and axis, or by altering the angle of the truncation.

Typical specification

7.80:8.00/9.40 × 9.00 −3.00 Add +2.00 Seg. height: 3.6 mm; 2.00

One-piece rigid bifocal (e.g. Tangent Streak)

Consists of two wide segments meeting at a straight horizontal junction (Figure 24.4), similar to an executive-type spectacle bifocal.6

Diagnostic lenses are necessary to assess movement of the lens and position of the segment line. A flat fitting is required so that the lens rides onto the lower lid between blinks. A low position is necessary to ensure sufficient clearance between the top of the lens and superior limbus for adequate translation, which is helped by truncation combined with 2 . The range of BOZRs is 7.42 mm (45.50 D) to 8.23 mm (41.00 D) and the initial lens is selected on the basis of Table 24.1.

The BOZD is a constant 7.80 mm, although there are two standard TDs of 9.90 × 9.40 mm and 9.40 × 9.00 mm. The position of the segment is observed with the ophthalmoscope or slit lamp. It is ideally 1.50 mm below the centre of the pupil in distance gaze. In downward gaze, the lens should translate for the

Horizontal diameter

Vertical diameter

Segment height

Optic zone = 7.80

Figure 24.4  Design of Tangent Streak one-piece rigid bifocal

Table 24.1  Tangent streak

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segment line to rise slightly above the pupil centre. If the lens is not pushed up by the bottom lid, then more prism or a flatter BOZR should be used. Where the lower lid is below the limbus, a lens should be used with larger TD, less truncation, and higher segment.

Example:

HVID: 12.00 mm Pupil size 4.00 mm ‘K’ 7.80 × 7.65 BOZR: 7.90 mm; TD 9.90 × 9.40; seg. height 5.20 mm

PRACTICAL ADVICE

•Excessive lens rotation with a blink is often associated with too steep a BOZR or an upswept lower lid.

•A lens positioning superiorly requires increased prism ballast or flattening of the peripheral curves.

•Poor lens translation can be helped by increased edge clearance.

•Poor distance vision due to a high riding lens or excessive movement can be helped by increased prism ballast or a larger TD.

•Poor near vision may require increased edge clearance to help translation.

Presbylite

Presbylite has a triangular segment reading area with an aspheric intermediate zone at the top of the triangular sector.7 Lenses are prism ballasted in three TDs: 9.00 mm, 9.30 mm, 9.60 mm. They are fitted empirically with a large BOZR in a full range of BVPs. The sectored near area allows up to 30° rotation without visual disturbance.

The second generation PresbyLite 2 has an aspheric back surface design.  It provides easier translation, needs only minimal prism for stabilization and rarely requires truncation. These factors are combined with the use of larger TDs.

Distance zone

Near zone sector

Figure 24.5  Presbylite

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