Section four Complex lenses

Concentric bifocals (e.g. Avision system)

The Avision lens has a CD concentric construction with alternating design principles. The optic is monocentric, so rotation is not a problem and the average lens mass is less than with many other designs. Any material can be used.

Lens position is controlled by a range of anterior bevels (designated P or M, from 1 to 4) to ensure the correct degree of lid attachment. P lenses promote more and M lenses less lid grip. Prism ballast can also be incorporated if necessary to encourage a lower lens position. A diagnostic set is available with the eight different edge types and a diagnostic ring to demarcate the 4.00 mm distance portion. Diagnostic lenses (with TD = 9.30 mm and AEL = 0.13 mm) allow assessment of centration, optic diameter and lens movement. The ideal lens has the size of the distance portion equal to that of the pupil diameter in ambient illumination.

The lens of first choice is selected on ‘K’ or 0.10 mm flatter than ‘K’ to give an alignment fitting with minimal mid-peripheral bearing and moderate edge clearance. The optimum edge type and fitting give a central position with 1 mm of rapid movement on blinking and a centre distance zone that provides full pupil coverage to avoid simultaneous vision effects. If the lens does not move by at least 2 mm to ensure translation for near vision, the TD should be reduced.

PRACTICAL ADVICE

•Take advantage of loan lenses where available.

•Use diagnostic lenses as near as possible to the anticipated BVP because lenses of different power behave differently on the eye.

•Err on the flat side for the initial lens.

•The larger TD is used more frequently.

•If the fluorescein pattern is irregular and difficult to interpret, judge the fit according to movement and translation.

•Lenses are supplied dry and require hydration for 24 hours before dispensing.

24.5 Fitting soft bifocals

24.5.1 Simultaneous types

Concentric bifocals (spherical)

Both CD and CN designs are available. CD designs tend to be more beneficial  for drivers, while CN are better for VDU operators. All designs are pupil-  dependent, so a diagnostic lens is necessary for both fitting and power determination.

The optimum fitting should give minimal movement with good centration. The initial lens, however, should be the flattest choice that is reasonable because older, presbyopic eyes have a reduced volume of tears and less free lens movement over the conjunctiva.

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The position of the central portion within the pupil area is more easily checked with either the retinoscope or ophthalmoscope than with the slit lamp. A decentred segment is unlikely to give good vision. It is often advantageous to use a different segment size in each eye, e.g. a smaller CN for the dominant distance eye and a larger CN as the reading lens.

Centre near (e.g. Simulvue, No. 7)

•The diameters available for the CN portion are 2.35 mm and 2.55 mm.

•Different segment sizes should be used, choosing the smaller near diameter for the dominant eye. It may be necessary to fit very small or very large pupils respectively with the 2.35 mm or 2.55 mm segments in both eyes. Pupils that vary widely may encounter problems in extreme lighting conditions.

•The near add should be at least +0.50 D more than the spectacle reading addition, myopes requiring more plus for near than hyperopes.

•There is a choice of two base curves (8.70 mm and 9.00 mm); adequate but not excessive movement is required.

•The CN zone needs to cover only 50% of the pupil diameter and does not need to be exactly centred to achieve good results.

•Against-the-rule astigmatism may cause lateral decentration of the CN portion. Increasing the near zone diameter may well give better acuity.

•Monocular over-refraction should be carried out, but not using   a phoropter.

•The distance over-refraction should be confirmed before checking the required reading add.

Typical specification

R.and L. 8.70:14.00 −3.00 Near add +2.50 Right segment 2.35 mm; Left segment 2.55 mm

Centre distance (e.g Acuvue Oasys for presbyopia)

The original Acuvue Bifocal was a combination of diffractive and refractive optics to achieve the bifocal correction. Using a series of concentric rings, the lens gave different levels of near and distance correction depending on the size of the patient’s pupil. The multiple ring configuration was intended to provide vision which is more specific to the available lighting conditions.

•The material is 58% Etafilcon A, as used for other Acuvue hydrogel lenses.

•Power range +6.00 D to −9.00 D

•Additions; +1.00 D, +1.50 D, +2.00 D, +2.50 D.

The concentric rings are not easily visible with the slit lamp but can be viewed using axial topography corneal mapping devices. The computer image allows centration of the lens to be compared with the corneal apex and patient’s line of sight. The distance power must not be over-minused and some adjustments may be needed to establish the optimal balance between distance and near vision. Success is increased by biasing one eye for distance and the other for near.8

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More minus

Figure 24.6  Acuvue for Oasys design

Acuvue Oasys for presbyopia is a silicone hydrogel development of the Acuvue bifocal design (Figure 24.6). It has aspheric rather than spherical rings and the width and diameter of the zonal design are different for each of the three reading addtitions. The back surface is aspheric, working in conjunction with the front surface zonal design.

Aspheric bifocals

Back surface aspheric (e.g. Elite, mark ’ennovy)

Progressive addition lenses of CD design with the periphery giving a near addition of up to +3.00 D provided by three series. Optimum performance is achieved with good centration under conditions of average illumination and pupil size. Lenses work better for all presbyopes because of the range of reading additions and can be made more successful with over-plussing in one eye or if fitted in combination with another lens type.

Front surface aspherics (e.g. PV200/250, Focus Progressives)

These are CN designs based on the overcorrection of spherical aberration. The power becomes relatively less positive towards the lens periphery. Distance vision is better with large pupils, whereas near is relatively unaffected by pupil size. Distance powers for this type of lens are generally available between

±10.00 D. Near powers have an effective range of up to +3.00 D depending on the manufacturer. Good centration is essential to give optimum vision. Where possible the lens of first choice is selected approximately 1.00 mm flatter than ‘K’; some designs are one-fit. Lenses are essentially fitted for near vision. The initial diagnostic lens is selected on the basis of best vision sphere (spectacle sphere with half the cylinder power) plus half the near addition. It is essential to

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assess acuity using binocular refraction with lenses within 0.50 D of the anticipated result. Success is often increased by under-correction in the dominant eye and over-plussing in the other, or by using lenses with different adds.

Example: Focus Dailies Progressives

Spectacle Rx: −2.00/−0.50 × 180; near add +1.00 Best sphere: −2.00 plus −0.25 = −2.25

−2.25 plus +0.50 = −1.75 −1.75 D

Multi-aspherics (Lunelle Variations 70+, Air Optix Aqua Multifocal, Proclear)

These are CN simultaneous vision lenses offering a range of power profiles for near correction within a progressively corrected zone of spherical aberration. They often have two to three power profiles up to +3.00 D, depending on the design.

The Air Optix Aqua Multifocal (CIBAVison) is a bi-aspheric CN lens. It utilizes a Precision Transition Lens Design which controls the aberration power profile. This extends the depth of focus to offset loss of accommodation so that myopic and hypermetropic power profiles show different designs. There are three adds, Lo, Med and Hi.

Typical specification: 8.60:14.2 −2.00 Med

The conventional Lunelle Variations 70+ (CooperVision) is a front surface aspheric with three power profiles: +1.25D, +2.00D and +2.75D.

Proclear EP is a multizonal design for early presbyopia with an effective addition of +1.25 D. The standard Proclear multifocal, however, utilizes modified monovision. Lenses are designated D or N for dominant or non-dominant eyes and are respectively centre distance and centre near. The power range is from +6.00 to −8.00 with reading additions from +1.00 to +2.50. Proclear lenses are also available as a back surface multifocal toric with an extended power range

±20.00 D and cylinders up to −5.75 D.

The greatest positive power acceptable to the patient is found. Success is increased by using the lowest addition profile in the dominant eye for distance. Hypermetropes tend to be more successful.

24.5.2 Alternating type

Alternating bifocals have not generally proved successful with soft lenses because of the difficulty of large lenses translating effectively. Lenses which have been produced include:

•BYO Royal Aspheric (Scotlens).

•Gelflex Triton Translating Bifocal (Gelflex).

The BYO Royal has a triangular shaped reading segment (based on the same principle as the PresbyLite RGP bifocal) which is located on the front surface in the inferior part of the lens (Figure 24.7). Two segment types are available, the full triangle (Distance Zone 0) and a ‘topless’ variant (Distance Zone 2) which is the standard version.7

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Section four Complex lenses

BYO Royal

Bifocal

Figure 24.7  BYO Royal Aspheric

Situated just below the reading segment on the front surface is an arched zone which, when aligned with the lower eyelid, enables the lens to be pushed upward when the gaze is directed downwards to make reading possible. Stabilization is achieved by a combination of prism and dynamic stabilization. The lens is available in a toric option with cylinders from 0.75 to 3.00 DC.

The Triton Translating Bifocal is manufactured with both truncation and segment.8 Location or marker dots are used to indicate centration in order to position the correct near power segment 1.5 mm below the geometric centre.

•Marker dots must locate at the pupil centre.

•If the marker dots sit low, a larger TD is required.

•If the marker dots sit high, a smaller TD is required.

•Fitting is in relation to the vertical palpebral aperture and the position of the lower lid relative to the limbus.

•Once the correct vertical size has been determined, the BOR and BVP are calculated empirically.

•There may be problems with lid awareness because of the lens bulk and and the truncation.

Enhanced monovision

A combination of one single vision lens and one bifocal, or two different designs of bifocal, increases the chance of success. The following combinations can be tried to give better distance vision to a dominant right eye:

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