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Maximising Multifocals: Individualisation

2CPD in Australia | 0.75G in New Zealand | 24 May 2016

This course has expired and is no longer available


Nicola Peaper

Changes in work habits and lifestyle necessitate clarity of intermediate and near vision. Fortunately, technological innovation means it is now possible to customise a lens design to suit individual requirements, even when a patient insists on one pair of spectacles to manage all of their visual requirements.

There have been some major changes in lifestyle and work habits since the turn of the century. We are aware that most of our patients are using multiple digital devices during their work and leisure time. There is no set working distance for an intermediate script because, for example, we use mobile phones between 25 and 30cm and we need to see desktop screens at 1m and beyond. We no longer read directly in front of us as tablets are held to the side, allowing us to type on the screen. We need enough width of clear vision to see two or more screens at a time. However, it is not simply the working distances and width required that have changed. Try looking at an Excel spreadsheet on a 13-inch laptop screen for long periods of time! Clarity of intermediate and near vision is now of paramount importance.

There are three ways we can use lens design to help our patients:

  • Choose a lens that delivers power accurately along the corridor. This includes delivering the refracted power to the eye, taking into account the position of wear and correcting for near astigmatism.
  • Choose a lens that can be customised for the frame chosen by the patient. We are all aware that Face Form Angle (FFA), Corneal Vertex Distance (CVD) and Pantoscopic Tilt (PT) affect lens performance and in standard lenses, averages are built into the design. Many lens designs now are available that give us the ability to factor the chosen frame measurements in.
  • Choose a lens that is customised to the patient’s lifestyle. We know that one lens will not necessarily cover all visual needs and in an ideal world an occupational lens solution should be prescribed. However, in cases where a patient insists on one pair for everything it is now possible to individualise a lens design to suit different requirements.

Retina Focus Principle

A spectacle lens is ordered using the powers found during refraction in circumstances where the trial lens is at right angles to the floor and the trial frame or phoropter head has no wrap. The multifocal lens produced will sit in a frame with both wrap and tilt and the CVD will differ for both distance viewing and near. Power must be varied accordingly to give the same resultant power at the retina as that measured during refraction.

If CVD, FFA and PT are measured then the compensation can be calculated using the actual frame characteristics rather than averages and so will be more accurate.

The compensated power supplied is the power that will be measured using a vertometer where the lens position is different again as the back surface of the lens lies flat on the vertometer lens rest.

Figure 1. Different circumstances of lens measuring.

From figure 1 it can be seen that during refraction the trial lenses are optically centred, small and thin. By the time sphere, cyl and add have been determined there will be up to three or more trial lenses.

The corrective lens produced is a single lens thicker than the trial lens.

  • When verified a multifocal lens is measured away from the optical centre, in the distance and near reference circles, with the back surface flat on the lens rest of the vertometer. Prism will be found in the distance and near reference circles due to the distance from the optical centre and prism thinning. When measuring the near script the light rays are incident at 90° and not as from a near object.
  • When in position of wear the lens is tilted and the CVD varies from the distance portion to the near. When observing a near object the light is incident at an oblique angle.

Most lens manufacturers supply the lens with documentation including both the ordered powers and compensated powers. The amount of compensation varies by manufacturer. Some merely compensate sphero /cyl powers and axis for position of wear and some also compensating ordered prism.

At this point the manner in which prism is measured during refraction should be considered along with the assumptions that are made by lens manufacturers. When placing a measuring prism in front of an eye the subsequent movement of the eye means that the eye is no longer looking through the OC of the lens. See figure 2 and 3.


Figure 2.  Minus lenses centred for refraction.

 

Figure 3. Following the addition of base out prism.

 

The trial lenses need to be moved nasally or the prism experienced will be the sum of the trial prism and that caused by the decentration of the eye with respect to the trial lens. As a rule of thumb, with a CVD of 13mm, the OC should be moved by 0.3mm for each one Prism D of power, away from the base of the prism.

If this is not done then the measuring prism does not match the amount of phoria present. Also the patient may experience aberration from looking through a point away from the OC of the lens.

It is essential that, when ordering a lens, assumptions that the lens manufacturer makes are understood:

  • That centration has been taken into account and the ordered prism and OC have been corrected.
  • That centration has not been taken into account and so the prism and OC must be compensated for in the calculation.

The lens manufacturer will detail any compensation and changes made.

Near Astigmatism

As stated in a previous article5 there is now patented technology that allows cyl power and axis to vary from the distance portion of the lens to near. In this way eye rotation and effective near astigmatism can be modelled and compensated for.

The third cause of near astigmatism that cannot be modelled is anatomical deviation. Anatomical deviations can be caused by:

  • Astigmatic accommodation due to an asymmetrical increase in curvature of the crystalline lens, in particular with high lens astigmatism.
  • Tilt of the crystalline lens on accommodation resulting in oblique astigmatism.
  • Positional change in the crystalline lens during accommodation.
  • Asymmetrical changes of the crystalline lens with presbyopia.

There have been several studies as to the incidence and magnitude of near astigmatism.

In 1985 Millodot et al1 studied 122 eyes refracted over five distances of fixation and found that while the cyl component of the script changed slightly, in 50 per cent of the eyes the cyl axis varied by at least 5°.

In 2007 Radhakrishnan and Charman2 studied 62 eyes and found that for most subjects in the group, astigmatism changed in the with-the-rule direction, the mean change being -0.036 DC per 1D of accommodation. They speculated that these changes might reflect increases in lens tilt about a horizontal axis, caused by the combined effects of a slacker zonule and gravity.

Finally, in 2012 a thesis study at the University of Applied Sciences at Jena3 discovered that, in a study of 144 eyes, approximately 45 per cent of the participants showed an amount of near astigmatism of at least 0.125 D and an axis change of at least 3°. The study concluded that “near astigmatism is not rare.”

A blurred retinal image due to uncorrected near astigmatism may cause accommodation fluctuations when looking for the best focus. This can lead to asthenopia related discomfort such as eye fatigue or headaches. Correcting near astigmatism in multifocal lenses will give a resolution to blur and provide clear and comfortable vision for prolonged, detailed visual tasks.

With that in mind, and the change in the near tasks that our patients are now faced with, it is surely a duty of care of the profession to provide the best near solution possible to help counter the all too common problem of near fatigue. Performing a near refraction and dispensing a suitable lens that will correct for near astigmatic changes can achieve this.

A near refraction need not be time consuming. With a trial frame set to near PD the near add can be determined using the usual method. Then all patients, including those without a cyl in their distance script, can be shown a near fan. (Figure 4.)

Figure 4. A near fan on an iPad app.




Figure 5. iPad app. Target for cross cyl exam.

Any deviation in clarity can then be checked and measured with a cross cyl at near to determine cyl axis and magnitude.

The near refraction will take into account effective near astigmatism. However, Listings Law for rotation will still need to be compensated for as during a near refraction the patient looks through the trial frame as centrally as possible without lowering of gaze.

Let us consider a patient with the following Right Eye script:

Distance +1.50 / -1.75 x 74
Near +4.00 / -1.50 x 86

The script shows a reduction in cyl value of 0.25D and an axis change of 12°, which results in an astigmatic difference of more than 0.7 D in near vision as shown in Figure 6.

Figure 6. Distance cyl -1.75 x 74; Near cyl -1.50 x 86; Resultant cyl 0.72D.

As the loss in visual acuity associated with a 0.72D cyl at near can be predicted, it is possible to produce the iso acuity plot of a multifocal lens that does not correct for near astigmatism. See figure 7. There will be three main effects:

  • At the near point there will be a reduction in near visual acuity.
  • There will be a monocular narrowing of the width of clear vision in the near and intermediate portions of the lens.
  • Binocular widths of clear vision will be adversely affected, as there will be asymmetrical arrangement of the near visual zones. In other words the eye is starting from a point where near VA is less than one. As the eye moves temporally the amount of aberration will increase quickly, further reducing VA. As the eye moves nasally VA will improve. For binocular vision the corridor needs to be symmetrical so as the eyes move across a page of print there are large overlapping areas of clear vision for right and left eyes.

Fig. 7: Representation of the relative visual acuity for each focal point for a multifocal lens without near astigmatism correction.

Fig. 8: Representation of the relative visual acuity for each focal point for a multifocal lens with near astigmatic correction.

If a lens is produced that has the full astigmatic correction for near in front of the pupil when the eye is lowered and converged to the near point, then VA will be improved significantly. The width of clear vision for intermediate and near will be increased as monocular widths will be increased and the corridor will be more symmetrical, again giving wide binocular fields of view. See figure 8.

Frame Parameters

As discussed previously the FFA, PT, CVD and PD all have an effect on the position of the corridor and subsequent perception of width of clear vision.5 In standard, non- individualised designs, averages of these are used and it is essential to know the values your lens manufacturer uses. As soon as the frame chosen by the patient differs from these averages, useful areas of the lens reduce and aberration has more effect, usually by increasing the amount of swim experienced. As a base result:

  • Difference in the PT will cause the height of the corridor and the way the near power comes in to alter from that intended by the lens design.

At a pantoscopic tilt of 0°, which is common with small frames or with sports eyewear, lenses calculated with a standard 7° PT have a performance of less than 50 per cent, while the performance is still 90 per cent in individual progressive lenses.

  • Difference in FFA will cause the corridor position to vary horizontally so the pupil will no longer follow the centre of the corridor. This will reduce the useful width monocularly and binocularly as overlapping areas are reduced.
  • As CVD increases the corridor width will reduce.
  • As PD increases central near vision may appear blurred. As PD decreases corridor width may appear to be narrow.

Most lens manufacturers now produce lens designs that can be optimised using individual frame parameters. These can be measured manually but increasingly electronic or digital devices are being used in practice. Indeed it is now becoming the norm for measuring devices to be used as a part of the dispensing process and is starting to be the expectation of patients.

Individualisation for Lifestyle

There are several ways to change a lens design to suit a patient’s way of life, generally by modification of the peripheral aberrations.

The oldest method to control aberration was to make the lens design relatively hard or soft. A hard design pushes the aberration towards the periphery of the lens and gives wider areas of clear vision. However, when the aberration is reached it comes in very quickly and produces noticeable swim. The opposite is a soft design, which spreads the aberration more over the lens. This will produce a narrower corridor, however, the aberration comes in more slowly and produces less swim.

The next method to control aberration is to move it vertically on the lens. Since multifocal lenses were first introduced, practitioners have been altering the fitting of a lens in an attempt to produce their ideal result by either:

  • Fitting slightly low to push aberration down the lens to preference distance vision by giving wider fields of view.
  • Fitting slightly high to sit the pupil in the progression and so preference intermediate and near.

The problem with doing this is that moving the corridor vertically also moves it relatively horizontally. When a lens manufacturer advises a fitting height it arranges the corridor to inset from that point. If the lens is fitted slightly low the eye will converge slightly before entering the corridor. The pupil will no longer follow the centre of the corridor adversely affecting VA and width of corridor.

Patented technology now exists that allows the distance design point, where the distance script is as ordered, to be placed above or below pupil. In this way either distance vision or intermediate and near can be preferenced without adversely affecting corridor position. See Figure 9 and 10.

Figure 9.  Distance design point is dropped 1.6mm below pupil to push aberration down the lens to give wide fields of view for tasks such as driving.

Figure 10. Distance design point is raised 1mm above pupil. This gives wider intermediate and near but reduces width of vision for distance. Suitable for office and near based tasks.

The near design point can then be moved to alter corridor length to allow the patient a different head position for intermediate and near. There are limitations upon how close the distance and near design points can be without limiting the width of the corridor.

Minkwitz theorised that as a lens increases in power by 1D, vertically unwanted astigmatism (aberration) will increase by 2D horizontally. Unwanted astigmatism associated with a given power change along a given distance can be redistributed but probably not reduced.4

By altering the relative positions of the distance and near design points the horizontal and vertical profiles of the lens can be controlled.

In this way figure 9, a design preferencing distance, can be represented by the graphs in figure 11. The distance design point (DDP) is 1.6mm below fitting cross and the iso VA plot shows the highest image stability for distance, with minimal swim and good peripheral view. As the DDP is below pupil, the corridor must be shortened to allow a reasonable head position for near. However, flexible design still allows control of how the near power comes in. An initial gentle transition allows the majority of aberration to be pushed into the lower portion of the lens that has the lowest usage. This lens is designed for a patient whose day is occupied by distance based tasks with some near work.

 

Figure 11. Design suitable for a patient whose lifestyle is dictated by distance tasks.

Figure 12. Design suitable for a patient whose lifestyle revolves around intermediate and near tasks.

A design preferencing intermediate and near, can be represented by the graphs in figure 12. The DDP is 1mm above the fitting cross and so the iso VA plot shows a slight reduction in width of stable distance fields of view with the pupil sitting in the corridor. The corridor can be longer while still maintaining head position and the power profile shows this as a gradual increase in power along the whole corridor. The aberration comes in more slowly, indicating a soft design. This lens is designed for a patient whose day revolves around intermediate and near tasks.

Lens design technology and manufacturing processes enable the practitioner to virtually design their own multifocal lens. There are multiple levels of personalisation, from PD and frame parameter optimisation, to near refraction and fully correcting for near astigmatism, to lifestyle preference design. In a market where competition is saturated at the price-based level it becomes ever more important for practitioners to improve levels of knowledge and ability to provide truly personalised, lifestyle based solutions for our patients.

A Quick Look at Higher Order Aberrations

Figure a. Diagrammatical representation of spherical aberration.

While it is beyond the scope of this article to discuss Higher Order Aberration (HOA) in detail, when discussing lens design and levels of individualisation it is important to mention it.

HOA such as spherical aberration, coma and trefoil tend to effect vision in low light levels reducing contrast and causing blur, halos and glare.

Practitioners have become increasingly aware of HOA as different procedures such as OrthoK and Lasik have been employed to alter the shape of the cornea.

One HOA that has a large impact on vision is Spherical Aberration. See figure a. The top diagram represents ideal aberration free imaging. The bottom diagram represents real imaging with spherical aberration.

Spherical aberration occurs because light passing through a spherical lens is not focused to a single point but rather to a zone of focus immediately anterior to the focal point. Light passing through the middle of the lens will be focused in one point (paraxial focus). However, light rays passing through the periphery of the lens will be refracted differently than paraxial rays, and in most cases more. In the human eye then, spherical aberration is related to pupil size.

During the daytime when the eye is exposed to bright light, the iris blocks light rays from the peripheral and mid-peripheral lens, and thus spherical aberration is minimised.

At night the pupil dilates and more peripheral rays are allowed through the lens and on to the retina. This phenomenon contributes to the decrease in the quality of vision that many patients notice at night.

A lens can admittedly not correct HOA; however, their influence on vision can be minimised by an adaptation of the sphero-cylindrical prescription.

Research allows lens manufacturers to calculate the average effect of HOA dependent on pupil size and build compensation into the lens design.

For a more individualised result an aberrometer can be used to measure the HOA of the patient’s eye and design compensation. This is the ultimate level of lens individualisation.


 

 


       Nicola Peaper qualified as an optometrist in the United Kingdom in 1985 and practised in private and corporate businesses for 20 years. Ms. Peaper moved to Australia in 2006 where she has worked in state and national training roles presenting the theory and practice of prescribing and fitting ophthalmic lenses. She is currently Professional Services Manager for Rodenstock Australia.


Material source:
Eye Lens Technology
Lenses of the Future - Step 2
Nicke, Katrin, Dipl. - Augenoptikerin / Optometristin (FH)
Welk, Andrea, Dipl.-Ing. (FH)
Schwarz, Ilka, Dipl.-Ing. (FH)
Esser, Gregor, M.Sc., Dipl.-Ing. (FH)
Research and Development
Rodenstock GmbH

References
1. M. Millodot, C. Thibault. Variation of Astigmatism with Accommodation and its Relationship with Dark Focus. Ophthal. Physiol. Opt., Vol. 5, NO.3, Pg, 297-301 (1985)
2. H. Radhakrishnan, W. N. Charman. "\Changes in astigmatism with accommodation. Ophthal. Physiol. Opt. 27, Pg. 275-280 (2007}
3. University of Applied Sciences Jena – Thesis Anders, Christin Epidemiology and refractive determination of near astigmatism. 2012
4. Sheedy JE, Campbell C, King-Smith E, Hayes JR. Progressive powered lenses: the Minkwitz Theorem. Optom Vis Sci. 2005 Oct;82(10):916-22.
5. mivision 110. Mar 2016. Maximising Multifocals: Intermediate and Near Vision.


' Many lens designs now are available that give us the ability to factor the chosen frame measurements in '