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Digital Single Vision and Multifocal Lenses: Resolving Non-Tolerance Issues

2 CPD in Australia | 0.5G in New Zealand | 5 October 2017

By Nicola Peaper

Non-tolerance to a spectacle appliance is only a small part of optical practice, with the research showing somewhere between 1.6 per cent and 2.3 per cent.1,2 However, the consequences of non-tolerance can be far-reaching in terms of wasted professional time and negative word of mouth. 

This article looks at the steps taken to problem solve various issues of non-adapt by working through several examples based on real cases experienced by Australian and UK practitioners.

Studies indicate1,2 that non-tolerance to the prescription is the greatest cause for patients to return. The next most common issue is from dispensing, such as frame issues, lens design or insufficient explanation of the lens solution being prescribed. The most common patient group returning with concerns is presbyopes in the 50–59-year age group.

Spherical changes that over plus (under minus) prescriptions cause more problems than any other script change, followed by changes to cyl power and then cyl axis. In many cases, over plusing of the correction can be explained away by using 3–4m projection charts inducing an over correction by +0.25D.

Interestingly a study in 2011,3 found that partial prescribing, or modification of refracted prescription, was more likely to occur the longer a practitioner had been qualified. Partial prescribing of a script change is three times more likely at the end of a forty year career than at the beginning. The study proposes that “the most likely cause of the strong link between years qualified and the likelihood of partially prescribing is increased experience, as with more exposure to patients who return unsatisfied with their spectacles, a greater appreciation of partial prescribing is achieved”.

When problem solving a non-tolerance case, it is important to consider the effect the combination of lens design and script change is having. For instance, a multifocal that prioritises intermediate and near vision will usually bring in the addition more quickly than a standard design. If this is coupled with a 50 year old myope, whose distance script has reduced by 0.25D, there may be adaptation issues. The reduction of script or dispensing of lens design may be fine in isolation but when put together, the reduction in minus power may be too much.

Many non-adapt remakes are ordered with small changes in heights, pupillary distances (PDs) and script. As already discussed, 0.25D changes can be significant with script, however, with examination and manufacturing tolerances, changing a cyl axis on a low powered cyl by a few degrees or altering PD or height by 0.5–1.00mm is probably insignificant.

Lens manufacturers’ warranties tend to cover design. In relation to PAL designs, most warranties allow a change to another design or to bifocals or single vision lenses. In other words, it is possible that with everything being correct – script, measurements and production – sometimes the patient may not adapt to a lens design. Once we, as practitioners and manufacturers, grasp this fact we open up a world of interesting problem solving!

Single Vision Issues: Aspherics

Case Study 1

“My patient feels like the floor is moving, straight lines are distorted.”

A patient called for a routine appointment wanting to update their frame. The script was unaltered at:

R +3.50 / -0.50 x 85
L +4.00DS
PD 63

Previous lens single vision distance – 1.5 index spherical design

New lens single vision distance – 1.6 aspheric as new frame is much larger than old.

Considerations 

  • The order quotes binocular PD and does not include a height
  • Frame face form angle (FFA), pantoscopic tilt (PT) and corneal vertex distance (CVD) have influence on design performance
  • The patient will encounter a different level of oblique astigmatism by changing design from spherical to aspheric
  • Magnification and distortion will differ from the spherical design.

Single vision lens design, like multifocal lens design, has been impacted by digital technology. Conventional spherical designs only guarantee optimum correction at the optical centre whereas freeform designs that have ‘multiple axes’ of asphericity to correct for oblique astigmatism, spherical aberration and distortion have larger areas of accurate power. Designs can include optimisation for a variety of things such as eye rotation as described by Listings Law, compensation for HOA and individual frame parameters.4

The first thing to consider is to dispense these lenses correctly. During the design process the manufacturer assumes that the optical axis of the lens will pass through the centre of rotation of the eye (Figure 1).

Figure 1. Correct fitting of aspheric single vision lens. O = optical centre; Z´ = centre of rotation of the eye

 

In basic terms, asphericity is the use of varying curves, across the surface of the lens, to reduce or try to eliminate aberrations, such as oblique astigmatism, encountered as the eye rotates away from the Optical Centre (OC). As such, at any point away from the OC the curvature should have changed just enough to counteract the oblique astigmatism that would normally be experienced at that point. If the lens is not fitted correctly, then the calculations will be incorrect and the asphericity will have an adverse effect potentially producing worse vision than the same power lens in a spherical form.

Because of this, accurate monocular heights (dropped by 1mm for every 2 degrees of PT) and monocular pupillary distances must be supplied with an order. Dropping the heights for PT simply ensures the eye looks through the correct portion of the lens in all directions of gaze.

It also must be considered that the aspheric curves need to be calculated knowing at what angle the light is incident upon the lens. During the lens design phase, an assumption must be made. In European designs, the assumption is generally a PT of 7 degrees and FFA of 5 degrees. When dispensing a freeform single vision aspheric lens, choosing a design that allows individual frame parameters to be compensated for will give an improved result.

Even if the script and measurements are all correct some patients will still report, “the floor is moving, straight lines are distorted”.

At this point, consider what changing from a spherical lens to an aspheric entails for the patient. The levels of oblique astigmatism experienced in the periphery are changed, in other words the patient is experiencing a different level of cyl power as the eye moves across the lens. The aberration may be of a lower level than with a spherical lens but it is still different to that which the patient is accustomed to.

Whenever a script change includes a change of cyl power and/or cyl axis, it is generally accepted that the patient is warned of adaptation issues: “Your vision will be clear but as you look at straight edges, they may appear curved…this will settle down over a short period of time”.

When changing a patient to an aspheric lens, a similar explanation given at the time of dispensing may reduce patients returning for non-adapt.

“The periphery of the lens is not sharp…”

As the eye moves towards the periphery of a lens, so the eye will rotate according to Listings Law. In cases of cyls over 1.00D the change in cyl axis required to compensate for this can be significant. If the patient is not achieving the degree of clarity expected at the periphery then choosing a lens that has eye rotation built into it may give improved peripheral clarity.

A final regular complaint, and reason given for not using aspheric lenses, is that the edge thickness at the upper rim in plus powers and the lower rim in minus powers is too thick. This occurs because the lens is no longer fitted along datum. As frames increase in size this problem is more prevalent. The only solution is to pick a frame where the pupil sits reasonably centrally in the frame. If the patient insists on an unsuitable frame the only recourse is to warn them of the consequences.

Conclusion

In conclusion, for aspheric single vision patients, consider the following: 

  • The measurements are correct so the eye always accesses the design as it should
  • Check PT and FFA are within the ‘normal’, or average, parameters set by your lens supplier. If not, consider a design that allows for individual frame parameters to be compensated for
  • For astigmatic patients, consider a design that compensates for eye rotation in the periphery as dictated by Listings Law
  • Choose a frame where the pupil sits as centrally as possible
  • Explain any adaptation issues that the patient may experience at the time of dispensing.

Multifocal Issues

Again, consider what the lens design is trying to achieve. As with single vision lenses, the usual aberrations of oblique astigmatism, spherical aberrations and distortion are compensated and optimised for.

Added to this is Minkwitz astigmatism that calculates that for every diopter in power a lens increases vertically there will be two diopters of unwanted cyl. This is the aberration that is generally described as distortion to patients. The major effect this has is on corridor width and swim experienced by patients.

Traditionally the aberration from the power change was dealt with either by:

  • Spreading the aberration over a large area, so called soft design. This meant although aberration was accessed quickly across the corridor it came in relatively slowly and reduced swim (Figure 2); or
  • Pushing the aberration out towards the edge of the lens, giving perhaps wider areas for distance and near. However, when the aberration was encountered it came in quickly to give peripheral swim (Figure 3).

With modern algorithms used in conjunction with digital surfacing, there is so much more that can be done with aberration. It can be pushed in to areas of the lens that have less significance to the user and matched so as one eye moves nasally and the other temporally the aberration encountered is similar. See Figures 4, 5 and 6. Figure 5 shows how, for intermediate and near use, aberration is pushed up and out of the corridor area compared to a general use lens. Figure 6 shows how the aberration is pushed down the lens to give wide distance fields of view. Cyl axis changes, due to eye rotation as the eyes converge, can be built into the algorithm, as can individual frame parameters.

Case Study 2

“The patient says the distance is blurred… It is clearer if I move my head or eye…”

Considerations

Accuracy of the measured/fitted PD
If the pupil is sitting in or regularly passing through a portion of the lens where there is aberration, the patient may complain of blur. For this to be an error of PD, it would have to be very inaccurate. If a small PD change of 0.5mm is found, there is probably something else wrong. (It is important to appreciate that fitting tolerance on PD is +/-1mm of that ordered.5 Changing a PD by less than 1mm can be insignificant)

Accuracy of the measured/fitted height
Again, as for PD, the height, measured or supplied, would have to be very inaccurate

Experience in practice
As stated, the longer a practitioner has been qualified the more likely it is that partial prescribing has taken place – specifically with the cyl power change. If the full-refracted cyl is not prescribed then the patient may, by moving their head to look through a different portion of the lens, find a preferred amount of cyl or axis. The symptom for this is that the patient will contort into a more visually comfortable position.

Figure 2. (left) Soft design (iso cyl lines indicate unwanted astigmatism) and Figure 3. (right) Hard design

Figure 4. Lens for general use

 

Figure 5. Lens prioritising intermediate and near use

 

Figure 6. Lens prioritising distance use

 

Effect of design
What effect does the design have? What happens if the patient has been dispensed with a lens that preferences intermediate and near over distance. In this case two things happen:

  1. The design of this type of lens either sits the pupil slightly low, inside the corridor, so lengthening it slightly, or the Add comes in more quickly.  Both of these mean the patient does not need to lift their chin so much to look at a desktop computer. The result of this is, especially in lower light conditions when the pupil is larger, the Add starts to come into play as the eye looks straight ahead. This generally can be tolerated, hence the success of this type of lens. However, coupled with the reduction in minus the patient may now feel blurred in the distance especially driving at night. Resolution may be to modify the script or to change the lens design.
  2. In preferencing intermediate and near, the corridor is widened by pushing the aberration into the distance portion of the lens. Again, the patient may be aware of this when performing tasks such as driving when the eye scans through these areas. A change in lens design may be necessary.

 

In both these cases, the patient may have been more open to adapt to the lens solution if all of the drawbacks of the design were discussed along with the benefits. Most lens manufacturers have Apps.or tools to help describe each of their designs.

If the design needs to be changed to one that has equal preference for all distances, remember to explain that this will probably result in a slightly narrower corridor, especially if the corridor is shortened to try and bring the intermediate and near higher up the lens.

Ideally, at the time of dispense, the patient should be advised that any multifocal designed for computer use is a compromise. If it works, great. If not it should be replaced with a general use lens and a second extended near focus pair of spectacles for the computer.

Figure 7. Patient PD is narrower than average and so convergence is not enough for the inset.

Figure 8. Patient PD is wider than average and so patient converges too much for the inset.

 

Case Study 3

“The intermediate and near portion are not clear… I cannot see the full width of a page”

Considerations

  • PD and height problems as above.
  • The corridor is not wide enough. There may be several causes:
  1. If accompanied by high levels of swim, consider the PT, FFA and CVD. As with aspheric designs, averages are built into PAL designs. A better result could be gained by replacing with a design that compensates for the individual frame parameters.
  2. A higher Add and/or a shorter corridor will produce a narrower corridor. In this example, the Add has gone up by +0.50D. On its own, this may be noticeable on corridor width. If, however the corridor has also been shortened, the percentage chance of non-adaptation will go up significantly. To resolve issues, look at maintaining the corridor length or using a design that prioritises intermediate and near.
  3. Another consideration is that the near script has gone up by about +0.75 overall. Reducing the Add to +2.25D will produce a wider corridor and still give a +0.50D difference on near power that is a significant change.
  4. A PAL that prioritises distance will push aberration down into the intermediate and near portion of the lens, which will narrow the corridor slightly. These lenses will also have the Add coming in more slowly and so the corridor may appear to be too long.
  5. The inset is not correct for the convergence of the patient. Any lens that solely uses variable inset to calculate the inset of a corridor will not give optimum performance across the full range of PDs. Inset is commonly calculated from the amount of prism a patient experiences due to the lens power, and from the working distance. As such the PD is only used as a positioning measure for putting the lens into the frame. In these cases, an average PD of around 63/64mm is used. Obviously if the patient’s PD is narrower than this they will not converge enough for the corridor inset and will follow the temporal edge. The result of this is that the corridor will appear to be too narrow (Figure 7). If the PD is wider, then the patient will over converge and will be blurred for both intermediate and near vision (Figure 8).

 

To resolve this, use a PD optimised design that takes into account the patient’s actual PD when calculating inset.

NB. In a very small minority of cases, taking a patient with a wide PD out of a standard design and putting them in a PD optimised design can cause problems. The patient may be able to tolerate a slight reduction in near clarity while benefiting from the apparent widening of the corridor.

6. The patient may not converge as expected. The most obvious example of this is a monocular patient who will generally have less convergence than a binocular patient, depending on the amount of time that they have been monocular. If the patient is long standing monocular then the chances are that he will not converge at all. If it is a more recent issue than the convergence will sit between normal and nil. Pick a lens that can vary inset by percentage i.e. zero inset, 50 per cent inset etc. Some lenses allow for a distance and near PD to be incorporated.

  • The power in the reading area is not clear due to uncorrected near astigmatism.

As the eyes look down and converge, they rotate by an amount governed by Listings Law for Near. The rotation is related to the PD and the distance that reading matter is held. With the PD of 65mm in the example, the cyl axis will rotate by 4 degrees when converging to 40cm. On a cyl power of 1.00D and above this is certainly significant.

Effective Near Astigmatism also plays a role. Even if the eye accommodates equally?in all meridians and therefore remains astigmatic by a fixed amount in all states of accommodation, the cylinder correction required for near work differs theoretically from that required for distance vision.6 On high cyls, the power deviation may be up to 0.50D.

As can be seen from Figures 9 and 10 compensating for near astigmatism not only ensures crisp near acuity but also produces a more symmetrical corridor. This gives the impression of a wider corridor.

  • The power for the task is not easily accessible. Causes include:
  1. The patient needs to lift their chin to access enough power for intermediate tasks such as desktop computer use. Having a short corridor will help marginally as this pushes the power up the lens. Certainly, if a patient has been successfully wearing a short corridor, changing them to a longer version as the modern frame depth increases can cause issues with head position.
  2. The patient has to hold reading matter too close to see clearly. Although PAL wearers tend to be more able to adapt to over plusing of the Add, consider the patient that only comes in once every six or seven years. Their distance script may have increased by +0.50D and their reading Add may have gone up by +0.75D totalling an extra +1.25D for reading. This patient has been having problems with all distances for years and has been resolving these issues by continually lifting their chin to access more power until there was not enough to be had. The habit is formed! They will be perfectly clear in the trial frame but will carry on lifting their chin in the new pair of PAL and so consequently nothing will be clear. Script modification may be the only solution to this.
  3. If there is a level of anisometropia generally over 1.00D, then prism will be encountered as the eyes depress to access the Add. This may cause, at the very least, discomfort for reading and may cause vertical diplopia. Consider a short corridor for these patients. Long standing anisometropes may be more tolerant to PAL so their use should not be automatically discounted.

 

Figure 9. Without compensation for near astigmatism

Figure 10. With compensation for near astigmatism

 

Conclusion

In conclusion for multifocal lens patients:

  • Supply the monocular PD and heights that are measured for the new frame. Remember that mono PD’s may change with respect to each frame but they should always add up to the same binocular PD. If the heights are different to each other order what you find. If the difference is split and equal heights right and left are ordered, the Add power will come in at a different rate for each eye and clarity of vision will be affected.
  • Check PT, FFA and CVD are within the ‘normal’, or average parameters set by your lens supplier. If not, consider a design that allows for individual frame parameters to be compensated for.
  • Consider the effect that script change and corridor length will have on head position for near and intermediate tasks and also on corridor width.
  • Understand where a lens design will position the aberration on a lens and consider if the eye will pass through these areas with visual tasks.
  • Consider a PD compensated design to make it more likely that the pupil will follow the centre of the corridor.
  • In cases where near vision is not as clear as expected, consider the effect of near astigmatism and choose a lens compensated for Listings Law for near and Effective Near Astigmatism.
  • Above all, consider the consequences of both script change and design and advise the patient of any differences they may encounter.

All examples are taken from cases I have encountered both in practice and working for various lens laboratories in Australia. With traditional design multifocals, bifocals and spherical single vision lenses, it seemed that less went wrong, although non-tolerance to lens design with multifocals sat at approximately 5 per cent. In reality, because we have so much choice when resolving patients’ visual problems, the thought process involved to choose a resolution is more complex. However, non-tolerance to lens designs is much reduced. 

 

 
 

 

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.

This CPD article was supplied by Rodenstock.


References
1. Hrynchak, P. (2006) Prescribing spectacles: reasons for failure of spectacle lens acceptance. Opthal. Physiol. Opt. 26: 111 – 115.
2. Freeman, C.E and Evans, B.J.W (2010) Investigation of the causes of non-tolerance to optometric prescriptions for spectacles. Opthal. Physiol. Opt. 2010 30: 1 – 11.
3. Howell-Duffy, C et al. (2011) Spectacle prescribing 2: practitioner experience is linked to the likelihood of suggesting a partial prescription. Opthal. Physiol. Opt. 2011 31: 155 - 167.
4. mivision 123. May 2017. Optimising Single Vision Lenses for Today’s Patient
5. AS/NZS ISO 21987:2011 Opthalmis Optics – Mounted spectacle lenses.
6. HH Emsley Visual Optics vol 1 Optics of Vision.

' partial prescribing, or modification of refracted prescription, was more likely to occur the longer a practitioner had been qualified '