Fit first--practical implications of multifocal soft lens design and fit: This article emphasises the importance of ensuring a suitable fit to achieve a successful visual outcome for soft multifocal contact lens wearers.
Multifocal soft lenses are a work of art: sophisticated pieces of precision in optics that attempt to provide patients with satisfying vision at all distances. To provide all of this with glasses in a frame is a challenge, but to provide it in a small piece of plastic is an even greater feat. This article explores the challenges that come with this modality. What is great about it, though, is that it brings together everything that makes our profession such a special and interesting one, combining optics of the eye and the lens with ocular shape and anatomical features such as pupil location and sagittal height make this work a challenge, but surely a challenge to embrace.
Apart from the essential technical considerations, it is the presbyope themselves that is interesting and challenging as well. In our institution, our students have always been taught that: 'presbyopes are a special breed; 0.25D plus or minus can make them love or despise you.' Sure enough, that is what I am experiencing as a patient now: +0.25D for either eye improved my reading, but decreased my distance vision--dramatically, in my opinion. This article explores both the technical knowledge needed to understand multifocal soft lens fitting success (and failure), and the experience from an interesting multifocal soft lens patient: myself.
Students are interesting to work with for a variety of reasons. Certainly, when it comes to teaching them contact lens fitting, students often seem to ask the right questions. The protocol at many optometry schools around the world is to take the central keratometry values and 'add something' to make the lens a certain amount flatter. That should be the ideal --or at least the first--trial lens to use. Sounds good for a start. But then students look for that lens, only to become somewhat disappointed to find that an '8.15mm' base curve lens is not available in the lens material they had in mind. In fact, most disposable lenses have two base curves at best. From various studies, we now know that one 8.6 base curve from one company is not the same as an 8.6 from another company (see Figure 1). (1) So, what next? We typically tell the students to look for 'the closest' lens they can find, somewhat undermining their belief in the strategy we as educators presented them with. We teach them what to look for (centration, movement and mobility) when the lens is on the eye. And if the lens doesn't comply with our terms and regulations --and we test them on this--they must search for an alternative. But as I have said: often, we cannot get an alternative because, if for instance the lens moves too much in this case, we cannot get a steeper lens design. Our fitting method does not have much credit at this point. Worse yet: our credibility as educators starts to deteriorate, too. Basically, it doesn't make sense to students. And if they wanted to choose a lens diameter based on the corneal diameter, same problem.
Switching to another manufacturer with other parameters doesn't do the trick from a fitting standpoint, either. The design of that lens could be very different. Many of the current disposable soft lenses on the market are monocurve designs, or monocurves with just a slight edge; others are true bicurve lenses, and some are aspheric. These designs all have their own specific characteristics with regard to lens behaviour on-eye. Moreover, lens edge shape may play a big role in lens fit and in lens perception (comfort). But other variables, including (and not limited to) lens material, lens thickness, front surface design, lens surface roughness and friction forces, and even the roughness of the palpebral conjunctiva, will play a role in the amount of lens movement on-eye as well. (2) In other words: lens movement does not seem to be a good predictor of the degree of alignment of the lens with the ocular surface. (3)
A recent study though by Sulley et al shows that it is rewarding to offer alternative lens strategies. (4) The study looked at factors in the success of new contact lens wearers in the UK and came to a couple of stunning conclusions. After 12 months from initial fitting, the retention rate (people still in lenses) was 74%. Of the dropouts, 25% discontinued during the first month, and 47% within 60 days. More surprisingly, for 71% of dropouts, no alternative lens or management strategy had been tried. And there was a wide variation in retention rates among sites: between 40-100%. So, in some practices the dropout rate was enormous (60%), whereas in others it was almost zero.
Beyond corneal borders
Instruments to measure eye shape and to better predict the fit of the soft lens on the eye are not widely available in our practices currently. However, a new generation of ocular surface topographers has entered the market that can help analyse the entire anterior ocular surface shape--beyond the corneal borders. (5) These instruments, based on profilometry (using fluorescein as a 'screen' to project height patterns), Scheimpflug systems or optical coherence tomography (OCT), can help determine limbal and anterior scleral shape.
Other alternatives include corneal topographers that take their data about the measured peripheral corneal shape, described as an angle in modern techniques, and extend that out further onto the periphery. (6) Studies at Pacific University in the US show that peripheral corneal angles can be predictive of anterior scleral shape (angles) which could help the eye care practitioner to some degree to predict eye shape beyond the corneal borders.
So, back to the presbyope: multifocal lens fitting is one of the most challenging modalities in which to succeed in contact lens practice. Multifocal soft lens fitting truly is multifactorial: anatomical, optical and physical properties of lens material are all important, as is corneal physiology, not to mention patient motivation and individual preference. But, understanding lens optics is a good starting point and an important part of successful multifocal soft lens fitting--something that may often be underutilised. (7)
Soft lens decentration
First to note is that soft lenses tend to decentre on the eye--potentially temporally because of the flatter shape of the nasal sclera that we now know exists. An article by Cathleen Fedke from the Brien Holden Vision Institute published in the journal Clinical Optometry states findings about soft lens fit in general and multifocal contact lenses specifically. (8) Several commercially-available multifocal contact lenses were tested in their study. Lens decentration, that is to say, the x--and y-differences between the contact lens and pupil centres, was objectively determined. On average, all lenses displayed a temporal-inferior decentration. From a clinical standpoint, it is also interesting to note that they concluded that certain multifocal contact lenses decentred more than others, which can primarily be attributed to differences in lens design and fitting parameters, according to the authors. Moreover, an association between multifocal contact lens decentration and higher-order aberrations was found. In other words, visual performance is negatively influenced by the lens decentration.
Numerous studies at Pacific University using different instruments have shown that the nasal part of the anterior ocular surface beyond the corneal borders is flatter compared to other parts, particularly the temporal portion of the eye; this typically causes larger diameter lenses, such as sclerals and soft lenses, to decentre temporally. In addition to this, soft lenses seem to decentre not just temporally, but also slightly inferiorly. The general thought is that vertical decentration may have to do with gravitational forces as well as eyelid pressure and tension, other than eye shape.
It should be obvious to everyone, though, that for a multifocal lens to function on the eye, the optics of the lens need to centre. If the optics of the lens are decentered over the eye, the effectiveness of the lens can decrease quite dramatically as higher-order aberrations are induced, degrading the quality of the visual performance of the lens system.
A little less known in the industry is that pupils on average are slightly displaced as well, typically nasally and superiorly. In a study by Wildenmann and Schaeffel on 4mm pupils (the decentration varies, depending on pupil size), the pupils were nasally decentered relative to the corneal centre by 0.18 [+ or -] 0.19mm in the right eyes and 0.14 [+ or -] 0.22mm in the left eyes. (9) Vertical decentrations superiorly were 0.30 [+ or -] 0.30mm and 0.27 [+ or -] 0.29mm, respectively, always in a superior direction. What this means is that for the standard eye--but individual differences appear--the error caused by temporal decentration of a soft lens because of the flatter nasal ocular shape is further magnified by the nasal decentration of the pupil (see Figure 2). And, the described error caused by inferior decentration of the soft lens may also be magnified by the superior decentration on average of the pupil (depending on where the lens shows its vertical decentration).
Line of sight
As a third variable, the line of sight is another phenomenon to be considered. Here it gets somewhat confusing, but the conclusion is quite simple. Basically, what we want as eye care practitioners is to align the multifocal lens over the patient's line of sight--and not over the geometric centre of the cornea or the pupillary axis. Certainly, this becomes relevant when it comes to multifocal intraocular lenses, but also in laser refractive surgery, it could be (or should be) of importance.
Unfortunately for us and our patients, most individuals have a positive angle kappa. This sounds good, but the negative part of a positive angle kappa is that the visual axis is nasal to the pupillary axis. In other words, this further adds to the effect of a relative temporal optical displacement of the lens.
In addition, lens flexure could interfere with our optics. (10) Studies in the Netherlands and at Pacific University have shown that for a soft lens fit to be successful, the shape of that lens needs to be somewhat 'deep' on the ocular surface. If a soft lens is fitted completely aligned with the ocular surface --for example, it has the same sagittal height as the sagittal height of the eye--then the soft lens fit, influenced by tear film, eyelid pressure and blink forces, will move excessively on the eye; this will result in an unsuccessful lens fit that would be very uncomfortable to wear. Our current understanding is that the sagittal height of a soft lens on-eye likely needs to be somewhere in the range of 200-300f<m 'deeper' to achieve a clinically successful lens fit. An increased difference in sagittal height from that could result in unwanted corneal changes, such as warpage, as shown in Figure 3. Less difference could, as said, typically result, (although dependent on many variables) in excessive lens movement, and more lens decentration. This may have consequences for the visual performance of that contact lens on-eye: lens flexure on the eye will cause a small change in lens optics from what the lens was designed to provide. For a simple spherical -2.50D correction contact lens, this does not seem to have a huge effect. But for multifocal lenses, and also for dual-focus myopia control lenses and wavefront-corrected lenses, this could indeed have an impact.
Recently, I had an interesting patient: myself. As an early presbyope, monovision did it for me up until recently. Then my vision started to bother me. Especially at conferences, in the back of the room in dim light conditions--the screen just wasn't sharp enough anymore. And with prolonged reading, I noticed that I was closing one eye (the distance eye, obviously), which was not very comfortable. In the search for a good alternative, a lesson or two were learned.
The case-in-point here is that the lens selected initially, based on the parameters of my existing prescription and on central keratometry values, didn't fit well, and the vision wasn't very good either. We could have made the crucial mistake of starting to change the power of the lenses right away at that point: the sphere power, the cylinder power, the axis, the reading addition or the centre-near or centre-distance nature of the lenses. But, if the lens fit is not optimal, then this doesn't make sense. My lenses moved too much and decentered, causing not only discomfort, but also visual disturbances, as described in the earlier paragraphs.
I concentrated on getting the right lens fit. After two different lenses with various sagittal heights and diameters, we settled for a lens that looked good on the eye in terms of lens behaviour and felt comfortable. From there, it was relatively easy to get the optics right.
Multifocal soft lens fitting truly is multifactorial. But understanding lens optics is a good starting point and an important part of successful multifocal soft lens fitting--something that maybe is underutilised in many optometry schools around the world.
A significant improvement in the quality of vision can be expected when multifocal optics are displaced nasally. In fact, if the eye care practitioner (or the patient) manually displaces the lens slightly nasally, the vision often 'clears up' in the patients' experience. This is true for the average eye, but ideally, as eye care practitioners, we would be better off measuring the different variables described to serve the individual patient best. Technically, this should be possible. This way, we can show our expertise, justify our fees and better help our presbyopic patients--one of the few areas in our field in which we often cannot provide satisfying solutions to patients.
In addition, this same concept becomes increasingly relevant when it comes to dual-focus optics soft lenses for children and myopia control. Part of the reason why the results are so variable, and that in some children the slowing of progression is so much more successful than in others, may be that we cannot align the optics of the lens--individually--with the optics of the eye and/or predict exactly what the optics of the eye are due to flexure.
We have a lot to learn, and it is challenging. But sure enough: it is an interesting challenge. And in addition, we have a large group of presbyopes with increasingly active lifestyles and prolonged life expectancy who count on us to provide a good alternative to spectacle lens wear.
We are so used to immediately altering and changing the lens power prescription if vision is suboptimal that we completely forget about the fact that lens fit can play a role in vision as well. Granted, previously we were limited to the parameter range in disposable soft lenses to make that difference. Today, these lenses are available in a monthly replacement modality, so we don't have that limitation.
As for the case report described in this article: I am happily wearing my custom made soft lenses now, in a larger-than-average lens diameter, as a monthly disposable toric multifocal lens--center-distance for the right (dominant) eye and centre-near for the non-dominant eye. But this case illustrates that it is not just about finding the right custom lens--it is about prioritising lens fit. Fit first! Then go for the right prescription and fine-tune the power and optics. For experienced eye care practitioners, this is something to remember.
For students and future practitioners, it is a bit of a different story. As a profession and certainly as educators, we need to come up with a better and more credible way of fitting soft lenses on-eye. O
Under the enhanced CET rules of the GOC, MCQs for this exam appear online at www.optometry.co.uk. Please complete online by midnight on 9 February 2018. You will be unable to submit exams after this date. Please note that when taking an exam, the MCQs may require practitioners to apply additional knowledge that has not been covered in the related CET article.
CET points will be uploaded to the GOC within 10 working days. You will then need to log into your CET portfolio by clicking on 'MyGOC' on the GOC website (www.optical.org) to confirm your points.
Visit www.optometry.co.uk, and click on the 'Related CET article' title to view the article and accompanying 'references' in full.
The author would like to give special thanks to Marielle van Goor for her input and help in the fitting process.
Dr Eef van der Worp BOptom, PhD, FAAO, FIACLE, FBCLA, FSLS
About the author
* Eef van der Worp is an educator and researcher from Amsterdam and is affiliated with the University of Maastricht as an associate researcher, an adjunct professor at the University of Montreal College of Optometry, and an adjunct assistant professor at Pacific University College of Optometry.
Course code: C-57823 Deadline: 9 February 2018
Be able to successfully manage presbyopic patients with multi-focal contact lenses (Group 5.3.2)
* Understand the factors that influence contact lens design selection for initial fitting (Group 5.1.2)
* Be aware of the differences between lenses supplied from a variety of manufacturers (Group 4.2.1)
* Be able to successfully manage presbyopic patients with multifocal contact lenses (Group 5.5.2)
Caption: Figure 1 Overview of the lowest (left) to highest contact lens sag of all lenses (spherical and toric) in the minus group (pm). Courtesy of Contact Lens & Anterior Eye, June 2015: Van der Worp, Mertz. Sagittal height differences of frequent replacement silicone hydrogel contact lenses
Caption: Figure 2 The relative difference of pupil displacement, geometric centre of the eye, line of sight and lens decentration. Image courtesy of Matthew Lampa, Pacific University College of Optometry, Oregon, US
Caption: Figure 3 Unwanted corneal changes in soft lens wear, resulting in subtle corneal warpage, imaged using a difference map with the Medmont E300 Corneal Topographer (Precision Technology Services Ltd., Vancouver, Canada)
|Printer friendly Cite/link Email Feedback|
|Title Annotation:||Multifocal contact lenses|
|Author:||van der Worp, Eef|
|Date:||Jan 1, 2018|
|Previous Article:||The therapeutic alliance: This article will explore the role of artificial intelligence decision support systems in providing a new, safer,...|
|Next Article:||Ocular side effects of systemic medication: This feature examines the practitioner's ability to recognize and interpret ocular side effects due to...|