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). Not very comfortable. In the search for a good alternative, I learned a lesson or two.
I’ve always taught my students that ‘presbyopes are a special breed: 0.25D plus or minus can make them love or despise you.’ Sure enough, that is what I was experiencing as a patient now. +0.25D for either eye improved my reading, but decreased my distance vision – dramatically, in my opinion.
Students are interesting to work with for a variety of reasons. But, certainly when it comes to teaching them contact lens fitting, they often seem to ask the right questions. The protocol at many optometry schools around the world, as I know first hand, 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, as we know, 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 another 8.6 from another company.
So, what then? We typically tell them to look for ‘the closest’ lens they can find, somewhat undermining their belief in the strategy we presented them with already. 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 said: often, we cannot get an alternative because only a flatter design is available, for instance, in that lens type (and we wanted a steeper design), or simply no other lens parameter is available within that brand at all. Our fitting method does not have much credit at this point. Worse yet: our credibility as educator starts to deteriorate, too. Basically, it doesn’t make sense to them. Well, they have a point. It doesn’t. And if they wanted to choose a lens diameter based on the corneal diameter, same problem.
“Basically, it doesn’t make sense to them. Well: they have a point. It doesn’t.”
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 bi-curve lenses, and others are aspheric. These 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, surely will play a role in the amount of lens movement on-eye as well. 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.
Instruments to measure eye shape and to better predict the fit of the soft lens on the eye are not 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. These instruments, based on profilometry (using fluorescein as a ‘screen’ to project height patterns) or Scheimpflug systems, 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. Studies at Pacific University (USA) 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.
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, which is another part of the game.
But understanding lens optics is a good starting point and an important part of successful multifocal soft lens fitting – something that may often be underutilized. First, soft lenses tend to decenter on the eye – potentially temporally because of the flatter shape of the nasal sclera that we now know exists. In addition, the line of sight is another phenomenon to be considered. 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 center of the cornea or the pupillary axis. 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. Studies in the Netherlands and studies at Pacific University have shown that for a soft lens fit to be successful, the shape of that lens needs to be somewhat ‘steep’ on the ocular surface. If a soft lens is fit completely aligned with the ocular surface (e.g. 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 a very unsuccessful lens fit that would be very uncomfortable. Our current understanding is that the sagittal height of a soft lens on-eye likely needs to be somewhere in the range of 200-300 microns ‘steeper’ to achieve a clinically successful lens fit.
This may have consequences for the visual performance of that contact lens on-eye: lens flexure on the eye of 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.
The case-in-point here is that the initial selected lenses, based on the parameters of my existing prescription and based on central keratometry values, didn’t fit well for me, 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 center-near or center-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. We first sat down to get the right lens fit. After two different lenses with various sagittal heights and diameters, we settled for a lens that both looked good on the eye in terms of lens behaviour and that felt good. From there, it was a relative piece of cake to get the optics right.
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 kind of limited to the parameter range in disposable soft lenses to make that difference. But now these lenses are available in a monthly replacement modality, so we don’t have that limitation anymore.
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 center-near for the non-dominant eye. But this case illustrates that it is not just about finding the right custom lens – it is about prioritizing lens fit. Fit first! Then go for the right prescription and fine-tuning the power and optics. For experienced eye care practitioners, this is something to remember maybe. For students and future practitioners, it is a bit of a different story. As a profession and surely as educators, we need to come up with a better and more creditable way of fitting and evaluating soft lenses on-eye, as the current method is surely insufficient.
Special thanks to Mariëlle van Goor for her valuable input and help in the fitting process.
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