How on Earth Do We 'Fit'
Soft Contact Lenses?


Editor Soft Special Edition Newsletter - Optometrist PhD FAAO FIACLE FBCLA FSLS

"How do we teach our students soft lens fitting?"


To answer the question raised in the title, and to fully understand soft lens behaviour on-eye, a few additional questions need to be raised and answered. The first question: Why is this important? The second one: What do we know about the ocular surface and (third question) about the shape of the lenses we fit? These two together define how a soft lens behaves on-eye. The final (fourth) question: Can we improve soft lens fitting? Four questions in a fast-paced, two-page article—here we go!

Question #1: Why is it important to understand soft lens fitting?

Any search into a difficult topic should start with the question ‘why’ in the first place. In this case, there are actually three main reasons why it is important to better understand soft lens fitting. First, we cannot explain it to our students anymore, that we measure central-k values and then ‘add something’ to come up with a desired first trial lens. Oftentimes, the lens that is thus calculated is simply not available in the stock-arena this is even more likely if adjustments need to be made to that first lens chosen. Moreover, there is virtually no relationship between central-k values and how a soft lens behaves on the eye. Our profession is ‘full’-on (and full of itself sometimes) ‘evidence-based’ practice and how ideally everything should be backed up by solid science. Well, big bummer—there unfortunately is not much science in this one, I am afraid.

The second reason is that acquisition of soft lenses increasingly happens through the internet—in some countries/regions more than others (including the Netherlands, where no prescription is needed to order lenses). Internet sales are frowned upon by the profession and industry. But let me play the ‘devil’s advocate’ here a little bit: what are we as ECPs doing differently than just selecting a ‘random lens’, throwing it on the eye and hoping for the best? I am exaggerating things a bit, but in essence, that is something to consider. Third, there is the huge group of dropouts (about 25% of soft lens wearers lapse lens wear). This topic is too big to cover here, but if we reduce dropout rates by about 3-4% on a yearly basis, we can as much as double the amount of lens wearers in 20 years. Indeed: double. And we may be able to do that just by looking at lens fit alone.

Question #2: What do we know about the ocular surface?

If we want to align the lens with the ocular surface over a diameter of, let’s say, 15mm (soft lens diameter is typically in the 14.2mm range on average, with about 0.3mm of movement on each side, which gives us a ‘workbench’ of about 15mm), then central-K values are not sufficient. Special scleral topographers and tomographers (profilometry, scheimpflug and OCT) can measure the full 15mm chord, but not everyone has those in their arsenal of instruments in their practice. The good news is that with standard Placido disc topography, measuring the corneal sagittal height—combined with measuring the peripheral corneal angle and extending that out onto the anterior sclera—provides a pretty good estimate of the overall sagittal height of the ocular surface (the OC-SAG). We know from various studies using these instruments that the OC-SAG of the average eye is about 3750 microns. If we calculate one standard deviation from the mean, then ‘standard eyes’ would be roughly between 3500 and 4000 microns—and 67% of eyes would fall into that category. This is what I call ‘Finding NEMOs’ (normal eyes measured ocular surfaces). This also means that eyes that are lower or higher in SAG than that value could potentially benefit from out-of-standard lenses, as standard stock lenses may not always fit well.

This is what we call ‘Finding NEMOs’: Normal Eyes Measured Ocular Surfaces.

Question #3: What do we know about the shape of the lenses we fit?

The inventor of the soft contact lens, Otto Wichterle, defined the very first soft lenses in the 1960s with ‘S1’ and ‘S’ values, representing the sagittal height of the contact lenses (the CL-SAG). In line with that, recent studies at Pacific University evaluated the CL-SAG values of 16 types of daily disposable lenses (DDs), 15 reusable lenses and 17 toric lens designs. One interesting finding from this was that there could be as much as 900 microns of difference between the highest and the lowest (flattest and steepest, if you wish) lenses in this lineup per category. Those are the tools we have ‘to play with’. A second interesting fact is that between one 8.4mm base curve lens from one company and another 8.4mm base curve lens from another company, there could be over 400 microns of difference. This again tells us that BCs (alone) don’t mean much. It also shows that lens substitution based solely on that parameter doesn’t seem desirable.

In our theoretical calculations at Pacific, if we look at normal eyes (based on OC-SAGs), combined with the available lenses that we have (the CL-SAGs), we think that (taking the average delta-SAG values into account to allow for soft lenses to ‘drape’ or ‘flex’ on the ocular surface) somewhere between 68% and 78% of eyes could be fit successfully with the standard lenses that we have based on eye shape. In other words, 25% or so would not fit well (enough).

Photo by David Clode on Unsplash

Question #4: Can we improve soft lens fitting?

That really depends on the eye, it seems. If we have eyes that fall into the ‘Finding NEMOs’ category, then the Pacific SAG charts may be somewhat helpful but, if we have the most average eye on earth, probably most standard stock lenses would fit well. In this category, other considerations such as replacement frequency (e.g., DDs versus reusable, etc), material considerations (friction coefficient, silicone, lens modulus) and even lens care products used are potentially crucial factors. This is the ‘thinking INSIDE the box’ category: a box of 30 lenses (or any other desired quantity) for a given eye would probably be the best option. We should remember that we have the best lenses ever on the market in the history of lens wear (in terms of replacement frequency, lens material choice and solution availability).

However, if we have more unique eye shapes (to the left or right of the spectrum), then out-of-standard or even custom-made lenses may come to the rescue. We see this almost daily in our practices. This is the ‘thinking OUTSIDE the box’ category, in which the quality of the lens design may be more important than the quantity of lenses provided.

Closing Remark

It seems that one thing is clear: we have to move beyond base curve if we 1) want to understand soft lens fitting better, 2) want to be able to explain it to our students better, and 3) want to improve the quality of the fit. The latter really depends on whether we have a ‘normal eye’ or not. Unfortunately, using keratometry is not going to help us ‘win that war’. So, let’s move away entirely from BCs and keratometry in the soft lens fitting procedure and start looking at the entire eye shape, for starters. Maybe we can gain back the skills involved in justifying our ‘fitting fee’, as currently there isn’t much ‘fitting’ in the soft lens fitting procedure.

Photo by Ying Ge on Unsplash