UNDERSTANDING SOFT LENS FITTING

Introduction

Our prime task and goal when fitting contact lenses is to align the lens’ back surface with that of the anterior ocular surface as optimally as possible. The purpose is to ‘first do no harm’ we want the corneal integrity to be impacted as little as possible, with no visible mechanical signs of lens wear and as little impact as possible on comfort of wear. In rigid lenses (corneal and scleral), this is an obvious necessity that requires a high level of accuracy. In soft lens fitting, there certainly is more leeway, but that does not mean ‘shape’ can be ignored altogether. It seems, though, like our profession sometimes forgets about the alignment factor in soft lenses. Does one size fit all? This article explains why one size fits some – or, more accurately, most – but not all.

Worldwide, an impressive number of about 150 million wearers use soft lenses. But the rate of people dropping out of lens wear is also ‘impressive’. The ‘pooled dropout frequency,’ according to Pucker and Tichenor, based on a combination of several studies that include 8,190 subjects, is estimated to be 21.7%. If we want to impact the corneal integrity as little as possible, with no visible mechanical signs of lens wear and as little impact as possible on comfort of wear, we may need to look closer at soft lens fitting. Studies have shown that soft lenses can have a marked influence on the ocular surface topography (causing corneal alterations), , and recent studies have also shown that lenses that do not match the ocular surface shape can give rise to a higher rate of corneal infiltrative events or microbial keratitis. So the question is: Can we do better with soft lens fitting than we currently do? And if so, how?

Normal Eyes

The best way to do this from a clinical standpoint is to identify whether we have a normal eye or not. This is difficult to determine based on central keratometry or even diameter alone – but the combination of the two (combined with other factors) can be summarized as the sagittal height (SAG) value. Recent developments in the last decade or so have been very instrumental (literally) in imaging a larger portion of ocular surface than just the corneal surface. Corneal topographers have been around for quite some time now, but they cover only a small portion of the ocular surface – typically about 6-9mm – while a soft lens on average is 14.2mm. If we take the maximum diameter that the corneal topographer reasonably can cover (9mm), then this would be 44mm² however, the full 14.2mm of a soft lens back surface would be 158mm,² more than triple. Modern technology including optical coherence tomography (OCT), profilometry and Scheimpflug imaging have given us much more insight into what the average ocular surface of a large diameter looks like. Figure 1 shows an overview of average 15mm sagittal height values over a 15mm chord. The 15mm chord has been traditionally chosen as a reference diameter for scleral lenses but would be applicable for soft lenses too (typical soft lenses are 14.2-14.5mm in diameter and move about 0.2-0.3mm on each side – hence the 15mm workbench). Based on the values in figure 1, a normal eye would be around 3720 microns on average, with a standard deviation of 174 microns. This means that most eyes that could be catered for with a normal lens (fig 2) would be roughly 3550 to 3900 microns in height and would represent 68% of eyes (e.g., the ‘normal eyes’).

Figure 1: Reported OC-SAG values over a 15mm chord in the horizontal meridian (in microns with SD)

Going Back to Base (Curve)

Now that we know a thing or two about the ocular surface, the next thing is to look at is the back surface shape of the lens: What are we putting on the eye? Traditionally, as inherited from corneal rigid lenses, soft lens’ nomenclature has included base curve (BC) and diameter as prime parameters. BC is seen by most eye care practitioners (ECPs) as the leading factor in soft lens fitting. Several studies have shown, though, that there is hardly any correlation between BC and soft lens behavior on the eye. Studies at Pacific have shown that BC is a very relative parameter a lens from two companies with a BC of, for instance, 8.6 can have sagittal depth values that vary quite drastically. All BC really tells us is that a lens with a BC of 8.7 is ‘flatter’ than an 8.3 of the same lens make (lens type/material, etc). But by how much is unclear – that is defined by the difference in SAG value between the lenses. On average, lenses that are offered in two BCs differ by about 275 microns in shape (more on this later). It is important to note that if we fit a flatter lens, then it is not actually the flatter BC that changes the lens fit, but the lower SAG value (caused by a flatter BC) that makes the difference in on-eye appearance.

In short, we must go back to base and redefine our soft lens parameters. Interestingly – and confusingly – there is no set rule to define the BC value on the package some companies calculate the ‘base curve equivalent’ based on the overall average base curve over the full lens diameter. Others arbitrarily use a 10mm chord, for instance. This may explain why some BC values don’t make sense or may act steeper or flatter than other lenses with the same BC. The conclusion here is that BC is an arbitrary value that is not very helpful in clinical practice.

"The conclusion here is that BC is an arbitrary value that is not very helpful in clinical practice."

Sagittal Depth – But Which One?

So, what do we know about the actual shape of soft lenses? And can we use this? To start with the first question: several studies have looked at soft lens shape, using both imaging systems (SHS Ophthalmic, Germany) and OCT-based systems (is830, Optimec UK). These studies all measured the sagittal depth of soft lenses in saline at 20°C by ISO (International Standards Organization) standards. What these studies showed is that there is a wide variety of lens sags (CL-SAGs) – but not unlimited. All major companies ‘cater’ toward the top of the bell-curve of normal eyes, which makes complete sense, of course.

It is obvious, though, that not all eyes of all patients can be satisfied with these standard lenses eyes that are quite flat or steep in nature (or, in the new terminology, have low or high sagittal height values) could potentially benefit from a higher or lower CL-SAG value. This may be achieved best by considering lenses that are situated on the left or right side of the graph (see fig 3) or by switching to an individually designed lens. This will give ECPs at least some grip on the process of soft lens ‘fitting’, which we have lost as a profession to a large degree.

Figure 2: Soft lens fitting flow chart

Matching

If we see soft lens fitting as a speed date, in which the goal is to align individuals on both sides of the dating table with each other as optimally as possible to achieve a positive match, it is obvious that on the ocular-surface side of the table in our analogy, we need to consider the full area covered by the soft lens (and ideally the average of all meridians). The soft lens then, by nature, will ‘drape’ over the ocular surface. What we know from our Pacific studies and from the work of Graeme Young and others is that a soft lens needs to be somewhat ‘steeper’ than the ocular surface to achieve a successful fit. Simply put, a soft lens needs ‘grip’ otherwise, the lens moves excessively under the influence of eyelid pressure, tear film variables, lubrication, etc, and is very uncomfortable. By how much a lens needs to be steeper (or higher in sagittal depth) is the clinical question that needs to be answered, but it is still a bit of a puzzle.

To start answering that question, we first need to take a closer look at CL-SAG values. As stated previously, ISO standards dictate to use 20°C for contact lens packaging, but is this really the best method? The lens goes on the eye, and the temperature of the ocular surface is estimated to be 34°C, so this may have an impact. The question is, does that make a huge (clinically significant) difference? Our most recent study shows that all soft lenses shrink when going from room temperature to eye temperature. On average, it is about a 3% decrease, but there are substantial differences between lens materials (fig 3). Typically, higher-water-content lenses shrink more. In some lenses, the difference exceeds 5%. It was not the goal of this paper to single out lenses or judge their performance. It is simply showing what the lens looks like that goes on the eye.

​Figure 3: Relative differences in CL-SAG for 20^oC (blue) versus 34^oC (orange) temperature

Figure 4: “New’ CL-SAG graph for daily disposables using on-eye temperature

Is 34°C is the new 20°C?

The question remains – is all this clinically relevant? The rule of thumb in our clinic is that everything over 100 microns of variation could become clinically relevant (less than that would be not observable/noticeable). The variation in CL-SAG in relation to temperature ranged from 48 microns to 211 microns. So for some lenses, the difference could become clinically relevant. Therefore, it could be argued that, at least from a theoretical standpoint, using the 34°C values in the future could potentially make more sense if we have all data available for all lenses on the market. If we single out the daily disposable lenses that we measured in this temperature study, the lens’ order slightly changed in the 34°C graph compared to the original 20°C graph previously generated using the ISO standard temperature (Figure 4).

To put things in perspective on what a certain difference in CL-SAG means, if companies produce lenses with two different BCs (of one lens type, same material and design), then the average difference between, let’s say, the 8.3 versus the 8.7 (and all other variables from different companies) is 275 microns. One can assume that the companies know what they are doing and that a 274-micron difference between lens A and lens B of the same material and design is considered a clinically significant difference. Some of the lenses out there may have temperature differences that come close to this.

The fact that a soft contact lens’ diameter decreases when it goes on the eye is not new Young et al described this in 2016 using traditional viewing techniques and rulers. In this study an OCT-based instrument was used, which most likely increased the accuracy. But moreover, it did look at other lens parameters as well, including BC and (for the first time) SAG. What is interesting, but also somewhat confusing, is that we found in our paper that BC in most (but not all) lenses steepened while the diameter decreased. With steepening of the BC, one would expect the sagittal depth to be higher, but the reverse was the case. This seems to indicate two things. First, diameter has a much bigger effect on CL-SAG than BC (in short, changing a lens from 8.3 to 8.7, as explained earlier, will change the CL-SAG by 275 microns on average, but changing the lens diameter from 14.0mm to 15.0mm causes the CL-SAG to be altered by 600-900 microns, depending on the design and curvature of the lens). Second, as mentioned earlier, BC is an arbitrary (and confusing) term that maybe we should not give much attention to in clinical practice.

Potentially, if we want to move forward and take soft lens fitting seriously, then the 34°C CL-SAG values could become the new norm as the ‘on-eye’ SAG, but the steepening of the BC is interesting and needs more investigation to determine whether these lenses are indeed ‘steeper’ on the eye. The interesting fact is that the current ISO standard for sagitta, or the CL-SAG, is ±0.05mm, or 50µm at 20°C. As can be seen in figure 2, pretty much all lenses analyzed exceed this going from 20°C to 34°C.

Closing Remarks

The main conclusion from this paper is that if we want to align the ocular surface better with our current arsenal of soft lenses, we need to know more about the shape of both. Using sagittal height could be a good (or at least better) starting point than BC. But if we want to ‘up our game’ on soft lens fitting, then using sagittal depth seems to be a step forward.

Ocular surface sagittal height, which can be measured or at least estimated by currently available instruments in clinical practice, is key in this. The next step is to better understand soft lens behavior on-eye. There is still a lot to learn and a lot to explore in that regard. One thing seems clear: if we have an eye with a low sagittal height value, (flat eye), it does not make sense to use a high or even medium CL-SAG from the right side of the graph. Picking a ‘left side of the graph’ lens makes more sense, maybe without taking the absolute numbers and sometimes small differences between lenses too seriously. To put it in perspective, other lens variables, such as material properties, lens design, lens edge design, and also lens dehydration during the day, all play crucial roles in lens comfort and success (fig 1). But maybe start by looking at ocular surface shape first, then combine that with the knowledge we have about the shape of the lenses on the market – that could make a great beginning.

Key points:

• Base curves are arbitrary values when fitting soft lenses.

• Both base curve and overall diameter influence the sagittal depth of a contact lens.

• These changes also vary with temperature, water content, and other physical properties.

• The 34^oC SAG values reflect the variation in the physical fit of the lens on-eye

• Selecting a CL-SAG value to better match a shallow or deep ocular sagittal height may be more efficient than relying solely on the lens base curve values.

References

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