Working with students is always interesting for a variety of reasons. Certainly, when it comes to teaching them soft contact lens fitting, they often seem to ask the right questions.

The protocol at many teaching institutions around the world is to take the central keratometry (K) values and “add something” to make the lens curvature a certain amount flatter. That should be the ideal, or at least the first, trial lens to use. That sounds good for a start, but when students look for that lens, they often become disappointed to find that an “8.15-mm” base curve lens is not available in the material they had in mind. In fact, most disposable lenses have two base curves at most and usually only one diameter. In addition, we now know that an 8.6-mm base curve from company A is not the same as an 8.6-mm base curve from company B.¹ So, what then?

CREDIBILITY

We typically tell our students to look for the closest lens they can find — in this example, an 8.3-mm base curve maybe — somewhat undermining their belief in the strategy we presented to them. We teach them what to look for (centration and movement) 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.

If a lens appears to be too loose on the eye, we may not have an alternative, perhaps because only a flatter design is available or no other lens parameter is available within that brand at all. At this point, our fitting method does not have much value. Worse yet, our credibility as educators begins to deteriorate. If the students wanted to choose a lens diameter based on the corneal diameter, the same problem exists. Basically, it doesn’t make sense to them. Well, they have a point. It doesn’t.

THE BEST LENSES EVER

We now have better lens materials than we’ve ever had and more frequent replacement of lenses than we ever could have dreamed. Replacing contact lenses once a day (daily disposables) is most likely the ultimate frequency. A recent study looked at replacing lenses more than once a day but found no benefits of doing that.²

Despite having the best materials ever available and the option for daily replacement of lenses, the contact lens industry faces a large proportion of dropouts. Most of these patients state that they stopped wearing their lenses primarily because of suboptimal comfort. In the western world, for every new contact lens wearer, another one stops wearing contact lenses. Hence, in terms of number of wearers, the contact lens market has been relatively stable for the last several years in many parts of the world, excluding some new, up-and-coming markets.

RETENTION RATE

A recent study looked at factors related to the success of new contact lens wearers in the United Kingdom and came to a few stunning conclusions.³ Twelve months from the lens fitting, the retention rate (people still wearing lenses) was 74%. Of the dropouts, 25% discontinued lens wear during the first month and 47% within 60 days. More stunningly, no alternative lens or management strategy had been attempted for 71% of dropouts. There was a wide variation in retention rates among sites, between 40% and 100%. In other words, in some practices the dropout rate was enormous (60%), while in others, it was almost zero.

Dropout rates and the reasons for dropout are complex, ranging from tear film components and dry eyes to lens material, lens surface friction, and edge design, as well as environmental factors.⁴ Eyecare practitioners have little or no control over many of these factors. What we do have control over is the lens fit. This article focuses on that topic, discussing how we can better control the fitting of a soft lens and how we can optimize lens fit to respect and follow the shape of the ocular surface.

WORKBENCH

If we want to improve soft lens fitting and educate our students concurrently, we need to examine our “workbench” (the shape of the ocular surface) and our tools (the lens designs that we have available).

Let’s begin with the workbench. For successful soft lens fittings, we need better ways to image and quantify the ocular surface shape. In daily practice, we individually and meticulously measure the precise power that the eye needs; so why not measure the ocular surface individually, too, to best match that shape of the eye?

Instruments to measure eye shape and predict the fit of a soft lens on the eye are not available in our practices currently. However, a new generation of ocular surface topographers that can help analyze the entire anterior surface shape beyond the corneal borders has entered the market. 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, with 360-degree coverage on the eye.

Other alternatives include corneal topographers that take data about the measured corneal shape, specifically the peripheral cornea described as an angle in modern techniques, and extend that out into the limbal region. Studies show that peripheral corneal angles can be predictive of anterior scleral shape, which could help us to some degree to predict eye shape beyond the corneal borders.⁵

From scleral lens fitting, we know that the ocular surface and the lens can be defined in sagittal height. The previously mentioned new instruments can measure the sagittal height of the ocular surface (OC-SAG) within 10 microns of accuracy. The average OC-SAG, using optical coherence tomography (OCT) and profilometry, is somewhere in the range of 3,750 microns (a micron is 1/1000 of a millimeter) based on a variety of measurements.^6-8 Figure 1 shows an overview of sagittal heights of 214 normal right eyes in the horizontal meridian over a 15-mm chord, illustrating the spread of normal eyes.

BETTER TOOLS

Let’s assume that we have this OC-SAG information available to us in our practices in the future. What do we know about the contact lens sagittal height (CL-SAG) of the lenses we fit daily? Not much, to be honest, as that information is currently unavailable to us. A relatively small study shed some light on this. The study aimed at measuring the CL-SAG of commercially available lenses, beginning with silicone hydrogel 2-week and 4-week replacement lenses.¹

These data show what difference is induced if an 8.8-mm base curve lens of one design is replaced with an 8.4-mm base curve lens of the same design, for example, this could be as much as 275 microns. It also shows that one 8.6-mm base curve design can be quite different from another (up to 170 microns difference), and that lens substitution is not advised, as described by Wolffsohn and colleagues, as well.⁹ It merely shows us something about what we are fitting, which could be insightful for students. Because the bottom line is that in the sample of lenses in the study, there was no more than 330 microns in variation from highest to lowest sagittal height of lenses to fit the overall ‘workbench’ of eyes with a range of 900 microns to 1000 microns, over the same chord.

Moreover, in clinical practice, when an eye with a spherical lens has an over-refraction of 0.75D cylinder, we typically reach for the toric version of that lens. According to our data, that means a change of lens fit of almost 500 microns (close to the thickness of the cornea) for one lens type, while for another lens type, this means no change at all. The investigators are not judging one over the other as potentially better, but it appears that we should be aware of this to, at minimum, better understand soft lens fitting. Figure 2 shows different CL-SAG heights in a sample of lenses.¹

STRAIN ON THE OCULAR SURFACE

If a soft contact lens is aligned with the ocular surface (i.e., it has the same sagittal height as the sagittal height of the eye), then that soft lens — influenced by tear film, eyelid pressure, and blink forces — will most probably move excessively on the eye. This will result in an unsuccessful fit that would be quite uncomfortable. Our current understanding is that the sagittal height of a soft lens on-eye needs to be somewhere in the range of 200 microns “deeper” to achieve a clinically successful fit. This amount, the difference between CL-SAG versus OC-SAG, is called the delta-SAG. The desired delta-SAG value likely varies per lens type and design.

The question then becomes, with the given workbench in OC-SAGs, and the given CL-SAG of lenses in standard silicone hydrogel 2-week and 4-week replacement, how many eyes can we serve with those lenses available in our standard arsenal? If the soft lens were rigid in nature, then with the available current lenses we would only be able to fit one-third of all eyes.

Obviously, a soft lens is not rigid; it allows for a certain amount of flexure. The amount is debatable, but studies indicate that somewhere between 100 and 250 microns in delta-SAG (increase in sagittal height from that of the ocular surface height) is likely to gain enough grip of the lens without creating excessive pressure and causing problems.^10,11 If we apply these realistic numbers to our database of normal eyes, then approximately 78% of lenses would fit. If the delta-SAG numbers were between 150 and 300 microns, then 68% of lenses would fit. It all depends on how much strain, or mechanical pressure, we are willing to accept on the ocular surface.

RELEVANCE

The steepness of the lens on-eye, defined as delta-SAG, may have consequences for the visual performance of that contact lens; lens flexure on the eye will cause a small change in lens optics from what the lens was designed to provide. Soft lenses drape to fit the cornea. Researchers found that the power of a soft contact lens on the eye is a function of its off-eye power, the way the lens flexes on the eye, lens hydration changes, and the corneal topography.¹² For a simple spherical –2.50D correction contact lens, this does not appear to have a great effect (probably less than 0.25D). For more complex designs, such as multifocal lenses and wavefront-corrected lenses, lens flexure could indeed have an impact, particularly as the delta-SAG amount is unknown for a given eye. The same is true for soft multifocal myopia control lenses. Before we are potentially fitting millions of children in soft lenses, we’d better get our act together on what we are actually fitting.

Furthermore, we know from clinical experience that unwanted corneal topographical changes occur under soft lenses. Corneal deformations resulting from suboptimal soft lens fittings are not as uncommon as many may think. If practitioners would remove patients’ soft lenses at every follow-up visit and perform corneal topography, as we often do with our students, the difference map could potentially indicate the amount of unwanted changes.

Until recently, this phenomenon was like undefined crop circles in wheat fields that nobody seems to be able to clarify. Where do they come from? Our current working theory now is that the steep lens appearance and, thus, the positive delta-SAG must have something to do with the topographical changes observed in the difference maps. It appears to be the effect of the mismatch between the soft lens shape profile and the ocular surface profile.

EDGE STRAIN

The previously described phenomenon appears to be in line with the edge strain mechanism.^13,14 A lens that is placed on the eye with a given steepness, by default, flexes on the eye. A mathematical model explains that because of this flexing, every lens causes a certain degree of edge strain, or mechanical pressure, on the ocular surface. A lens that is placed on the eye increases in size on-eye, because of the described flexure. The described optimum amount of increase in size and therefore edge strain appears to be 3%.¹³ This would mean that a 14-mm lens on the finger would increase in size to 14.4 mm on the eye. This also has implications for changing the base curve of the lens. If we change the base curve from 8.4 mm to 8.8 mm, the overall diameter decreases on the eye (as the delta-SAG decreases). In theory, this would mean we would have to add 0.2 mm to the lens diameter if we wanted to keep the same on-eye lens diameter.

To make it slightly more complex — but, again, this is something we must teach our students — another study showed that the reported diameters (on the lens blister pack) of soft contact lenses are conventionally measured at room temperature (20° C); however, all soft lenses shrink when raised to eye temperature (34° C).¹⁵ It would be better, in terms of lens fit, to mark the on-eye lens diameter of soft lenses, at minimum, in addition to the theoretical lens diameter at room temperature.¹⁶

HOW TO IMPROVE SOFT LENS FITTING

“Everything should be made as simple as possible, but not simpler” is a famous quote by Albert Einstein. For educators, this is the essence of our profession, and this certainly applies when it comes to soft lens fitting. Using just sagittal heights will not solve our fitting issues, as it is too simple. Lens design and geometry also play a role. In theory, a monocurve, a bicurve, or an aspheric lens design could all have the same sagittal height but potentially show different on-eye behavior. Many other factors contribute to how a lens fits on the eye, including edge lift design, lens thickness, lens roughness, and front surface lens design. In addition, they all can influence lens movement on the eye, which is a well accepted method of lens evaluation. This brings us to another important point, although beyond the scope of this article: lens movement may not be a good predictor of the amount of steepness (or flatness) of the lens on-eye.

In addition, when looking at the OC-SAG we should look at the 360-degree value, not the horizontal OC-SAG only, as has been performed primarily to date. Studies are currently being performed, comparing the 360-degree OC-SAG value to the CL-SAG values (which are spherical in nature, and a 360-degree value by nature).¹⁰

The horizontal OC-SAG may be different from the vertical OC-SAG. This would mean that often we are fitting a spherical lens to an oval surface independent of any corneal astigmatism. At least in theory, bisagittal soft lenses (with two different CL-SAGs in principal meridians) would make more sense in those cases potentially.

SURROGATE MEASURES

Contact lens fitting can be described as using different sagittal heights. Although certainly not perfect, students and eyecare professionals alike should be aware that both base curve and diameter are surrogate measures for lens sag, and that increasing sag reduces movement and consequently improves comfort within the limits of acceptable fitting.³

In order to answer the question, “How can we improve soft lens fitting,” it may be preferable to separate soft lens fitting into different groups. To optimally respect the shape of the cornea in soft lens fitting, let’s begin by recognizing three different categories of soft lenses, adapted from a classification by Lampa & André.¹⁷

FINDING THE NORMAL EYE

Stock lenses are basically off-the-rack designs; these lenses have a fixed shape and the eyecare professional basically attempts to find a lens that fits a given eye. The more variety there is in stock lenses, the more likely we are to find a lens that best matches a given cornea. Likewise, materials can have varying properties (e.g., modulus) that influence fit. These lenses would probably best serve the normal, standard eye or the entire top part of the bell curve of eyes. In most cases, a frequent replacement lens can serve the normal eye perfectly, likely better than any other lens.

How do we know we are dealing with a normal eye? As eyecare professionals we should be able to determine that. And we can. New technology can help us find these standard eyes, and companies are now starting to embrace this.

Using a database of 10,000 eyes, a company in the Netherlands has developed software to determine whether we are working with a standard eye, based on corneal topography and extending out the information peripherally. Thus, we are estimating the overall (15 mm) sagittal height of the ocular surface. The logarithm then recommends either a standard lens or a specially designed lens. Artificial intelligence is entering our profession and most probably will make a big difference going forward.

OUT-OF-STANDARD LENSES

This brings us to the second category: out-of-standard lenses. These lenses are recommended if the eye is not part of the top of the bell curve. They have a fixed geometry (or maybe a number of geometries, but they are still considered standard lens designs), but they have a parameter range that falls outside that of the standard stock lens range. Examples of this category include higher sphere or cylinder powers, large or small diameters, etc. They may be manufactured individually (lathe-cut or cast-molded), but their design choice is still somewhat limited.

A number of companies are offering several lens diameters and different base curves, if desired. These can be ordered for a monthly replacement schedule and in either conventional or silicone hydrogel materials. One company in the United States even offers a daily disposable lens in three different diameters.

Following up on the discussion of sagittal height, it can be stated that diameter and sagittal height are more closely correlated than base curve radius and sagittal height. If anything, diameter would be a better alternative than base curve. Measuring corneal diameter accurately is not particularly simple or accurate, though, as we are typically measuring the horizontal visible iris diameter (HVID). Researchers found the average corneal diameter based on OCT and on the actual transition in shape between the cornea and the limbus to be 13.4 mm instead of the 11.8 mm to which we typically adhere when using HVID.^18,19

From a clinical perspective, however, if you have a larger than average corneal diameter (whatever way that is measured) and the central curves are steep, this indicates we are dealing with a large overall sagittal height, and a standard lens most probably will not fit. Out-of-standard lenses would be a viable option.

CUSTOM-MADE LENSES

These lenses are made individually and specifically for each eye. With these lenses (typically lathe-cut), there are no limits to shape, power, and design, and they may be made in a silicone hydrogel material to benefit from the latest technology in material development.

A true tailored fit can be achieved to accommodate the shapes and patterns that may be present on the ocular surface. Some companies, for example in conjunction with academia, have developed effective eye modeling based on ocular surface profilometry to develop the best suitable (literally) soft lens fit.

CLOSING REMARKS

As one of my former students said, “You can’t measure a toe to determine shoe size.” The limitations in soft lens fitting are clear, but unfortunately this article doesn’t deliver the answers to many of the questions and issues raised. This article is more about understanding soft lens behavior on-eye and maybe more about finding the right question in a way. What is it that we want to achieve? Or as a famous quote, widely believed to come from Mark Twain reads, “It ain’t what you don’t know that gets you into trouble. It’s what you know for sure that just ain’t so.”

From an educational standpoint, it seems we need to go back to the drawing board to redefine how we teach our students in a plausible way. It appears that in the near future we will see more soft lens parameters from a variety of sources. Without trying to be over-dramatic, I believe, to some degree, the future of our profession as contact lens fitters is at stake. In other words, it is about the survival of the fitting. Many questions remain. It’s a learning curve, or perhaps more accurately, it is beyond (base) curves. Microns of height and elevation may be better ways to educate our students to understand soft lens fitting and on-eye behavior. CLS

REFERENCES

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