Article

HOW IMPORTANT IS THE SOFT LENS FIT ANYWAY?

New tips and tricks can help practitioners predict fit, reduce fit failures, and increase patient satisfaction.

One of the many things that you may anticipate when patients ask whether or not they are suitable for soft lens wear is how the lenses will potentially fit. You may even already have your “Top 3” contact lenses in mind. There could be many reasons why these are the preferred ones, and it may be that they’ve achieved the greatest frequency of success with your patients.

One of the factors that contributes to soft lens wear success is a good fit. Fortunately, fitting soft lenses is a relatively simple task. Unlike GP lenses, for which several parameters must be tailored for patients, many soft lenses provide choices of, at most, two different base curves and a single diameter. Their pliability and flexibility allows to them to fit virtually the entire population, making them nearly one-size-fits-all.

Studies show that a single base curve of 8.4mm managed a “good or better” fit in approximately 90% of individuals,1 and base curves of 8.4mm and 8.6mm together encompassed 98% of individuals.2 A more recent study mathematically determined that the highest rate of fitting success was achieved when the base curve/diameter combinations ranged between 8.6mm/14.0mm and 8.8mm/14.6mm.3 It is no surprise then that the parameters for most commercially available soft lenses for the mass market fall within that range. Table 1 displays these parameters for some commonly worn soft contact lenses.

TABLE 1 AVAILABLE BASE CURVES AND DIAMETERS FOR SOME COMMONLY WORN SOFT LENSES
LENS BASE CURVES AVAILABLE (mm) DIAMETERS AVAILABLE (mm)
1 8.6 14.0
2 8.6 14.1
3 8.6 14.2
4 8.4, 8.6 13.8
5 8.6 14.0
6 8.5, 9.0 14.2
7 8.4, 8.8 14.0
8 8.7 14.2
9 8.7 14.0
10 8.7 14.2
11 8.5, 9.0 14.2

By employing an optimal base curve and diameter, these mass-marketed soft lenses have a high probability of achieving a good fit when prescribed for a random individual in the population. This convenience is quite advantageous for practitioners because it can substantially reduce the time it takes to find a good-fitting soft lens. Patients can also benefit from this by avoiding the need to endure round after round of trialing and re-trialing several different pairs. However, there is a cost for this convenience. When contact lenses fit reasonably well on almost everyone, the act of fitting and assessing the fit of a contact lens becomes an overlooked and ignored process. When this occurs, contact lenses themselves become viewed as a commodity rather than as a medical device.

In this day and age, it is extremely easy for existing and potential contact lens patients, or even non-patients, to obtain contact lenses and contact lens education online. There are many distributors that sell them online without ever requiring a prescription from the buyer. There also are numerous resources available on popular video hosting sites, some of which unfortunately feature unqualified and untrained persons who erroneously educate the public on how to use contact lenses.

This “bad information” may relate to wearing schedules, application, removal, and hygiene. Some of these videos have literally been viewed, at minimum, hundreds of thousands of times, suggesting their wide and far-reaching impact. Although, to be fair, there are some videos that advise properly and emphasize the importance of visits to an eyecare professional. However, in the former, the spread of misinformation is a disservice to the general public. Interestingly, many of these videos fail to discuss how a contact lens should fit on the eye or what to do if the contact lens does not fit.

For practitioners, it may be easy to neglect properly assessing the fit of a soft lens because it is expected to fit well. While parameters are quite easy to choose from, we should not expect that any soft lens can be applied on the eye and perform well. In fact, one study found that 21% of contact lens wear discontinuation was related to poor practitioner judgement, and 33% of those cases were due to improper soft lens fitting.4 This data should serve as a reminder that one-size-does-not-fit-all; improper fits can impact comfort and physiology of the ocular surface and, ultimately, cause discontinuation of lens wear.

It is a well-known fact that comfort is a major driving force for contact lens success, with discomfort being the primary reason why patients discontinue lens wear.5-7 One of the reasons why comfort is poor could be due to a poorly fitting lens.

A soft lens that does not have sufficient coverage may irritate the cornea with its edge. Soft lenses that are poorly fitted may result in decreased ocular surface integrity.8 Lenses fitted too loosely exhibit excessive movement, which causes discomfort and visual fluctuations. Tight-fitting lenses, although appearing to fit more comfortably,2,9,10 when excessively tight will reduce tear exchange, leading to a buildup and trapping of metabolic waste and toxins under the lens. For these reasons, the process of contact lens fitting and the assessment of contact lens fit are both vital to healthy contact lens wear.

BASE CURVE IS JUST A NUMBER

The fitting of soft contact lenses essentially comes down to selecting a lens with the appropriate sagittal height. It was thought that steeper corneas theoretically exhibited a higher sagittal height and should therefore be fitted with steeper lenses. Likewise, it was thought that flatter corneas with the same given diameter exhibited lower sagittal height and should therefore be fitted with flatter lenses (Figure 1).

Figure 1. Two different curvatures (r1, r2) with identical chords (d), exhibiting two different sagittal heights (h, and h+x). The idea is similar with contact lenses, where r1 and r2 would be base curves and d would be lens diameter.

This led to a belief that central corneal curvature was a major dictator of sagittal height, but this idea was disproven in 1991.11 As it turns out, other factors such as corneal asphericity, corneal diameter, scleral radius, and scleral shape factor all contribute to sagittal height in some way. In fact, that study found that normal variation in population corneal asphericity and corneal diameter separately contribute more variability to sagittal height than does apical corneal radius of curvature.11 Despite that fact, fitting soft contact lenses today still involves measuring central keratometry and using that information as the basis for initial lens selection.

To complicate matters, the labeled base curve of a lens may not truly reflect the actual radius of curvature of the back of the lens. There are many reasons why this is the case. Manufacturers often incorporate modifications (e.g., flattening of the lens periphery) to obtain a better fit in the periphery; this ultimately impacts the overall curvature of the lens. Environmental factors such as dehydration,12 temperature,13,14 and osmolality15 can also impact lens shape.

Because base curve is a poor predictor of sagittal height and is a poor representation of lens curvature, information from central keratometry does not really contribute anything related to selecting the initial base curve or how well the initial lens will fit. This idea is supported by a study conducted by Young et al16 in which they examined the various physical ocular variables that affect soft lens fit and found that “…there was no significant difference in mean K between those subjects best fit with the steeper base curve and those subjects best fit with the flatter base curve.” This essentially states that the quality of fit is independent of central corneal curvature.

If central keratometry does not predict the base curve nor the sagittal height of the potential lens, then what does? And what information does a clinician now use to best select a lens?

IMPROVING THE PREDICTABILITY OF LENS FITS

With the advent of corneal topographers and optical coherence tomography (OCT) imaging, we can gain a complete characterization of the shape of the anterior ocular structures. With OCT, a number of additional parameters that were not readily available to us can now be measured. This includes the profile and angle of the corneoscleral junctions, sagittal height at various depths, corneal diameter, corneal eccentricity, and scleral radius. The question that remains is: How does knowing all of this help us to predict the fit of a lens or select the best-fitting lens? Hall et al17 used OCT imaging to characterize various aspects of the eye and showed that some of the above anatomical parameters play a significant role in predicting clinical lens performance on the eye.

One of the findings was that measuring the horizontal corneal diameter using OCT was much more reliable compared to measuring horizontal visible iris diameter (HVID) at a slit lamp. The HVID has previously been known to be unreliable because it depends largely on the rate of transparency loss of the tissue, and while it correlates with true corneal diameter, it is almost always an underestimation.18 The true corneal diameter does not end where the transparency stops, but instead ends at the external scleral sulci.19 From this anatomical location, the sagittal height of the cornea can be reliably measured.

Another interesting finding is that the eye is not rotationally symmetrical in the sense that the profiles of the corneoscleral junctions differ from each other in each of the four quadrants. The differences in the corneoscleral junction angles were vastly greater in the temporal-nasal meridian than in the superior-inferior meridian. The nasal corneoscleral junction angle was sharpest, followed by that in the inferior, temporal, and superior aspect.17 The scleral radius also is much steeper in the nasal aspect compared to the temporal, but the scleral curvatures of the superior and inferior were similar.

The differences in the elevations and profiles in the meridians may explain why soft lenses tend to decenter temporally, which may even suggest that the perfect-fitting lens may not be “round.” Figure 2 shows the difference between the temporal and nasal corneoscleral junction angles in a 40-year-old Asian male. In addition, the difference in horizontal corneoscleral junction angles is associated with lens tightness on push up in a silicone hydrogel lens, making this a potentially important parameter to predict future lens fits.

Figure 2. (Left) The nasal corneoscleral junction angle was measured to be 166.6°. (Right) The temporal corneoscleral junction angle was measured to be 169.2°. This small difference may have a profound effect on contact lens behavior on the eye. These images were captured with a custom-built OCT developed at the University of Alabama at Birmingham.

This information can be combined with data from slit lamp measurements. For example, variation in palpebral aperture explained up to 18% of post-blink movement in a traditional hydrogel lens, but after including differences in corneoscleral junction angles, up to 24% of post-blink movement can be explained for a silicone hydrogel lens.17

An aspect of lens fitting that is often overlooked is patient demographics. Age and sex have a large effect on various ocular parameters, including HVID, palpebral aperture size, corneoscleral junction profiles, and scleral curvatures.20 There are also differences in corneal diameter, sagittal height, HVID, palpebral aperture size, and corneal asphericity based on different ethnicities.20,21 As age and ethnicity can impact these parameters, they have an indirect way of impacting how soft lenses may fit on the eye. For example, Caucasians and males tend to have a higher sagittal height compared to Asians and females. This piece of information suggests that soft lenses may fit relatively more loosely for Caucasian and/or male patients than for Asian and/or female patients.

There is still much to learn about what factors affect lens fit. Troung et al22 predicted that about 75% of variability in lens behavior on the eye is still unaccounted for. Understanding these factors could provide some clues or steps for improving and honing the fitting process and may even have implications for future lens design.

ASSESSING THE LENS FIT

Once the lenses have been successfully applied onto the eye, they need to be assessed. Some of the lens fit parameters are directly related to each other, and using a reticle to measure these parameters is valuable in determining the quality of the fit.

It is important to consider the time of lens assessment. The lenses should not be assessed immediately after application because they take time to settle. Lenses tend to show much mobility immediately after application, but this movement slowly decreases over the first half-hour and then increases again throughout the day.23 For a hydrogel lens, the fit of the lens at the five-minute post-application mark has been shown to be predictive of the fit at the eight-hour mark. Therefore, the five-minute mark is a good time for soft hydrogel lens assessment.23 For silicone hydrogel lenses, it may take a little longer. Post-blink movement and tightness at 10 to 20 minutes post-application was similar to that at the eight-hour mark.24

Once the lenses have settled, one of the first parameters to examine is lens centration. Check to see that the limbus is covered in all four quadrants by at least 1.0mm.25 It is not necessary that the lens be concentric with the cornea, but you must ensure that the lens is not excessively decentered. Decentration of more than 0.3mm is a good indicator that the lens may be too loose; and, keep in mind that tight lenses can also show excessive decentration.26

There needs to be sufficient lens movement so that tear exchange can occur. At the same time, there should not be excessive movement, as it would cause discomfort. The best comfort occurs when the post-blink lens movement is between 0.1mm and 0.4mm; exceeding that range is linked to symptoms of discomfort and dryness.22 Using the reticle, the movement of the lens can be quantified by seeing how much the lens moves after a patient blinks.

Lens movement is associated with lens tightness, in the sense that an excessively tight lens will almost always show no movement. A good indicator of a tight lens is that post-blink movement is less than 0.15mm.26 The movement of a loose-fitting lens can range widely, showing excessive to no lens movement. Therefore, lens movement is not a specific test or a surrogate measure for lens tightness.22,26

Lens tightness can be assessed using the digital push-up test, which is considered to be a good, reliable, and repeatable test for tightness.26,27 It is performed by pushing up the lower eyelid margin with the thumb to move the lens upward. Tightness is then graded on a continuous scale from 0 to 100 based on the ease with which the lens moves; 100 is no movement at all, and 0 is the lens separating from the cornea and falling out (even without the manipulation of the lower eyelid). According to Troung et al,22 the highest frequency of comfort occurred when tightness values were between 40 and 55. Lenses that are fitted too tightly or too loosely tend to result in increased corneal staining, and lenses fitted too loosely also resulted in increased hyperemia.8

Upgaze lag and version lag were both generally not considered to be strongly diagnostic of lens fit. However, excessive version lag is sensitive for loose fits, and zero upgaze lag is more sensitive for tight fits.26

THE FUTURE OF SOFT LENS FITTING

The future for soft lens fitting is exciting, and there is still much we could learn about lens fits. In the meantime, there are many new technological breakthroughs that we can use to enhance our ability to fit soft lenses. This will ultimately help practitioners better predict the fit, reduce the chance of fit failures, increase patient satisfaction, and raise the quality of care. Perhaps it is also time to start including more meaningful information on contact lens products, as it has been shown repeatedly over the past two decades that base curve values have almost no purpose in the soft lens fitting process. CLS

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