Natural Optics Balaguer and PhD candidate at the University of Alicante (Spain)

Introduction

For soft contact lenses, the “one-size-fits-all” philosophy seems to be the predominant trend globally. However, the actual fitting of soft lenses has recently gained more attention.[1] Despite having the ability to choose from the best materials in terms of oxygen supply, surface coatings, physiological response and safety,[2-4] we still encounter high dropout rates.[5] Discomfort seems to be the main culprit.[6]

In Clinical Practice

As an eyecare practitioner in a private practice, I would guess that I’m not alone in facing the problem of discomfort daily in the contact lens consulting room. I am pretty sure that when a soft lens patient comes into your practice complaining about discomfort, your contact lens complications “scan” is activated to search for conditions such as uncorrected astigmatism, tarsal response, dry eye and solution-induced staining, among others. Once the fitting has been identified as the source of discomfort, my “scan” goes a step further to identify some common patterns. Centration of the lens, or rather decentration, is one of the key elements for which to scan, in addition to movement. Figure 1 (below) shows an inferior-temporally decentred lens, which barely moves after blinking and with the push-up test, as a good example of an unsuccessful fit.

Figure 1

The Soft Lens Navigation System

After a preliminary evaluation, many eyecare practitioners switch their “scan” to advanced mode to search for corneal parameters that do not meet the standard. If the corneal parameters are outside the norm, then the “navigation system” may suggest you take the extended parameter/custom path. If this is the case, this is where your “navigation system” needs an update: an extended function is needed to track the limbus and anterior sclera and to obtain true ocular sagittal measurements (OC-SAG) beyond the cornea. Sometimes, an eye with out-of-the-norm corneal parameters (Figure 2) may have not so out-of-the-norm OC-SAG values at chord diameters between 14mm and 15mm (Figure 3). In such cases, another standard or “one-sized” soft lens can still work. The sagittal height values of soft contact lenses available on the market are published.[7] Adjusting the sagittal height of the lens for such an eye using these values is a good strategy in these cases (Figure 4).

The opposite situation is also possible: an eye with relatively regular corneal parameters (Figure 5) but not-so-standard OC-SAG values (Figure 6). In our practice, this leads us to fit an extended parameter/custom soft lens more often than in the previous situation.

Figure 2, 3 and 4

At a Junction

These clinical observations made me curious about how we calculate the OC-SAG beyond the cornea (although the term “extrapolate” would be more suitable here).[8] Others, such as Bandlitz et al,[9] recently have looked at this and have published on the topic as well. In essence, there are statistically and clinically significant differences between OC-SAG values calculated through corneal parameters alone and those calculated with information about the corneo-scleral profile and shape.[8,9] The corneo-scleral junction (CSJ) angle seems to be the source of most of these differences, our recent findings suggest.[8]

When the CSJ angle is isolated from corneal parameters, even small CSJ angle variations have a clinically considerable impact on the OC-SAG values.[10] Alejandra Consejo’s work about the shape of the human limbus and the CSJ intra- and inter-variability in normal healthy eyes is clarifying in this regard.[11,12] Her description of the CSJ in 360 degrees raises the question: How does CSJ intra-eye variability affect OC-SAG and on-eye soft lens performance? A first step is to assume that if you base your soft lens fitting decisions exclusively on the corneal profile, important information is missed that may be critical to these decisions.

In other words, we are at a junction. And an important parameter to consider in soft lens fitting is, in fact, the corneo-scleral junction. Once the corneo-scleral profile information has been uploaded into our navigation systems, an algorithm is needed to identify eyes that cannot be fitted with standard/“one-sized” soft lenses. According to the previous discussion, the measured OC-SAG value could be a more valuable tool than corneal parameters for this purpose.

Figure 5 and 6

“..scanning for corneal parameters that do not meet the standard: this is where your “navigation system” may need an update…”

Summary & Future Directions

So, while identifying the OC-SAG value is a good first step, we need to use all (not just corneal) parameters – as the ocular surface shape is comprised of many variables such as corneal radius, corneal eccentricity, corneal diameter, and also importantly the CSJ angle and the scleral profile. This comprehensive method of describing the ocular surface shape could provide a hypothetically better prediction to identify out-of-the-norm eyes from a soft lens fitting perspective. But the next step would be to allow extra “input” into our navigation system – almost like adding information on road blocks and traffic jams – to optimize our performance. Let us see what modern macro data science can add to that. Our current studies at the University of Alicante and in my private practice here in Spain are geared in that direction. Although perhaps controversial, this approach might help to identify some patterns in which the “one-size-fits-all” philosophy does not work and to determine the ideal 𝛿-sag with standard/“one-sized” and extended parameters/custom soft lenses.

References

1. Van der Worp E, Wolffsohn JS, Jones L. When was the last time you fitted a soft lens? Cont Lens Anterior Eye 2020 Oct43(5):415-417.
2. Bonanno JA, Clark C, Pruitt J, Alvord L. Tear oxygen under hydrogel and silicone hydrogel contact lenses in humans. Optom Vis Sci 2009 Aug86(8):E936-42.
3. Szczesna-Iskander DH. Comparison of tear film surface quality measured in vivo on water gradient silicone hydrogel and hydrogel contact lenses. Eye Contact Lens 2014 Jan40(1):23-7.
4. Chalmers RL, Hickson-Curran SB, Keay L, et al. Rates of adverse events with hydrogel and silicone hydrogel daily disposable lenses in a large postmarket surveillance registry: the TEMPO Registry. Invest Ophthalmol Vis Sci 201556:654–63.

5. Young G. Why one million contact lens wearers dropped out. Cont Lens Anterior Eye. 2004 27: 83–85.

6. Nichols KK, Redfern RL, Jacob JT, Nelson JD, Fonn D, Forstot SL, Huang JF, Holden BA, Nichols JJ. The TFOS International Workshop on Contact Lens Discomfort: Report of the Definition and Classification Subcommittee. Invest Ophthalmol Vis Sci 2013 Oct 1854(11):TFOS14-9.

7. van der Worp E, Lampa M, Kinoshita B, Fujimoto MJ, Coldrick BJ, Caroline P. Variation in sag values in daily disposable, reusable and toric soft contact lenses. Cont Lens Anterior Eye 202144(6):101386.

8. Rojas Viñuela J, Piñero DP. Burgos M. Comparing sagittal heights calculated using corneal parameters and those measured with profilometry. Cont Lens Anterior Eye 2022 In Press.

9. Bandlitz S, Lagodny M, Kurz C, Wolffsohn JS. Prediction of anterior ocular surface sagittal heights using Placido-based corneal topography in healthy eyes. Ophthalmic Physiol Opt. 202200:1–9.

10. Rojas Viñuela J, Stortelder R, Piñero DP. A theoretical approach to estimate the impact of the corneo-scleral junction angle on ocular sagittal height. Poster at AAOpt (San Diego, October 2022).

11. Consejo A. Llorens-Quintana C. Radhakrishnan H, Iskander RD. Mean shape of the human limbus. J Cataract Refract Surg. 201743(5):667-672.

12. Consejo A, Rojas Viñuela J, Sebastian Carmona J, Ezpeleta J, Piñero DP. Mean corneoscleral junction angle in healthy eyes assessed objectively. Sent for publication September 2022.

13. Hall L, Young Graeme, Wolffsohn JS, Riley C. The Influence of Corneoscleral Topography on Soft Contact Lens Fit. IOVS 201152(9): 6801-6806.

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