It’s difficult not to observe that the recent industry focus on scleral contact lenses—whether pertaining to clinical education, research and development, and/or practice management—has definitely outpaced the attention paid to corneal gas permeable lenses.

While we enthusiastically embrace the continuing developments in scleral lens technology, it’s critical that we likewise devote educational curricula, professional resources, and industry support to ensure that corneal GP lenses remain an equally viable and valuable tool in our management of patients requiring rigid lens designs.¹

Contact lens fitting in general involves a high level of clinical decision-making and, perhaps even more when it pertains to complex refractive errors, underlying pathology, and/or irregular corneal or scleral shape.

While we were invited to present a point-counterpoint, we find it difficult to choose between two lens modalities, as we are deeply convinced that each one has its place in current contact lens practice. Here, we present key considerations and a clinical rationale for deciding between scleral lenses and corneal GP designs.

HISTORICAL PERSPECTIVE

Scleral lenses were the first vision-correcting lenses in the 19th century, but for a long time they were limited by technology. They could not be manufactured in a relatively reproducible way, and their design could not be too complex, as they were made with blown-glass techniques or by hand with tools that were much less precise than the machines we have today.

The primary challenge in their use was the physiological response to wearing them. With low oxygen permeability and thick lens designs, scleral lenses were often associated with significant levels of hypoxia that were detrimental to the patient’s condition.²

Donald F. Ezekiel, AM, Dip Opt WA, DCLP, reintroduced the concept using modern manufacturing methods, including the use of higher oxygen permeability materials.^3,4 Over the last 10 years, technological advances in materials and designs, and a better understanding of the ocular surface, have made scleral lens designs more refined and indispensable.

The first all-plastic polymethylmethacrylate (PMMA) corneal lenses were developed in the late 1940s. They were smaller than the originally designed scleral contact lenses of the 1930s and early 1940s, which were made of glass or a glass/plastic combination. While these new smaller lenses greatly improved comfort and wearing time due to their thinner and lighter profile, there remained a major clinical concern for corneal health due to glass’s lack of oxygen permeability. The negative and potentially serious side effects of corneal hypoxia encouraged further research and development, which led to the introduction of new oxygen permeable, rigid materials by the late 1970s.

The U.S. Food and Drug Administration (FDA) approved the first GP material, cellulose acetate butyrate (CAB), in 1978.⁵ The Polycon lens (Syntex) followed—a much thinner design due to the greater stability of its silicone acrylate polymer. These early materials still exhibited low oxygen permeability, though they were better than pure PMMA.

The majority of today’s GP lenses are made of either fluoro-silicone/acrylate or fluoro-siloxanyl/styrene. The addition of fluorine yields a better mix of stability, flexibility, and oxygen permeability while reducing the hydrophobic silicone component.

Despite those technological improvements, the market showed a slow decline in corneal GP lens usage. When Nathan Efron, AC, DSc, PhD, predicted the demise of GP lenses as a common vision correction modality in 1994, a shock wave hit to the optometric and scientific community.⁶ He predicted that GP lenses would represent less than 1% of all fitted lenses by 2010.⁷ By then, the annual U.K. practitioner survey revealed that his prediction was not so far off the mark with only 2% of fitted lenses remaining rigid.⁷

There were several reasons why GP lenses might become infrequently used sooner or later: new soft lens technologies that could effectively correct complex refractive error or presbyopia, better-performing soft lens materials that provide comfort and moisture retention during wear, etc.

Fortunately, however, GP lenses are anything but “dead.” Advances in manufacturing platforms have taken the process from the lenses being handmade and polished to modern-day lathes with which the lenses are cut with lasers to computer precision. This evolution in materials, manufacturing, and digital technology currently provides us with the opportunity to design corneal GP lenses in a virtually unlimited range of sizes, shapes, and refractive parameters.

The most recent data compiled indicates that corneal GP lenses (including hybrids) account for nearly 10% of all fitted lenses.⁸ Considering that there are 140 million wearers worldwide, this is still 14 million people, including nearly 1 million in the United States. Of these, the vast majority remain spherical corneal GP lens wearers (70%), followed by scleral (14%), orthokeratology (10%), and hybrid (6%). Some countries have a higher use of GP lenses; for example, Holland reports that 50% of wearers are in GP lenses, followed by Colombia with 48%, Germany with 32%, and France with 31%.⁹

This being the case, and despite the advances in scleral lenses in recent years, it must be admitted that the two types of lenses still coexist to this day. So how can we determine which option is better? Let’s apply a few criteria that can guide us in our thinking.

PHYSIOLOGICAL CONSIDERATIONS

The primary purpose of scleral lenses is to restore the ocular surface, creating a microenvironment that is conducive to restoring ocular health.¹⁰ Their large diameter allows them to cover a large portion of the exposed ocular surface in open-eye conditions.

The scleral reservoir can contain not only unpreserved saline but also preservative-free tears formulated to nourish and protect the cornea, or plasma-derived tears, as well as medication. The possibilities are endless, and there are many case reports proving the benefit of using scleral lenses in the presence of ocular surface disorder.

Another use of sclerals is to restore vision in patients who have irregular or ectatic corneas.¹¹ Again, the tear reservoir plays an important role in smoothing the surface, allowing for better optical correction. There is only one downside here. Caroline and Andre identified that corneal GP lenses should be preferred to sclerals when the difference between the highest and lowest point along the meridian with the greatest difference is less than 350µm.¹² Under such circumstances, the patient has an 8 out of 10 chance of being best fit with a corneal GP lens.¹³

More recent usage extends the use of scleral lenses to the vision and comfort problems experienced by patients wearing soft or corneal GP contact lenses, but with a normal cornea. In these latter cases, the lens diameter need not be as large as in the former cases, and the use of small-diameter sclerals has become more common.¹⁴

Some will favor an intermediate approach between corneal GP and scleral lenses, favoring the fitting of corneoscleral lenses. This approach is not very popular due to the small number of designs available as well as the limitations in use for physiologically compromised corneas. Vision may be improved over the standard scleral, but the lack of a significant fluid reservoir greatly limits the benefits of using corneosclerals.

Hypoxia, or the insufficient delivery of oxygen to the cornea, has been the subject of ongoing debate over the past 10 years. From the research undertaken and the results published, it is clear that the subject is more complex than it appears.¹⁵ One must consider the oxygen permeability of the scleral lens material and its thickness, both central and peripheral.

The fluid reservoir thickness must also be considered. Fluids have poor gas permeability and research has identified the reservoir as the key element in the passage of oxygen to the cornea.¹⁶

If oxygen transmission is inhibited, it is theoretically possible that limbal vessels diffuse into the peripheral cornea and allow corneal homeostasis to be maintained at this level.¹⁷ Centrally, most studies show that scleral lens wear is always associated with hypoxic stress, equivalent to 2% to 3%, depending on the group tested.

This cannot be compensated for by tear exchange, which is nonexistent after a few minutes following application of the scleral lens,¹⁸ nor by tear mixing, which is limited, unless the reservoir is shallow (less than 150µm). Corneal edema is therefore present and chronic, but not visible with the slit lamp.

This level of edema has often been regarded as benign, as it is similar to or milder than basic physiological closed-eye edema. This is a mistake, as physiological edema disappears rapidly in the open-eye condition and the cornea can recover. In the case of scleral wear, edema appears within the first 15 minutes of wear, peaking at 90 minutes post-application.¹⁹ It is mostly central and stromal in nature, and is maintained throughout the wear period, often until bedtime.¹⁹ The cornea never has time to recover before the eyes remain closed overnight.

The current consensus is that scleral lenses should be fitted with a lens thickness between 250µm and 350µm, using the material with the highest Dk.²⁰ The reservoir should initially be about 300µm to 350µm, which will reduce during the day by about 10µm to 125µm, with about half of the loss occurring in the first 30 minutes of wear.²¹

My experience is that physiological response is also improved if the prescriber takes care to adapt the scleral peripheries to the significant irregularities of the ocular surface. Fortunately, technological advances over the past 10 years have increased the availability of instruments that measure the surface shape beyond the cornea.

Conjunctival scans and topographies have revealed that the vast majority of patients have highly irregular, asymmetric ocular surfaces with non-periodic variations between the quadrants.²² Studies have also revealed that conjunctival toricity and irregularity between the quadrants increases as one moves away from the limbus, especially in ectatic corneas.²³

WHY CHOOSE SCLERAL LENSES?

Why and when are scleral lenses the preferred alternative? The simplest and most direct reason is comfort. Scleral lenses vault the cornea and, therefore, do not touch this very sensitive part of the human body (except for corneoscleral lenses). They rest on the conjunctiva and the sclera, tissues that are not as highly innervated as the cornea.

Initially, the patient will be aware of the lenses without experiencing any frank discomfort. Within a brief time period, when the scleral lens is well fitted and does not cause compression, the lenses are comfortable to the wearer, unlike corneal GP and even hybrid lenses. This unique experience allows prescribers to offer a user-friendly option at the initial trial, which greatly improves the patient experience.

The gain in comfort is sometimes greater than the gain in visual acuity. Some patients who migrate from corneal GPs to sclerals will readily accept the loss of some letters while gaining greatly improved comfort.

Scleral lenses are not as frequently associated with corneal desiccation (3 o’clock and 9 o’clock staining) as corneal GP lenses are.²⁴ This avoids discomfort but also hyperemia, neovascularization, and even the development of vascularized limbal keratitis.²⁵ Scleral lenses are often not associated with the development of papillae and follicles, as the smaller-diameter corneal GP lenses are.

Scleral lenses do have some limitations, however. In addition to the previously mentioned issues surrounding corneal oxygenation, another potential drawback of scleral lenses is that they often generate unusual optical aberrations compared to other correction methods.²⁶ While it is still possible to use decentered optics to reduce the aberrations and improve acuity, the fitting process is more complex than it is with corneal GPs or hybrids.

Midday fogging is still a concern for many scleral lens wearers.²⁷ Cloudiness of the fluid reservoir, over time, was found to be multifactorial in nature.²⁸ Managing ocular surface disorders, increasing lubrication, better aligning the scleral lens landing on the conjunctiva, limiting the reservoir thickness (especially over the limbus), and using a more viscous solution to fill the reservoir have proved to be effective strategies to alleviate debris accumulation, in my experience.

Additionally, recent studies have shown that scleral lens wear may be associated with increased intraocular pressure during wear.^29,30 Although the conclusions are not definitive, as the results of the studies are sometimes contradictory, they suggest that the prudent approach is to closely monitor the visual field and optic nerve changes in patients who have suspected or existing glaucoma. For the latter, the use of other modes of correction could be fully justified, with the scleral being kept as a last resort.

WHY CHOOSE CORNEAL GP LENSES?

Globally, as of 2021, eyecare professionals reported 78% of new fits and refits as corneal GP lenses, with sclerals comprising the remaining 22%.⁹ We continue to prescribe corneal GP lenses for diverse clinical indications, with only 38% in standard spherical designs.⁹

Corneal GP lenses provide several benefits that may not be found in alternative designs. The first is their unique optics, delivering unparalleled visual acuity, especially in cases of severe refractive error, moderate-to-severe corneal astigmatism, and presbyopia.³¹ The ability to modify all corneal GP lens parameters, including quadrant-specific adjustments, allows the lens/cornea relationship to be optimized while minimizing physiological compromise.³²

Furthermore, they promote tear exchange, an important component in maintaining a healthy ocular surface,³³ by eliminating pathogens from the corneal surface and delivering more oxygen to the tissue. Corneal GP lenses are also associated with fewer inflammatory adverse events than soft lenses, due in part to the low attraction of the materials to pathogens.³⁴

Corneal GP lenses may be a first choice for patients who are dissatisfied with their vision with their current soft lens correction or who have moderate-to-high refractive errors, including corneal astigmatism.³⁵ Presbyopes in particular are better served with corneal GP lenses than with any other technology, especially in the presence of refractive and corneal astigmatism.³¹

And while scleral lenses will often be a first choice in the management of patients who have moderate-to-severe corneal irregularity and ocular surface disease, corneal and intralimbal lenses are still an excellent option for a wide range of corneal and refractive needs. These designs may be considered for patients who have failed with soft lenses and for those who have mild, centrally located irregularities secondary to conditions such as corneal ectasia and scarring.

Compared to scleral lenses, corneal GP lens designs present fewer challenges related to lens application and removal from both patient training and manual dexterity perspectives. The fitting of corneal GP lenses is less time-intensive and often requires fewer follow-up visits. With lower material costs and less chair time, the fitting and lens replacement costs may be considerably more favorable to both the clinician and the patient.

Given their smaller diameters, corneal lenses may be the best, or often the only, choice for patients who have naturally small apertures or anatomic alterations secondary to surgical tarsorrhaphy or pathologic cicatricial changes. Furthermore, conjunctival and/or scleral irregularities such as pinguecula, pterygia, chalasis, and filtering blebs will not complicate the fitting or be subject to potential interference from lens wear. While advanced scleral lens manufacturing technologies, including topographically assisted digital designs and/or fabrication from direct ocular surface impression, are good clinical options, access to them may be limited for both the general practitioner and patient.

Corneal GP lenses resting directly on the cornea without a thick post-lens tear layer, with movement to deliver additional oxygen through tear exchange, provide a potentially better physiological environment in cases of surgically altered corneas and high degrees of refractive error. As previously mentioned, corneal GP lenses are the better choice for glaucoma patients, for whom there may be concern of impediment to aqueous outflow, or when chronic usage of topical medications is needed.

The extensive variety of bifocal and multifocal corneal GP lens designs makes them a great choice for satisfied current GP lens wearers, multifocal soft lens failures, and visually demanding new wearers. Simultaneous and translating designs will accommodate higher degrees of corneal toricity as well as enable customization to various intermediate distance visual tasks. Finally, corneal GP lenses, available in several materials having an indication for overnight wear, are uniquely suited for pediatric aphakia, adult overnight orthokeratology, and myopia management.

There are, of course, challenges in corneal GP lens prescribing. The two primary roadblocks to successful corneal GP lens fitting and wear are comfort and lens displacement or ejection. Certainly, if corneal shape or lid interaction precludes stable and consistent lens positioning, alternatives such as hybrid or scleral lenses remain the better option. Instilling a topical anesthetic upon first application works well to control reflexive tearing and promote easier application and removal training.³⁶ Initial adaptation to corneal GP lenses can take up to several weeks and demands patience and a positive attitude on the parts of both the practitioner and the patient.³⁷

Finally, piggybacking is a good option, albeit a less convenient approach, to reduce corneal sensitivity as well as lid sensitivity. A soft lens carrier also helps to improve lens centration and stability, which improves visual quality and overall comfort. In my opinion, a low convex silicone hydrogel lens is preferred for piggybacking. The use of a spherical soft lens provides 20.9% of its marked power,³⁸ so modification of the GP lens power is typically not necessary.

WHICH IS BETTER?

Tables 1 and 2 provide a summary of pros and cons of each lens modality. When asked whether scleral or corneal GP lenses are better, we submit that the answer is neither. As presented here, both modalities—with their individual and complementary indications, versatility, and safety profiles—are equally essential and valuable tools in our management of patients who will benefit from the vision, comfort, and physiological advantages of GP lenses. Continued clinical educational focus on each alternative is imperative.

With additional advances in ocular surface analysis and measurement of higher order aberrations, we look forward to even more sophisticated product design. Continuing trends in empirical fitting will facilitate greater convenience, lower cost, and faster outcomes, promoting greater patient access and ensuring even more success for practitioners and patients alike. CLS

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