Topical review: Twenty-five years of silicone hydrogel soft contact lenses

SIGNIFICANCE:

The impact of silicone hydrogel (SiHy) contact lens materials on clinical contact lens practice and patient care over the past 25 years is reviewed, along with identifying areas for future innovation in material science and clinical practice to further improve outcomes for contact lens wearers.

A quarter of a century since the launch of SiHy contact lens materials, with many current eye care professionals having never practiced without them as an option, this literature review reflects how this significant change in soft lens material technology has impacted routine clinical contact lens practice and patient care. SiHy lenses now account for approximately 75% of all new daily wear soft lenses fitted and they are available in a wide variety of modalities, replacement frequencies, and designs, including daily disposable, torics, and multifocals. From the physical properties of SiHy materials and their adoption to their use in helping meet patient needs, conclusions can be made where historical clinical issues have been solved, and where innovation in material science and evolution in clinical practice are still required to deliver the best outcome for contact lens wearers. SiHy materials have largely eliminated hypoxia as a complication seen in contact lens clinical practice, and when used for daily wear, in particular as daily disposable lenses, they provide an exceptional option for vision correction that is minimally invasive, comfortable, and effective. This review helps with the understanding of how the eye care profession has adopted the use of these lenses over the last 25 years, and questions what comes next for these widely used family of materials and the opportunities that exist for them to continue contributing to patient satisfaction and to growing the contact lens market.

Considering the staggered launch over a 2-year period across 1998 to 1999, silicone hydrogel (SiHy) contact lens materials hit their 25th anniversary in 2024. A quarter of a century later, it feels appropriate to take a moment to reflect on how this step change in soft lens material technology has impacted routine clinical contact lens practice and patient care. This literature review summarizes the physical properties of SiHy materials and the significant stages in the development of various SiHy contact lenses. The adoption of this family of materials is reviewed, along with their clinical impact. Finally, the use of SiHy materials is considered in the context of patient needs, allowing conclusions to be drawn over those areas where historical issues have been solved, and where innovation in material science and evolution in clinical practice are still required to deliver the best outcome for contact lens wearers.

In the broad domain of health care, it is said that it takes an average of 17 years for new clinical practices and technologies to be fully adopted.¹ Indeed, for any eye care professional who commenced their career at the turn of the last century or later, it is true to say that they have never practiced without the option of prescribing SiHy contact lenses. Analyzing the uptake of SiHy materials therefore can be helpful as a barometer of change. With the increasing choice of materials and optical designs over the last 2 decades, how easily does contact lens practice change? In a time where the contact lens profession is attempting to realize the vast potential of both, helping manage the myopia epidemic via myopia management for children and the untapped population of presbyopes that would benefit from contact lenses, appreciating how quickly and how well clinical practice evolves is useful to understand.

One assessment of global contact lens prescribing suggests that SiHy lenses account for approximately 75% of all new daily wear soft lenses fitted,² and they are available in a wide variety of modalities, replacement frequencies, and designs, including daily disposable, torics, and multifocals. This review helps with the understanding of how the eye care profession has arrived at this point after a 2.5 decades of availability, and questions what comes next for this widely used family of materials.

SILICONE HYDROGEL MATERIAL DEVELOPMENTS

The unique properties of SiHy materials can be broadly broken down into those related to the bulk of the lens versus the lens surface.

Bulk properties

Oxygen transport

The oxygen permeability (Dk) of conventional, nonsiloxane-based hydrogel materials is directly related to the amount of water that the polymer can hold since the oxygen dissolves into the water phase of the material and diffuses through the lens from the anterior to the posterior lens surface. The Dk increases logarithmically with increasing water content of the material and can be determined from the published water content, using the boundary and edge-corrected Morgan and Efron formula (Dk = 1.67 × 10⁻¹¹ e^0.0397WC),³ in which “WC” is the quoted water content of the material concerned. The units of Dk are 10⁻¹¹ (cm²/s) (mLO₂/mL × mmHg) or “Barrer.”

The dependence on increasing water content to maximize Dk was a limiting factor for the development of soft lenses since water has a Dk value of only 80 Barrer.⁴ Using the Morgan and Efron formula,³ poly 2-hydroxyethyl methacrylate (polyHEMA) (with a water content of 38%) has a Dk of only 7 to 8 Barrer. To increase the Dk of a conventional hydrogel contact lens material, it is necessary to incorporate monomers that will bind more water into the polymer. These higher water content materials frequently use monomers such as N-vinyl pyrrolidone or methacrylic acid, which typically increase the water content to 55 to 75%, providing maximum Dk values of approximately 35 Barrer.⁵

Determining the minimum level of oxygen required by the cornea for normal physiological performance is complex⁶ and hampered by the fact that patients exhibit widely different corneal metabolic requirements.⁷ In addition, the minimum levels reported will depend upon the criteria used to determine whether metabolic activity is compromised. Two major criteria to determine minimal oxygen requirements have been described. The first relies upon observing and measuring clinical signs such as corneal swelling. Using this as a criterion, Holden and Mertz reported that the minimum acceptable oxygen transmissibility (Dk/t) to prevent clinical complications was 24 × 10⁻⁹ units for daily wear and 87 × 10⁻⁹ units for overnight wear.⁸ More recent studies, using limbal redness as the clinical criterion, reported that a level of around 125 × 10⁻⁹ units for overnight wear would be more appropriate, with 55 × 10⁻⁹ as a minimum target.⁹ The second criterion for assessing corneal oxygen requirements is based upon mathematic modeling of the effects that a contact lens would have on the cornea. Using such an approach, it has been estimated that the minimal Dk/t requirements are 35 × 10⁻⁹ units for open eyewear and 125 × 10⁻⁹ units for the closed eye.¹⁰ Further work has questioned whether a reliance on Dk/t estimates is appropriate when trying to model the minimal oxygen requirements of the cornea when considering materials that have very high Dk values.^11,12 Brennan¹³ has suggested that a more appropriate methodology would be to use a mathematical model to look at the oxygen flux through such materials. Using an eight-layer modeling approach, it was suggested that the minimum requirements are much closer to the values originally described by Holden and Mertz in the mid-1980s.^11,12

Whatever the criteria chosen, it is clear that conventional polyHEMA-based hydrogel materials provide woefully inadequate oxygen transmissibilities for edema-free overnight wear in all wearers. Indeed, for patients with thicker lenses (due to either their refractive error or the use of multifocal or toric lens designs), there may even be a shortfall in oxygen delivery on a daily wear modality, particularly if the patient has a high corneal oxygen requirement. This awareness of the shortcomings of conventional hydrogel materials resulted in the development of SiHy materials that would provide increased amounts of oxygen to the corneal surface. SiHy materials combined the hydrophilic properties of hydrogels with the oxygen transmission of siloxane species previously used in rigid gas-permeable materials.¹⁴ The process of combining these two vastly different materials proved to be an enormous challenge, and it took over 20 years of considerable intellectual and scientific input and financial resources for these materials to be created.¹⁵

In the late 1990s, both CIBA Vision and Bausch + Lomb were able to overcome these difficulties and first-generation SiHy lenses became a commercial reality and were launched as the “holy grail of lens wear” for a month of continuous wear. In terms of its bulk properties, lotrafilcon A, used to manufacture Focus Night & Day (formerly CIBA Vision, now Alcon Inc., Fort Worth, Texas), employs a co-continuous biphasic or two-channel molecular structure, in which two phases persist from the front to the back surface of the lens.¹⁶ Balafilcon A, used in PureVision (Bausch + Lomb Inc., Rochester, New York), is a homogeneous combination of the silicone-containing monomer polydimethylsiloxane (a vinyl carbamate derivative of tris[trimethylsiloxy] silyl propyl methacrylate) copolymerized with N-vinyl pyrrolidone.¹⁷ Following these initial two materials, Johnson and Johnson Vision released a higher water content SiHy (galyfilcon A used in Acuvue Advance, which is no longer available),¹⁸ closely followed by senofilcon A (Acuvue Oasys, Johnson & Johnson Vision, Jacksonville, Florida).¹⁹ Soon thereafter, comfilcon A from CooperVision became available through the release of Biofinity (CooperVision Inc., San Ramon, California).²⁰ The first SiHy daily disposable was launched in 2008 (narafilcon A, 1 day Acuvue TruEye, Johnson & Johnson Vision), although this is no longer commercially available. In 2012, a substantially different SiHy concept was commercialized by Alcon via their “water gradient” materials. Delefilcon A (Dailies Total1, Alcon Inc.) has a SiHy core, with a high water content (>80%) outer surface that is approximately 5 to 6 µm thick.^21–23

On review of commercially available hydrogel and SiHy materials (Table 1 and Appendix Table A1, available at http://links.lww.com/OPX/A832), it is clear that lower water content SiHy materials exhibit higher Dk values, as opposed to hydrogels, in which higher water content results in materials with a higher Dk. Given the vehicle for oxygen transportation through SiHy is silicone, not water, this inverse relationship for SiHy materials as demonstrated in Fig. 1 is somewhat expected. The relationship for most SiHy materials is strong, with a Pearson correlation of −0.65 (p=0.01).

TABLE 1.

Examples of widely prescribed hydrogel materials (spherical lenses)

Material (USAN)	Brand name	Manufacturer	Replacement frequency; modality	Proprietary technology	Wetting agent	Water content (%)	Oxygen transmissibility, Dk/t (−3.00 D)	Modulus (MPa)	UV blocking
nelfilcon A	DAILIES AQUA COMFORT PLUS	Alcon	Daily disposable; DW	Blink-activated moisture	Hydroxypropyl methylcellulose (HPMC), Polyethylene glycol (PEG) and Polyvinyl alcohol (PVA)	69	26	0.76	No
polymacon A	Soflens 38 (Optima 38)	Bausch + Lomb	1 mo; DW			38	13	0.5	No
nesofilcon A	Biotrue ONEday		Daily disposable; DW	Hypergel	PVP (polyvinyl pyrrolidone); Poloxamer 407	78	42	*	Yes†
omafilcon A	Proclear 1 day	CooperVision	Daily disposable; DW	PC technology	Phosphorylcholine (PC)	60	28	0.3	No
ocufilcon D	Biomedics 1 day		Daily disposable; DW			55	26	0.5	Class 2
etafilcon A	Acuvue 2	Johnson & Johnson Vision	2 wk; DW			58	25.5	0.27	Class 2
etafilcon A	1 DAY ACUVUE MOIST		Daily disposable; DW	Lacreon	Polyvinyl pyrrolidone (PVP)	58	25.5	0.26	Class 2

Information was gathered from manufacturer’s websites and materials where available. Modulus values are from manufacturer’s websites where available; where not available, values have been used from Bhamra and Tighe²⁴ and Sulley et al.²⁵

^*No publicly available modulus value.

^†No level of UV-blocking class stated. DW = daily wear.

FIGURE 1.

Graph of water content (WC) versus oxygen permeability (Dk) for a number of SiHy (solid symbols) and hydrogel lens (open symbols) materials. A clear trend of reducing Dk with increasing WC can be seen for the SiHy materials. SiHys Pearson correlation r = −0.65; p=0.01; hydrogels Pearson correlation r = 0.97, p<0.01. All hydrogel Dk figures are calculated from manufacturer’s stated water content using the formula outlined by Morgan and Efron³ and SiHy Dk values are gathered from various public sources.

Likely due to the need to balance mechanical properties and the increasing use for daily wear, it is noteworthy that over time, Dk values for newer SiHy materials have marginally reduced and water contents have broadly increased.²³

Mechanical properties

SiHy materials are generally “stiffer” (have a higher modulus) than hydrogel materials due to the incorporation of siloxane species and therefore lower water content. The modulus of the first two marketed SiHy materials (lotrafilcon A and balafilcon A) was significantly greater than low modulus hydrogels such as etafilcon A (Fig. 2) and that of more recently launched SiHy contact lenses.^26,27

FIGURE 2.

Graph of water content (WC) versus modulus for a number of SiHy (solid symbols) and hydrogel lens (open symbols) materials. A clear trend of reducing modulus with increasing WC can be seen for all materials. Pearson correlation was r = −0.73; p<0.01. Note that no one standard measurement for determining modulus exists and values vary depending on methodology. Modulus values are from manufacturer’s websites where available; where not available, values have been used from Bhamra and Tighe²⁴ and Sulley et al.²⁵ SiHy = silicone hydrogel.

Increased modulus has some advantages, in that the lenses are typically easier to handle and can be valuable for wearers who exhibit poor handling capabilities, which is a major reason for new wearers to cease lens wear.²⁸ Of note, is that while this concept of easier handling with SiHy materials is often quoted, it appears to be mainly anecdotal, with only a conference abstract published to date to support this belief (Read M, et al. OVS 2024;101:AAO E-abstract 245346), aside from the fact that too low a modulus may be clinically undesirable.²⁹ Indeed, increased stiffness can result in some notable clinical complications. Initially, this increased rigidity and the limited availability of base curves and designs with early SiHy materials resulted in reduced initial comfort compared with patients’ hydrogel materials.³⁰ The presence of mucin balls³¹ and several mechanical issues such as giant papillary conjunctivitis, superior epithelial splits, and occasional epithelial erosions were commonly reported.^26,32 However, as manufacturers have become aware of these issues, greater attention to lens design, the availability of lens materials with lower moduli, and more base-curve options have significantly reduced the number of these mechanical complications observed with SiHy materials.

Compared to the first-generation SiHy materials, later-generation SiHys have a lower modulus (Table 1), due to the fact that they have higher water contents and thus less silicone within the base material.²³ Analysis of Fig. 2 indicates that there is a strong relationship between water content and material modulus, with the materials having the highest ratio of silicone (and thus lower water contents) being the stiffest (Pearson correlation = −0.73; p<0.01).

Surface properties

Silicone is far more hydrophobic than polyHEMA and one of the impediments to the commercialization of SiHy materials related to manufacturers needing to overcome this inherent issue with the wettability of SiHy materials.

To make the surfaces of the initial two SiHy materials hydrophilic and more wettable, techniques incorporating plasma into the surface processing of the lens were developed.^15–17,33 The surfaces of lotrafilcon A and lotrafilcon B are modified in a gas plasma reactive chamber to create a permanent, ultrathin (25 nm), high refractive index, and continuous hydrophilic surface.^16,34–36 Balafilcon A lenses are surface treated in a gas plasma reactive chamber that transforms the silicone components on the surface of the lenses into hydrophilic silicate compounds.^15,17,37,38 Glassy, discontinuous silicate “islands” result,^36,38 and the hydrophilicity of the transformed surface areas bridge over the underlying hydrophobic balafilcon A material. The flow of oxygen and fluids through the lenses is not impeded by these surface modifications.³⁹ Both surface treatments are an integral part of the lens and are not surface coatings that can be removed from the base material during daily handling and cleaning.

Despite these surface treatments providing wettable materials, this two-stage process of surface treatment in the dry state and then subsequent hydration is both costly and time-consuming. Galyfilcon A was the first nonsurface-treated SiHy material to be commercialized, closely followed by senofilcon A. Both materials use an internal wetting agent based upon polyvinyl pyrrolidone, which is designed to provide a hydrophilic layer at the surface of the material that “shields” the silicone at the material interface, thereby reducing the degree of hydrophobicity typically seen at the surface of SiHy materials.^36,40,41 Comfilcon A is a naturally wettable SiHy material that uses long- and short-silicone chains with hydrophilic arms that attract and anchor water molecules.^20,36 Most recently, delefilcon A and other water gradient materials attach an overlying very high-water⁴² content “gel” over an underlying SiHy core to enhance surface wettability.²¹

Wettability

Wettability is a highly important property of contact lens materials, governing the interaction between the ocular surface and eyelid with the contact lens surface. Lenses that exhibit poor wettability have a tendency to be uncomfortable, as surface drying between blinks results in hydrophobic areas that irritate the lid as it moves over the lens surface.⁴³

Wettability may be assessed in a number of ways, using both in vivo techniques and laboratory-based in vitro techniques.^44,45 Analysis of the surfaces of both balafilcon A and lotrafilcon A has shown that the applied surface treatments only partially mask the silicone, with the lenses having significantly more silicone exposed at the surface than conventional hydrogel lenses^46–48 and a more hydrophobic surface (as evidenced by the presence of higher advancing water contact angles).^5,42,49–52

Coefficient of friction

Contact lens discomfort (CLD) continues to be a major problem associated with contact lens wear, with up to 50% of patients reporting CLD.⁵³ CLD is influenced by many factors, including those related to the patient and contact lens material.⁵⁴ Previous studies suggest that there is a relationship between the coefficient of friction (CoF) measurements of hydrogel contact lenses and subjective comfort, with low CoF values resulting in increased comfort.^43,55,56 Measuring the friction properties of soft lenses has gained a significant amount of attention over the past few years and researchers have employed a number of differing methods. Determining the friction properties of soft contact lenses is not trivial and the literature has reported widely differing results for identical lens materials. Coefficient of friction, or lubricity, which is the term often used in the literature, is not an intrinsic material property (like Dk) but is a “system property” that depends upon many factors of the measurement system, including the counter-surface substrate, contact mode, pressure, movement speed, and lubricating fluid, among other factors.⁵⁷

Recent studies have shown that modern SiHy materials do indeed exhibit very low levels of CoF^42,58–63 and ongoing studies continue to look at determining whether these values result in increased lens comfort.

Uptake and early use of SiHys around the world

As described, the first SiHy materials to reach a wide market were balafilcon A and lotrafilcon A across Europe in 1999, following a “soft” launch of the latter material in Mexico the year before.⁶⁴ These lenses were released in the United States in 2001, after gaining US Food and Drug Administration approval.⁶⁴ With these launches signaling the most important development in soft lens materials since the first soft lenses received US Food and Drug Administration approval in 1971,⁶⁵ there was considerable interest and excitement in this new era of contact lens options. These new products offered levels of oxygen permeability that represented a significant leap from that available in conventional hydrogels at the time.⁶⁶ Both lenses were approved for extended wear and indeed, this was seen as the main utility for this new generation of materials, with a significant amount of marketing to both eye care professionals and the general population around the oxygen improvement and extended wear potential of these materials.

Despite the significant improvement in oxygen permeability, the prescribing of SiHy lenses remained minimal for the first 4 years after launch, with the exception of Australia (Fig. 3).²

FIGURE 3.

Prescribing of SiHy lenses, presented as a proportion of all soft lens fits from 2000 to 2024. Data are courtesy of the International Contact Lens Prescribing Survey Consortium and for markets reporting at least 15 years of data and for years with at least 100 reported fits per country. See Efron et al.² for details of methodology of data capture. SiHy = silicone hydrogel.

The marked increase in the use of SiHys compared with other markets in Australia was likely due to the increased interest in that country around the launch of the new materials due to much of the fundamental research having been conducted in Sydney as part of the See3 project,⁶⁷ and was followed with a considerable educational effort across the country by researchers and academics. Nevertheless, the early response to the prescribing of SiHy lenses in the wider marketplace was relatively muted.

Modality shift and new materials

Fig. 3 shows that a significant increase in the prescribing of SiHy lenses occurred from about 2004, coincident with the launch of a lens manufactured from galyfilcon A. Unlike its predecessors, this lens was specifically approved and marketed for daily wear⁶⁸ and it is evident that this pivot in the options for SiHy lenses was well received by many countries around the world. This launch was followed by many others in subsequent years, with daily wear as the key focus for all later lens materials. Indeed, with the exception of a small number of markets, the launch of SiHy lenses—initially promoted as a solution to the problems seen previously with the overnight use of contact lens—did not lead to a significant expansion in extended wear worldwide, with this modality continuing to be rarely prescribed.^69,70

Despite the importance of increasing corneal oxygenation during contact lens wear, there has not been a “Dk arms race” over the 25 years of SiHy product launches. In fact, one of the two initial SiHy lenses to become commercialized (lotrafilcon A) is only surpassed in its Dk by one other lens material of the 25 shown in Fig. 4. Early arguments that increasing material Dk presented a situation of “diminishing returns” with respect to levels of actual corneal oxygenation^11,13 plus the technical challenges of continually improving chemistries to increase Dk appear to have led to SiHy Dk levels reaching a practical maximum of about 140 Dk units.

FIGURE 4.

Twenty-five SiHy contact lens materials launched since 1999 (with a few no longer commercially available). Note how there is no clear increase in Dk values over time, with all SiHy materials offering Dk values between 55 and 150 units. SiHy = silicone hydrogel.

SiHy lens replacement

The increased use of SiHy lenses for daily wear from the middle of the first decade of the 21st century was for lenses replaced on a 2-weekly or monthly replacement basis (Fig. 5), with wider use of the materials in a daily disposable format commencing about 5 years later. This disparity was likely due to the earlier availability of optical designs (e.g., torics and multifocals) and parameter ranges (e.g., higher spherical powers, cylinder power options, cylinder axes) with reusable lenses compared to those for daily disposables. By 2023, the design and parameter ranges were essentially comparable for these two lens categories, but SiHy lenses still remain more widely used for reusable lenses. Anecdotally, this in part may be due to the belief that both SiHy materials and daily disposable lenses offer overlapping ocular “health” benefits and so for some patients, prescribing a hydrogel (i.e., a non-SiHy) daily disposable lens is deemed to be clinically adequate without the perceived need for this lens type to also be in a SiHy material. The notion that daily disposable prescribing is also related to affluence may also account in part for the lower proportion of daily disposable prescribing compared with reusable SiHy materials.⁷¹

FIGURE 5.

The proportion of SiHy lenses used when lenses are prescribed for monthly replacement or daily disposable. Data are courtesy of the International Contact Lens Prescribing Survey Consortium and for markets reporting at least 15 years of data and for years with at least 100 reported fits per country. See Morgan and Efron⁶⁹ for details of methodology of data capture. SiHy = silicone hydrogel.

Notwithstanding these considerations related to the uptake of daily disposable SiHy lenses, it is evident from Figs. 3 and 5 that the use of SiHy materials overall at the time of their 25th anniversary has reached a stage of minimal growth, along an S-curve, in line with classic “diffusion models” of the adoption of new technologies.⁷² The continued use of the original hydrogel family of materials for about 30% of global contact lens fits in 2023⁷⁰ may be due to observations that SiHy lenses are not associated with improved wearer comfort^56,73,74 and for many wearers, that eye care professionals do not see too many clinical signs of hypoxic-related complications.⁷⁵

The replacement intervals for SiHy lenses vary significantly across different markets (Fig. 6). Daily disposables account for around a third or more of all SiHy lenses prescribed in countries such as Italy, Taiwan, and the United Kingdom, but for fewer than 10% of fits in the Netherlands and Bulgaria. The reasons for such differences can be difficult to ascertain but probably relate to local issues of prescribing culture, education, and commercial factors.^2,76

FIGURE 6.

SiHy replacement frequency for different markets. Data are courtesy of the International Contact Lens Prescribing Survey Consortium and for markets reporting at least 15 years of data and for years with at least 100 reported fits per country. See Morgan and Efron⁶⁹ for details of methodology of data capture. Abbreviations are for Italy, Taiwan, United Kingdom, Australia, Japan, Canada, Israel, Denmark, Lithuania, Norway, Hungary, Spain, New Zealand, Sweden, United States, Greece, Czechia, Portugal, and Bulgaria, respectively. SiHy = silicone hydrogel.

Clinical impact of SiHy materials

In the early 1990s, the range of complications confronting practitioners was dominated by those perceived to be caused by hypoxia. In an excellent review published at the time, more than 30 abnormal signs were associated with contact lens wear, of which about half were linked to hypoxia or hypercapnia.⁷⁷ Against this background, it is easy to understand the desire for lenses that would deliver more oxygen to the anterior surface. Further momentum was added by the widely held belief that the most serious contact lens-related complication (microbial keratitis) was also related to corneal hypoxia.^78,79 This view was based partly on evidence from animal models^80,81 but also because the risk associated with eye closure during overnight wear was known to be almost five times greater than for daily wear.⁸² These factors nurtured a fear of corneal infection, which hindered the popularity of extended wear, an otherwise convenient and attractive way to use contact lenses. Consequently, reducing hypoxia and, by extension, improving the safety profile of extended wear were major drivers for the research and manufacturing activities that culminated in the launch of the first SiHy lenses a few years later.

Hypoxia

In the first few years after their launch, evidence quickly emerged of the effectiveness of SiHy materials in eliminating hypoxia-related, clinical signs. For example, typical extended wear-associated corneal changes such as microcysts, polymegethism, and striae were no longer observed.^83–87 Likewise, peripheral neovascularization, always a worrying indicator of compromise in corneal metabolism, was greatly reduced in both prevalence and severity.^86,88,89 Another vascular response that some thought of as a precursor to vascularization^90,91 was limbal hyperemia. Hitherto, this had been regarded as an unavoidable consequence of both daily and extended soft lens use,⁹² but the advent of SiHy lenses demonstrated that the phenomenon was, in fact, oxygen mediated.⁹³ As such it could be effectively eradicated by using lenses with higher transmissibility.^{75,84,86–88}

Mechanical interactions

Despite these undoubted successes, some clinical problems were seen with the first-generation SiHy materials, which were unrelated to hypoxia. The new materials were relatively stiff, with modulus of elasticity values roughly 2 to 3 times higher than typical hydrogels.^26,27 Some wearers, habituated to regular hydrogels, reported initial awareness that was worse than expected³⁰ and others manifested superior epithelial arcuate lesions.^94,95 While altering the lens fit often provided an effective solution in both situations,³⁰ the need to provide a range of base curves was an indication that lowering the modulus of elasticity would be beneficial. Subsequent generation lenses responded to this feedback and, as evidenced in Fig. 2, moduli for many currently available materials approaches that of conventional hydrogels.^26,27,96

Microbial keratitis

As the uptake of SiHy materials increased, the realization slowly emerged that the hoped for decline in corneal infections^97,98 was not occurring. Studies conducted in different parts of the world all agreed that infection rates with SiHy lenses were unchanged relative to before their introduction.^78,99–101 These data strongly implied that factors other than hypoxia were mainly responsible for contact lens-related infections during overnight wear,¹⁰² and with that awareness, clinicians again began to pivot away from fitting lenses on an extended wear basis.⁷⁰

A recent multicenter case–control study on risk factors for bacterial keratitis and severe disease in hydrogel contact lens wearers found no significant associations between contact lens material (SiHy vs. other), nor replacement frequency (daily disposable vs. reusable), and occurrence of bacterial keratitis.¹⁰³

Solution interaction

Disappointing though it was, the decline in extended wear confidence was countered by increased popularity for SiHy daily wear.² The change in modality brought a need to prescribe a nightly disinfection regime and this created an almost instant demand for care products. Understandably, the existing range of disinfecting solutions was pressed into service, but this led to unexpected issues of apparent incompatibility. Several studies have shown that the uptake and subsequent release of biocides are markedly different between hydrogel and SiHy materials,^104–107 potentially impacting the interactions between these materials when disinfected with certain preserved solutions and the ocular surface.

Reports appeared describing episodes of asymptomatic, transient, superficial, epithelial staining,¹⁰⁸ which subsequently became known as “solution-induced corneal staining” (SICS).^109–111 The fact that the problem varied in severity depending on the solution/lens combination in use,^112,113 and potentially had an inflammatory component,¹¹⁴ raised questions of safety in the minds of some clinicians and manufacturers. Ultimately, these fears proved to be unfounded, as no link between SICS and corneal infection was ever reported. Further work examined the uptake of fluorescein into cells in vitro to help increase understanding of the potential mechanisms of action of this clinical finding.¹¹⁵

Corneal infiltrates

The inflammatory impact of contact lenses in general, and SiHy materials in particular, has been a matter of ongoing interest and the associations with corneal infiltrative events (CIEs) have been well studied.¹¹⁶ Although less serious than microbial keratitis, CIEs remain a cause of concern, given that they must be differentiated from active, infectious disease and can result in sequelae that impact the patient, such as reduced days of lens wear and corneal scarring.

During reusable daily wear, SiHys have been shown to carry a CIE risk approximately twice that of conventional hydrogels.^117–119 Following a daily disposable schedule is strongly protective, however, and dramatically reduces the risk to very low levels in both material types.^118,120 For hydrogel daily disposal, the annualized CIE rate is effectively zero, while that for SiHy materials is slightly higher at 0.4%.¹²⁰ It thus appears that while some residual effect associated with lens material remains, factors such as eliminating care solutions, contact lens cases, and reducing the daily bioburden interaction with the ocular surface are more important in limiting inflammatory signs.¹¹⁸

Limbal stem cell deficiency

Contact lens-induced limbal stem cell deficiency is caused by disruption of corneal epithelium stem cell precursors and typically manifests as conjunctivalization of the superior cornea in the region of the limbus, though other locations can be affected.^121–124 The condition has a multifactorial etiology, with long-wearing times, solutions/preservative toxicity, mechanical trauma, and hypoxia being implicated, among several other factors.^121,122 This condition has been reported in wearers of SiHy lenses¹²⁵ and potential causes include their higher modulus and differential uptake of solution components compared with hydrogel materials.¹²² Given their increased oxygen transport, in cases presumed to be due to hypoxia it might be expected that SiHy lenses, especially those with lower moduli, would be a useful option, both from a prophylactic and therapeutic perspective. However, apart from a couple of small, case series that have suggested the latter,^123,126 little effort appears to have been made to verify this potential.

Deposition

It has been widely reported that SiHy materials tend to deposit more lipid, though less protein, than hydrogels.^127–129 In general, however, there do not appear to have been any adverse clinical consequences of this situation,^87,130 with one study even proposing that additional lipids may have a protective role with respect to discomfort.¹³¹ The increasing prevalence of daily disposability⁷⁶ is likely to further decrease any risk of complications or discomfort associated with deposition, as well as being a management option for those occasional individuals who present with problems related to excessive deposition.

Comfort

Subjective comfort is an important aspect of contact lens wear, irrespective of modality. The early issues with insertion awareness of first-generation SiHys have already been mentioned and have subsequently been solved with newer materials and lens designs. However, discomfort and sensations of dryness experienced over the rest of the wearing cycle are widespread problems affecting many wearers.⁵³ The hypothesis that increased oxygenation would be beneficial in comfort terms was among the original reasons for pursuing the development of SiHy materials, but whether or not it is true remains controversial. Reviews published 10 years ago found most of the available studies to be flawed, biased, or both^56,74 and evidently, the situation has not improved up to the time of writing. For example, a recent Cochrane review considered relevant studies to offer only low-certainty evidence of any improvement in comfort with SiHy materials. Apart from being associated with a high risk of bias, the majority of these studies did not assess comfort using a validated instrument.¹³² There is, therefore, no clear evidence to suggest that SiHy materials differ substantially from conventional hydrogels in terms of subjective comfort, and this tends to support the view that oxygen transmissibility is not an influential factor in that response.

Comfort differences between lens types are thus likely to be due to other considerations, such as surface friction,⁴³ lens design, and care system.⁵⁶ As with other aspects of performance, use as a daily disposable appears to maximize wearer comfort¹³³ and in general, modern SiHy materials offer excellent levels of comfort often approaching those reported by nonwearers.¹³⁴

Overall performance

The ways in which SiHy materials are worn have evolved considerably over the last 25 years. From their launch as an extended wear device, they are now frequently employed as either a frequently replaced (1- or 2-weekly, or monthly) daily wear or a daily disposable lens, and a range of options exist that permit their use for almost any refractive or therapeutic, bandage application. This heterogeneity among materials, lens types, and wear schedules makes it a challenge to discern something of their overall clinical impact. A recent attempt took clinical data from current contact lens wearers, a large proportion of whom would be using SiHy lenses, and compared them with similar data acquired 20 years previously, before SiHy lenses were widely available. A group of neophytes, with no experience of contact lenses, was also included as a control.⁷⁵ The analysis showed that, when examined with a biomicroscope, the eyes of contemporary contact lens wearers appeared largely similar to neophytes, the implication being that wear of modern contact lenses is minimally invasive. With respect to limbal redness, this finding represents a significant performance improvement, as there had been manifestly more hyperemia among contact lens wearers than neophytes, 2 decades previously. The data also showed that very low levels of corneal staining were associated with contact lens wear both currently and 20 years ago, but there was a significant upward spike during the period 2009 to 2014, which probably reflects the impact of the SICS episode described earlier.

Patient impact

Patient satisfaction and drop out

It is challenging to find accurate estimates of the size and growth of the contact lens market in published peer-reviewed literature. While the total value of the market is increasing over time with more expensive products available, especially in relation to the increasing use of daily disposables,⁷⁶ actual wearer numbers appear to tell a different story. Often quoted values—although noting they are several years old now—suggest there are 140 million contact lens wearers worldwide, with little to no growth in those numbers year-on-year.⁵³ The relatively static nature of overall wearer numbers can be understood by looking at drop out rates. A review of contact lens drop out established a pooled mean frequency of 21.7% (range 12.0 to 27.4%), with the most common reason for drop out in established wearers being discomfort, and in neophyte wearers, vision.¹³⁵ Along with vision and comfort issues, another common reason for discontinuation in new wearers is difficulties with lens handling,^28,136 and in established wearers, problems with handling have been correlated with overall levels of dissatisfaction with contact lens wear.¹³⁷

Oxygen performance

The relevant question then is to consider whether SiHys have had an impact on patient satisfaction in contact lenses and, importantly, on drop out from lens wear. As summarized earlier, the fundamental difference between SiHy materials compared with hydrogels is their significantly increased oxygen transmissibility, with the associated almost total elimination of hypoxic-related sequelae, including limbal hyperemia.^{85,93,138,139} This is a clear difference in terms of clinical performance and it translates to a tangible benefit when it comes to enabling patients to wear their contact lenses for as many hours in the day as their lifestyles demand. Limiting wear time to manage corneal neovascular ingrowth, which was a common management approach over 2 decades ago, is no longer a concern with SiHy materials. No data are available on how this has impacted patient satisfaction, but this improvement in maintaining corneal physiology certainly removes a potential limitation of the way some patients were able to wear their hydrogel lenses.

Further, while no contact lens has an approved indication for the kind of short-term, closed-eye wear that occurs when people nap, it is known that up to 87% of wearers admit to doing so.^140–143 While patient education about best wearing practices is crucial, being able to prescribe a contact lens material that delivers high amounts of oxygen to the cornea is also helpful in mitigating for patient’s inherent occasional tendency to sleep or nap in their contact lenses.

Comfort

The impact of SiHys in relation to the three main reasons for drop out, discomfort, poor vision, and difficulty with handling, is interesting to review. Discomfort is the most common reason that drives drop out. The review in this manuscript of the clinical impact of SiHys in terms of comfort demonstrates there is no clear difference between the two soft lens material types in terms of comfort performance. The fact is that both contact lens and overall ocular comfort are multifactorial in nature, with efforts in the contact lens and dry eye disease space still pushing forward to further understanding and find solutions for symptomatic patients.¹⁴⁴

Vision

Dissatisfaction with vision leads to drop out, with an increased chance of this occurring with neophyte astigmats and presbyopes.²⁸ This makes the availability of toric and multifocal optical designs important, along with encouraging proactive use of these options with patients. While both hydrogel and SiHy material groups offer a wide range of prescriptions for these corrections, innovation and investment in toric and multifocal optical designs have primarily happened with SiHy base materials over the last few years, and with toric multifocal options now available. This suggests a trend over time toward greater availability of the widest range of prescription correction options in SiHy rather than hydrogel materials.

Handling

In relation to drop out, the way the lenses are handled is important, with difficulties with handling driving early drop out in new wearers and lower satisfaction in established wearers. Overall handling experience includes the opening and removal of the lens from its blister packaging, checking the lens for correct orientation, ease of application, and ease of removal of the lens to and from the eye. Recent evidence suggests that for minus lens powers at least, users find removing a lens from its blister and application onto the eye to be easier with SiHy lens brands than with hydrogels. Handling differences are likely to be due to the increased modulus of SiHy materials compared with hydrogel brands and especially with earlier SiHy materials. Although much more work is required in this area,¹⁴⁵ it is clear that there is a general perception that SiHys are easier to handle than hydrogels, and as such, they are often recommended to neophytes.

Convenience

A further patient need relates to convenience. The use of SiHy materials accelerated with their development for daily wear. Further innovation drove even greater choice from 2008 onwards when the first daily disposable SiHy lens became available. In addition to the flexibility of wear that daily disposables offer, the clinical benefits of combining SiHy material with a 1-day replacement wearing schedule have been comprehensively reviewed elsewhere, with clear conclusions being drawn about reduced adverse event rates and better compliance with wear and care regimens compared with reusable contact lenses and increased long-term health advantages compared with hydrogels.¹⁴⁵

DISCUSSION

In the quarter of a century, since SiHys were introduced, soft contact lens practice has changed substantially. The use of this material now dominates clinical practice overall, with greater popularity for reusables and growing use in daily disposables. Eye care professionals have a wide range of SiHy materials and optical designs to choose from in both daily and reusable replacement frequencies. It is of note that while hydrogel materials are still fitted to a significant minority of patients and can perform well clinically, most recent product introductions and innovations have been with SiHy materials and may suggest the growing use of such lens types in the future. Given the evolution of materials, designs, and clinical performance over the last 2 decades, the profession can feel confident in this change. SiHy materials have all but eliminated hypoxic complications of contact lens wear, have been shown to handle well, are just as comfortable as hydrogel materials, and are available to correct almost all prescriptions. However, it would be helpful to be able to better validate the outcomes of these combined properties of SiHy materials. In addition to the lack of published evidence on presumed handling benefits, current data on patient satisfaction and drop out from contemporary SiHy lenses specifically are almost totally absent and investment in research in these areas would be welcome.

However, there are areas for improvement and innovation. First, the need to recognize that the introduction of neither SiHys nor daily disposable replacement schedules has been able to significantly drive an increase in overall numbers wearing contact lenses around the world. The reasons for drop out among new and established wearers are well understood. Some of those can be proactively addressed in clinical practice today: ensuring the wearer is confident with handling contact lenses and ensuring they are offered the most appropriate correction for their prescription, especially important for astigmats and presbyopes. Some areas require further innovation, most obviously to tackle discomfort. For those wearers who cannot wear their lenses comfortably for as many hours as they would like, alternative SiHy materials can be offered, but this is not always enough. Future innovation to deliver even greater levels of comfort for this specific group of patients would be welcome. Finally, the thorny issue of infection remains. The 25 years of SiHy use has demonstrated that oxygen delivery was not the answer to reducing the incidence of microbial keratitis, although it does appear to have reduced the severity. Many of the risk factors for this serious ocular infection are modifiable, but the creation of a contact lens that can be worn safely overnight without increasing the chance of infection currently remains elusive.

CONCLUSIONS

The impact of SiHys has been to largely eliminate hypoxia as a complication seen in contact lens clinical practice. While the promise of safer extended wear has not been realized, when used for daily wear, especially as a daily disposable, they provide an exceptional option for vision correction that is minimally invasive, comfortable, and effective. Considerable numbers of eye care professionals have never practiced without them, and likewise have rarely seen signs of hypoxia such as neovascularization with SiHy wearers. Eye care professionals who have been around longer have direct experience of how the twin innovations of both affordable and reliable daily disposable lenses combined with SiHy materials have fundamentally changed the landscape of soft contact lens use since the turn of the century. Reviewing the impact of SiHy materials over this period demonstrates an ability for the eye care profession to change and adopt new technologies and fitting practices; a change that is in line with the often quoted 17 years that is required to fully adopt changes in healthcare practice.¹ This is encouraging, given the specific opportunities that exist to grow the contact lens market and provide even more patients with the option of contact lenses; both at the start of their journey with myopia management and towards the latter part as they become presbyopic via the fitting of multifocal designs.