Abstract
Daily disposable (DD) contact lenses first came to the market approximately 25 years ago and eye care professionals (ECPs) started prescribing silicone hydrogel (SiH) contact lenses, primarily for extended or continuous wear, approximately 20 years ago. It has now been over ten years since SiH DD contact lenses have been available, and while SiH materials are routinely prescribed by ECPs for reusable daily wear, hydrogel materials are still frequently selected for the DD modality of contact lens wear. This article reviews the evidence to support the benefits of both a DD modality and SiH materials and how patients’ needs may be met with SiH DD contact lenses, with respect to clinical performance, health outcomes, satisfaction, compliance and convenience. Factors which may enable or constrain ECPs from prescribing SiH DD contact lenses, as opposed to hydrogel DD and reusable contact lenses, for more of their patients are discussed with the objective of providing ECPs with a greater understanding of the advantages that can be afforded by prescribing SiH DD contact lenses to both their new and existing contact lens wearers.
1
Introduction
Daily disposable (DD) contact lenses have now been available for 25 years [ , ], and are being increasingly prescribed by eye care professionals (ECPs) for a high proportion of their patients [ ]. The first silicone hydrogel (SiH) contact lenses were introduced to the market 20 years ago in the late 1990s [ ]. Although they were initially intended for extended or continuous wear, the many benefits afforded by their high oxygen transmissibility has resulted in them being increasingly prescribed for daily wear [ ]. The first SiH DD lenses were launched in 2008 [ ].
There has been a steady upward trend in DD contact lens prescribing worldwide since DD contact lenses were first introduced in the mid 1990s [ ]. According to the latest worldwide prescribing data, an average of 45 % of soft lenses prescribed in 2019 were DDs; however, this varies significantly by country, with a high of 78 % in Denmark and only 10% in Mexico [ ]. Fig. 1 summarizes the soft contact lens prescribing data collected each year by the International Contact Lens Prescribing Trends Consortium, and published annually in Contact Lens Spectrum [ ]. This figure shows the steady decline in the proportion of patients fitted with reusable daily wear hydrogels and the corresponding increase in reusable SiH soft lenses, which then plateaued around ten years ago. Further inspection shows that prescribing of hydrogel DD lenses has remained relatively constant over the past 20 years, but there has been a steady increase in the prescribing rates of SiH DD lenses since they were first introduced in 2008, and for the past three years SiH DD lenses have been fitted more frequently than hydrogel DD lenses [ ]. Interestingly, while there was an increase in the rates of prescribing of soft contact lenses for extended wear in the early 2000’s, corresponding with the introduction of SiH materials, the overall rate of soft lenses prescribed for extended wear has dropped to around the same proportion that it was in 2000, before SiH materials were introduced.
It has been estimated that there are 175 million full or part-time contact lens wearers worldwide [ ]. In 2016, the number of contact lens wearers in the United Kingdom and the United States were reported to be 4.2 million and 45 million respectively [ , ]. While the global market value for soft contact lenses has been reported to be growing by at least 4–6% over the past years, and to be in excess of $8.5 billion in 2018 [ ], it is not clear whether the actual number of contact lens wearers is increasing at the same rate. There are a considerably wider range of materials, designs, prescription availability and modalities for wear available today, and ECPs are fitting more patients with contact lenses to meet their visual demands and active lifestyles; but despite this there continue to be individuals who lapse from contact lens wear as a result of symptoms of discomfort, poor vision and difficulties with handling [ , ].
In order to satisfy their patient requirements with respect to continued comfort and vision, and to grow their practices, it is important for ECPs to seek the best contact lens technologies and replacement modalities that are available. This can be achieved by providing their patients with the convenience of a DD modality in conjunction with the newest technology with respect to materials, surfaces and designs. Fortunately parameter restrictions are no longer a barrier to prescribing DD lenses, and many manufacturers are able to offer designs that have been optimized for both patients with astigmatism, and patients with presbyopia. It is interesting that worldwide, 72 % of all soft lenses prescribed have been reported to be SiH materials, but the proportion of DD contact lenses that are prescribed in SiH materials is considerably lower [ ].
This article will review the current evidence in the literature to support the many benefits of a DD modality, in contrast with reusable contact lenses. Comparisons between the ocular response to SiH and hydrogel materials will also be appraised with the objective of providing ECPs with a greater understanding of the advantages that can be afforded by prescribing DD SiH contact lenses to both their new and existing contact lens wearers.
2
ECP opinions relating to silicone hydrogel daily disposable contact lenses
In order to gain a better understanding of ECP’s perceptions concerning the benefits of, and possible barriers to, prescribing SiH DD contact lenses, a survey was conducted among 300 ECPs in the United States, United Kingdom and Japan [ ]. The aspects evaluated in the survey were broadly categorized into those relating to health, comfort, patient experience, and standard of care; in addition a series of other aspects were evaluated, including oxygen requirements, lens design, cost etc. Health benefits were reported to be the most important factor when deciding to prescribe SiH DD contact lenses, followed by their patients’ experiences and overall satisfaction with their lenses. Indeed, 90 % of ECPs agreed that “Silicone hydrogel is the healthiest lens material for my daily disposable patients”, 91 % that “Silicone hydrogel 1 day lenses provide better long term eye health for my patients than hydrogel 1 day lenses”, and 90 % that “Silicone hydrogel 1 day lenses provide a better wearing experience for my patients than hydrogel 1 day lenses”.
Despite the majority of ECPs completing this survey agreeing that SiH lenses should be the standard of care for the DD modality, the main factor which constrains them from prescribing these lenses to more of their patients continues to be the generally higher cost of SiH DD contact lenses; 95 % of the survey respondents agreed that “If cost was equivalent, I would choose silicone hydrogel over hydrogel for my 1 day patients” [ ]. This is interesting because ECPs may not even be presenting the cost of these newer technology contact lenses to their patients, but simply assuming that they would not be prepared to pay a higher price for a premium product. A recent study has reported that the frequency with which discussion relating to treatment cost occurs between patients and their health care providers is extremely variable [ ]. There may be several reasons for this, including concern with respect to upsetting the patient and jeopardising their retention in the practice. In actual fact, patients may be frustrated that they are not being recommended contact lenses that are best able to meet their demanding lifestyles and provide a superior wearing experience. This is supported by data from an online survey that was recently conducted among 1520 adult contact lens wearers [ ]. In this survey 68 % of respondents said that they would expect their ECP to recommend the contact lens that provides 100 % of the oxygen their eyes need regardless of cost, and 75 % of respondents indicated that they would follow their ECP’s contact lens recommendation regardless of the cost of the lens.
3
Benefits of a daily disposable modality: convenience, physiological response and overall satisfaction
DD contact lenses are able to provide many benefits to contact lens wearers when compared with a reusable lens modality [ ]. These include the convenience of not having to clean and disinfect lenses after each use and the ability to have spare, replacement contact lenses readily available in the event of loss or damage. When a new lens is worn each day, there is significantly less spoliation from lens deposits [ ], and no opportunity for deleterious lens deposit and care solution interactions with the corneal epithelium or tarsal conjunctiva [ ]. There is also less opportunity for non-compliance with contact lens and contact lens case cleaning and care procedures [ ]. Improved comfort, vision and relief from allergies have also been reported with the use of DD contact lenses [ , , ]. In addition, DD contact lenses are the ideal choice for part time or occasional wearers [ ].
A major benefit of single use, daily wear lenses is fewer adverse events, as compared with reusable modalities. Although infiltrative events can occur with DD contact lenses [ ], more recent evidence supports a single use modality improving ocular health outcomes. Chalmers et al published the results of a multicentre case control study in 2012 [ ]; the purpose of the study was to evaluate association between symptomatic soft contact lens related corneal infiltrative events (CIEs) with lens material, lens care products and other risk factors. A significantly lower proportion of cases with CIEs were wearing DD lenses, when compared with their matched controls (2% vs. 13 % controls), signifying that DD lenses were protective against the development of CIEs and reduced their risk by 75 %. A possible reason for this protective effect is that replacing a lens every day results in lower levels of bioburden on the lens and there is also no opportunity for contamination from the storage case [ , ]. In a subsequent study, CIE rates of 0.4 % for SiH DD and 0% for hydrogel DD contact lenses were reported, both of which were significantly lower than previously reported rates for reusable SCLs of 3%–4% per year, indicating improved ocular health outcomes with DD lenses [ ]. Further evidence to support these findings has been reported by Saliman et al. in their study that was designed to investigate the subclinical inflammation of the ocular surface that occurs with soft contact lens wear [ ]. Results from tear sample analysis (cytokines), in vivo confocal microscopy and impression cytology showed only a minimal subclinical inflammatory response following one week of wear with a SiH DD contact lens when compared with no lens wear, in contrast to an upregulated inflammatory response with reusable contact lenses.
DD lenses are an excellent option for children and several studies have reported success with this modality of wear [ ]. The incidence of CIEs with DD lenses has been reported to be no higher in children than in adults [ ]. In a recent retrospective review of 581 myopic children fitted with hydrogel DD contact lenses in six randomised clinical trials, no serious or significant adverse events were reported; two cases of non- significant CIEs were documented, giving an incidence of 0.3 (95 % CI: 0.1–0.9) infiltrative events per 100 patient years [ ]. Likewise, no serious ocular adverse events were observed during a 3-year prospective clinical trial of children wearing DD contact lenses, and only four asymptomatic non- significant CIEs were reported [ ]. The absence of serious or significant ocular adverse events supports the growing acceptance that soft contact lenses are safe for use by children. When ease and convenience of use are also taken into consideration, the soft contact lens of choice of ECPs when fitting children is therefore usually a DD. Furthermore, in a group of almost 100 teenagers, DD contact lenses were reported to be suitable for vision correction and offered improvements in Quality of Life measures (appearance, satisfaction, activities, and peer perceptions) when compared with spectacle wear [ ].
At least two studies have been conducted in which the ocular response and adverse events occurring in spectacle wearers and DD wearers have been compared over a period of one year [ , ]. In both studies individuals were randomly assigned to either the spectacle wearing group or the contact lens wearing group and the DD wearers had no prior contact lens wearing experience at the start of the study. In the first study, conducted by Sankaridurg et al., the DD lenses were hydrogels (etafilcon A, 58 % water content, Johnson & Johnson, Jacksonville, FL) [ ]. No serious adverse events occurred in either the DD or the spectacle wearing groups, and the incidence of significant adverse events (CIEs and contact lens associated papillary conjunctivitis) was extremely low in the DD group. In the second study, conducted by Morgan et al., the DD lenses were SiH (narafilcon A, 1 DAY ACUVUE® TruEye®, Johnson & Johnson Vision) [ ]. In this study, clinical performance in the two groups was equivalent (subjective vision, comfort, bulbar conjunctival hyperaemia, limbal hyperaemia, corneal staining and papillary conjunctivitis). Conjunctival staining was slightly higher in the DD wearing group: this was considered to be most likely related to the edge design of the narafilcon A contact lens, and was not at a level that would require clinical management. There was also one significant CIE reported in the DD group. Together these studies support the premise that spectacle wearers can be successfully fitted with DD contact lenses that can provide the wearers with lifestyle and visual benefits over spectacles, with minimal response of the ocular tissues and an extremely low risk of adverse events.
4
Compliance with contact lens replacement and contact lens care and wear
In the past decade, several studies have been conducted to evaluate compliance with contact lens wear, replacement and care, and these studies have shown that DD wearers appear to be more compliant than reusable lens wearers. In 2009, Dumbleton et al. published a study in which patient and ECP compliance with reusable SiH and hydrogel DD lens wear and replacement was evaluated in the United States [ ]. Surveys were completed by 1,654 patients 16 % DD, 45 % 2 week reusable SiH, 39 % 1 month reusable SiH) and the contact lens types worn and instructions for replacement, were provided by their ECPs. Patients were most compliant with replacement of DD lenses (88 %) followed by 1 month lenses (72 %) and 2 week lenses (48 %). The most frequently reported reason given for over wearing of contact lenses was “forgetting which day to replace lenses” (51 %), followed by “to save money”. Better communication between the ECP and the patient was found to be associated with greater compliance with recommended replacement frequency. Very similar results were reported when the study was repeated in Canada [ ].
In 2011, Morgan et al. reported compliance with lens wear and care among 4021 wearers in 14 countries [ ]. While compliance with all aspects of wear and care was rare, it was reported to be significantly better for DD wearers. Case cleaning and rubbing and rinsing with care products were the behaviours associated with the lowest levels of compliance, both of which are not required when wearing DD lenses. Similarly poor compliance with cleaning of reusable contact lenses and cases, and replacement of cases has also been reported in other studies [ , , ].
In 2013, Dumbleton et al. published the results of a survey conducted in The United Kingdom, the United States, Norway and Australia, to specifically investigate compliance with DD contact lenses [ ]. In this survey of 805 DD wearers, 91 % were compliant with replacing their lenses every day. SiH DD lenses were worn by 14 % of respondents and no differences in compliance were found with respect to lens material (SiH versus hydrogel), age, sex, frequency of lens wear (part time versus full time) or number of years of contact lens wear. The principal reasons given for reusing DD lenses were to save money (60 %) and simply running out of lenses (47 %). Re-use of DD contact lenses was associated with inferior comfort on the second day that they were worn. This finding could be as a result of lens spoilage and/or the absence of the comfort enhancing agents, which are included in the blister pack solution of many DD contact lenses [ ].
A more recent survey has been conducted to evaluate compliance with DD replacement in Italy, and in addition to test which specific wearer attitudes towards contact lens use may affect the compliance rate. [ ]. In this survey 23 % of the 354 DD wearers reported non-compliance with daily replacement, and once again the principal reason given was to save money. Consistent with the findings from earlier studies, there was no association between compliance and demographic factors including age, gender or number of years of wear. Psychological factors may however play a role and the authors recommended that ECPs warn DD contact lens wearers of the negative consequences that can occur when DD lenses are reused and how these can be prevented. The latest survey investigating compliance with contact lens wear was published in 2019 and included almost 300 contact lens wearers attending a science museum [ ]. Once again, DD contact lens wearers were found to be significantly more compliant with recommendations for lens replacement than reusable contact lens wearers.
Another aspect of non-compliance with contact lens wear and care, which has been shown to be a risk factor for infection, is exposure of the contact lenses to water [ ]. A relatively uncommon, but devastating ocular infection that can occur as a result of exposure of contact lenses to water is Acanthamoeba keratitis [ , ]. Unfortunately exposure to water while wearing contact lenses is widespread; in a survey of 542 soft contact lens wearers aged 12–33 years, up to 65 % reported “always or fairly often” showering while wearing their contact lenses [ ]. Another survey of approximately 1000 contact lens wearers in the United States, 85 % reported having showered, and 61 % reported having swam, while wearing their lenses [ ]. Perhaps even more concerning, is the high proportion of contact lens wearers who report having rinsed (up to one third) or stored their soft contact lenses in water (up to 17 %), at least sometimes [ , ]. In a recent prospective case-controlled study, the exposure to water, and compliance with lens hygiene were investigated in a group of individuals with contact lens related adverse conditions and a control group of contact lens wearers presenting for evaluation without contact lens related adverse events [ ]. The authors reported that half of the disease load could be eliminated by modifying water contact behaviour and lens hygiene compliance in this cohort of contact lens wearers and concluded that improving hygiene compliance and minimizing contact with water may reduce the risk of contact lens related adverse events. A DD modality can reduce the risk of exposure to water, since no cleaning or case are required, and lenses can be readily replaced after exposure to water. However, even with a DD modality it is important to reinforce water avoidance and a “no-water” campaign has been launched to try to address this issue [ ]. Therefore compliant wear with DD contact lenses should reduce the prevalence of serious corneal infection [ , , ], although there is currently no evidence in the literature to support this supposition.
In summary, non-compliance with DD contact lens replacement is much lower than it is for re-usable lens types, and there does not appear to be a difference in compliance between hydrogel and SiH materials. The use of DD contact lenses may also reduce the risk of adverse events that are caused by exposure of the contact lenses or storage cases to water. It is important, however, that DD CL wearers be reminded regularly of the risks of re-using DDs, wearing non-disinfected lenses and exposure to water, to ensure maximum compliance and to minimise the risk of infection.
5
Oxygen delivery and requirements for successful contact Lens Wear
The cornea is unique in that is an avascular tissue which is primarily dependent upon the atmosphere for its oxygen supply, when the eye is open during the day. Contact lenses constrain the supply of oxygen and can lead to reduced oxygenation, or hypoxia, which results in corneal swelling or oedema [ ]. Chronic hypoxia can result in complications including limbal hyperaemia, corneal vascularisation, epithelial microcysts, refractive error changes, corneal distortion and endothelial changes [ ]. Oxygen supply to the cornea, while wearing a contact lens, is determined by the oxygen permeability (Dk) of the material and the thickness of the contact lens (t) [ ]. Oxygen transmissibility (Dk/t) quantifies how much oxygen can pass through the contact lens. In hydrogel contact lenses, the Dk is determined by the water content of the material and therefore any change in water content will alter the Dk. Temperature and pH will affect water content of a contact lens, but these are unlikely to change enough during lens wear to have a significant impact; however, hydrogel lenses can dehydrate on the eye and Efron and Morgan demonstrated that hydrogel lenses could lose up to 6 per cent of absolute water content and therefore reduce Dk/t [ ]; however some hydrogel materials have also been reported to exhibit very little dehydration [ , ]. Polarographic measurement of Dk/t can also be affected by boundary and edge effects [ , ] and therefore the correct values for Dk and Dk/t must take account of these effects.
In the 1980s, Holden and Mertz conducted a series of studies to determine the minimum Dk/t that would be required to limit corneal oedema to the level that would occur with no contact lens during eye closure overnight (87 barrers/cm), and when the eye is open during the day (24 barrers/cm) [ ]. While Dk/t is the most widely used method to describe the delivery of oxygen through a contact lens to the cornea, it has been criticized because it is simply a physical quantity of the contact lens and does not take into account the physiology and interfaces or boundaries between the tear film, contact lens and cornea. As a consequence, oxygen flux and oxygen consumption rates have also been proposed as alternative techniques for assessing the amount of oxygen that is needed to prevent corneal oedema [ , , ]. In addition, Dk/t is not a linear measurement, and particularly with SiH contact lenses, there is a law of diminishing returns in that relatively large increases in Dk/t values provide relatively small differences in the actual amount of oxygen reaching the cornea [ , , ].
The majority of corneal swelling studies conducted have reported on overnight swelling and have only measured the central corneal response. Since soft contact lenses are most frequently worn on a daily wear basis (91 % of soft contact lenses prescribed worldwide in 2019 [ ]), it is also important to measure corneal swelling in open eye wear. Furthermore, measurements should not only be made centrally, but also peripherally, particularly for contact lens designs that may be thicker in the mid-periphery and periphery. Morgan and Efron conducted a study to determine the central and peripheral Dk/t thresholds required for soft contact lenses in order to avoid corneal oedema during open eye wear [ ]. Central and peripheral (4 mm inferior) corneal thickness was measured using the Pentacam Scheimpflug imaging device (Oculus Optikgeraete GmbH, Wetzlar, Germany) before and after 3 h of open eye wear with five reference lenses (four hydrogel and one SiH) and a control (no lens). From these data, they concluded that “to avoid swelling across all regions of the cornea during the open eye wear of soft contact lenses, a Dk/t of about 33 barrers/cm or greater is required across the entire lens for a typical wearer”. Of course there is no way of knowing whether a patient can be considered a typical wearer since not all corneas swell to the same degree [ ]. Moezzi et al. conducted a study designed specifically to investigate the distribution of central corneal swelling for 29 individuals following eight hours of closed eye wear with silicone hydrogel lenses with various Dk/t values and in eyes without lenses [ ]. They concluded that although mean values of central corneal swelling are important for comparisons between lenses, they cannot predict an individual’s corneal swelling response.
The degree of corneal swelling, both centrally and peripherally varies according to the thickness of the contact lens worn, and therefore may also vary significantly according to lens power and design. Moezzi et al. reported a significant effect of lens power in their study with +6.00 diopter (D) powered lenses inducing significantly greater central swelling than the -3.00D and -10.00D lenses [ ]. Lira et al. conducted a study that was designed to calculate the central and peripheral thickness of several hydrogel and SiH contact lenses with different powers (-12.00D to +6.00D), and to analyze how the variation in thickness across the lenses affected the Dk/t [ ]. Among the DD lenses (three hydrogel and one SiH) evaluated the Dk/t values ranged between 9.5 ± 0.5 and 178.1 ± 5.1 barrers/cm. These results clearly support the importance prescribing contact lenses with high Dk/t values, even for daily wear, particularly when patients’ prescription requirements demand thicker lens designs.
As discussed earlier, oedema is not the only consequence of a reduced oxygen supply to the cornea and it is important for clinicians to understand the overall physiological impact of contact lenses on the cornea and its associated tissues. Bulbar and limbal hyperaemia, and neovascularisation have also been used as metrics for measuring hypoxic response [ ]. In 2014 Papas published an extensive literature review to establish the significance of oxygen during contact lens wear and summarised the minimum Dk/t values (barrers/cm) that had been reported to be required to avoid a range of contact lens induced changes in both the open and closed eye situations [ ]. More recently, Yeung et al. investigated the peripheral Dk/t and central Dk/t in soft contact lenses that would be needed to prevent corneal neovascularisation in healthy, myopic daily soft contact lens wearers [ ]. They reported that peripheral Dk/t values of 30–40 barrers/cm should protect most soft contact lens wearers from developing neovascularisation. This value is similar to that reported by Morgan et al. when reporting on the Dk/t required across a soft contact lens to prevent corneal oedema in open eye wear [ ].
It must be recognized that SiH materials are not always needed to meet the minimum oxygen requirements for daily contact lens wear, and that hydrogel contact lenses can provide sufficient oxygen to prevent corneal oedema, neovascularisation and limbal and bulbar hyperaemia for open eye wear in many patients [ ]. Indeed equivalence in biomicroscopy findings has been reported between SiH and hydrogel DD contact lenses in some recent studies [ , ]. Nonetheless, ECPs know that not all their “daily wear” patients are compliant with the product indications or directions for lens wear that they have been given, and that napping and sleeping while wearing contact lenses is extremely prevalent among adults, even in DD wearers. In the earlier referenced survey conducted by Dumbleton et al., 75 % reported having napped and 28 % having slept overnight, at least once in the preceding month, while wearing their DD lenses [ ]. The proportions wearing lenses overnight were highest in those aged 18–24. Other studies have reported napping while wearing lenses in 35%–87% of adult wearers [ , ]; and sleeping overnight while wearing lenses in 23%–50% of adult wearers [ , , ]. Even after a period of only one hour of closed eye wear, significantly greater corneal swelling has been reported with hydrogel contact lenses [ ]. Fortunately, non-compliance with respect to over-wear of DD contact lenses in children is expected to be relatively low, since wearing schedules are more likely to be closely monitored by parents, and this belief has been supported in a recent clinical study [ ].
In summary, signs of hypoxia are not generally observed as a significant complication of DD wear; however, in conventional hydrogel materials, thicker designs of contact lenses and over-wear, even with DDs, could potentially induce signs of hypoxia. Fortunately the introduction of SiH contact lenses, regardless of wearing modality, have been reported to effectively eliminate hypoxia for most patients [ ].
6
Material and surface properties of silicone hydrogel contact lenses
The development of SiH contact lens materials arose from the desire to eliminate the hypoxic responses that can result with hydrogel materials. The silicone component of these lens materials provides extremely high Dk, while the hydrogel component facilitates fluid transport and lens movement. The disadvantage of including siloxy groups is that they are hydrophobic [ , ]. Consequently, a number of different approaches have been used in an attempt to overcome the inherent wettability issues that are associated with hydrophobic materials. Since the first SiH contact lenses were launched in 1999, there have been marked changes in the properties of these lenses, particularly with respect to water content and modulus [ ]. Furthermore, hydrophobicity of the contact lens surfaces can result in increased lipid deposition on some SiH materials [ ] and although there may be less protein (lysozyme) deposition when compared with hydrogel lenses [ , ], a greater proportion of the protein deposited may be denatured [ ]. Contact lens deposits can significantly influence on-eye performance and lipid deposits in particular can destabilise the tear film [ ]; however, lysozyme does not appear to have an impact on contact lens wettability [ ].
In general, SiH contact lenses are of higher modulus (stiffness) than their hydrogel counterparts [ , ]. Higher modulus materials can cause greater shear forces which have been shown to be relevant to in-eye contact lens behaviour [ ]. This may also account for the higher incidence of mechanical complications that were reported with the first generation of SiH contact lenses. These complications include superior epithelial arcuate lesions, corneal erosions, contact lens papillary conjunctivitis, mucin balls, conjunctival flaps and changes in corneal curvature [ ]. Over time the properties of the SiH contact lens materials have evolved, with a general trend to reduce the modulus of these materials to a level that is more consistent with conventional hydrogel materials. Soft contact lenses with extremely low moduli are also not desirable, since these can be associated with greater fragility and difficulty with handling [ ].
During soft contact lens wear, there are mechanical interactions between the contact lens and the ocular tissues. These interactions are influenced by both the tear film, and the mechanical properties of the contact lens, including elasticity and friction [ ]. As explained previously, SiH materials can create greater sheer forces than hydrogels, due to their greater elasticity; however, they may also have lower coefficients of friction which can to some extent offset this elastic effect [ ]. A detailed explanation of the surface and mechanical properties of SiH contact lenses, and their role in compatibility with the ocular tissues during contact lens wear, is beyond the scope of this manuscript, however the reader is directed towards the publications by Tighe, and Bhamra and Tighe, for more information [ , ].
Wettability is the ability for fluid to adhere to the contact lens surface and good wettability is important to reduce deposition and provide clear vision and maintain comfort during wear [ ]. Once again, a detailed review of the wettability of SiH contact lenses is beyond the scope of this manuscript, but a poorly wetting contact lens has been reported to result in an unstable tear film [ , ] and many studies have shown that contact lenses reduce the stability of the pre-lens tear film [ ]. The strongest link to comfort during contact lens wear has been reported to be tear film stability [ ]. Significant efforts have been made to minimize the impact of SiH materials on the tear film by making the surfaces more wettable; to date, these include surface treatments, internal and external wetting agents, water gradient surfaces and the incorporation of hydrophilic monomers [ ]. As a result, the on-eye wettability of SiH materials has been shown to be very similar to that typically observed with hydrogel materials [ , ].
In summary, current SiH contact lenses have, for the most part, overcome the hydrophobic nature of these materials: however, lipid deposition can still be an issue. A DD modality is therefore ideal for SiH contact lenses and the lower modulus of the newer generation of SiH DD lenses has also helped to reduce mechanical complications, while still maintaining ease of handling for contact lens insertion and removal. Furthermore, it is widely recognized that excellent wettability is required to maintain a stable tear film, for both comfort and clear vision, and currently available SiH DD lenses are able to afford this characteristic.
7
Myth busting with respect to silicone hydrogel materials
A number of myths continue to persist with respect to SiH contact lenses. The first relates to comfort; the reader will no doubt have read or heard that “SiH lenses may be less comfortable because of their higher modulus”, or that “SiH lenses provide more oxygen and are therefore more comfortable than hydrogel lenses”. Several factors have been reported to be associated with improved contact lens comfort including edge design, tightness of fit and possibly coefficient of friction, among others [ ]; however, the evidence with respect to differences in comfort between SiH and hydrogel reusable materials has been equivocal [ , ]. This is largely due to there being so many confounding variables, which are almost impossible to overcome when comparing comfort with different lens materials and designs [ ].
There have been few studies conducted in which the comfort of SiH DD contact lenses has been specifically compared with hydrogel DD contact lenses; however in a study conducted by Shah et al., significantly higher overall comfort and less dryness were reported with stenfilcon A (SiH) than with etafilcon (hydrogel) DD contact lenses [ ]. A more recent study evaluated comfortable wearing time and comfort during the day with delefilcon A (SiH) and nelfilcon A (hydrogel) contact lenses and reported superior comfort when the SiH lenses were worn [ ]. The findings from these studies are not in agreement with a more recent retrospective analysis of data collected during a series of studies, where no difference in comfortable wearing time, or comfort on insertion, during the day or at the end of the day was found between wearers of SiH and hydrogel DD contact lenses [ ]. Another recent study compared the short-term comfort of SiH and a hydrogel DD contact lenses with similar water contents and also reported no difference with respect to comfort [ ].
In the previously described survey that was conducted among 300 ECPs in the United States, United Kingdom and Japan, 88 % agreed with the statement “Silicone hydrogel 1 day lenses provide better long term wearing comfort for my patients than hydrogel 1 day lenses” and 80 % agreed with the statement “Silicone hydrogel 1 day lenses are more comfortable than hydrogel 1 day lenses” [ ]. So the perception by ECPs appears to be that SiH DD contact lenses can offer superior comfort to their hydrogel counterparts, but to date, the evidence in the literature does not strongly support this belief.
A second common myth relating to SiH contact lens materials is that some patients may be allergic to the silicone in SiH contact lenses. Indeed 57 % of ECPs in the aforementioned survey disagreed with the statement: “There is no such thing as allergy to silicone in silicone hydrogel materials”, suggesting that this “urban myth” still exists, among some ECPs. Despite its widespread used in many fields, including medicine, true allergies to silicone are extremely rare and there is no strong evidence in the literature to support allergic responses to SiH contact lenses [ ]. It is more likely that the inflammatory responses that have been reported to occur in some patients when wearing SiH contact lenses are actually attributable to inflammatory reactions occurring in response to antigens accumulating on or within these silicone biomaterials [ ].
8
Meeting patient needs with contemporary contact lens materials and modalities
There are currently at least six SiH DD contact lenses available on the market, delefilcon A (spherical and multifocal designs), narafilcon A (spherical design), senofilcon A (spherical and toric designs), somofilcon A (spherical, toric and multifocal designs), stenfilcon A (spherical and toric designs) and verofilcon A (spherical design). The properties of these lenses are summarized in Table 1 , which has been adapted from the version originally published in Contact Lenses, sixth edition, edited by Phillips and Speedwell [ ]. This table is not meant to represent an exclusive listing of SiH DD contact lenses and additional products have been introduced in some countries. Initially there was restricted availability of parameters of SiH DD contact lenses, which may have prevented ECPs from selecting these lenses when fitting their patients, but now spherical, toric and multifocal lens designs are available in a wide range of parameters.
