It has been two decades since the introduction of daily disposable (DD) contact lenses and it seems that eye care practitioners have finally spoken, judging by the trends in prescribing habits. According to a recent survey DD accounted for at least 40 per cent of worldwide contact lens sales in 20121 and Morgan et al reported that DDs comprised of 33 per cent of all lens fits.2 This was the average of the 36 countries compared to 17 per cent in the US.1
For years now DD prescribing in the US seems to have lagged mysteriously behind countries such as the UK and Norway and Japan. This could be attributed to the popularity of Silicone Hydrogel (SiH) lenses in the US which until fairly recently were not available in DD form. However that might change quite substantially in the near future as the survey conducted by Nichols reported that 64 per cent of respondents anticipate prescribing SiH DD this year.1
Reusable SiH lenses account for at least 50 per cent of all prescribed soft lenses.2 Walker3 reported that SiH DD lenses only represented 11 per cent of the DD market in Europe about three years ago which is understandable as there was only one SiH DD lens available, but with the availability of four SiH DD lenses now, expect both SiH and DD lens categories to grow substantially.
DD silicone hydrogel lenses
Any contact lens should fulfil the following criteria: it should be physiologically compatible with the eye (cause no or minimal ocular changes), should not soil, be safe, provide excellent vision, should be comfortable for all wearing hours and of course convenient for patients to use. Lofty goals? This short description will act as a report card of how well DD lenses fulfill those criteria.
Physiological compatibility
Holden and Mertz suggested that day time corneal swelling can be eliminated by using lenses with an average oxygen transmissibility of at least 24x10-9ref4 or according to Harvitt and Bonanno 35x10-9 using stromal oedema as an index.5 While there are many other physiological compatibility considerations that are unrelated to hypoxia, eliminating low levels of chronic corneal swelling may minimise the risk of secondary complications such as endothelial, refractive and corneal topography changes. One clinical report which indirectly casts some doubt on the 24x10-9 value is by Normura et al who found a 30 per cent prevalence of corneal neovascularisation in disposable hydrogel lens wearers (Etafilcon and Nefilcon lenses) compared to 7 per cent of RGP lens wearers.6 Although the details of the lenses or the lens-wearing history of these patients are not provided, the Dk/t of the hydrogel lenses must have been close to the 24 value.
The material properties of four SiH DD lenses have been described previously.3,7 For any given patient, oxygen transmissibility (Dk/t) is the more applicable parameter, not barrers or permeability. On the assumption that the thickest lens for any of these four materials might have an average transmissibility of 20 per cent less than the Dk values in Table 1, they would still exceed the minimum values of 24 or 35 Dk/t for daily wear.
One of the few reports on DD SiH lenses, compared the clinical response of neophytes wearing narafilcon A (1 Day Acuvue TruEye) lenses after one year, to a non-wearing control group.8 It showed that bulbar and limbal hyperaemia, corneal staining, and papillary conjunctivitis were clinically equivalent for the two groups with only conjunctival staining being higher in the lens wearing group. If the same results are achieved with other SiH DD lenses, this category will be considered as a game changer.
Compliance and convenience
A repeatable feature of patient noncompliance is failure to replace lenses as recommended. Dumbleton et al’s noncompliance rates were lowest for DD (12 per cent), compared to 28 per cent for one month replacement schedules and 52 per cent for two-week replacement.9 The ratios reported by Nichols1 and Yeung10 were very much the same. One would have to assume that eliminating the need for and provision of cleaning, disinfecting solutions and storage cases, the use of DD lenses would contribute to increasing compliance because of convenience.
Safety
The term safety is most often used in the context of ocular health. Contact lens wear has the propensity to cause corneal pathology such as infiltrates and corneal infections. One strategy to minimise the incidence of infiltrates is by prescribing DD lenses. Although the evidence for this statement is scant, it is compelling and almost obvious. Chalmers et al found in a case control study that the relative risk of developing corneal infiltrates (the infiltrate category was broadly inclusive) was 12.5 times greater when using re-usable daily wear soft lenses compared to DD lenses.11 While this is an illuminating result, it is not an incidence study, so further epidemiological research is urgently required. Some indirect evidence is provided by the findings of Stapleton et al that storage case hygiene, infrequent storage case replacement and solutions increased the risk of moderate and severe microbial keratitis.12 The implication is that DD modality will eliminate these risk factors provided that patients are
compliant. Less encouraging is the Stapleton and Carnt analysis that DD lenses carry the same risk of infection as other daily wear soft lenses but at least the disease is less severe with DD lenses.13
Lens comfort
Researchers, clinicians and lens manufacturers have been trying to fathom the cause of discomfort induced by lenses and more specifically end of day discomfort. One example of the attempts to understand the problem is the Investigative Ophthalmology and Visual Science November 2013 special issue publication of the Tear Film and Ocular Surface Society workshop on Contact Lens Discomfort.
There have been a few studies which have demonstrated an improvement in comfort at the end of day. In the paper by Morgan et al mentioned earlier they found that comfort scores were equivalent between the DD lens wearers and non-lens wearers and surprisingly there was no decrement in comfort towards the end of the day in either group.8 The results of a registry trial presented by Chalmers et al also showed favourable results for DD lenses in that comfortable wearing time increased from re-usable habitual soft lens wear when patients were switched to DD lenses, but not for all categories.14 While these two results appear encouraging, both were confined to one type of DD SiH lens and judgment should be reserved until other types of DD SiH lenses are studied in randomised, masked and controlled clinical trials.
This report card must be a resounding pass for DD lenses as they are approaching panacea status. The materials contain sufficient silicone, resulting in a balance between oxygen permeability and modulus of elasticity to make the lens easy to handle. At the very least, minimising complications and improving compliance should far outweigh reservations that some practitioners may have about the cost and the potential of losing patients because of the perception that they do not need to return for eye care
References
1 Nichols JJ. Contact Lenses 2012. Contact Lens Spectrum, 2013; 28 (1): 24-29,52.
2 Morgan PB et al. International Contact Lens Prescribing in 2012. Contact Lens Spectrum, 2013; 28 (1): 31-38,44,
3 Walker J. Achieving success with daily disposable lenses. Optician, 2013; 246 (6426): 33-36
4 Holden BA, Mertz GW. Critical oxygen levels to avoid corneal edema for daily and extended wear contact lenses. Invest Ophthalmol and Vis Sci, 1984; 25:1161–7.
5 Harvitt DM and Bonanno JA. Re-evaluation of the oxygen diffusion model for predicting minimum contact lens Dk/t values needed to avoid corneal anoxia. Optom Vis Sci, 1999;76:712-719.
6 Nomura, K. Nakao, M, and Matsubara K. Subjective symptom of eye dryness and lifestyle factors with corneal neovascularization in contact lens wearers. Eye & Contact Lens. 2004; 30: 95-98.
7 Shah D, Richardson P and Vega J. Fitting MyDay into practice. Optician, 2013; 246:6418 12-16.
8 Morgan PB, Chamberlain P, Moody K, Maldonado-Codina C. Ocular physiology and comfort in neophyte subjects fitted with daily disposable silicone hydrogel contact lenses. Cont Lens Anterior Eye, 2013;36:118-25.
9 Dumbleton K, Woods C, Jones L et al. Patient and practitioner compliance with silicone hydrogel and daily disposable lens replacement in the US. Eye & Contact Lens, 2009;35:164-171.
10 Yeung K, Forister J, Forister E, et al. Compliance with soft contact lens replacement schedules and associated contact lens-related ocular complications: The UCLA Contact Lens Study. Optometry, 2010;81:598-607.
11 Chalmers RL, Keay L, McNally J, Kern J. Multicenter case-control study of the role of lens materials and care products on the development of corneal infiltrates. Optom Vis Sci, 2012;89:316-25.
12 Stapleton F, Edwards K, Keay L, et al. Risk factors for moderate and severe microbial keratitis in daily wear contact lens users. Ophthalmology, 2012;119:1516-21.
13 Stapleton F, Carnt N. Contact lens-related microbial keratitis: How have epidemiology and genetics helped us with pathogenesis and prophylaxis. Eye, 2012;26:185-93.
14 Chalmers RL, Hickson-Curran S, Keay L, et al. Struggle with soft contact lens wear is addressed by refitting with daily disposable lenses: 4 Month Follow-up from the TEMPO Registry. Invest Ophthalmol Vis Sci, 2013; 54: E-Abstract 5458.
? Desmond Fonn is Distinguished Professor Emeritus at the School of Optometry and Vision Science and Founding Director, Centre for Contact Lens Research, University of Waterloo
