C L I N I C A L
A N D
E X P E R I M E N T A L
OPTOMETRY EDITORIAL
Why are we still fitting reusable soft contact lenses? Clin Exp Optom 2014; 97: 386–388
Nathan Efron PhD DSc Institute of Health and Biomedical Innovation and School of Optometry and Vision Science, Queensland University of Technology, Kelvin Grove, Australia E-mail: [email protected]
Whether the first daily disposable soft contact lens to enter the market in 1994 was the Premier lens (Award Technology, Scotland, UK; subsequently purchased by Bausch & Lomb, Rochester New York, USA) or the 1-Day Acuvue lens (Johnson and Johnson Vision Care, Jacksonville, Florida, USA) has long been a matter of bitter dispute1 but whatever the answer, this year marks the 20th anniversary of the launch of this modality of lens wear. The concept of daily disposability has been fully embraced by the contact lens industry. All of the major global contact lens companies and many smaller regional companies now manufacture a variety of types of daily disposable lenses. They are produced in hydrogel and silicone hydrogel materials and are available in spherical, toric and multifocal designs. At the present time in Australia, 25 daily disposable lens products are available—19 spherical, four toric and two multifocal designs.2 Daily disposable lenses are manufactured in a wide range of parameters; for example, 1-Day Acuvue Moist for Astigmatism (Johnson and Johnson Vision Care, Jacksonville, Florida, USA) is available in spherical powers from +4.00 to -9.00 D, four cylindrical powers from -0.75 to -2.25 DC and 10 cylinder axes.2 As such, only a tiny minority of potential contact lens wearers would have refractive errors that fall outside these currently available daily disposable lens parameters. The fifth largest global contact lens company—Sauflon Pharmaceuticals (Twickenham, United Kingdom)—has recently repositioned its strategic direction by almost exclusively manufacturing daily disposable lenses.3 This has been achieved by consolidating all of their daily disposable lens manufacturing into a large-scale facility
DOI:10.1111/cxo.12170
in Hungary,3 which every day produces over 150 million silicone hydrogel daily disposable lenses in spherical, toric and multifocal designs. Despite the capacity for virtually all ametropic contact lens wearers to be fitted with daily disposable lenses, an international survey of contact lens prescribing in 31 markets during 2013 revealed that these lenses constitute only 31 per cent of fits4 (38 per cent in Australia4). In some markets, the vast majority of fits are with daily disposable lenses; for example, in Denmark, 65 per cent of all soft lens fits are with daily disposable lenses—a value that rises to 87 per cent if only daily wear spherical lenses are considered.4 To my way of thinking, the benefits of daily disposable lenses compared to reusable lenses are overwhelming. This leads to the obvious questions—why are we still fitting reusable soft contact lenses? Or considering the antithesis—why are we not fitting all contact lens wearers with daily disposable lenses? Let me first consider the benefits of daily disposable lenses and then I shall return to this question.
CONVENIENCE FOR LENS WEARERS The convenience that daily disposability offers to lens wearers is perhaps the most obvious benefit and does not really need to be spelled out to the informed readership of this journal, but I shall restate the key issues for completeness. Wearers of daily disposable lenses do not need to concern themselves with tedious lens care, as is required with reusable lenses, notwithstanding the fact that lens maintenance procedures have become somewhat simplified with multipurpose solutions. The requirements for possessing and caring for lens cases are obviated. Daily disposable lenses are easy to discard (‘any time, any place, without a case’). They are excellent for monovision correction of presbyopia, as it is easy to
Clinical and Experimental Optometry 97.5 September 2014
386
alternate between various lens combinations. Daily disposable lenses are also especially useful for part-time wear, such as for sporting activities. Compliance is easier because there are fewer instructions to remember. Daily disposable lenses are convenient for travel, as there is no need to carry bulky lens-care solutions. Packaging of daily disposable lenses has become more compact in recent times, with a significant advance being the wafer-thin ‘flat pack’ (Miru, Menicon, Japan), which is 12.5 per cent of the thickness and 40 per cent of the volume of a conventional disposable lens pack.5
ADVANTAGES FOR PRACTITIONERS There are many advantages of daily disposable lenses over reusable lenses from the standpoint of the practitioner. Less ‘chair time’ is required for patient education, as virtually no advice needs to be offered concerning lens care. Less chair time is incurred due to unscheduled visits relating to problems with lens care solutions (for example, solution-induced corneal staining) or to patient non-compliance with the use of lens care systems. Less ancillary staff time is required because there is no need for discussions and sales advice relating to lens care products. Lens wearers can be supplied with an initial starter set of five pairs of lenses. Practitioners can work with contact lens manufacturers to effect timely and efficient lens supply either via the practice or directly to the lens wearer’s home.
ENHANCED VISION The early literature showed that with the prolonged wear of reusable soft lenses, deposits such as protein and lipids gradually build on the lens surface despite daily lens cleaning.6 The greater the extent of these deposits, the greater is the amount © 2014 The Author
Clinical and Experimental Optometry © 2014 Optometrists Association Australia
of visual degradation,6 although it is true that reduced vision over four weeks of lens wear—which today is essentially the longest time for lens wear prior to lens replacement—would be difficult to detect with a letter chart. Nevertheless, it stands to reason that if vision decreases with increased wearing time, then vision can be optimised by more regular lens replacement, with daily disposability representing the optimum replacement frequency.
OCULAR HEALTH Early controversial reports of adverse ocular reactions to daily disposable lenses7 are now thought to have been confounded by skewed practitioner prescribing approaches and distorted consumer demand behaviour that can arise when new and exciting health products enter the market. When daily disposable lenses were introduced in 1994, they were initially available in limited supply and priced at a premium. Practitioners may have adopted a conservative approach and selected difficult or problem patients, who perhaps were considered to derive what was perceived to be the added health benefits of such a product. As well, lens wearers who are eager to try new untested products can be considered to be ‘risk takers’. Such persons may have accompanying personality and behavioural traits that make them more susceptible to be non-compliant with the principles underlying hygienic and meticulous lens handling and wear.8 These factors could explain the initial counter-intuitive findings that daily disposable lenses induced a higher rate of adverse reactions than reusable lenses.7 Perhaps the most sensitive indicator of an adverse ocular reaction to contact lens wear is the appearance of corneal infiltrative events, which represent a proinflammatory reaction. A more recent 2012 study, in which the potentially confounding effects of ‘problem-patient prescribing’ and ‘risk-taking early adopters’ are obviated, has demonstrated distinct health advantages of daily disposability. Chalmers and colleagues9 identified cases with symptomatic corneal infiltrative events in a retrospective, multi-centre case-controlled study at five academic eye care centres in the USA. They reported that reusable soft lenses carried a 12.5 times greater risk of developing such events compared with daily disposable lenses.
SO WHY NOT FIT DAILY DISPOSABLE LENSES TO EVERYONE? With the clear benefits of lens wearer convenience, ease of lens fitting and supply, avoidance of lens deposition and visual degradation, and superior ocular health, why bother fitting reusable soft lenses? ‘Cost’, I hear you shout! There is a common perception that daily disposable lenses are generally more expensive than equivalent reusable lenses, which limits the extent to which such lenses are fitted. Well, let us consider the cost issue from two perspectives—the broad perception of cost and its impact on prescribing daily disposable lenses, and whether these lenses really are more expensive than reusable lenses. A strong positive correlation has been demonstrated between the rate of prescribing of daily disposable lenses in different nations and the gross domestic product of those nations.10 The gross domestic product of a nation is a financial construct that essentially indicates the average personal wealth of individuals in that nation. Thus, the wealthier people are in a country, the more daily disposable contact lenses are fitted in that country. In fact, this analysis showed that 55 per cent of daily disposable lens prescribing can be explained by cost considerations.10 So, although other factors are involved in the decision to fit daily disposable lenses, cost is a major influence. Is it true that daily disposable lenses are more expensive than reusable lenses? This question has been tested with the aid of a ‘cost-per-wear’ model11 and the results of this analysis are revealing. Simply put, the ‘costper-wear’ model describes the cost of wearing any specified lens brand each time lenses are worn, in relation to the number of times lenses are worn, on average, each week. The model, which assumes that the lens wearer is compliant with the specified schedule of lens replacement and solution usage (if reusable lenses are being worn), takes into account all direct costs, including the cost of the lenses, lens care solutions and professional fees. The primary outcome of the model is that daily disposable lenses cost the same as reusable lenses when lenses are worn five days per week, and are less expensive when worn fewer days per week.11 Given that 55 per cent of those using daily disposable lenses wear their lenses five or less days per week, it is clear that for the majority of those wearing this modality, the costs of lens
© 2014 The Author Clinical and Experimental Optometry © 2014 Optometrists Association Australia
wear are actually less than for those wearing reusable lenses. So what about the increased cost to the minority of daily disposable lens wearers using lenses six or seven days per week (which can be described as ‘full time’ wear)? I have a simple answer for this. Compared to reusable lenses, the additional daily cost incurred as a result of full-time daily disposable lens wear is equivalent to the price of a cappuccino coffee served up at your local café. I would argue that this is a price worth paying for the sheer convenience, improved vision and superior ocular health of this form of lens wear. DISPELLING OTHER CONCERNS I have been espousing the virtues of daily disposibility at practitioner conferences for many years. Often, I pose the following question to an audience: ‘Putting aside the issue of cost and assuming that a lens wearer falls within the available range of parameters of currently available products, can you think of any reason why you might prescribe anything other than daily disposable lenses?’ This question is generally met with stony silence, although one issue of concern that has occasionally been raised is environmental impact. This refers to the notion that the amount of waste material entering the environment as a result of wearing daily disposable lenses is greater than that with reusable lenses. A study of the environmental impact of contact lens wear12 has confirmed that slightly more waste is generated from usage of daily disposable lenses compared with reusable lenses; however, the amounts we are dealing with are small, representing less then 0.5 per cent of average domestic household waste and insignificant when compared to environmental waste generated by major activities/projects, such as large-scale manufacturing, civil engineering programs (constructing roads and railways) et cetera.12 Some cite the threat of unregulated lens supply, such as via the internet, as a reason to avoid fitting daily disposable lenses. Of course, unregulated lens supply is of great concern in view of the potential for lens wearers to develop asymptomatic ocular pathology, which may remain undetected and lead to more serious ocular health problems.13 However, this issue is not specific to daily disposable lenses and therefore is not a valid reason for avoiding fitting of these lenses.
Clinical and Experimental Optometry 97.5 September 2014
387
CONCLUSION Contact lens practitioners and lens wearers alike derive far more benefit from daily disposable lenses than reusable lenses. When considered in terms of ‘cost per-wear’, the overall cost of daily disposable lenses to the lens wearer is not that much more, and in many instances is considerably less, than the cost of reusable lenses. I believe that it will not be long before the four major global contact lens companies follow the lead set by Sauflon Pharmaceuticals and gear up their production to exclusively produce daily disposable lenses. I also believe that it is now time for contact lens practitioners to follow the lead set by our Danish colleagues and fully embrace the concept of daily disposability, relegating reusable lenses to the annals of history. REFERENCES 1. Meyler J, Ruston D. The world’s first daily disposables. Optician 2006; 231 (6053): 12. 2. Contact Lenses Product Guide. Carlton, Australia: Optometrists Association Australia, 2013. 3. Brogan R. An eye for business. Optician 2010; October 1: 22–23. 4. Morgan PB, Woods CA, Tranoudis IG, Helland M, Efron N, Teufl IM, Grupcheva CN et al. International contact lens prescribing in 2013. Contact Lens Spectrum 2014; 29: 30–35. 5. Kell M. Discovering the revolutionary flat-pack contact lens. Mivision 2013; 86: 58–59. 6. Gellatly KW, Brennan NA, Efron N. Visual decrement with deposit accumulation on HEMA contact lenses. Am J Optom Physiol Opt 1988; 65: 937–941. 7. Dart JK, Radford CF, Minassian D, Verma S, Stapleton F. Risk factors for microbial keratitis with contemporary contact lenses: a case-control study. Ophthalmology 2008; 115: 1647–1654. 8. Carnt N, Keay L, Willcox M, Evans V, Stapleton F. Higher risk taking propensity of contact lens wearers is associated with less compliance. Cont Lens Anterior Eye 2011; 34: 202–206. 9. 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: 316–325. 10. Efron N, Morgan PB, Woods CA. The International Contact Lens Prescribing Survey Consortium. An international survey of daily disposable contact lens prescribing. Clin Exp Optom 2013; 96: 58–64. Erratum: 2013; 96: 250. 11. Efron N, Efron SE, Morgan PB, Morgan SL. A ‘cost-per-wear’ model based on contact lens replacement frequency. Clin Exp Optom 2010; 93: 253–260. 12. Morgan SL, Morgan PB, Efron N. Environmental impact of three replacement modalities of soft contact lens wear. Contact Lens Ant Eye 2003; 26: 43–46. 13. Wu Y, Carnt N, Stapleton F. Contact lens user profile, attitudes and level of compliance to lens care. Cont Lens Anterior Eye 2010; 33: 183–188.
Clinical and Experimental Optometry 97.5 September 2014
388
© 2014 The Author Clinical and Experimental Optometry © 2014 Optometrists Association Australia
A N D
E X P E R I M E N T A L
OPTOMETRY EDITORIAL
Why are we still fitting reusable soft contact lenses? Clin Exp Optom 2014; 97: 386–388
Nathan Efron PhD DSc Institute of Health and Biomedical Innovation and School of Optometry and Vision Science, Queensland University of Technology, Kelvin Grove, Australia E-mail: [email protected]
Whether the first daily disposable soft contact lens to enter the market in 1994 was the Premier lens (Award Technology, Scotland, UK; subsequently purchased by Bausch & Lomb, Rochester New York, USA) or the 1-Day Acuvue lens (Johnson and Johnson Vision Care, Jacksonville, Florida, USA) has long been a matter of bitter dispute1 but whatever the answer, this year marks the 20th anniversary of the launch of this modality of lens wear. The concept of daily disposability has been fully embraced by the contact lens industry. All of the major global contact lens companies and many smaller regional companies now manufacture a variety of types of daily disposable lenses. They are produced in hydrogel and silicone hydrogel materials and are available in spherical, toric and multifocal designs. At the present time in Australia, 25 daily disposable lens products are available—19 spherical, four toric and two multifocal designs.2 Daily disposable lenses are manufactured in a wide range of parameters; for example, 1-Day Acuvue Moist for Astigmatism (Johnson and Johnson Vision Care, Jacksonville, Florida, USA) is available in spherical powers from +4.00 to -9.00 D, four cylindrical powers from -0.75 to -2.25 DC and 10 cylinder axes.2 As such, only a tiny minority of potential contact lens wearers would have refractive errors that fall outside these currently available daily disposable lens parameters. The fifth largest global contact lens company—Sauflon Pharmaceuticals (Twickenham, United Kingdom)—has recently repositioned its strategic direction by almost exclusively manufacturing daily disposable lenses.3 This has been achieved by consolidating all of their daily disposable lens manufacturing into a large-scale facility
DOI:10.1111/cxo.12170
in Hungary,3 which every day produces over 150 million silicone hydrogel daily disposable lenses in spherical, toric and multifocal designs. Despite the capacity for virtually all ametropic contact lens wearers to be fitted with daily disposable lenses, an international survey of contact lens prescribing in 31 markets during 2013 revealed that these lenses constitute only 31 per cent of fits4 (38 per cent in Australia4). In some markets, the vast majority of fits are with daily disposable lenses; for example, in Denmark, 65 per cent of all soft lens fits are with daily disposable lenses—a value that rises to 87 per cent if only daily wear spherical lenses are considered.4 To my way of thinking, the benefits of daily disposable lenses compared to reusable lenses are overwhelming. This leads to the obvious questions—why are we still fitting reusable soft contact lenses? Or considering the antithesis—why are we not fitting all contact lens wearers with daily disposable lenses? Let me first consider the benefits of daily disposable lenses and then I shall return to this question.
CONVENIENCE FOR LENS WEARERS The convenience that daily disposability offers to lens wearers is perhaps the most obvious benefit and does not really need to be spelled out to the informed readership of this journal, but I shall restate the key issues for completeness. Wearers of daily disposable lenses do not need to concern themselves with tedious lens care, as is required with reusable lenses, notwithstanding the fact that lens maintenance procedures have become somewhat simplified with multipurpose solutions. The requirements for possessing and caring for lens cases are obviated. Daily disposable lenses are easy to discard (‘any time, any place, without a case’). They are excellent for monovision correction of presbyopia, as it is easy to
Clinical and Experimental Optometry 97.5 September 2014
386
alternate between various lens combinations. Daily disposable lenses are also especially useful for part-time wear, such as for sporting activities. Compliance is easier because there are fewer instructions to remember. Daily disposable lenses are convenient for travel, as there is no need to carry bulky lens-care solutions. Packaging of daily disposable lenses has become more compact in recent times, with a significant advance being the wafer-thin ‘flat pack’ (Miru, Menicon, Japan), which is 12.5 per cent of the thickness and 40 per cent of the volume of a conventional disposable lens pack.5
ADVANTAGES FOR PRACTITIONERS There are many advantages of daily disposable lenses over reusable lenses from the standpoint of the practitioner. Less ‘chair time’ is required for patient education, as virtually no advice needs to be offered concerning lens care. Less chair time is incurred due to unscheduled visits relating to problems with lens care solutions (for example, solution-induced corneal staining) or to patient non-compliance with the use of lens care systems. Less ancillary staff time is required because there is no need for discussions and sales advice relating to lens care products. Lens wearers can be supplied with an initial starter set of five pairs of lenses. Practitioners can work with contact lens manufacturers to effect timely and efficient lens supply either via the practice or directly to the lens wearer’s home.
ENHANCED VISION The early literature showed that with the prolonged wear of reusable soft lenses, deposits such as protein and lipids gradually build on the lens surface despite daily lens cleaning.6 The greater the extent of these deposits, the greater is the amount © 2014 The Author
Clinical and Experimental Optometry © 2014 Optometrists Association Australia
of visual degradation,6 although it is true that reduced vision over four weeks of lens wear—which today is essentially the longest time for lens wear prior to lens replacement—would be difficult to detect with a letter chart. Nevertheless, it stands to reason that if vision decreases with increased wearing time, then vision can be optimised by more regular lens replacement, with daily disposability representing the optimum replacement frequency.
OCULAR HEALTH Early controversial reports of adverse ocular reactions to daily disposable lenses7 are now thought to have been confounded by skewed practitioner prescribing approaches and distorted consumer demand behaviour that can arise when new and exciting health products enter the market. When daily disposable lenses were introduced in 1994, they were initially available in limited supply and priced at a premium. Practitioners may have adopted a conservative approach and selected difficult or problem patients, who perhaps were considered to derive what was perceived to be the added health benefits of such a product. As well, lens wearers who are eager to try new untested products can be considered to be ‘risk takers’. Such persons may have accompanying personality and behavioural traits that make them more susceptible to be non-compliant with the principles underlying hygienic and meticulous lens handling and wear.8 These factors could explain the initial counter-intuitive findings that daily disposable lenses induced a higher rate of adverse reactions than reusable lenses.7 Perhaps the most sensitive indicator of an adverse ocular reaction to contact lens wear is the appearance of corneal infiltrative events, which represent a proinflammatory reaction. A more recent 2012 study, in which the potentially confounding effects of ‘problem-patient prescribing’ and ‘risk-taking early adopters’ are obviated, has demonstrated distinct health advantages of daily disposability. Chalmers and colleagues9 identified cases with symptomatic corneal infiltrative events in a retrospective, multi-centre case-controlled study at five academic eye care centres in the USA. They reported that reusable soft lenses carried a 12.5 times greater risk of developing such events compared with daily disposable lenses.
SO WHY NOT FIT DAILY DISPOSABLE LENSES TO EVERYONE? With the clear benefits of lens wearer convenience, ease of lens fitting and supply, avoidance of lens deposition and visual degradation, and superior ocular health, why bother fitting reusable soft lenses? ‘Cost’, I hear you shout! There is a common perception that daily disposable lenses are generally more expensive than equivalent reusable lenses, which limits the extent to which such lenses are fitted. Well, let us consider the cost issue from two perspectives—the broad perception of cost and its impact on prescribing daily disposable lenses, and whether these lenses really are more expensive than reusable lenses. A strong positive correlation has been demonstrated between the rate of prescribing of daily disposable lenses in different nations and the gross domestic product of those nations.10 The gross domestic product of a nation is a financial construct that essentially indicates the average personal wealth of individuals in that nation. Thus, the wealthier people are in a country, the more daily disposable contact lenses are fitted in that country. In fact, this analysis showed that 55 per cent of daily disposable lens prescribing can be explained by cost considerations.10 So, although other factors are involved in the decision to fit daily disposable lenses, cost is a major influence. Is it true that daily disposable lenses are more expensive than reusable lenses? This question has been tested with the aid of a ‘cost-per-wear’ model11 and the results of this analysis are revealing. Simply put, the ‘costper-wear’ model describes the cost of wearing any specified lens brand each time lenses are worn, in relation to the number of times lenses are worn, on average, each week. The model, which assumes that the lens wearer is compliant with the specified schedule of lens replacement and solution usage (if reusable lenses are being worn), takes into account all direct costs, including the cost of the lenses, lens care solutions and professional fees. The primary outcome of the model is that daily disposable lenses cost the same as reusable lenses when lenses are worn five days per week, and are less expensive when worn fewer days per week.11 Given that 55 per cent of those using daily disposable lenses wear their lenses five or less days per week, it is clear that for the majority of those wearing this modality, the costs of lens
© 2014 The Author Clinical and Experimental Optometry © 2014 Optometrists Association Australia
wear are actually less than for those wearing reusable lenses. So what about the increased cost to the minority of daily disposable lens wearers using lenses six or seven days per week (which can be described as ‘full time’ wear)? I have a simple answer for this. Compared to reusable lenses, the additional daily cost incurred as a result of full-time daily disposable lens wear is equivalent to the price of a cappuccino coffee served up at your local café. I would argue that this is a price worth paying for the sheer convenience, improved vision and superior ocular health of this form of lens wear. DISPELLING OTHER CONCERNS I have been espousing the virtues of daily disposibility at practitioner conferences for many years. Often, I pose the following question to an audience: ‘Putting aside the issue of cost and assuming that a lens wearer falls within the available range of parameters of currently available products, can you think of any reason why you might prescribe anything other than daily disposable lenses?’ This question is generally met with stony silence, although one issue of concern that has occasionally been raised is environmental impact. This refers to the notion that the amount of waste material entering the environment as a result of wearing daily disposable lenses is greater than that with reusable lenses. A study of the environmental impact of contact lens wear12 has confirmed that slightly more waste is generated from usage of daily disposable lenses compared with reusable lenses; however, the amounts we are dealing with are small, representing less then 0.5 per cent of average domestic household waste and insignificant when compared to environmental waste generated by major activities/projects, such as large-scale manufacturing, civil engineering programs (constructing roads and railways) et cetera.12 Some cite the threat of unregulated lens supply, such as via the internet, as a reason to avoid fitting daily disposable lenses. Of course, unregulated lens supply is of great concern in view of the potential for lens wearers to develop asymptomatic ocular pathology, which may remain undetected and lead to more serious ocular health problems.13 However, this issue is not specific to daily disposable lenses and therefore is not a valid reason for avoiding fitting of these lenses.
Clinical and Experimental Optometry 97.5 September 2014
387
CONCLUSION Contact lens practitioners and lens wearers alike derive far more benefit from daily disposable lenses than reusable lenses. When considered in terms of ‘cost per-wear’, the overall cost of daily disposable lenses to the lens wearer is not that much more, and in many instances is considerably less, than the cost of reusable lenses. I believe that it will not be long before the four major global contact lens companies follow the lead set by Sauflon Pharmaceuticals and gear up their production to exclusively produce daily disposable lenses. I also believe that it is now time for contact lens practitioners to follow the lead set by our Danish colleagues and fully embrace the concept of daily disposability, relegating reusable lenses to the annals of history. REFERENCES 1. Meyler J, Ruston D. The world’s first daily disposables. Optician 2006; 231 (6053): 12. 2. Contact Lenses Product Guide. Carlton, Australia: Optometrists Association Australia, 2013. 3. Brogan R. An eye for business. Optician 2010; October 1: 22–23. 4. Morgan PB, Woods CA, Tranoudis IG, Helland M, Efron N, Teufl IM, Grupcheva CN et al. International contact lens prescribing in 2013. Contact Lens Spectrum 2014; 29: 30–35. 5. Kell M. Discovering the revolutionary flat-pack contact lens. Mivision 2013; 86: 58–59. 6. Gellatly KW, Brennan NA, Efron N. Visual decrement with deposit accumulation on HEMA contact lenses. Am J Optom Physiol Opt 1988; 65: 937–941. 7. Dart JK, Radford CF, Minassian D, Verma S, Stapleton F. Risk factors for microbial keratitis with contemporary contact lenses: a case-control study. Ophthalmology 2008; 115: 1647–1654. 8. Carnt N, Keay L, Willcox M, Evans V, Stapleton F. Higher risk taking propensity of contact lens wearers is associated with less compliance. Cont Lens Anterior Eye 2011; 34: 202–206. 9. 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: 316–325. 10. Efron N, Morgan PB, Woods CA. The International Contact Lens Prescribing Survey Consortium. An international survey of daily disposable contact lens prescribing. Clin Exp Optom 2013; 96: 58–64. Erratum: 2013; 96: 250. 11. Efron N, Efron SE, Morgan PB, Morgan SL. A ‘cost-per-wear’ model based on contact lens replacement frequency. Clin Exp Optom 2010; 93: 253–260. 12. Morgan SL, Morgan PB, Efron N. Environmental impact of three replacement modalities of soft contact lens wear. Contact Lens Ant Eye 2003; 26: 43–46. 13. Wu Y, Carnt N, Stapleton F. Contact lens user profile, attitudes and level of compliance to lens care. Cont Lens Anterior Eye 2010; 33: 183–188.
Clinical and Experimental Optometry 97.5 September 2014
388
© 2014 The Author Clinical and Experimental Optometry © 2014 Optometrists Association Australia
