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Contact Lens Solutions: Time to Talk to Patients?

2M CPD in Ausrtalia | 1G in New Zealand | 8 December 2018

By Joshua Clark

With around 40 per cent of contact lens patients in Australia and New Zealand wearing fortnightly or monthly contact lenses1, thorough contact lens cleaning protocols are essential to reduce the risks of ocular infection and vision loss. To provide the best advice, we need access to up to date knowledge of the various disinfecting solutions available in order to understand which solution to recommend to individual patients, based on the contact lens they wear and their specific ocular health needs.

LEARNING OBJECTIVES

  1. Understand how multipurpose solutions are tested.
  2. Gain awareness of recent studies that have compared popular MPSs.
  3. Gain knowledge to educate patients about how to prevent infection and vision loss when using contact lenses.
  4. Understand the different contact lens cleaning solutions available, which to recommend and why.
  5. Realise the advantages of prescribing a particular contact lens with a particular disinfection solution.

 

It seems as though there are always new contact lenses coming onto the market. In particular, there is a plethora of daily disposable lenses that we can choose from.

With all of the daily disposable options available, it begs the question: why should we talk to patients about fortnightly or monthly disposable contact lenses? Furthermore, why should we talk about the difference in contact lens multipurpose disinfection solutions (MPSs)?

When considering these questions, I searched my contact lens patient database and found that approximately 40 per cent were using fortnightly or monthly lenses. I was quite surprised with this figure. Even though we may think that we mostly prescribe daily contact lenses, the truth is that fortnightly or monthly contact lens wearers make up approximately 40 per cent of the Australian and New Zealand contact lens market.1 Therefore, understanding our options with contact lens solutions becomes vitally important to our practices and our contact lens patients.

The next question is, which contact lenses should we choose? I am sure that we have all seen corneal staining tables that compare different contact lens and MPS combinations, but do we understand the mechanics of the different solutions? For example, Opti-Free Puremoist, in combination with Acuvue Oasys, gives 2 per cent corneal staining at two hours, while Opti-Free Puremoist, in combination with Biofinity lenses, gives 1 per cent cornea staining at two hours.2 Why is there a difference? What combinations should we prescribe to our patients? I think we would all know that there is difference between MPSs and hydrogen peroxide solutions. However, do we know the differences between the main MPSs on the market? A section of this article will explain the mechanism behind Bausch and Lomb’s cleaning solution Biotrue. By understanding the solution, we as optometrists can be more informed as to whether or not we should recommend it to our patients.

Disinfection Efficacy

The efficacies of multipurpose solutions are tested through the US Food and Drugs Association (FDA) and the International Standards Organisation (ISO) standalone protocol.3,4 The solution passes if there is a 90 per cent (1-log) reduction in fungi activity and 99.95 per cent (3-log) reduction in bacteria at the end of the minimum soak time.4

The US FDA and ISO both use the American Type Culture Collection (ATCC), which consists of three bacteria and two fungi to measure disinfection ability. The organisms tested are 2 gram negative bacteria (Pseudomonas aeruginosa, Serratia marcescens), 1 gram positive bacteria (Staphylococcus aureus), yeast (Candida albicans) and mould (Fusarium solani). For a contact lens solution to have an approvable level of disinfection, it must pass the standalone test or the regimen test.5

In Focus MPS: Biotrue from Bausch and Lomb

Biotrue MPS is the latest cleaning and disinfecting solution from Bausch and Lomb. Biotrue draws inspiration from the biology of the eyes by matching the pH of healthy tears, using a lubricant found in eyes and keeping certain beneficial tear proteins active.6

pH

The pH of different contact lens solutions is vital to promote comfort upon insertion. Biotrue uses sodium borate and boric acid to set the pH of the solution at 7.5, which matches the pH of healthy tears. These two chemicals provide chemical stability in resisting pH changes, which helps to maintain the eye’s natural condition. The pH of some contact lens solutions on the market falls outside the range of 7.3 –7.7, which can cause stinging upon insertion and discomfort throughout wearing.6

Contact Lens Hydration

The interaction of the contact lens with the tear film and eyelids is vital for a patient’s comfort when wearing contact lenses.6,7 Biotrue uses Hyaluronan (HA), the same moisture-loving molecule found in our eyes, to hydrate a lens.6 Considering that HA is the eye’s natural lubricant, it makes sense to use it in a MPS. HA is a naturally occurring glycosaminoglycan, and has also been shown to have anti-inflammatory properties, play a role in wound healing and have a protective effect against oxidative damage.6,8 The degree of hydration is shown by the fact that each molecule can hold up to 1,000 times its weight in water. HA therefore keeps corneal epithelium hydrated and stabilises the preocular tear film. When the eye is open, tears are more viscous and spread over the cornea, promoting tear film stability and increasing tear break up time. The blink then spreads HA out amongst the tear film.6

Figure 1. Percentage of protein activity in various contact lenses

 

 

Figure 2. Mean log reduction of clinical isolates of S. aureus, MRSA and F. solani

 

There are many benefits of using HA in Biotrue. By forming a hydrating network on the lens surface, HA attracts water molecules to improve initial comfort, end of day comfort and match the tear film pH.7 This is a similar concept to Alcon’s Hydraglyde moisture matrix in Opti-Free Puremoist, which promotes wettability of the contact lens.9

Another benefit of HA is that it helps the contact lens stay moist for a longer period of time. It’s stated that Biotrue keeps the lens moist and gives a low surface tension for up to 20 hours.6 A low surface tension allows the tear film to spread more easily over the contact lens. We know that 16 per cent of contact lens users drop out; predominantly due to dryness in 2017.10 Having a contact lens cleaning solution, which wets the lens for up to 20 hours, could make a huge difference to patients struggling with their lenses.11,12

Biocompatibility

Our tear film is made up of around 400 different proteins, which form part of the eye’s defence against microorganisms.6 Of these proteins in the tear film, lactoferrin and lysozymes are the highest in concentration.13 When these proteins stay in their natural state, they keep their antimicrobial properties.6 Lysozymes are particularly potent against streptococcus and staphylococcus, while lactoferrin is protective against both gram-positive and gram-negative bacteria.13,14 Lactoferrin inhibits the growth of many bacteria such as E. coli, H. influenza, Streptococcus spp, Staphylococcus spp and Pseudomonas spp.6

The design of Biotrue aims to mimic the natural tear film. It attracts, lifts and dissolves denatured protein deposits on the contact lens surface. Biotrue also protects healthy proteins, such as lysozymes, in the tear film so that they can retain their antimicrobial properties.6 Of note, Biotrue keeps more than 90 per cent of lysozymes active, compared to around 5 per cent in other solutions.15

Disinfection Properties

Biotrue uses a dual-disinfection system of Polyhexamethylene biguanide (PHMB) and Polyquaternium-1 (PQ-1).6 PHMB has excellent efficacy against bacteria, fungi and acanthamoeba.16,17 PHMB targets the negative phospholipids of cell membranes, causing disruption and cell death. PQ1 has excellent anti-bacterial efficacy and targets cell membranes, leading to leakage of intracellular contents and cell death.6,18

A study tested Biotrue solution on 7,212 eyes with 72,000 contact lens exposures and found no adverse events. In this study, all lenses were soaked for four hours as per the manufacturer’s guidelines. Of note, the minimum soak time of contact lenses in Biotrue is four hours, compared to six in many solutions.6,19 In my opinion, a minimum four hour soak time is one of Biotrue’s greatest advantages.

The method for testing Biotrue’s efficacy against microbes is mentioned previously.3

MPS Disinfection System Comparisons

The above section explains how Biotrue disinfecting solution works. Other multipurpose disinfecting solutions on the market include Alcon’s Opti-Free PureMoist, Bausch and Lomb’s Renu Fresh and Abbott’s Complete RevitaLens. All of these solutions have different disinfecting systems. As mentioned previously, Biotrue uses PHMB and PQ-1.7 Renu Sensitive, Renu Fresh and Complete Easy Rub also use PHMB. Opti-Free Puremoist uses Polyquaternium -1/polidronium chloride (Polyquad) and Myristamidopropyl dimethylamine (Aldox).9

Below are three case studies, which compare the efficacy of different multipurpose cleaning solutions. The studies illustrate that all solutions have their advantages and disadvantages.

Study One

A study by David in 2009 compared the disinfection properties of Biotrue to Opti-Free Replenish, Opti-Free Express, Complete Easy Rub and Clear Care. In line with the FDA and ISO testing, bacteria, yeast and mould were tested. Biotrue achieved consistently high log reduction against S. Aureus, C. Albicans, S. Marcescens, P. Aeruginosa and F. Solani.20 Most solutions performed very well against all organisms. The organism with the greatest range of log reduction in combination with the multipurpose solutions was C. Albicans. Complete Easy Rub showed a 0.3 log reduction while Clear Care showed a 4.2 log reduction against C. Albicans.

Another study by David in 2010 has also shown that Biotrue has a high log reduction against acanthamoeba (trophozoites and cysts).21,22 Of note, Clean Care performs very well against acanthamoeba, while Opti-Free Express, Opti-Free Replenish and Complete Easy Rub do not perform well against A. polyphagacysts.

Study Two

A study by Walsh in 2011 compared the microbial efficacy of six different contact lens solutions, including hydrogen peroxide, against six different organisms for 60 minutes. The study tracked the efficacy of each solution at 10, 20, 30 and 60 minutes. The five solutions tested in this study were made up of different active ingredients, which included Polyaminopropyl biguanide (PHMB), polyquarternium – 1 (PQ), myristamidopropyl dethylamine, alexidine (AL) and hydrogen peroxide (H2O2). Microbes used were three different bacteria, two fungi and yeast.23

The study used the ISO 14729 standard for biocidal efficacy, which has been mentioned previously.3,4 This states that bacteria need a minimum 3.0 log reduction and yeast and fungus require a 1.0 log reduction at the manufacturer’s recommended soak time to illustrate efficacy.4

The results of the study showed that different solutions were useful against different types of organisms. Of note, some MPSs required longer to be protective against an organism, while some were simply not protective against that organism. For example, efficacy against C. albicans was observed at 60 minutes for only four of the six solutions. Unfortunately, PQ-MPS-1 and PHMB-MPS-2 were not effective against C.albicans. On the other hand, efficacy against F.solani was observed by all solutions at 60 minutes. With F.solani, PHMB-PQ-MPS-1 was effective after five minutes while PQ-MPS-1 was effective after 30 minutes.22

Another example is that against Haemophilus Influenza. It took 10 minutes for both PHMB-PQ-MPS-1 and PHMB-MPS-1 to achieve a five log unit reduction (which did not improve after 10 minutes), while it took 60 minutes for PQ-MPS-2 to achieve the same log reduction. This was not a simple case, in that all solutions reached the desired efficacy within 60 minutes.

The results of this study demonstrate that the rate of disinfection of a given MPS can vary, depending on the challenge organism. It also begs the question as to why more organisms should be used to test new solutions.22

Study Three

An article by Lapple reiterated the importance of contact lens disinfection and compared Biotrue’s disinfection properties against Opti-Free Replenish and RevitaLens. This study was performed in-vitro and followed the FDA/ISO stand-alone procedures. This study also used an ‘organic soil’, which helped the organism adhere to the contact lens, making it easier for the organism to survive. The study tested the three solutions against F. Solani, C. Albicans, S. Aureus, P. Aeruginosa and S. Marcescens.

The results of the study illustrated that Biotrue was superior in disinfection of C.Albicans when compared to Replenish and RevitaLens. Of note, the disinfection efficacy of the other three solutions were similar when compared with the other four organisms. Lapple notes that while there is a difference between MPDSs and MPSs, there is not necessarily a difference in disinfecting or cleaning efficacy.3,23

Lapple made two interesting points when comparing Biotrue to Replenish and RevitaLens. As mentioned previously, Biotrue only requires for the contact lens to be stored for four hours, compared to six hours in many other MPSs. Secondly, the concentration of PHMB and PQ-1 in Biotrue is four times less than in Opti-Free Replenish and RevitaLens. In other words, Biotrue disinfects with a lower concentration in a lower soak time.23

 

Figure 3. Log reductions of clinical isolates of Acanthamoeba polyphaga trophozoites and cysts

 

Figure 4. Rate of disinfection of various multi-purpose solutions and hydrogen peroxide solution against clinically relevant bacterial and fungal species

 

Effect of Fluorescein Corneal Staining

In general practice, when it comes to determining the success of contact lens and MPS combinations, I often look at fluorescein corneal staining as my first port of call. However, what does corneal staining really mean and how effective is it in assessing corneal performance for contact lens wearers?

An article by Morgan highlighted that there is actually little evidence-based research underpinning the mechanisms involved in the interaction of the superficial ocular cells and the dye itself that leads to the various ocular surface fluorescence patterns that we routinely observe. There is a clear need for more research into this area. For example, why do some solutions with higher molar mass agents have higher levels of fluorescein staining with certain materials? There have been a few studies to look into the nature of solution induced corneal staining (SICS), but there is still a lot of research required.24

Furthermore, a literature review by Ward showed that the mechanism of action and importance of corneal fluorescein staining was poorly understood.25 For example, does fluorescein staining mean compromised corneal integrity? What’s the difference between gross corneal staining and superficial punctate staining? If anything, the conclusion was that superficial punctate corneal staining does not reflect corneal injury or toxicity. Similar to Morgan’s article, this shows that more work is required to understand the underlying mechanism behind superficial punctate staining.24,26

Both studies by Morgan and Ward have illustrated that there is not enough evidence to show that corneal staining is detrimental for the eyes or enough to judge a MPS. Both studies say that there is a need to monitor the effects of different solutions over a greater period of time.23,24 Furthermore, a paper by Carnt et al suggested that there is not a clear relationship between corneal staining and contact lens induced infection.26

Personally, I find the results of these studies to be confronting as I have always used corneal staining as a strong indicator to judge the performance of the contact lens and MPS combination. This begs the questions: how should we judge the performance of the contact lens and MPS combination? Patient comfort, risk of infection, log reduction against different organisms…?

Figure 5. a. Disinfection performance of multipurpose solutions (based on FDA/ISO stand-alone testing protocol modified with addition of organic soil) b. Comparative product overall disinfection concentrations

 

Tear Film

The role of the tear film is vitally important in a patient’s contact lens experience. As well as interacting with the contact lens, the tear film is also responsible for hydrating the cornea, flushing debris from the ocular surface and maintaining corneal integrity.27

A study investigated the difference in composition of eight contact lens disinfecting solutions compared to the normal tear film. The idea behind this study was that a MPS with similar properties to the tear film would improve the comfort of the contact lens when on the eye. The pH, osmolality, viscosity, and surface tension of eight contact lens disinfecting solutions were measured. The accepted measurements were pH 7.3–7.7, osmolality of 244–344mOsm/kg7, viscosity of 1–10cP8 and surface tension of 4246mN/m. Of interest, only three (one of which was Biotrue) of the eight solutions gave results that have been found to exist in the tear film. Further studies are needed to see how these properties affect the patient’s wearing experience. As mentioned, it would be a logical conclusion that matching a contact lens solution to the property of tears would improve initial comfort after insertion, decrease dryness and potentially make the experience more positive.27 On the other hand, mimicking the pH of natural tears will make no difference to patient’s comfort! More testing is required.

pH

A study by Yamada measured the mean pH of tears to be 7.5 (+/-0.23). The study tested 40 eyes from 20 healthy volunteers. Of note, this almost directly correlates to the previous study, which found Biotrue’s pH to be 7.50.28 Once again, the effect of this on a patient is unknown.28

Case Studies

The three case studies below indicate the complexity of managing our contact lens patients. The case studies discuss issues such as wearing schedule, cost, coexisting dry eye, modalities and choosing the ‘right’ cleaning solution.

Study One: Blepharitis

Ms. X had been wearing daily contact lenses for many years. She had tried every lens on the market and was close to dropping out of contact lenses (like 20 per cent of users) and having laser surgery. Mrs. X also felt as though she needed to constantly use lubricating drops while using her contact lenses. In particular, she felt as though her eyes stung when putting her contact lenses in. Slit lamp examination revealed small amounts of fluorescein corneal staining in both eyes. Both eyelids had blepharitis. I educated Ms. X about blepharitis and recommended lid scrubs and warm compressors on a daily basis. I spoke to her about the chronic nature of blepharitis and how she would have to treat it on a continual basis in the future. I advised Ms. X to continue using daily disposable lenses as the underlying blepharitis was the problem.

I also advised Ms. X that we could consider monthly lenses in the future, but only after the blepharitis was being treated. I explained that there are now new MPS which can minimise symptoms on insertion. I mentioned Biotrue MPS and how the pH of 7.5 could improve Ms. X’s symptoms of her eyes stinging when she inserted the contact lens. I believe giving M.s X another option made her feel much more comfortable about my future management.

To my delight, at a two week review, Ms. X’s symptoms dramatically improved and there was reduced corneal staining in both eyes. This example shows that treating the underlying issue, in this case blepharitis, is very important in contact lens success.

Study Two: Which Solution Should I Use?

Mr. Y presented to clinic for a general opinion on his contact lens regime. Mr. Y was wearing Acuvue Oasys lenses and was using a hydrogen peroxide cleaning solution. He had previously been switched to a hydrogen peroxide solution due to protein build up on his lenses and when he was lazy, he would simply use saline to clean his lenses. He was frustrated as he had recently accidentally flicked some peroxide solution in his eyes, which was very painful. With AOSept cleaning solution, it takes six hours for the peroxide to neutralise.

Slit lamp examination revealed a minimal amount of protein on the anterior surface of both contact lenses. All eyelids looked healthy and there was only a small amount of inferior SPK on both corneas. Considering the relatively healthy nature of Mr. Y’s lids, cornea and anterior surface of the contact lenses, I decided to change his solution from hydrogen peroxide to MPS. In this case I changed Mr. Y to Biotrue, though other MPS were also options. I was slightly nervous in changing Mr. Y from a hydrogen peroxide disinfection solution back to a MPS, but I decided clinically that his anterior eye was suitable.

I saw Mr. Y two weeks later. He was delighted with the change in solution and he didn’t notice any difference in his wearing experience. If anything, he noted a slight decrease in glare when driving at night. I pointed out that this could have been due to a decrease in dry eye symptoms. Under slit lamp, I could not see any observable difference in protein build up or corneal staining compared to using the hydrogen peroxide solution. I advised Mr. Y to continue using Biotrue, which is simpler to use compared to the hydrogen peroxide system. He told me that he wasn’t tempted to use saline again… he also admitted that he now knows saline has
no disinfection properties.

Study Three: Change in Modality Options

Ms. Z presented to clinic for an opinion on her contact lens wearing schedule. I had seen her for many years and she has been comfortable wearing daily disposable lenses five to six days per week. Due to changes in her life, Ms. Z asked me if there were more cost-efficient ways to wear contact lenses. In preparation for writing this article, I spoke to Ms. Z about the option of monthly contact lenses and asked her to try PureVision 2 monthly lenses with four different MPSs. To make the experiment as accurate as possible, I asked Ms. Z to use four unopened PureVision 2 lenses for a week with the four different MPSs. I did not specify the solutions and simply labelled them as A, B, C and D. I saw Ms. Z after a month and I was intrigued to find that she strongly preferred two solutions. She found one solution improved comfort on insertion (Biotrue) and another solution was slightly more comfortable after wearing her lenses for 10 –12 hours (Opti-Free PureMoist). I acknowledge that different patients will have varied experiences with different contact lens and MPSs combinations, however, I think Ms Z. illustrates that the type of MPS does make a difference for some patients. As optometrists, I feel it is important to recommend a specific contact lens solution with a specific contact lens to each of our patients. This shows our knowledge of products on the market and illustrates that we genuinely care for the patient. 


   

Joshua Clark B Optom (Hons), GradCertOcTher completed his Bachelor of Optometry (Hons) in 2008 and Ocular Therapeutics in 2012 from UNSW. He works in Northern Sydney for EyeQ optometrists and has an interest in contact lenses, orthokeratology and Aboriginal eye care. He has lectured around Australia on contact lenses, dry eye management and the direction of our profession.

 

References
1. Morgan et al, International Contact Lens Prescribing in 2016. Contact Lens Spectrum 2017
2. Andrasko G, Ryen K, Corneal staining and comfort observed with traditional silicone hydrogel lenses and multipurpose solution combinations. Optometry 2008; 79(8):444-454.
3. Lapple, J. The multipurpose disinfection solution with unsurpassed disinfection – an easy recommendation for all soft lenses, 2011.
4. International Standards Organization ISO 14729. Ophthalmic Optics – Contact Lens products. Microbiological requirements and test methods for products and regimens for hygienic management of contact lenses, 2001.
5. Food and Drug Administration. Guidance for Industry. Premarket notification (510(k)) guidance document for contact lens care products. 1 May 1997. Available at: http://www.fda.gov/downloads/MedicalDevices/DeviceRegulationandGuidance/GuidanceDocuments/UCM080218.pdf
6. Nichols J et al, Bringing New Inspiration to Contact Lens Care, Contact Lens Spectrum, 2010.
7. Buch J et al, The Tear Film and Contact Lens Wear, Contact Lens Spectrum, 2016.
8. Dutta D. Soft Contact Lens-Care Solutions: Navigating the options, Mivision 116 September 2016.
9. Opti-Free Puremoist Product Information 2011.
10. The 2017 Study Of The US Consumer Contact Lens Market. Multi-Sponsor Surveys. September 2009.
11. Scheuer C et al, Retention of conditioning agent hyaluronan on hydrogel contact lenses, British Contact Lens Association, 2010 (33S)S2-S6.
12. Merchea M et al, Patient Satisfaction with Multipurpose Solution in over 4000 subjects, Poster, 2010.
13. Ng A et al, Impact of tear film components on the conformational state of lysozyme deposited on contact lenses, Journal of Biomedical Materials Research, 2013, 101B(7):1172-1181.
14. Jahani S et al, The Antimicrobial Effect of Lactoferrin on Gram-Negative and Gram-Positive Bacteria, Int J Infect, 2015 2(3): e27954.
15. Barniak V et al, Comparative evaluation of multi-purpose solutions in the stabilization of tear lysozyme, British Contact Lens Association, 2010 Biotrue
16. Butcher, M PHMB: an effective antimicrobial in wound bioburden management, British Journal of Nursing, 2012 21(12): S16-S21.
17. Elsztein C et al, The resistance of the yeast Saccharomyces cerevisiae to the biocide polyhexamethylene biguanide: involvement of cell wall integrity pathway and emerging role for YAP1, BMC Molecular Biology 2011 12(38):1-10.
18. Codling C et al, Aspects of the antimicrobial mechanisms of action of a polyquaternium and an amidoamine, Journal of Antimicrobial Chemotherapy 2003, 51:1153-1158.
19. Orsborn G et l, Assessing the Physical Compatibility and Biocompatibility of a New Multi-Purpose Solution, 2011 Global Specialty Lens Symposium.
20. David B et al, Evaluation of multipurpose solutions for in vitro biocidal efficacy against a challenge of ocular clinical isolates including methicillinresistant Staphylococcus aureus and methicillin-resistant Staphylococcus epidermis strains. Paper presented at: Annual Meeting of the American Academy of Optometry; November 11-14, 2009; Orlando, Florida, USA.
21. David B et al, Walsh P, Norton S, Evaluation of Acanthamoebicidal efficacy of multi-purpose solutions using ISO 14729 standard for bacteria and fungi. Poster presented at: 2010 Meeting of the Association for Research in Vision and Ophthalmology; May 2-6, 2010; Lauderdale, Florida.
22. Walsh P et al, Rate of Disinfection of Various Multi-purpose Solutions and a Hydrogen Peroxide Solution Against Clinically Relevant Bacterial and Fungal Species, Poster presented at: 2010 Meeting of the Association for Research in Vision and Ophthalmology; May 2-6, 2010; Lauderdale, Florida.
23. Results of in vitro study following FDA/ISO stand-alone procedures performed with the addition of organic soil to create a more rigorous test condition. Primary criteria for effective disinfection are defined as a reduction in the number of bacteria by a minimum of 3 1ogs (99.9%) and a reduction in the number of mould and yeast by a minimum of 1 log (90%) within the recommended disinfection time. Graphs depict mean log reduction measured after manufacturers' recommended disinfecting time (soak only). Biotrue Biotrue, OPTI-FREE RepleniSH, and RevitaLens results from separate in vitro studies performed following identical test procedures.
24. Morgan PB et al, Corneal staining: do we really understand what we are seeing? Contact Lens Anterior Eye, 2009, 32(2):48-54.
25. Ward KW, Superficial punctate fluorescein staining of the ocular surface, Optom Vis Sci, 2008 85(1):8-16.
26. Carnt N, et al, Solution toxicity in soft contact lens daily wear is associated with corneal inflammation. Optom Vis Sci 2007; 84:309–15.
27. Scheuer CA, Physical Properties of Contact Lens Disinfecting Solutions. Vision Care R&D, 2010, Bausch & Lomb Inc, Rochester, NY, USA.
28. Yamada M et al, Fluorophotometric Measurement of pH of human tears in vivo, Current Eye Research. 1997, 16:482-6.
This article was sponsored by Bausch and Lomb Australia BIO.0026.AU.18

' fortnightly or monthly contact lens wearers make up approximately 40 per cent of the Australian and New Zealand contact lens market '