Contact Lens Materials

Dublin Institute of Technology From the SelectedWorks of Claire McDonnell May, 2013 Contact Lens Materials Claire McDonnell Available at: https://works.bepress.com/claire_mcdonnell/1/

CET CONTINUING EDUCATION & TRAINING Sponsored by 1 CET POINT Contact lens materials Claire McDonnell FAOI, PGDE 54 The material that a particular contact lens is made from is possibly not something to which every optometrist gives a great deal of thought. This article discusses the different soft and RGP materials available and when a patient may need to be changed from one type of material to another. Course code C-31513 Deadline: June 14, 2013 Know how to change a patient s soft lens material and/or modality if that patient is experiencing deposition or modulus-related problems (Group 5.2.1) Know how to change a patient s RGP lens material if that patient is experiencing deposition problems and know how to refit a PMMA wearer (Group 5.2.2) Understand how the contact lens fitter or optometrist refits a PMMA lens wearer into an RGP lens and any potential problems this may create (Group 5.1.1) Be able to recommend an appropriate lens care system for a patient based on the lens material they are using (Group 5.2.1) Understand how the contact lens fitter or optometrist might change a patient s soft lens or RGP material and/or modality if that patient is experiencing deposition problems (Group 5.2.2) Know how to recommend an appropriate lens care system for a patient based on the lens material they are using (Group 5.2.1) Know how to change a patient s RGP or soft lens material and/or modality if that patient is experiencing deposition problems (Group 5.4.2) Know how to refit a PMMA lens wearer into an RGP lens (Group 5.4.2) About the author Claire McDonnell works in private practice and lectures BSc Optometry undergraduates at the Dublin Institute of Technology in the areas of contact lenses, advanced clinical techniques and refractive surgery. She has given CET workshops and presentations in the UK, Ireland and at the European Conferences. My Academy A unique online resource, offering personalised education to meet individual needs and interests.

Sponsored by History The first hydrogel contact lenses became available in 1957. In 1971 Bausch and Lomb produced the first soflens made from polymacon (a low water content, non-ionic material). Originally Otto Wichterle (the inventor of hydrogel lenses) recommended that the lenses be cleaned by boiling them in physiological saline every two weeks. By 1985, as more companies began to develop newer hydrogel materials and proper lens care systems, it was necessary for the American FDA to categorise soft lenses into distinct groups. 1 The groups were initially designed to deal with how different lenses interacted with various lens care products and to differentiate between lenses which attracted varying amounts of protein deposits from the tear film. Table 1 shows the original four FDA groupings. While there are still some hydrogels available in low water content materials, the most popular disposable hydrogels mostly fall into either groups II or IV, that is high water content nonionic or ionic. Deposition The ionic hydrogels tend to have a negatively charged surface which makes them sensitive to changes in ph and osmolarity and more likely to attract tear proteins such as lysozyme (which is positively charged). Non-ionic hydrogels, however, are treated to remove the negative charge. Studies have found that the ionic lenses (in particular those which contain methacrylic acid, for example 1-DAY ACUVUE ) tend to attract more protein deposits than the non-ionic lenses. 2 Natural protein from the tears is colourless and easily removed by normal lens cleaning. In fact some deposition of proteins on lenses may be beneficial due to their antimicrobial effect. 3 However, if the same proteins become denatured, they lose their antimicrobial efficacy. Denatured proteins can then cause clouding of the lens and, as the body no longer recognises them, can lead to inflammatory responses the most common one being contact lens papillary conjunctivitis (CLPC). Denaturing of protein can occur as a result of interaction with different lens materials, but a study in 2007 showed that, while ionic hydrogel lens materials with methacrylic acid can attract a considerable amount of protein deposition, very little of that protein actually becomes denatured. Other lens materials can FDA Group Water content Ionicity Example I Low (<50%) Non-ionic The original SofLens 38 II High (>50%) Non-ionic Biotrue ONEday III Low (<50%) Ionic IV High (>50%) Ionic 1-DAY ACUVUE MOIST Table 1 Original four FDA groupings Lens Material Hydrogel or SiHy Modulus Biotrue ONEday Nesofilcon A Group II hydrogel 0.49 MPa 1-DAY ACUVUE MOIST cause greater denaturation but they only deposit very small amounts of protein. 4 Multipurpose contact lens solutions are relatively effective at removing natural protein, but denatured proteins bind much more tightly to lenses and are far more difficult to remove. Biotrue multipurpose solution contains protein stabilisation agents which have been shown to minimise denaturation. 5 Other solutions aim primarily to remove natural deposited protein before it can become denatured. With the popularity of disposable lenses protein has very little opportunity to build up on lenses. The exception here is group IV lenses, but even these lenses show very little denaturation of protein. As a result, protein deposition tends not to be a significant source of patient discomfort. If it is thought to be a problem, changing the patient to a group II (non-ionic) lens or changing their solution to Biotrue may help to improve their symptoms. Oxygen transmission The water content in hydrogel lenses is an important factor in determining oxygen permeability. This is because the oxygen is able to pass through water while being unable to pass through the material itself. Oxygen permeability increases logarithmically with an increase in water content. However, refractive index decreases with an increase in water content because the refractive index of water is normally lower than that of the material itself. A lower refractive index means a thicker lens Etafilcon A Group IV hydrogel 0.3 MPa PureVision 2 HD Balafilcon A 1st generation SiHy 1.1 MPa ACUVUE OASYS Senofilcon A 2nd generation SiHy 0.7 MPa Biofinity Comfilcon A 3rd generation SiHy 0.8 MPa Table 2 The moduli of common hydrogel and SiHy lenses which then offsets some of the gain in oxygen transmission. Concern was raised about the effects of contact lens induced corneal hypoxia as early as 1967. 6 Interest in the possibility of overnight wear began in the early 1970s and by the 1980s there was a considerable demand for extended wear lenses. These two factors combined meant that contact lens manufacturers needed to develop materials with much higher oxygen transmission. Manufacturers knew that silicone has excellent oxygen transmission and silicone elastomer lenses had been available since the 1970s, but silicone is intrinsically hydrophobic. So, although this material can have Dks in the 200s, it still has problems with excessive lipid deposition and corneal adherence. The fact that it took over 20 years to eventually combine silicone and hydrogel together to produce a viable contact lens material is a testament to the enormity of the task. Finally, in 1999, Ciba and Bausch and Lomb brought the first commercially available silicone hydrogel (SiHy) lenses to market. In 2001 the FDA approved the lenses for 30 days continuous wear. These first generation lenses had to have a special surface treatment to make them hydrophilic. Previous hypoxia-related problems were eliminated with these new lenses but some new problems were to appear. SiHy lenses can be made with much lower water contents than hydrogels because the oxygen 55 Find out when CET points will be uploaded to the GOC at www.optometry.co.uk/cet/upload-dates For the latest CET visit www.optometry.co.uk/cet

1 CET POINT CET CONTINUING EDUCATION & TRAINING 56 Figure 1 Contact lens papillary conjunctivitis Figure 2 Superior epithelial arcuate lesion Figure 3 Lipids deposited on a silicone hydrogel lens permeability of the material is not dependant treatments or intrinsic wetting agents. These lenses which is bound to the corneal epithelium. on water content. However, water is what gives have a different polymer which is intrinsically Therefore, it is not a true pathological condition the material some of its flexibility and silicone hydrophilic. in which fluorescein stains the actual epithelial is a relatively inflexible material. The new SiHy All SiHy lenses attract lipid deposition from cells themselves or pools within disturbances in lenses showed a much higher modulus (rigidity) than the older hydrogels (see Table 2, page 55, for the moduli of some common hydrogel and SiHy lenses). This increase in modulus (compared to the tears because silicone is intrinsically lipophilic (Figure 3). Researchers have compared the amount of lipid deposition which occurs on the three different generations of SiHy lenses but they have come up with conflicting results. In one paper, the epithelium. 13 There is evidence to support this theory in that the staining is asymptomatic. 14 Another paper from 2012 found that ocular surface shedding with contact lens wear was similar in all eyes regardless of levels of SICS observed. 15 hydrogels) led to an increase in some contact lens researchers found that second generation lenses On the opposing side of the argument, yet related problems such as contact lens papillary adsorbed more lipids, while another paper found another paper from 2012 found that eyes with conjunctivitis (CLPC) (Figure 1) and superior that first generation lenses adsorbed more. 9,10 What SICS were more likely to show higher levels of epithelial arcuate lesions (SEALs) (Figure 2) in some researchers can agree on is that hydrogel lenses upper and lower palpebral hyperaemia and tarsal cases. 7,8 Patients exhibiting these problems should deposit fewer lipids than SiHy and group IV deposit roughness both of which would suggest that be changed to a lower modulus material. There are less than group II. If excessive lipid deposition is there is some pathological basis to SICS. 16 It seems advantages to having a stiffer lens in that it allows a problem the simplest solution is to change the that researchers have yet to reach a consensus for slightly more tear exchange underneath the patient into a hydrogel lens or, if this is not possible regarding SICS, and therefore for practitioners it is lens and a stiff lens can mask some mild corneal (due to oxygen requirements), then change the probably better to err on the side of caution and irregularities. patient to a one day disposable SiHy lens. change patients with obvious signs of SICS into one Initially, SiHy lenses were placed in one of the original four FDA groups (mostly group I), but in 2007 the FDA made a fifth group specifically for SiHys. SiHy lenses are also divided into three different generations. Each generation has a different treatment to make the material hydrophilic and a different polymer. The first generation SiHy lenses have surface treatments to make them hydrophilic. They tend to have high Dks and correspondingly high moduli. The second generation materials do not have a surface treatment. Instead, they have poly vinyl pyrrolidone (PVP) as an intrinsic wetting agent. The third generation lenses do not have surface SICS and PATH debate Shortly after the first SiHy lenses became commercially available, researchers and practitioners started to notice what became known as solution induced corneal staining (SICS) (Figure 4), which seemed to be most prevalent with polyhexanide preserved multipurpose solutions. 11 A decade on and the debate about SICS continues. In 2012, Nathan Efron produced a paper where he gives SICS a new definition and title. 12 He calls it preservative-associated transient hyperfluorescence (PATH), which he describes as a temporary hyperfluorescence of fluorescein of the hydrogen peroxide based solutions, which generally produce little or no SICS in combination with either SiHy or hydrogel materials. 17 RGP materials The first hard lenses were made from PMMA, which is an optically excellent material but allows negligible oxygen transmission. Although gas permeable materials first became available in the late 1970s, there are still patients wearing PMMA lenses today. PMMA has almost no oxygen transmission and is quite inflexible and so long term wear usually causes corneal warpage (Figure 5) and problems related to hypoxia and My Academy A unique online resource, offering personalised education to meet individual needs and interests.

Sponsored by Group Description Material Example I Contains no silicone or fluorine Cellulose acetate butyrate II Contains silicone but no fluorine Silicone acrylate Boston IV III Contains silicone and fluorine Fluorosilicone acrylate Fluoroperm Figure 4 Solution induced corneal staining (SICS) Figure 5 Distorted keratometry mires reflected from a cornea showing contact lens related warpage therefore these patients should be refitted with RGPs. When refitting these patients, the recommendation is to fit with a lens which has parameters that match their original PMMA lens as closely as possible. Choose a low Dk material as this will keep flexure of the lens to a minimum. The lowest acceptable Dk/t is approximately 23. This is taken from work done by Harvitt and Bonanno, which showed that this is the Dk/t required to ensure delivery of oxygen to the basal epithelial cells under open eye conditions. 18 Morris also recommends prescribing extra minus with this initial refit as the patient s myopia is likely to increase as their cornea loses some of the flattening induced by the PMMA lenses. 19 These patients need to be carefully counselled that their RGP lenses are unlikely to last as long as their old PMMA lenses and that they may be more aware of the RGP lenses initially as corneal sensation returns with a reduction in hypoxia. IV Contains fluorine but no silicone Fluorocarbon Table 3 Modern RGP materials, divided into four groups based on their silicone and fluorine content Modern RGP materials are divided into four groups based on their silicone and fluorine content (see Table 3). Group I lenses were mainly cellulose acetate butyrate (CAB). They had poor oxygen transmission and were prone to warping. The advantage of these lenses was that they were non-sensitising and so they were used for abnormal conditions such as keratoconus. With the advent of more modern materials with higher Dks, this material is rarely used now. Group II lenses are mainly silicone acrylates. These lenses were made from silicone and methacrylic acid polymers. The silicone provides increased oxygen permeability while the methacrylic acid is present for wettability and optical clarity. These materials had some advantages over earlier PMMA and CAB such as increased oxygen permeability and less warpage (compared to CAB). The disadvantages were that the lenses had poorer wettability (than PMMA) and tended to deposit significant amounts of protein. Group III lenses are mainly fluorosilicone acrylates. These would normally be the material of first choice in a new RGP fit. These are fluorinated monomers combined with silicone acrylates. The fluorine helps to improve wettability while maintaining high oxygen transmission. These materials show less protein deposition but can be prone to lipid deposits. Group IV lenses are mainly fluorocarbons. These are rarely used now. When the oxygen transmission of a material is increased, there is inevitably a loss of wettability and, therefore, most RGP lenses have a surface treatment to improve their wettability. Lenses treated in this way should not be cleaned with alcohol as the alcohol can break down the treatment. Replacement of RGP lenses is recommended every six to 12 months. 20 the higher the Dk, the sooner replacement is required. Although older RGP lenses can be polished to remove scratches and possibly prolong their lifespan, this polishing will also damage the surface treatment which will reduced the surface wettability of the lens. 21 Another problem with RGPs is the tendency for the material to flex with time. Flexure is when a rigid lens bends because of pressure from the lids and because the back surface of the lens does not conform perfectly to the front surface of the cornea. Thin RGP lenses can flex approximately one-third of the corneal toricity, with increased chance of flexure with steeper and larger lenses and increasing oxygen transmission. To avoid problems with flexure, choose materials with slightly lower Dks and keep the total diameter as small as possible. Conclusion Many contact lens related problems can be very effectively addressed with a change of material. It is to the practitioner s advantage to be familiar with the various available materials and their respective advantages and disadvantages. 57 MORE INFORMATION References Visit www.optometry.co.uk/clinical, click on the article title and then on references to download. Exam questions Under the new enhanced CET rules of the GOC, MCQs for this exam appear online at www.optometry.co.uk/cet/exams. Please complete online by midnight on June 14, 2013. You will be unable to submit exams after this date. Answers will be published on www.optometry. co.uk/cet/exam-archive and CET points will be uploaded to the GOC on June 24, 2013. You will then need to log into your CET portfolio by clicking on MyGOC on the GOC website (www.optical.org) to confirm your points. Reflective learning Having completed this CET exam, consider whether you feel more confident in your clinical skills how will you change the way you practice? How will you use this information to improve your work for patient benefit? Find out when CET points will be uploaded to the GOC at www.optometry.co.uk/cet/upload-dates For the latest CET visit www.optometry.co.uk/cet