Lens types and materials 7 Chapter 
lowing manufacture by conventional methods, lenses are immersed for 120 minutes in a 10.6% solution of sulphuric acid which creates a surface layer of OH groups. These hydroxyl groups are permanently bonded to the polymer and are not reactive.
A different approach is adopted by Aquasil RSP and the Hybrid FS and FS Plus from Contamac where the latter’s ‘fluid surface technology’ combines fluorosilicon acrylate material with a hydrophilic component. Prior to hydration, the hydrophilic molecules are distributed throughout the polymer matrix so that, after lens manufacture, the surface is guaranteed to contain a number of hydrophilic sites. After hydration the hydrophilic components attract water molecules which then improve both comfort and oxygen permeability. No additional treatment is necessary and lenses are compatible with all standard solutions. The Hybrid FS has a refractive index of 1.4465 in dry and wet states, a Dk of 23 and a water uptake of less than 0.94%. The Hybrid FS Plus has the same refractive index but a Dk of 60 and a water uptake of less than 0.85%.
The Millennium Lens (Vista) is produced differently, by the polymeric grafting of hydrophilic polymers onto a fluorosilicon acrylate material with the surface covalently bonded to the entire lens surface.
PRACTICAL ADVICE
•Surface-treated lenses are fitted according to rigid lens criteria and assessed in the normal way with fluorescein.
•They may be cleaned and soaked with nearly all conventional rigid lens systems.
•Solutions which contain alcohol, and aggressive cleaning solutions containing particulates (e.g. Boston cleaner), should be avoided.
7.2Polymethyl methacrylate
Polymethyl methacrylate (PMMA, perspex) has been in use since the 1940s, first as a replacement for the earlier glass scleral lenses and, subsequently, as the material of choice with the development of corneal lenses. There are many patients who have worn PMMA successfully for over 40 years, although it has now almost completely fallen into disuse with the advent of modern rigid lenses, by comparison with which its permeability is negligible. Nevertheless, its original merits of inertness and stability mean that PMMA may retain a place for very occasional new patients as well as a small minority of existing wearers. There are still a few long-standing patients who exhibit neither signs nor symptoms and are best left without refitting, although their corneas should be carefully monitored (see also Chapter 30). Perspex is one of the few rigid lens materials available in a comprehensive range of tints (see Section 25.2.2).
7.3 Soft lenses
Soft lenses are classified according to the system shown in Table 7.11 and Example 2 on page 97 shows how the system is applied to Acuvue 2. Lenses are
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generally discussed according to the interrelated properties of water content, Dk and material type.
Water content and uptake
The definitions for water content and uptake are given in Section 1.3.2. Care must be exercised when interpreting brand names which include a numerical suffix because these do not always accurately reflect the true water content.
Ionic and non-ionic polymers
Polymers are categorized into four groups, linking water content to ionic properties. The classification applies to both hydrogels and silicone hydrogels and relates to materials rather than clinical considerations. In this context, high water content is defined as greater than 50%. Ionic polymers contain more than 0.2% methacrylic acid and these high water content lenses may therefore contain the negatively charged carboxylic acid. The polymers are more sensitive both to temperature and the composition of care products, so lens parameters show greater variability with environmental factors. The materials attract higher levels of deposit from the tears, particularly protein, and are generally more suitable for disposable lenses where life span is less important.
The four groups include both hydrogel and silicone hydrogel materials:
1.Low water content, non-ionic polymers, e.g. Polymacon (Bausch & Lomb HEMA, 38%); Lotrafilcon A (Night & Day 24%). Materials generally show lower levels of protein deposit.
2.High water content, non-ionic polymers, e.g. Vasurfilcon A (Precision UV, 74%); Omafilcon A (Proclear, 62%). Heat and sorbic acid should be avoided for disinfection because of the risk of lens discoloration.
3.Low water content, ionic polymers, e.g. Balafilcon A (Purevision 36%)
4.High water content, ionic polymers, e.g. Etafilcon A (Acuvue, 58%); Vifilcon A (Focus, 55%). These polymers show the highest level of protein deposition and, as with group 2, heat and sorbic acid should be avoided for lens disinfection.
7.3.1 Clinical implications of hydrogel lens water content
Hydrogel materials have been produced with water contents from 18% to 85%. The majority of modern lenses are manufactured with high water contents, 50% and above. Nevertheless, some are still HEMA-based lenses in the region of 38–46%.
Advantages of low water content lenses
•Greater tensile strength.
•Less breakage.
•Longer lifespan.
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•Smaller swell factor during manufacture.
•Better reproducibility.
•Easier to manufacture.
•Can be made thinner.
•Less dehydration on the eye.
•Less discoloration with age.
•Fewer solutions problems.
Disadvantages of low water content lenses
The disadvantages of low water content hydrogel lenses relate mainly to their modest Dk values.
•A greater tendency to cause corneal oedema.
•A long-term tendency with thicker lenses (e.g. with high powers) to cause vascularization.
Advantages of high water content lenses
Most high water content hydrogel materials have Dks much greater than HEMA. Apart from their obvious application in oedema cases, they have several other advantages:
•Better comfort because of material softness.
•Faster adaptation.
•Longer wearing time.
•Easier to handle because of greater thickness.
•Better vision because of greater thickness.
•Better for intermittent wear.
Disadvantages of high water content lenses
Despite these good features, there are, nevertheless, disadvantages with conventional high water content lenses which preclude their use in some cases. Most of these problems are relatively unimportant with disposable lenses.
•Shorter lifespan.
•Greater fragility.
•More deposits, especially white spots.
•More discoloration.
•Reproducibility less reliable.
•More difficult to manufacture by lathing.
•Greater variation with environment.
•Fitting requires longer settling time.
•Greater variability in vision.
•More solutions problems.
•Lens dehydration.
•Corneal desiccation.
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7.3.2 Clinical implications of soft lens thickness
The typical centre thickness for a ‘standard’ corneal diameter HEMA lens of power −3.00 D is in the region of 0.10 mm–0.14 mm. Lenses below 0.10 mm may be regarded as thin; those below 0.07 mm as ultrathin and those thinner than 0.05 mm have been termed superthin or hyperthin.They represent one way of increasing transmissibility (Dk/t) and improving physiological performance as well as giving an inherent safety factor for patients who accidentally fall asleep while wearing their lenses. Low plus and aphakic lenses cannot truly be considered ultrathin because of their necessarily greater centre thickness. Nevertheless, ‘thin’ plus lenses give a more satisfactory overall performance.
Oxygen performance for a lens cannot be judged solely in relation to its specified centre thickness but must be considered for the entire lens. If an ‘average’ or ‘mean’ thickness is used, this itself requires definition to avoid error and give valid comparison.6
Advantages of thin lenses
•Lower incidence of oedema.
•Reduced lid sensation because of thinner edges.
•Reduced limbal irritation because of thinner edges and larger total diameter.
•Different fitting characteristic may provide better centration than standard lenses.
•Easier to fit because fewer fitting steps are necessary.
•Safer if patients accidentally fall asleep.
Disadvantages of thin lenses
•Handling is more difficult, especially in low minus powers below about −2.00 D.
•Higher breakage rate than standard thickness lenses.
•Lifespan is shorter.
•Visual acuity may be less good with toric corneas.
•Greater tendency to dehydrate on the eye and disturb precorneal tear film.
7.3.3 Dehydration of soft lenses
One of the main reasons for the clinical success or failure of a particular lens on the eye relates to its dehydration characteristics.
Effects of lens dehydration
•Change in parameters and fitting.
•Reduction in comfort.
•Reduction in Dk.
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