Lens types and materials 7 Chapter

B = USAN series suffix, indicating the second formulation of this polymer. III = Group suffix, indicating a material containing both silicon and fluorine 3 = A Dk in the 31 to 60 range of ISO units.

Example 2. ISO classification applied to a soft lens, Acuvue 2

Etafilcon A IV 1

Eta = USAN prefix.

filcon = Stem, indicating a soft lens material ≥10% water by mass.

7.1 Rigid gas-permeable lenses

Rigid gas-permeable lenses are available in a wide range of materials and Dks. Oxygen considerations, however, must take into account:

•The barrier effect, which reduces the Dk on the eye to approximately 55% of that measured in air in the gas/gas situation.2

•Centre and average lens thickness.3,4

For physiological reasons, lenses should be as thin as possible, although, in practical terms, making them too thin is counter-productive since they are very likely to distort throughout the power range and also become too brittle. In most cases, a realistic minimum centre thickness is 0.14 mm, even for high minus powers.

Although Dk is important, there are several other considerations which affect comfort, vision and lifespan. These include:

•Surface wetting properties.

•Lens design.

•Fitting method.

•Manufacturing technique.

•Mechanical stability.

•Optical quality.

7.1.1 Cellulose acetate butyrate

Cellulose acetate butyrate (CAB) was one of the first non-polymethyl methacrylate (PMMA) materials, introduced in 1977. By modern standards, its Dk (between 4 and 8 × 10−11) is extremely low and the material is now rarely used. Its main difficulty when manufactured by traditional lathing methods was dimensional instability. However, when manufactured by moulding, this

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problem was largely overcome, and lenses such as Conflex and Persecon E can give good clinical results for occasional problem solving.

Advantages of CAB

•Good wettability.

•Relatively inert.

•Does not attract protein.

•Low breakage rate.

•Very low incidence of contact lens-induced papillary conjunctivitis (CLIPC).

•Relatively good for 3 and 9 o’clock staining.

Disadvantages of CAB

•Low Dk.

•Moulding necessary for dimensional stability.

•Limited range of lens designs.

•Scratches easily.

•Attracts lipids from the tears.

•Corneal adhesion in some cases.

•Lens flexure and distortion on toric corneas with tight lids.

7.1.2 Silicon acrylates (siloxanes)

Silicon acrylates are copolymers in varying proportions of acrylate (PMMA), which provides lens rigidity, and silicon, which controls the degree of oxygen permeability. Also included are cross-linking agents to improve the strength of the material and wetting agents such as methacrylic acid to improve the naturally hydrophobic properties of silicon. A reasonable range of materials is available with differing physical properties and Dk values (Table 7.3). They give

Table 7.3  Silicon acrylates5

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Lens types and materials 7 Chapter

adequate oxygen and physiological performance and many have stood the test of time in terms of dimensional stability, optical and mechanical results. They are routinely fitted for daily wear but no longer for extended wear.

Advantages of silicon acrylates

•Good range of materials available.

•Wide range of designs with practitioner control.

•Low to medium Dks available.

•Good dimensional stability.

•Good vision with limited lens flexure.

•Good scratch resistance.

Disadvantages of silicon acrylates

•Attract protein from the tears.

•Some materials are brittle with a breakage problem.

•3 and 9 o’clock staining.

•Some incidence of CLIPC.

7.1.3 Fluorosilicon acrylates

Fluorosilicon acrylates are composed of fluoromonomers and siloxy acrylate monomers. The addition of fluorine atoms to replace some of the hydrogen present in methacrylate monomers improves surface wettability, tear film stability and deposit resistance5 as well as increasing oxygen permeability. The solubility of oxygen in fluoro-materials is enhanced and so higher Dks can be achieved (Table 7.4). Alternatively, moderate Dks can be produced with a lower siloxy acrylate content and hence provide improved wettability. The silicon content ranges from 5 to 7% with Boston ES to 16 to18% with Fluoroperm 90.

Advantages of fluorosilicon acrylates

•Very high Dks possible.

•Suitable for flexible extended wear.

•Better wettability.

•Fewer deposit problems.

•Lower incidence of CLIPC.

•Modification possible.

Disadvantages of fluorosilicon acrylates

•Brittle if too thin.

•Require careful manufacture.

•Dimensional stability depends on material and manufacture.

•Corneal adhesion in some cases.

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Table 7.4  Fluorosilicon acrylate materials5

GENERAL ADVICE

•Fluorosilicon acrylates are suitable for almost all straightforward patients and resolve many of the problems found with lower Dk materials. They have also proved suitable for flexible extended wear.

•Silicon acrylates are also suitable for most straightforward patients and give good dimensional stability. They are suitable for normal daily wear and for most problem solving with the exception of CLIPC.

•CAB lenses might be considered where a low Dk is sufficient and where CLIPC and 3 and 9 o’clock staining are a problem with other materials.

7.1.4 Hydrophilic rigid gas-permeable lenses

Silicon acrylate lenses with an improved hydrophilic surface are sometimes used to assist initial comfort, surface wettability and deposit resistance.

The Novalens (Ocutec) has a Dk of 55 × 10−11 and a ‘soft’ coating of OH groups which gives the surface characteristics of a hydrophilic lens. The lens itself does not absorb water but has good wettability and improved comfort.

Aquasil (No. 7) has a Dk of 50 and a surface containing OH groups similar to HEMA. The refractive index is 1.463 and water absorption less than 2%. Fol-

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