Lens types and materials 7 Chapter

•Disruption of tear film.

•Corneal desiccation and staining.

•Increased deposits.

•Reduction in vision.

Factors influencing lens dehydration

Ocular factors

•Volume of tears.

•Quality and stability of tear film.

•Osmolarity of tears.

•Blinking habits.

•Size of palpebral aperture.

Other factors

•Lens material.

•Lens thickness.

•Temperature changes.

•Relative humidity.

•Drafts and wind.

•Systemic drugs.

•Alcohol.

The water content contained within the polymer matrix consists of bound water directly attached to hydrophilic sites by hydrogen-bonding Van der Waals forces and free water, which is more readily lost by evaporation. The higher the bound water (e.g. with biocompatible polymers), the less any particular material will dehydrate on the eye.7

Generally, most water loss occurs within the first few minutes and high water content materials give greater dehydration. Tear film stability is better with thicker lenses and low water contents; it is worse with ultrathin and high water content lenses.

7.4 Silicone hydrogels

Silicone hydrogels were introduced in the UK during 1999 and represented a major advance in materials technology by combining silicone rubber (see Section 7.6) with hydrogel monomers. These materials also use the classification suffix –filcon. The silicone constituent permits very high oxygen permeability while the hydrogel component ensures that the lenses are soft and comfortable and, in addition, provides fluid transport through the lens.

The first silicone hydrogels (Night & Day and Purevision) had a relatively high modulus so that, despite their excellent permeability, they sometimes proved uncomfortable or caused CLIPC. Subsequent lenses such as Acuvue Advance or Biofinity have a reduced modulus to overcome these problems and have become known as second generation silicone hydrogels. More recent lenses have both a

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reduced modulus and a higher water content and have been termed third generation.

When considering the Dk values, there is a significant difference from hydrogel lenses where the Dk value is directly liked to the water content. With silicone hydrogels, because the majority of the oxygen flow takes place within the silicone components, a lower water content can have a higher Dk value.8 There are now several varieties of lens available.

Silicone-based materials are inherently hydrophobic and require either surface treatment or the addition of wetting agents to the polymer in order to render finished lenses hydrophilic and comfortable.9 The treatment must first have no effect on the oxygen transmission and, secondly, become an integral part of the lens so that it cannot be removed with use and handling, or interaction with disinfecting solutions. The following three examples show different approaches to rendering silicone hydrogels hydrophilic.

The Bausch & Lomb Purevision (Balafilcon A) is a homogeneously co-  polymerized combination of the hydrophilic monomer N-vinyl pyrrolidone and the silicone monomer polydimethylsiloxane, which is itself a vinyl carbamate derivative of trimethylsiloxy silane (TRIS). The resultant material has a water content of 36% and a Dk of 110 × 10−11 Fatt units. Purevision lenses are treated in a gas plasma chamber to modify the surface to hydrophilic silicate compounds

– so called glassy silicate islands.

The CibaVision Focus Night & Day (lotrafilcon A) has a biphasic molecular structure. It consists of a fluoroether macromer co-polymerized with TRIS and N, N dimethylacrylamide (DMA). The fluorosiloxane phase allows the storage and transmission of oxygen while the hydrogel phase transmits both oxygen and water. Lotrafilcon A has a water content of 24% and a Dk of 140 × 10−11 Fatt units.

Acuvue Oasys with Hydraclear adopts a different approach. The principal monomers are mPDMS + DMA + EGDMA + HEMA + siloxane macromer + PVP. Instead of employing a surface treatment, the PVP is included throughout the polymer matrix as an internal wetting agent which also coats the lens surface.

Advantages of silicone hydrogels

•Very high Dks available.10

•Suitable for extended wear.

•Rapid adaptation.

•Suitable for patients with vascularization.

•Good dehydration characteristics.

•Easier handling because of lens rigidity.

•Good tensile strength with low breakage rate.

•Low uptake of fluorescein during aftercare.

Disadvantages of silicone hydrogels

•Limited range of complex designs (torics, bifocals).

•Mainly available only as disposables.

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•Greater incidence of CLIPC.

•Greater incidence of arcuate staining (see Section 19.5.1).

•Frequently exhibit mucin balls (seeSection 19.5.1).

•More expensive.

See Chapter 19 for further details of silicone hydrogels.10

7.5 Biocompatible lenses

7.5.1 Biocompatible lenses

Biocompatibility has been defined as ‘the ability of a material to interface with a natural substance without provoking a biological response’. The advantages of biocompatible materials are their good physiological response with reduced evaporation of tears, corneal desiccation and deposits. Current lenses and materials include Proclear, Sauflon Bioclear and Benz 5f-x.

Proclear materials include phosphorylcholine (PC), a naturally occurring component in the cell membrane of red blood cells, which has a high affinity for water and is resistant to protein adsorption. Similar technology is used for intraocular lenses as well as cardiac, orthopaedic and other health care products. The range of contact lenses includes both conventional, manufactured by lathing, and disposable, produced by cast moulding.

7.5.2 Biomimetic lenses

In biomimesis, the principles of the complex structure and chemistry of nature are emulated by much simpler scientific means which, nevertheless, achieve the same results. The Vistagel PLUS material is based on the idea of biomimesis.

In its main features, Vistagel PLUS is modelled on the structure of the cornea which experiences no deposit problems within the environment of the tear film. This logic was applied to the composition of the lens material. Two versions of Vistagel PLUS are available with water contents of 40% and 55%.

7.5.3 Collagen lenses

Collagen lenses are also produced from biological polymers and show good biocompatibility. Their main applications have been therapeutic. They dissolve after a number of days and are therefore never permanent.

7.6 Silicone lenses

7.6.1 Silicone rubber lenses (elastomers)11

Silicone lenses differ from rigid lenses in several ways. They can be flexed, stretched and turned inside out. They have excellent elastic properties, partly

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conform to the shape of the cornea in wear, and have extremely high Dks in the region of 200 × 10−11. They are also unlike hydrophilic lenses because their natural state is dry and they are extremely tough. Since they do not absorb water to any significant extent, fluorescein can be used in their fitting and they do not need disinfecting in the same way as soft lenses.

Because of the amorphous nature of the silicone rubber raw materials, lenses are produced by a moulding and vulcanization technique, which also assists in maintaining good reproducibility. The main difficulty with silicone is that its natural surface is extremely hydrophobic and it has been necessary to devise methods of rendering the surface permanently hydrophilic without interfering with any of its optical or physical properties. The final stage of manufacture is therefore surface treatment by ion bombardment.

Because of the following advantages and disadvantages, silicone is very much a minority lens with limited therapeutic applications (see Section 32.9).

Advantages of silicone lenses

•Very high Dk.

•Better and more stable vision than many soft lenses.

•Little variation in comfort or fitting with environmental factors.

•Low risk of loss or damage.

Disadvantages of silicone lenses

•Difficult to fit, requiring as much precision as rigid lenses.

•A negative pressure effect producing lens adhesion, particularly if not correctly fitted.

•Breakdown in surface coating and difficulties with wetting.

•Build-up of deposits.

•Foreign bodies, especially with loose fittings.

References

1.Hough T. A Guide to Contact Lens Standards. London: British Contact Lens Association; 2000.

2.Holden BA, La Hood D, Sweeney DF. Does Dk/L measurement accurately   predict overnight edema response? American Journal of Physiological Optics

1985;62:95.

3.Fatt I. Some comments on methods used for measuring oxygen permeability (Dk) of contact lens materials. Contact Lens Association of Ophthalmologists Journal

1986;11:221–6.

4.Brennan N, Efron NA, Holden BA. Oxygen transmissibility of hard gas permeable and hydrophilic contact lenses. American Journal of Physiological Optics

1986;63:4.

5.Fatt I. A new look at fluoropolymers. Optician 1985;190(5015):25–6.

6.Sammons WA. Contact lens thickness and all that. Optician 1980;180(4467):11–18.

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7.Hart DE. Surface interactions on hydrogel contact lenses: scanning electron microscopy (SEM). Journal of the American Optometric Association 1987;58:962–74.

8.Sweeney DF, editor. Silicone Hydrogels: the Rebirth of Continuous Wear Contact Lenses. London: Butterworth-Heinemann, Oxford/BCLA; 2000.

9.Jones L, Subbaraman L, Rogers R, Dumbleton K. Surface treatment, wetting and modulus of silicone hydrogels. Optician 2006;232:28–34.

10. Sweeney D. Oxygen through silicone hydrogels. Optician 2006;July:7.

11. Hill RM. Effects of a silicone rubber contact lens on corneal respiration. Journal of the American Optometric Association 1966;37:1119–21

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