Section ONE Preliminaries

6.6 Contact lens management

6.6.1 Hydrogel lenses

Hydrogels which may improve tolerance with dry eyes are:

•Bound water materials – contain less free water and lose their   moisture content more slowly over time (eg Proclear (CooperVision),   Benz GX).

•Thicker high water content lenses – give a stable tear film on their   surface and encourage reflex tearing on blinking (e.g. Lunelle ES70 (CooperVision).

•Thin low water content lenses – have less free water to lose but as they

need to be very thin (tc: 0.06 mm) for physiological reasons do not promote a stable tear film on their surface so may exacerbate the dry eye problem if the patient has marginal dry eye (eg Hydron Zero 6, CooperVision).

•Lenses with an internal wetting agent:

•PVA is used for added wettability and comfort but leaches out   during the course of the day (eg CIBA Dailies Aqua Comfort Plus, CIBAVision).

•Hydroclear Plus is the internal wetting agent in Acuvue Moist (Johnson & Johnson) which is not released during lens wear.

•Safegel 1 day and 2 weekly (No. 7) has Sodium Hyaluronate-Gel as its continual slow release internal wetting agent.

6.6.2 Silicone hydrogels

Silicone hydrogels have proved helpful with dry eyes: they have a lower  water content so that there is less moisture to lose on dehydration; some materials are modified to increase their wettability and lubricity to promote better comfort.

First generation silicone hydrogels have a low water content with their  surfaces modified to help wetting (eg PureVision, Bausch & Lomb, Night  and Day, CIBAVision) (See Section 7.4). Second generation materials have an internal wetting agent to improve comfort in addition to having a higher water content (e.g. Acuvue Advance and Oasys with Hydraclear, Johnson & Johnson). One day disposables are also available and are particularly aimed at patients requiring comfortable social wear (eg TruEyes, Johnson & Johnson, and Clariti, Sauflon).

6.6.3 Rigid gas-permeable materials

Surface modified materials can give better wetting and increased comfort (e.g. Millenium, Hybrid FS), (See Chapter 7).

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The tear film and dry eyes 6 Chapter

PRACTICAL ADVICE

•Non-preserved rewetting drops are preferable with hydrogel and silicone hydrogel lenses (e.g. Refresh Contacts, Blink Contacts).

•Rewetting drops should be used before the symptoms start rather than trying to alleviate them once evident.

•Routine rewetting, such as mid morning and early evening, can reduce or prevent symptoms.

References

1.Mishima S. Corneal thickness. Surv Ophthalmol 1968;13:57–96.

2.Argueso P, Gipon IK. Epithelial mucins of the ocular surface: structure, biosynthesis and function. Exp Eye Res 2001;73:281–9.

3.Veys J, Davies I. Basic contact lens practice, Part 4 – Assessment of the tear film. Optician 1995;209(5496):18–24.

4.O’Toole L. The tears and lacrimal gland. Diagnosing sight and life-threatening eye disease. Differential Diagnosis of Ocular Disease Module 9, Part 3. Optometry Today

2006;March:42–7.

5.Guillon J-P, Godfrey A. Chap 5 Tears and Contact Lenses. Contact Lenses 5th ed. B-H; 2007.

6.McMonnies CW, Ho A, Wakefield D. Optimum dry eye classification using questionnaire responses. Advances in Experimental and medical Biology

1998;438:835–8.

7.Schiffman RM, et al. Reliability and validity of the Ocular Surface Disease Index. Arch Ophalmol 2000;118:615–21.

8.Little SA, Bruce AS. Repeatability of the phenol-red thread and tear thinning time tests for tear film function. Clin Exp Optom 1994;77:64–8.

9.Johnson M. Tears and dry eye: Part 3: Diagnosis and management. Optician 2008;5 September.

10. Korb DR, Greiner JV, Herman J. Comparison of fluorescent break-up time measurement reproducibility using standard fluorescent strips versus the Dry Eye Test (DET) method. Cornea 2001;20(8):811–15.

11. Gilbert J. Nutrition and the eye. Dry eye and the role of nutrition. Optometry Today 2004;June.

12. Harvey W. Tear Sampling in practice. Optician 2008;5 September.

13. Kokke K, Morris J, Lawrenson J. Oral omega-6 essential fatty acid treatment in contact lens associated dry eye. Contact Lens and Anterior Eye 2008;31(3):141-6.

14. Guillon J-P, Godfrey A. Chap 5 Tears and Contact Lenses. Contact Lenses 5th ed. B-H; 2007.

15. Mann E. Drugs used in the treatment of dry eye syndrome, anti-inflammatory drugs and topical anti-allergy drugs. Optometry Today 2007;November.

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Section

Preliminaries ONE

Lens types and CHAPTER7 materials

The International Standard method for the classification of both rigid and soft contact lens materials is BS EN ISO 11539: 1999. This is a published European Standard and so has the status of and replaces the former British Standard for contact lenses.

Each material is classified by a six part code (Table 7.1):1

•Prefix.

•Stem.

•Series suffix.

•Group suffix.

•Dk range.

•Surface modification code.

The oxygen permeability (Dk range) part of the code is a numerical designation that categorizes the oxygen permeability in ISO Dk units at intervals which are considered significant in contact lens wear. For both lenses and materials, the oxygen permeability is measured according to ISO 9913-1 or ISO 9913-2 and the Dk range is then denoted by numbers as indicated in Table 7.2.

Example 1. ISO classification applied to a rigid gas-permeable material, Paragon HDS

Paflufocon B III 3

Paflu = USAN prefix.

focon = Stem, indicating a rigid lens material.

©2010 Elsevier Ltd, Inc, BV

DOI: 10.1016/B978-0-7506-7590-1.00011-X

Section ONE Preliminaries

Table 7.1  Material classification

After Hough.1

Table 7.2  Oxygen permeability (Dk range). Dk units are: (cm2/s) · [ml O2/

(ml · hPa)]

0<1 Dk unit

11–15 Dk units

216–30 Dk units

331–60 Dk units

461–100 Dk units

5101–150 Dk units

6151–200 Dk units

7, … Add new categories in increments of 50 Dk units

After Hough.1

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