Soft lens specification and verification 20 Chapter

ficult because the small difference in the refractive index of the solution largely disguises any surface flaws or cracks.

20.3.7  Water content and material

Water content can be estimated with a refractometer because of its inverse relationship with refractive index. An instrument developed for contact lens measurement is the Atago CL-1 refractometer.4 Assessing the lens material from the water content alone is far from certain, so identification may depend upon other clues such as style of engraving (see Section 25.1), handling tint or type of edge bevel.

20.3.8  Deposits

Deposits on the lens surface are best seen with the slit lamp where dark field illumination and high magnification can be achieved.5 Some lens deposits fluoresce with ultraviolet light.

PRACTICAL ADVICE

• The easiest methods for practitioner verification are:

•Wet cells are a source of possible cross-infection and require regular cleaning and disinfection.

•Particular care is required when flattening lenses against a glass surface (e.g. microscope stage or refractometer) because of the difficulty of removal and the risk of damage.

References

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

2.Cagnolati W. Lens checking: soft and rigid. In: Phillips AJ, Speedwell L, editors. Contact Lenses. London: Butterworths; 2007. p. 355-74.

3.Poster MG. Hydrated method of determining dioptric power of all hydrophilic lenses.

Journal of the American Optometric Association 1971;43:287–99.

4.Efron N, Brennan NA. The soft contact lens refractometer. Optician 1987;94(5115):29–41.

5.Killpartrick MR. Soft lens contaminant detection by dark field illumination. Optician 1987;193(5083):34–7.

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Section

Hydrogel and silicone hydrogel fitting three

Extended wear 21CHAPTER

Soft lenses have been routinely fitted for extended wear since the early 1970s with the introduction of Permalens and Sauflon PW. Improvements have since occurred not only in lens design and materials but also in understanding the physiological requirements of the cornea necessary to achieve extended wear with relative safety. More recently, high Dk rigid lenses have been used but the introduction of silicone hydrogels in 1999 has revolutionized the concept of extended wear in practice and made them the first choice for most patients who desire overnight use.

21.1 Physiological requirements

21.1.1 Oxygen requirements of the cornea

For any lens to be successful for extended wear, it must satisfy the physiological requirements of the cornea under closed eye conditions. The main effect is a reduction in the available atmospheric oxygen from 155 mmHg to the 55 mmHg derived from the palpebral conjunctival capillaries. In addition, the cornea’s demand for oxygen increases along with an increase in lens temperature and an acidic shift in the pH of the tears. The consequence of these changes in a normal eye not wearing a contact lens is overnight corneal swelling of about 4%. The effect of most contact lenses is to increase this swelling still further.

©2010 Elsevier Ltd, Inc, BV

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

Section three Hydrogel and silicone hydrogel fitting

Limiting any hypoxic effect on the cornea is essential for successful long-term wear of any contact lens. In practical terms, the following physiological requirements must be met (using Fatt units):1

(i.e. to achieve no greater swelling than that normally encountered overnight without a contact lens).

None of the previous generation of purely hydrogel lenses is able to fulfil these critera, since the theoretical maximum Dk is approximately 40 × 10−11, limited by the water content of the polymer. Extended wear with these materials, therefore, almost always risks compromising corneal health and should only be undertaken with extreme caution.

Silicone hydrogel polymers have very high transmissibility and effectively overcome the problem of corneal swelling. Examples are PureVision (Balafilcon A, Dk/t = 110 × 10−9, Bausch & Lomb), Air Optix Aqua Night & Day (Lotrafilcon A, Dk/t 175 × 10−9, CIBAVision) and Biofinity (Comfilcon A, Dk/t 116 × 10−9 CooperVision) (see Chapter 19).

Rigid gas-permeable materials can also provide four or five times the permeability of even high water content hydrogel lenses before taking into account any tears pump effect. Lenses with a Dk in the region of 100 provide approximately 15% EOP. The tears pump adds about 2% oxygen to give a total EOP of 17% for open eye conditions and 4% for closed eye conditions. This is sufficient to avoid epithelial compromise in extended wear for most patients.2 A Dk in the region of 150–200 provides even better physiological performance.3

21.1.2 Effects of insufficient oxygen

Extended wear causes chronic cumulative hypoxia to a much greater extent than daily wear because of continuous eye closure during sleep, with sometimes inadequate time for corneal deswelling during the day. Between 10% and 15% oxygen is needed to avoid oedema and measurable corneal changes.4

•Epithelial changes include decreased glycolysis,5 decreased mitosis,6 decreased cell adhesion7 and reduced sensitivity.8

•The stroma shows oedema which eventually leads to stromal thinning.5

•Endothelial cells show distinct polymegathous changes similar to those seen in the corneal exhaustion syndrome.5

21.1.3 Tears exchange and osmolarity

There is a normal increase in corneal thickness overnight, mainly due to a change in tear osmolarity causing swelling. This rapidly disappears on eye opening. Soft lens materials may become tighter with these changes and lens adhesion may be found in the morning. Lenses usually start moving after a few blinks and this movement is very important to allow tears exchange to eliminate overnight debris from beneath the lens. This might otherwise cause a toxic effect

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