Silicone hydrogels 19 Chapter

Conjunctival epithelial flaps

Conjunctival epithelial flaps (CEF) are a more recently reported phenomenon, more commonly associated with silicone hydrogel extended wear but also observed with rigid lenses. Flaps represent delamination of conjunctival tissue and may give rise to dry eye symptoms and an increased risk of inflammatory events. CEFs are a mechanical effect on the bulbar conjunctiva within about 5 mm of the limbus and relate to the contact lens edge shape. They are observed with fluorescein pooling at the affected area.5

Lens deposits

Those silicone hydrogels with surface treatment are relatively unaffected by white spots and other deposits. Lipid formation is sometimes encountered and lenses should be cleaned by rubbing after removal from the eye. Deposits are rarely a serious problem since lenses are replaced on a regular basis.

Lens prescription

Where extended wear lenses have been worn with earlier generations of relatively low Dk hydrogel materials, there may well be a reduction in myopia between 0.25 D and 0.50 D after 1 or 2 weeks. The lens prescription therefore needs careful verification at aftercare checkups.

GENERAL ADVICE

•Particularly with extended or continuous wear, the importance of aftercare must be stressed to all patients.

•Extended wear patients, in particular, should be discouraged from obtaining lenses from postal or internet sources because they may be seen less frequently for aftercare examinations.

References

1.Efron N, Morgan PB, Hill EA, Raynor MK, Tullo AB. Incidence and morbidity of hospital-presenting corneal infiltrative events associated with contact lens wear.

Clinical and Experimental Optometry 2005;88:232–9.

2.Andrasko G, Ryen K. A series of evaluations of MPS and silicone hydrogel lens combinations. Review of Cornea and Contact Lenses 2007;March:36–42.

3.Fleming C, Austen R, Davies S, Bolis S, Papas E, Holden BA. Pre-corneal deposits during soft contact lens wear. Optometry and Vision Science 1994;71(suppl): 152–3.

4.Pritchard N, Jones L, Dumbleton K, Fonn D. Epithelial inclusions in association with mucin ball development in high-oxygen permeability hydrogel lenses. Optometry and Vision Science 2000;77(2):68–72.

5.Graham AD, Truong TN, Lin MC. Conjunctival epithelial flap in continuous contact lens wear. Optometry and Vision Science 2009;86(4):318–23.

Further reading

Dumbleton K, Jones L. Introducing silicone hydrogel lenses. Part 1 – Material properties and patient selection. Optician 2002;223(5836):16–22.

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Section three Hydrogel and silicone hydrogel fitting

Dumbleton K, Jones L. Introducing silicone hydrogel lenses. Part 2 – Fitting procedures and in-practice protocols. Optician 2002;223(5840):37–45.

Sweeney DF. (2000) The rebirth of extended wear. The Optician, Contact Lens Monthly, 1 October 1999–4 February 2000.

Sweeney DF, editor. Silicone hydrogels: the rebirth of continuous wear contact lenses.

Oxford: Butterworth-Heinemann; 2000.

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20.1 International standards and tolerances1

Soft lens specification is currently based on BS EN ISO 8321-2:2000 (BS 720824:2000) Ophthalmic optics – Specifications for materials, optical and dimensional properties of contact lenses – Part 2: Single-vision hydrogel contact lenses.

20.2 Soft lens specification (Tables 20.1, 20.2)

A typical soft lens specification takes the following abbreviated form: 8.60:14.00  −3.00  silicone hydrogel  36% water content

where 8.60 = back optic zone radius (BOZR)

   14.00 = total diameter (TD)

   −3.00 = BVP

The majority of lenses are made according to predetermined laboratory designs and it is not feasible for the practitioner to specify parameters such as thickness, lenticulation and peripheral curves.

20.3 Soft lens verification

Soft lenses in theory require verification for the same reasons as rigid lenses (see Section 13.3), although checking is considerably more difficult because parameters vary with:

•Degree of hydration.

•Temperature.

•pH and tonicity of storage solution.

©2010 Elsevier Ltd, Inc, BV

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

Section three Hydrogel and silicone hydrogel fitting

•Time taken if measurement is in air.

•Method of supporting lens.

With disposables it is rarely desirable to break the sterility of the manufacturer’s blister pack to verify lenses in advance of dispensing or supply to the patient. Modern mass production methods by moulding have considerably improved reproducibility although, in some instances, it becomes necessary  to measure a lens where its performance on the eye is not as anticipated.  Generally, this is only in respect of power where a resonable assessment can be made.

Table 20.1  Recommended tolerances for soft lenses

Table 20.2  Material property tolerances

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Soft lens specification and verification 20 Chapter

20.3.1 Back optic zone radius

Spherometers

Many common methods of radius checking rely on the principle of spherometry, where the sagittal depth is measured for a given chord diameter. The majority of instruments employ a wet cell, but this is not essential.

Wet cell instruments (e.g. Optimec)

•The lens is rinsed and placed in a wet cell containing 0.9% saline solution.

•The lens is centred on a support device.

•A probe is advanced towards the concave surface until it just touches (established by viewing lens movement or electric contact) and the radius is read from the instrument scale.

Base curve comparator

A simple device consisting of a series of spheres of known radius on which the contact lens is placed and compared by straightforward observation in air.

Keratometer

This can be used in conjunction with a wet cell.2 Measured radius, however, requires conversion to actual radius and observation is difficult because the light intensity is considerably reduced.

20.3.2 Total diameter

Projection magnifiers

An optical system projects a magnified image of the lens on to a calibrated screen. Instruments may use a wet cell (e.g. Optimec) or support the lens in air on a microscope stage.

Comparison

Comparators consisting of a series of annuli of known diameter can be used as a rapid in-air method.

20.3.3 Power

The most accurate method of measuring power is by power profile mapping. This is generally only employed by manufacturers because of the very high cost of the instrumentation. The usual methods used by practitioners are:3

Air measurement

•Rinse the lens with saline.

•Shake off surplus fluid and dry carefully with a lint-free tissue.

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Section three Hydrogel and silicone hydrogel fitting

•Place lens concave surface down on a reduced aperture focimeter stop.

•Read power directly from focimeter scale.

Wet cell measurement

•Place lens in wet cell.

•Mount on focimeter.

•Read power from focimeter scale.

•Convert to approximate power in air by multiplying by 4.

PRACTICAL ADVICE

•Power is more easily and more accurately measured in air.

•Any error in wet cell measurement is magnified fourfold (an error of 0.25 D 1.00 D in air).

•Power in air is reasonably constant for up to about 1 minute.

•Best results are obtained with a projection focimeter.

20.3.4 Thickness

Projection magnifiers

The easiest method of measuring thickness is by means of a millimetre scale calibrated for the screen of a projection magnifier (e.g. Optimec).

Drysdale’s method

A rigid lens radiuscope can be used to determine lens thickness:

•Place the lens, concave surface down, on a convex sphere (this should have a curvature steeper than the lens to be measured to ensure contact at the centre).

•Focus the target on the surface of the sphere with the lens in place.

•Set the scale to zero.

•Focus the target on the front surface of the lens.

•Read the distance travelled from the scale.

•Multiply by the refractive index of the material to give the actual centre thickness.

20.3.5  Edge form

Edge form and integrity are best observed with projection magnification but a hand loupe or the slit lamp can also be used.

20.3.6  Surface quality

Surface quality is best assessed with the slit lamp either on or off the eye. Projection magnification can also be used but observation with a wet cell is more dif-

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