Section THREE Hydrogel and silicone hydrogel fitting

-3.00 D

-4.00 D

-5.00 D

Figure 17.1  Bausch & Lomb Soflens. The front surface radius was constant, variation in power being achieved by altering the posterior apical radius: the steeper the radius, the higher the negative power

Lens geometry

•The front surface gave the ‘series’, ‘base curve’ or PAR, determined by the constant curvature of the mould during manufacture. This is the opposite of lenses manufactured by conventional lathing or modern techniques of moulding.

•The back surface was aspheric and governed the power (Figure 17.1).

•Minus series generally had a constant sag and centre thickness.

•The higher the minus power, the tighter the fitting.

The original Bausch & Lomb lens now survives only as the disposable SofLens 38 which is still available in some parts of the world.

17.7 Unusual lens performance

Fitting sometimes gives unexpected or unreliable results. If the lens is:

Too tight

•Stored in hypotonic solution, giving temporary osmotic adhesion.

•Causing initial irritation with excessive lacrimation and a hypotonic shift in the patient’s tears.

•Ultrathin with a temporarily distorted shape because it has adhered to the storage vial or blister pack.

Too loose

• Stored in hypertonic solution.

210

Other soft lens fitting considerations 17 Chapter

•Inserted inside out.

•Damaged.

References

1.Gasson AP. Soft (hydrogel) lens fitting. In: Phillips AJ, Stone J, editors. Contact Lenses. London: Butterworths; 1989. p. 382–439.

2.Bennett AG. Power changes in soft lenses due to bending. Ophthalmic Optician 1976;16:939–45.

3.Sarver MD, Ashley D, Van Every J. Supplemental power effect of Bausch and Lomb Softlens contact lenses. International Contact Lens Clinic 1974;1:100–9.

4.Wichterle O. Changes of refracting power of a soft lens caused by its flattening. In: Girard LJ, editor. Corneal and Scleral Contact Lenses, Proceedings of the International Congress. Paper 29. St Louis: Mosby; 1967. p. 247–56.

211

18.1 Frequent replacement lenses

Considerably fewer clinical difficulties arise when lenses are replaced on a regular basis. Deposits are avoided and there are significantly reduced risks of discomfort, CLIPC, infections and red eyes. The distinction is imprecise but disposable is generally used to describe lenses replaced monthly or more often, and frequent replacement for lenses disposed of on a planned basis of over 1 month. Conventional now refers to lenses replaced on an annual or unplanned basis.

It has always been difficult to define the lifespan of a soft lens, since it depends on a variety of factors:

•Water content and material.

•Lens thickness.

•Method of disinfection and cleaning.

•Wearing time.

•Daily or extended wear.

•Tear chemistry.

•Handling ability.

•Environment.

A reasonable average time after which lenses needed either replacement or thorough professional cleaning used to be considered about 12 months. This

©2010 Elsevier Ltd, Inc, BV

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