Eye Care Blog

The immediate appeal of contact lenses is social. The desire of spectacle wearers to be ‘normal’, to appear in public without a facial contraption, is very real. In the USA, Europe and Japan most young short-sighted people have, by the age of twenty-five, at least tried contact lenses if their vision requires the daily or continual use of spectacles. Some patients will even persist through considerable discomfort in order to achieve ‘normalization’. And as more and more people adopt contact lenses, it becomes less and less normal to wear glasses. It is as well therefore that, apart from its cosmetic advantages, the contact lens also has some optical advantages. The contact lens moves with the eye and therefore corrects vision wherever the eye moves. Used to correct short-sightedness, it results in a larger image, and therefore gives better vision than spectacles even when the eye is looking straight ahead. The greater the degree of short sight, the more dramatic the difference. It is even said, by some practitioners as well as patients, that contact lenses can arrest the progressive development of short-sightedness. But while there is no evidence to suggest that contact lenses prevent the eye from gradually elongating, it may well be that some beneficial effect is produced on the cornea. Alternatively, it may be that the contact lens wearer does not read so much.

When a spectacle wearer changes over to contact lenses there are usually alterations in his or her behaviour and personality. A short-sighted person in contact lenses is often a more extroverted individual. It is small wonder that contact lenses are held in such high repute. People will talk about them, openly and without encouragement, in a way they never did about glasses.

Some ophthalmologists and optometrists have put forward theories and some proof that contact lens wearers with short sight have less need to accommodate because of the larger image size, or because the internal ocular pressure becomes slightly decreased when contact lenses are being worn. In clinical practice it does appear that there is slower progression of short sight in contact lens wearers in the age group 8-16. Also, the need to change contact lenses, or at any rate their prescription, becomes less since the optical tolerances of the device are cruder than is the case with spectacles. Thus though the short-sighted eye may have changed by half a dioptre, the patient still sees the small letters on the distance chart, whereas with glasses there is no such tolerance of change.

The contact lens in its small form is a modern invention, dating from the 1950s. Contact lenses did exist before that time, but they were much larger, covering the whole of the front of the eye. And it was not until the late 1960s that softer materials were used (`soft’ contact lenses), and only in very recent years have soft coloured and very thin optical forms been available.

At present there are three ways of classifying a contact lens:

Classification according to size

Scleral (haptic): 13-26 mm in diameter Corneal: 7.5 to 13 mm

(most contact lenses are of the second, smaller size)

Classification according to material

Hard or rigid

Soft hydrophilic (water content over 60 per cent) Soft silicone rubber

Classification according to gas permeability

High gas (or oxygen) Low gas (or oxygen) No gas

By and large, hard contact lenses cover only part of the cornea over the centre of the iris and can be gas-permeable, while soft contact lenses cover the whole iris, or cornea, and are gas- permeable in varying degrees. Both sorts of contact lens are worn daily, but taken out at night, though a recent development has produced a semi-permanent soft contact lens that stays in contact with the cornea for periods up to six months. More recently still, in Japan, a semi-permanent gas-permeable hard lens has been manufactured. These latter varieties, though they have obvious user advantages, carry with them certain risks, mainly that they can cause damage to the cornea and lids. Some practitioners will not prescribe them, or will only prescribe them if the patient provides a written undertaking that the practitioner will not be held responsible for any injury incurred. They do, however, have a real application for certain kinds of eye disease, and for individuals unable to handle lenses on a daily basis; and they are the logical goal for perfection in contact-lens performance. The ordinary soft contact lens is also reputedly more comfortable to wear, though it requires a good deal more attention in its care (at night they have to be specially cleaned, sometimes heated, and allowed to soak in a prepared solution), whereas the hard contact lens, which takes more getting used to, needs only to be cleaned. The hard contact lens has a much longer life, indeed it may last indefinitely, and so is less expensive than the soft contact lens, which generally must be renewed every one to two years. In advising you which sort of contact lens to adopt, hard or soft, the practitioner may well make an undisclosed assessment of your character, whether or not you are the sort of person who takes good care of things, and to a lesser degree he may assess your financial well-being.

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