What types of contact lenses are there?
In linguistic usage one differentiates between "hard" and "soft" contact lenses. Both forms have advantages and disadvantages.
Hard contact lenses
Modern hard contact lenses are made of extremely lightweight plastic, which is well oxygen permeable and generally tolerated without problems. They are stable for 2 to 4 years, easy to clean and easy to handle. These gas-permeable, dimensionally stable contact lenses are smaller than the cornea and feel "hard" when touched.
They have very good optical properties, can compensate for high diopter numbers and, above all, a corneal curvature or irregularity. However, they need to be adjusted very precisely and trigger a foreign body sensation rather than soft contact lenses.
Soft contact lenses
Soft contact lenses consist of so-called hydrogel. This material absorbs water like a sponge. As a result, many dirt particles can store, so special care is needed. Alternatively, soft silicone lenses are offered.
Soft lenses can be worn for an average of one and a half years. They are bigger than dimensionally stable lenses. The acclimatization time is shorter, the wearing comfort often much better than with hard lenses. They adhere securely to the eye and slip less frequently than hard lens. Since they adapt very well to the ocular surface, they usually do not cause a foreign body sensation. However, they cause complications more often.
Both lens types must be taken out of the eye before sleeping, cleaned and stored in a special cleaning solution - they must not dry out! In addition, there are soft lenses that can be worn continuously from one week to a month. However, these lenses cause more complications than regular contact lenses and are therefore only recommended in exceptional cases.
For particularly sensitive people, computer workers or on vacation, there are also soft disposable lenses, 2-week or 4-week lenses and quarter lenses and especially oxygen-permeable models with high water content.
Orthokeratology lenses (OK lenses)
Since 2003, special hard, highly breathable lenses have been available on the German market for myopia up to -4.5 diopters, the so-called OK or Ortho-K lenses (orthokeratology lenses). These are worn only at night for 8 hours, during the day, the affected then need no visual aid. The principle is based on the fact that the lens flattened by gentle pressure, the cornea, which protrudes a bit too far in short-sighted.
There are currently only a few ophthalmologists and opticians in Germany who are familiar with the OK lenses and have completed special training for them. The individual initial adjustment is not cheap, the familiarization often difficult and there must be regular checks at the optician (at the beginning several times a week, then about every three months) are performed.
In the evening, many people gradually return to sight, as the eye returns to its old form. Pressure damage to the cornea and damage due to the lack of care of the eye with oxygen and tear fluid are currently not completely ruled out.