Types of contact lenses
There are two main types of contact lens: soft lenses made of water-containing plastic; and gas permeable (or 'rigid gas permeable') lenses which are less flexible.
Soft contact lenses ('hydrophilic' or 'hydrogel')
These lenses, as their name suggests, have a soft structure, a bit like a piece of thick clingfilm, making them very comfortable to wear. In most cases they are larger in size than their gas permeable counterparts, cover the whole of the iris (the coloured part of the eye) and cornea (the transparent front part of the eye), and rest on the sclera (the white of the eye). This is the most common type of contact lenses fitted today.
Soft lenses are often described by their replacement frequency or wearing schedule. Replacement may be daily, two-weekly, monthly, or in some cases three-monthly or six-monthly. The lenses may be used on a daily-wear basis or sometimes for up to 30 days of extended (or 'continuous') wear. The most commonly fitted soft lenses in the UK are daily disposable contact lenses. Soft contact lenses come in a wide variety of materials, fittings, powers and designs to correct almost all types of vision. Soft lenses incorporate water, much like a sponge, and must be kept in contact lens solution to prevent them from drying out.
Advances in materials have led to a new generation of soft contact lenses called silicone hydrogels, which allow much more oxygen to pass through to the cornea than previous soft lens materials, making them healthier for the eye. Originally intended for extended wear, these materials are now used for all types of soft lenses.
Contact lenses for astigmatism ('toric' lenses), bifocal and multifocal lenses are all available in soft materials. Coloured and special-effect soft lenses to change the colour or appearance of the eyes rather than correct eyesight are known as 'zero-powered' (or 'plano cosmetic') lenses and can also be used by specialists to mask eye injury or disfigurement. Many soft contact lenses incorporate a UV (ultraviolet) inhibitor to help protect the eye.
Gas permeable contact lenses ('rigid gas permeable' or RGP)
Rigid contact lenses have been available for longer than soft contact lenses, although many improvements have been made over this time to allow more oxygen to pass through the material. These lenses are smaller than soft lenses and usually rest within the corneal area.
Rigid gas permeable (RGP) lenses come in an extensive range of materials, fittings, power and designs. Despite a decline in the use of rigid lenses, some contact lens practitioners believe RGP lenses provide a healthier option for long-term full-time wear than soft contact lenses. They are better at correcting irregularly shaped eyes than soft lenses, and are more durable so are usually replaced every six or 12 months. They take a little longer to get used to than soft lenses, but regular wearers find them comfortable.
Contact lenses for astigmatism ('toric' lenses), bifocal and multifocal lenses are all available in gas permeable materials. RGP lenses are normally used for daily wear but recently there has been renewed interest in a technique called orthokeratology ('corneal reshaping' or 'overnight vision correction') in which specially designed RGP lenses are worn overnight and removed during the day. The aim is alter the shape of the cornea in order to reduce or correct short sight, thereby alleviating the need for contact lenses or spectacles during waking hours.
Your eyecare practitioner will help you to decide which type of contact lens is best suited to your needs.