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. They are larger 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, six-monthly or less often. 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 wear, monthly replacement lenses, followed by daily disposable single-use lenses, worn for a day then thrown away.
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, 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 – called 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 are also used to mask eye injury or disfigurement. Some soft contact lenses also incorporate a UV (ultraviolet) inhibitor to help protect the eye.
Gas permeable contact lenses (‘rigid gas permeable’ or RGP)
RGP 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. RGP lenses represented only five per cent of contact lens market in 2011. This is probably because they take a little longer to get used to than soft lenses, although regular wearers find them comfortable.
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 thought to be better at correcting irregularly shaped eyes than soft lenses and are also more durable so are usually replaced every six or 12 months, making them a very cost-effective option.
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 a technique called orthokeratology (‘corneal reshaping’ or ‘overnight vision correction’) is gaining in popularity. This is where specially designed gas permeable lenses are worn overnight and removed during the day. The aim is to alter the shape of the cornea in order to reduce or correct short sight.
Contact lens solutions
Most contact lenses need to be cleaned and disinfected with solutions in order to provide optimal performance and, in the case of soft lenses, to keep them hydrated. The exception is daily disposable lenses, worn for a day and thrown away, which do not require solutions and should never be stored once opened.
The aim of a disinfecting solution is to reduce the number of microorganisms that accumulate on the lenses with wear and to minimise the risk of infection with contact lenses. Cleaning solutions can also improve comfort by conditioning the lens surface and making it more wettable. A ‘rub and rinse’ step with solution is also recommended as part of your lens care regime, before you put the lenses on your eyes and before you store them. This step helps to physically remove some of the substances that deposit on the lens surface during wear.
All lens care procedures are important but some hygiene measures are crucial. Make sure you empty the lens case of solution after each use, clean it with fresh solution and air dry it, then refill with fresh solution each time the lenses are stored. Your lens case should be replaced on a regular basis, usually every month. Poor lens case hygiene can increase the risk of problems with your lenses.
There are many types and brands of contact lens solution, each with different ingredients and for different lens types. Some lens/solution combinations are not compatible so it’s important to use the solution recommended to you by your contact lens practitioner and always to follow instructions carefully. It is good practice for your practitioner to record the recommended solutions on your contact lens specification. Contact lens solutions carry an expiry date and the discard date is normally 2-6 months after opening. If you don’t use your lenses every day, it’s a good idea to mark the date of opening the solution on the bottle so you know when to throw it away.
Multipurpose solutions are the simplest and most convenient type of contact lens solution and are designed to clean, rub, rinse and store lenses. The minimum storage time to ensure adequate disinfection varies but is generally 4-6 hours. Hydrogen peroxide solutions work in a different way and incorporate a metallic disc in the case, or tablet to add to the solution, or different solution to use, to neutralise the disinfectant before wear. The minimum storage time is usually 6 hours. With these solutions it’s particularly important to follow instructions to avoid discomfort.
Other lens care products include special cleaning drops and tablets, usually for rigid lenses, and also comfort and re-wetting drops which can be used with the lenses on the eye. Saline solutions are sometimes recommended for rinsing lenses before they are applied to the eye but should not be used for storing lenses. Only use these products if advised to do so by your contact lens practitioner. No other solutions, including any type of water, or saliva, should come into contact with your lenses.