In OA Corner last month, we looked at the advantages and disadvantages of contact lenses in general. This time, we’ll consider the different types of contact lenses available in more detail.
Contact lenses can be classified into the following types:
Soft hydrogel contact lenses are larger than RGP lenses and are designed to cover the cornea and limbus. The lenses are soft and flexible but will become brittle and unusable if they dry out, and so must be stored in solution when not in the eye. If the patient only wants them for occasional use, they may be a better option as the adaption time is faster than RGP lenses. Solution allergies are possible, and deposits from the tears can adhere to the lens, causing irritation and discomfort. Nowadays, frequent replacement lenses, such as monthly or daily replacement, are recommended to avoid this.
Silicone hydrogel contact lenses are soft lenses but made from a material that allows more oxygen through the lens, to the eye. This means there are less likely to be complications for the patient, and they can wear the lenses for longer. The lenses can feel thicker than hydrogel lenses, which can cause initial adaption problems for some patients.
Soft lenses are available as daily disposable lenses, two weekly and monthly lenses. Daily disposables are convenient and healthy for the eyes as they do not to be stored in solution and are worn once and then thrown away. This makes them ideal for patients who do not want to wear lenses on a full-time basis, and ideal for travelling.
Two weekly lenses are more cost-effective than daily disposables, and there is less packaging and less waste. Contact lens leaning and storage solution is needed, and the patient must clean and store the lenses every day correctly to ensure problems do not develop.
Monthly contact lenses are the most cost-effective lenses, but there can be a higher risk of infection and complications as the lenses are kept and handled for longer, and so lens hygiene and patient compliance is very important.
Scleral contact lenses are large, inflexible RGP lenses – between 22mm and 25mm in diameter – and are designed to cover the cornea and sclera. The lenses are held in place by the lids, and so cannot become displaced from the eye. They can be useful for certain pathological conditions, and can correct larger amounts of astigmatism, including corneal and irregular astigmatism. As scleral contact lenses cover a large part of the eye, it is very unusual for foreign bodies to become trapped underneath the lens. They are used less commonly in practice than other lens types.
RGP contact lenses are smaller lenses, between 8.5mm and 10.5mm in diameter. They are designed to rest on the tear layer of the eye and can correct some corneal astigmatism by the tears underneath the lens filling in the irregular corneal shape. They have good oxygen transmission to the eye, and so can present fewer long-term complications than soft lenses. They are generally harder to adapt to than soft lenses and the wearing time has to be built up more gradually. The lenses will last longer than soft lenses, and do not have to be replaced as frequently, lasing up to 18 months.
Generally, the shorter the replacement time, the more expensive the lens, but there are less likely to be compliance issues. There are other factors that determine lens type and frequency of lens replacement, which we will look at next month.
Sue Deal FBDO R is a practising dispensing optician, ABDO College examiner, senior tutor and supervisor for dispensing opticians. She is also a practice visitor and external moderator for ABDO. She was recently awarded the ABDO Medal of Excellence for her outstanding services to the profession.