Advantages and Disadvantages of wearing Silicone Hydrogel lenses
- silicone hydrogel lenses
- the benefits of silicone hydrogel lenses
- special silicone hydrogel lenses
- the risk of keratitis
- allergies to silicone hydrogel lenses
- who are silicone hydrogel lenses suitable for?
Silicone hydrogel contact lenses are sophisticated soft lenses that allow more oxygen to pass through the lens to the cornea than regular soft hydrogel lenses. In fact, this type of lens allows up to five times more oxygen to reach the cornea than classic hydrogel lenses.
Silicone hydrogel and regular hydrogel lenses are both made of plastic that is hard in a dry state but absorbs water fairly easily and becomes soft, much like a gel when moistened.
If you've ever let a soft or silicone hydrogel lens dry out, you know it loses its shape and becomes hard and fragile. But if you soak the dried gel in a lens solution for a few minutes, it becomes soft and pliable again.
In 2013, silicone hydrogel lenses already accounted for 66% of all lenses worn in the United States, compared to about 24% for regular soft lenses, 8% for gas-permeable lenses, and 2% for hybrid contact lenses.
Popular brands of silicone hydrogel lenses include:
- Acuvue Oasys (Johnson & Johnson.)
- Air Optix Aqua (Alcon)
- Biofinity (CooperVision)
- PureVision2 (Bausch + Lomb)
Silicone hydrogel (not silicon hydrogel) contact lenses
Silicone hydrogel lenses are sometimes mistakenly called silicon hydrogel lenses. Silicon is a common mineral. In fact, ordinary sand is mainly composed of silicon dioxide (silica).
This well-purified substance is used to make semiconductors, which is why the southern part of the bay of San Francisco, where Apple and many other high-tech and computer companies are located, has earned the nickname Silicon Valley.
Silicone is the name of a group of flexible, plastic-like materials that contain silicon, carbon, oxygen, and other chemicals. In addition to its use in increasing the oxygen-permeable properties of contact lenses, silicone is also found in breast implants, medical tubing, and other medical devices.
Silicone hydrogel lenses are soft lenses, but silicone is also used in the production of many rigid gas-permeable lenses to improve the oxygen-permeable properties of the lenses.
The benefits of silicone hydrogel lenses
All lenses reduce to some extent the amount of oxygen that reaches the upper surface of the eye. When the oxygen supply to the retina is severely reduced, a condition known as hypoxia, a number of problems can occur, such as red eyes, swelling of the retina, blurred vision, and discomfort when seeing.
This condition can also increase the risk of developing a range of eye infections.
Problems related to this condition in the eye became a major topic in the 1990s as wearing lenses for extended periods became increasingly popular. As the number of people wearing their lenses overnight and continuously for several days increases, unfortunately, so does the number of infections that can be associated with wearing lenses.
Silicone hydrogel lenses were specifically introduced in the hope of reducing the problems associated with this condition (Hypoxia) and increasing the safety of both people who wear their soft lenses daily and for extended periods.
Increasing the oxygen supply to the eye is potentially a good thing for all lens wearers, especially when you consider that many wearers do not always closely follow the specialist's instructions regarding the correct wearing and replacement of the lenses.
According to one of the major companies, Alcon, about one-fifth of all soft lens wearers wear their lenses during sleep, and of those patients who sleep with their lenses at night, two out of three use lenses that are not actually suitable for overnight wear.
Special silicone hydrogel lenses
The increasing permeability of oxygen in silicone hydrogel lenses makes them particularly suitable for special designs of contact lenses that, if made with conventional soft hydrogel materials, could cause problems for the eyes.
This includes all designs that require a larger lens mass, such as toric contact lenses for astigmatism, bifocal lenses, lenses for eyes that are difficult to fit, and custom-made lenses, such as soft lenses for keratoconus.
The risk of keratitis (corneal inflammation) when wearing soft silicone hydrogel lenses
Do silicone hydrogel lenses reduce the risk of keratitis and other eye infections? Yes and no.
A study published in the British Journal of Ophthalmology concluded that wearers of soft silicone hydrogel lenses had a significantly lower risk of developing a severe form of keratitis, but a higher risk of a less severe form of this condition.
The study was conducted in a hospital over a period of 12 months in the United Kingdom.
The study showed that the annual occurrence of the non-severe form of this condition was low in both groups, with a total of 14.1 per 10,000 wearers or 0.14% being affected with the condition in wearers of hydrogel lenses and 55.9 per 10,000 wearers or 0.56% in wearers of silicone hydrogel lenses.
There were no cases of severe keratitis in patients who wore silicone hydrogel lenses daily, whereas those who were assigned the regular hydrogel lenses for daily use developed the condition in 6.4 cases out of 10,000.
The risk of the condition was significantly greater in both groups of soft lens wearers who also wore their lenses overnight.
Continuous wear of hydrogel lenses caused 48.2 cases of non-severe keratitis and 96.4 cases of severe keratitis per 10,000 wearers.
Silicone hydrogel lenses for extended use caused 98.8 cases of the first condition and 19.8 cases of the second condition per 10,000 wearers.
The researchers concluded that there is a significantly higher chance of severe keratitis in people who sleep with their contact lenses in compared to those who only have their lenses in when they are awake.
They therefore recommend that people who want to wear their contact lenses continuously should be advised to switch to silicone hydrogel lenses. This type of lens appears to be much safer for nighttime use than regular hydrogel lenses.
Critics of silicone hydrogel lenses have seized on this study as evidence that silicone hydrogel lenses may be associated with a higher risk of non-severe keratitis compared to classic hydrogel lenses.
It should be noted that the study did not evaluate how many people who wore silicone hydrogel lenses and had non-severe keratitis switched back to conventional hydrogel lenses prior to the study period because they had previously experienced similar problems with wearing hydrogel lenses.
It should also be mentioned that during this study, the only silicone hydrogel lenses allowed in the United Kingdom were designed for a monthly replacement, whereas several brands of classic hydrogel lenses are available for a more frequent replacement every two weeks.
Today, there are already several brands of silicone hydrogel lenses in the United States and Europe that are designed to be replaced every two weeks or every month. There are even silicone hydrogel lenses available that you can throw away and replace every day.
Allergic reaction to wearing silicone hydrogel lenses
A series of blogs, websites, and online forums can be found with numerous complaints from lens wearers who have developed allergic reactions to wearing silicone hydrogel lenses.
The most common complaints are red eyes, discomfort, itching, and a greater awareness of the presence of the lens or symptoms of dryness. It should be mentioned that these symptoms can also be caused by dry contact lenses or sensitivity to a new solution used for their maintenance.
Silicone hydrogel lenses do allow more oxygen to reach the surface of the eye, but the added silicone can reduce the moisture of the surface of the lenses, potentially making it more difficult for them to stay sufficiently moist on the eye.
To counteract this tendency, new contact lens solutions have been developed that contain special elements to keep the silicone hydrogel lenses moist throughout the day.
The moisture of the lenses will not be a problem for most users, but people with particularly dry eyes may still experience a feeling of dryness and discomfort when wearing silicone hydrogel lenses that they did not have with their previous classic hydrogel lenses.
This discomfort, which is mainly related to dryness, can be mistakenly seen as a symptom of an allergy to silicone hydrogel lenses.
Because silicone hydrogel lenses allow more oxygen to reach the eye, some wearers will become more aware of the fact that they are wearing lenses for another, rather paradoxical reason.
One factor why soft lenses are so comfortable for many users is precisely the reduced supply of oxygen to the front of the eye because wearing the lenses reduces the sensitivity of the surface of the retina.
When more oxygen is available by wearing these highly oxygen-permeable silicone hydrogel lenses, the cornea may retain its normal sensitivity, which makes the wearer more aware that there is a lens on his or her eye.
In a study published at the end of 2010 in a specialized journal for ophthalmology and visual science, researchers discovered that patients who were fitted with silicone hydrogel lenses or replaced classic hydrogel lenses with silicone hydrogel lenses experienced a clear increase in the sensitivity of their retina.
The study also found that changes in the sensitivity of the retina in wearers of silicone hydrogel lenses occurred when different types of lens care products were used.
The findings of the study led the scientists to the ultimate conclusion that the effect of wearing soft lenses on the functioning of the sensitivity nerves on the top of the eye appears to be more complex than initially assumed.
The authors of the studies therefore recommended additional studies to investigate how the sensitivity of the surface of the eye can change after long-term use of silicone hydrogel lenses and whether the sensitivity of the retina and surrounding tissues contributes to a feeling of discomfort at the end of the day that is often reported by wearers of hydrogel and silicone hydrogel lenses worldwide.
Regarding the possible allergies that people may have to silicone hydrogel lenses: there is currently no real formal evidence for this statement available.
At the time of writing, there appear to be no confirmed cases of allergic reactions to silicone hydrogel lenses according to reports from the US Food and Drug Administration FDA or from the similar institute for medical and health concerns in the United Kingdom. Also, no clinical reporters have been published in leading medical newspapers or journals in any country.
Are silicone hydrogel lenses suitable for you?
Most ophthalmologists and eye specialists agree that the most obvious beneficial effect of silicone hydrogel lenses is the reduction of problems that we can relate to hypoxia that are associated with soft lenses made of regular hydrogel materials.
This has clearly resulted in a decrease in the number of cases of red eyes due to wearing lenses, a feeling of discomfort and serious keratitis problems when wearing contact lenses for an extended period of time.
It is also plausible that due to the increased transmission of oxygen, silicone hydrogel lenses have allowed many people to wear their lenses in a comfortable way, and especially for longer, than with classic hydrogel lenses.
Silicone hydrogel materials have also allowed lens manufacturers to offer a wider choice of lens designs that ensure a healthy amount of oxygen to reach the retina in order to wear the lenses comfortably during the day or even continuously.
Yet, the new silicone hydrogel lenses have not been able to solve all the problems related to discomfort when wearing contact lenses.
For example, some people feel more like they are wearing a lens when they switch to silicone hydrogel lenses than when they would wear classic hydrogel lenses.
Some studies have found that silicone hydrogel lenses are more sensitive to certain types of deposits on the lenses than classic hydrogel lenses, and some contact lens solutions may be less effective on silicone hydrogel lenses than on regular soft lenses.
Also, due to their increased permeability of oxygen, silicone hydrogel lenses are associated in some studies with a greater risk of inflammation of the retina and other complications in some patients, compared to classic hydrogel lenses.
The reason for these remarkable problems remains unclear for the time being.
Some experts say that there is no hard evidence that increasing the supply of oxygen in the eye when wearing silicone hydrogel lenses contributes to better wearing comfort at the end of the day, compared to wearing classic hydrogel lenses.
Of course, there are always pros and cons for any contact lens material or design.
The best way to determine whether silicone hydrogel lenses are the best choice for you is to first discuss all possible contact lens options with your eye doctor.
During the consultation, your doctor will first perform a thorough examination of your eyes, analyze the results, and then, based on these findings, make a proposal of which type of lenses best suits you and your specific needs, upon which you can then make the right decision yourself.