Silicone hydrogel contact lenses
Silicone hydrogel contact lenses are advanced soft lenses that allow more oxygen to pass through the lens to the cornea than regular soft ("hydrogel") contacts.
In fact, silicone hydrogel lenses enable up to five times more oxygen to reach the cornea than regular hydrogel lenses.
Silicone hydrogel and regular hydrogel lenses both are made of plastics that are hard when dry but readily absorb water and become soft and gel-like when hydrated.
If you've ever let a soft or silicone hydrogel lens dry out, you know that it becomes deformed, hard and fragile. But if you soak it for a few minutes in contact lens solution, it becomes soft and pliable again.
Silicone hydrogel contacts account for the majority of contact lens fittings currently performed worldwide.
Silicone hydrogel (not "silicon" hydrogel) contacts
Silicone hydrogel contact lenses sometimes are erroneously called silicon hydrogel lenses.
Silicon is a very common mineral. In fact, ordinary sand is composed primarily of silicon dioxide (silica).
Highly-purified silicon is used to manufacture semiconductors, which is why the southern part of the San Francisco Bay area, home of Apple and many other high-tech and computer companies, is nicknamed 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 to increase the oxygen permeability of contact lenses, silicone is used to make breast implants, medical tubing and other medical devices.
Silicone hydrogel lenses are soft lenses, but silicone also is used in the production of many rigid gas permeable contacts to improve oxygen permeability of the lenses.
Benefits of silicone hydrogel lenses
All contact lenses reduce the amount of oxygen that reaches the front surface of the eye to some degree.
When the cornea's oxygen supply is significantly reduced (a condition called hypoxia), a number of problems such as red eyes, corneal swelling, blurred vision and eye discomfort can occur. Hypoxia also can increase contact lens wearers' risk for a number of eye infections.
Hypoxia-related eye problems became a significant issue when extended wear contact lenses first gained popularity. As the number of people who wore contacts overnight (and continuously for several days) increased, so did the number of contact lens-related eye infections.
Silicone hydrogel lenses were introduced in hopes of decreasing hypoxia-related problems and increasing the safety of both daily wear and extended wear of soft contact lenses.
Increasing the oxygen supply to the eye is potentially beneficial for all contact lens wearers, especially considering that many wearers don't follow their eye care professionals' instructions regarding proper lens wear and replacement.
Specialty silicone hydrogel lenses
The enhanced oxygen permeability of silicone hydrogel contacts makes them a great choice for special contact lens designs that might cause hypoxia problems when made with conventional soft hydrogel materials.
These include any designs that require greater lens mass, such as toric contacts for astigmatism, bifocal contact lenses, contacts for hard-to-fit eyes, and custom contact lenses, including soft contact lenses for keratoconus.
Risks of silicone hydrogel contacts
Do silicone hydrogel contact lenses reduce the risk of keratitis and eye infections? Yes and no.
A study published in the British Journal of Ophthalmology found a significantly lower risk of severe keratitis among silicone hydrogel users, but a higher risk of non-severe keratitis.
The 12-month, hospital-based study conducted in the U.K. showed the annual incidences of non-severe keratitis were low in both groups, affecting 0.14% of those wearing conventional hydrogel lenses and 0.56% of those wearing silicone hydrogel contact lenses.
There were no cases of severe keratitis among patients using silicone hydrogel contact lenses for daily wear, whereas 0.06% of patients wearing regular hydrogel lenses on a daily basis (6.4 per 10,000) developed severe keratitis.
The risk of keratitis was greater for both types of soft contacts among patients who used their lenses for extended wear.
Non-severe keratitis occurred in:
48.2 per 10,000 among those wearing regular hydrogel lenses overnight
98.8 per 10,000 among those wearing silicone hydrogel lenses overnight
Severe keratitis occurred in:
96.4 per 10,000 among those wearing regular hydrogel lenses overnight
19.8 per 10,000 among those wearing silicone hydrogel lenses overnight
The study authors concluded there is a significantly higher incidence of severe keratitis in wearers who sleep in contact lenses compared with those who only use contact lenses during the waking hours. They recommended that people who want to wear contact lenses continuously should be advised to wear silicone hydrogel lenses, which appear to be much safer for overnight wear than regular hydrogel lenses.
Critics of silicone hydrogel contact lenses have used this study as evidence that silicone hydrogels are associated with a greater risk of non-severe keratitis compared with regular hydrogel lenses. It is worth noting, however, that the study did not evaluate how many of the silicone hydrogel lens wearers who exhibited non-severe keratitis were refitted into silicone hydrogel lenses from conventional hydrogel lenses prior to the study period due to similar problems while wearing hydrogel lenses.
Also, at the time of the study, the only silicone hydrogel lenses approved for use in the U.K. were designed for monthly lens replacement, whereas there were several brands of regular hydrogel lenses available for more frequent two-week replacement.
Today, there are several brands of single-use silicone hydrogel lenses worldwide that are designed for daily disposal and replacement, reducing the risks of keratitis associated with reusable lenses.
Allergic response to silicone hydrogel lenses
A number of blog sites and online forums contain complaints among contact lens wearers that they have developed allergy-like symptoms from wearing silicone hydrogel lenses.
The most common complaints are redness, discomfort, itchy eyes and greater lens awareness or dryness symptoms.
However, these symptoms can also be caused by contact lens dryness or sensitivity to a new contact lens solution or care regimen.
Though silicone hydrogel lenses allow more oxygen to reach the surface of the eye, the added silicone may cause them to dry out more easily.
To combat this tendency, new contact lens solutions have been developed that include special agents to keep silicone hydrogel lenses well-hydrated for all-day wear.
Though silicone hydrogel and regular hydrogel lenses are equally comfortable for most wearers, people with marginal dry eyes might notice some dryness discomfort from silicone hydrogel lenses. This dryness-related discomfort might be mistaken as an allergy symptom.
Because silicone hydrogel lenses allow more oxygen to reach the eye, some wearers might become more aware of their lenses for another reason:
Wearing regular hydrogel lenses that reduce the amount of oxygen reaching the cornea can decrease the normal sensitivity of the corneal surface. When more oxygen is supplied to the cornea by wearing highly oxygen-permeable silicone hydrogel lenses, it's possible that normal corneal sensitivity returns, making the wearer more aware of the silicone hydrogel lenses.
In fact, in a study published in Investigative Ophthalmology & Visual Science, researchers found that when patients were refitted with silicone hydrogels to replace regular hydrogel contacts, the sensitivity of their cornea increased.
The study also found changes in corneal sensitivity among silicone hydrogel wearers when different lens care regimens were used.
As for possible allergies to silicone hydrogel contact lenses, there is no scientific evidence that this occurs.
Are silicone hydrogel contact lenses for you?
Most eye care professionals agree that the most obvious benefit of silicone hydrogel lenses is the reduction of hypoxia-related problems associated with soft lenses made of regular hydrogel materials.
This likely has resulted in fewer contact lens-related red eyes, discomfort and serious keratitis problems from wearing contacts for extended periods of time.
It's also likely that due to their increased oxygen transmissibility, silicone hydrogel lenses have enabled many people to safely wear contacts for longer hours than they could have with regular hydrogel lenses.
Silicone hydrogel materials also have enabled contact lens manufacturers to create a wider variety of lens designs while enabling a healthy amount of oxygen to reach the cornea for safe and comfortable daily or continuous wear.
However, silicone hydrogel lenses have not solved contact lens discomfort problems altogether, and, in fact, some people may experience more lens awareness when wearing silicone hydrogel lenses than when wearing regular hydrogel lenses.
Some studies have shown that silicone hydrogel lenses are more prone to certain types of contact lens deposits than regular hydrogel lenses, and some contact lens solutions may be less effective on silicone hydrogel lenses than they are on regular soft lenses.
Also, despite their increased oxygen permeability, silicone hydrogel lenses have been shown in some studies to be associated with a greater risk of corneal inflammation and other complications in some patients, compared with regular hydrogel lenses. Reasons for these apparent problems remain unclear.
Some experts also say that there is no hard evidence that the increase in oxygen supply to the eye when wearing silicone hydrogel lenses improves end-of-day wearing comfort, compared with wearing regular hydrogel lenses.
Because there are pros and cons to every contact lens material or design, the best way to determine if silicone hydrogel lenses are the right choice for you is to discuss all possible contact lens options with your eye care professional during your contact lens exam and consultation.
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Page published on Wednesday, 16 March 2022
