Introduction
Silicone hydrogel contact lenses represent a breakthrough over traditional hydrogel soft contact lenses. Silicone hydrogel contact lenses first appeared commercially in 1998 and since then have shown phenomenal growth, with increase of sales by 109% in 2006 over 2005. Initially developed for the extended wear market, Silicone hydrogel has become the darling of eye-care specialists for both overnight and daily wear use. By 2009, silicone hydrogel lenses accounted for over 50% of U.S. soft contact lens sales.
Silicone Hydrogel Contact Lenses: A Milestone in Contact Lens Technology
For years, eye care specialists and patients feared hypoxia (lack of oxygen) while wearing contact lenses overnight. Now this fear has been eliminated with Silicone Hydrogel Contact Lenses, making it a milestone in contact lens history. High Dk (denoting high oxygen diffusivity to the cornea) silicone hydrogel contact lenses have given patients a convenient, comfortable, and healthy alternative to other types of corrective lenses. Global contact lens studies have shown that corneal edema levels with silicone hydrogels are dramatically lowered when used overnight compared to other disposable soft lens materials. Moreover, indicators of hypoxic stress, such as contact lens-induced corneal striae, microcysts and contact lens-induced endothelial polymegethism are rarely if ever seen with silicone hydrogels compared to extended wear disposable contact lenses.
*Note: When opticians/optometrists talk of Dk, they are actually implying Dk/t, where “t” is the contact lens thickness. A very thick contact lens will obviously show less value of Dk/t, signifying less oxygen transmission than a thinner contact lens of the same material. Therefore, “t” is a variable and Dk becomes a convenient form of referring to the amount of oxygen made available to the eyes by any particular contact lens.
Brief History of Soft Contact Lenses and Extended Wear Contact Lenses
In 1971, Soft Contact Lenses (polymacon lens, made of hydrogel) entered the picture. This lens caused no spectacle “blur” because it was very flexible; it did not reshape the cornea, but took on the cornea’s shape. However, this feature became a drawback; soft lenses did not have the visual acuity of the harder lenses. Nonetheless, due to their soft flexibility, comfort and reduced adaptation time, soft lenses edged out the harder lenses and quickly became the lens of choice.
Increased comfort is due to the thin design of the lens as well as its higher water content. The higher water content makes it necessary for the lenses to soak in a contact lens solution when they are not in the eye. If left out of the solution, the moisture will evaporate in minutes and the lens will become a dry, crisp sphere, leading to possible lens damage. However, those who wear daily soft contact lenses (the same lenses every day for a year or more) must now also perform weekly cleaning for these lenses in an enzyme solution.
Arrival of Extended Wear Contact Lenses
In 1981 extended wear soft lenses became available. The basic difference in the soft lens designs was the water content. While there was 38.5 % water by weight in a standard soft contact lens, there was 55% to 70% water by weight in the extended wear contact lens when each lens was immersed in a sodium chloride solution. In 1982, the first bifocal soft contact lens was available. In 1987, disposable soft contact lenses were introduced and in 1991 planned replacement and daily-wear two-week replacement lenses became available.
Demise and Resurgence of 30-Day Extended Wear Contact Lenses
The 1980s also saw the demise of the extended wear soft lens due to health and safety issues. The lenses were made of hydrogel polymers having low oxygen permeability. This overruled them for long wearing-periods. Besides, their high water content made them dry out after long periods of wear, allowing build up of deposits, growth of bacteria and causing blurred vision, ultimately leading to corneal damage.
With continuous research and development in contact lens technology, the 30-day extended wear lens is back in full force in the new millennium. Of the approximately 32 million contact lens wearers in the United States today, 20% wear extended-wear contact lenses approved for overnight wear.
Silicone Hydrogel Lenses - the Latest Generation of Extended Wear Contact Lenses
Silicone hydrogel lenses, the latest generation of extended wear contact lenses, have been a major contributor to the successful fitting, dispensing and profitability of contact lenses. Silicone hydrogel lenses, because of their high oxygen transmission, are becoming ever more desirable to eye-care practitioners due to their growing range of vision correction lenses and versatility in potential uses.
With their design and range of parameters and powers, silicone hydrogel lenses are often the first choice in new fits and refits for the correction of myopia, hyperopia, astigmatism, presbyopia and therapeutic uses. The construction of the lenses makes for easy handling. Their comfort, convenience and durability of silicone hydrogel lenses make them an excellent choice for vision correction. They are optician-friendly as they are easy to make optometric calculations.
Study on Extended Wear Silicone Hydrogel Contact Lenses: Eye Infection
A study by CIBA Vision of the safety of 30-day Extended Wear Silicone Hydrogel contact lenses showed that the incidence of microbial keratitis, an eye infection that can cause vision loss, was less than .18%. This is higher than in daily wear, but is still considered safe by those who want to wear extended wear lenses. Other studies indicate that the incidence of keratitis is several times more likely in those who sleep wearing standard hydrogel lenses than in those who sleep wearing silicone hydrogel lenses.
FDA-Approved Extended Wear Silicone Hydrogel Contact Lens BRANDS
Night & Day from CIBA Vision are approved for 30-day extended wear. These are spherical and available only in sphere powers, but can correct astigmatism up to 1.50 D. CIBA’s Night & Day are also approved for therapeutic use as a bandage for after surgery, corneal erosions, edema and injuries.
O2 Optix from CIBA are approved for a maximum of 6 nights extended wear or 2 weeks daily wear.
PureVision from Bausch & Lomb is available as Toric, Multi-focal and Spherical contact lens; approved for 30-day extended wear. They claim that 94% of wearers of their segment-leaders, Soflens 66 Toric contact lenses, can migrate to the Purevision Toric lens without a prescription change.
Acuvue Oasys from Johnson & Johnson are approved for a maximum of 7 days extended wear or 2 weeks daily wear.
Acuvue Advance from Johnson & Johnson is approved for 2-weeks daily wear. Vistakon claims to have the number one silicone hydrogel in their Acuvue Advance and the number one new-wearer toric in the Advance for Astigmatism brands.
FDA Approved Cleaning Aids for Silicone Hydrogel Contact Lens
The Food and Drug Administration (FDA) has approved Aquify, Optifree Express and ClearCare multipurpose solutions for silicone hydrogel extended-wear contact lenses. AOSept hydrogen peroxide based system is also acceptable. However, the hydrogen peroxide system and abrasive surfactant cleaners may not be appropriate for all silicone hydrogel lenses. Checking with the manufacturer’s recommendations is the best bet for finding a care system.
Side Effects of Silicone Hydrogel Contact Lenses
Side effects of wearing silicone hydrogel lenses include blurring, itching, burning, watering, pain, decreased comfort, feeling of foreign body in eyes, discharge, redness, poor visual acuity, haloes, rainbows, photophobia, dryness, Giant Papillary Conjunctivitis (GPC), keratitis and conjunctivitis.
Do Not Wear Silicone Hydrogel for the following Conditions (Contraindications)
Occasionally, contact lenses cannot be worn. The following are conditions under which contact lenses are contraindicated: When there is Inadequate tear flow; temporary setbacks such as abrasions, corneal swelling and infection can be aggravated by wearing contact lenses; when the cornea or conjunctiva is diseased, or when any systemic disease affects the eyes.
Ideal Candidates for Silicone Hydrogel Contact Lenses:
The ideal candidates for wearing silicone hydrogel lenses are those who: regularly sleep wearing contact lenses; have high prescriptions; have dry eyes, redness or discomfort after wearing contacts all day; tend to over-wear their lenses; need more oxygen to their cornea than standard soft lenses can provide.
Conclusion: The Future
Silicone hydrogel lenses have sparked a revolution in the contact lens world. With constant innovations in contact lens technology, virtually all eye physiologies or vision problems are expected to be handled in future by silicone hydrogel lenses.
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