Types of Contact Lenses
If you don’t like wearing glasses, then contact lenses are an excellent choice for you. Nowadays, contact lenses are more popular than ever, especially because they come in different forms, materials, wearing time, and various replacement schedules.
Types of Contact Lens Materials
Based on the type of materials that lenses are made of we can divide contact lenses in five types: soft lenses, silicone hydrogel lenses, gas permeable, hybrid, and
PMMA contact lenses.
• Soft contact lenses are very thin and pliable and conform to the front surface of the eye because they are made from gel-like, water-containing plastics called hydrogels. These lenses are also more popular because they are usually immediately comfortable. In addition, soft lenses are bigger and they usually move less meaning that they are more stable and thus ideal for sports with risk of impact. The use of these lenses does not usually deform the cornea. On the other hand, some of the downsides of these lenses are lower duration and they are more difficult to manipulate. They have less gas permeability than modern rigid materials, also not all optical powers are manufactured. Cleaning and maintenance of soft lenses is more complex and expensive and additionally, they produce more allergic problems and may cause more eye dryness than rigid lenses.
• Silicone hydrogel lenses are an improved type of soft contact lenses that are more porous than regular hydrogel lenses and allow even more oxygen to reach the cornea. Moreover, these lenses enable up to five times more oxygen to reach the cornea than regular hydrogel lenses.Just like soft lenses, they are made of plastics that are hard when dry but readily absorb water and become soft and gel-like when hydrated. Additionally, increasing the oxygen supply to the eye is potentially beneficial for all contact lens wearers, especially considering that many wearers are not complying with their opticians’ instructions regarding proper lens wear and replacement.Some of disadvantages of silicone hydrogel lenses are that the silicone material tends to attract more lipid deposits, which may cause blurry vision and discomfort, in some patients. Also, they are generally more expensive than non-silicone lenses, so a more price-conscious consumer may not prefer this option.
• Gas permeable lenses, also known as GP or RGP lenses, are made of durable plastic that transmits oxygen. Most GP lenses incorporate silicone, which makes them more flexible. Also, silicone is oxygen permeable, so oxygen can pass directly through GP lenses to keep the cornea healthy without having to rely solely on oxygen-containing tears to be pumped under the lens with each blink. These lenses provide excellent vision, and a short adaption period since they are very comfortable to wear. GP lenses correct most vision problems and are relatively easy to put on and care for. They are available in tints (for handling purposes) and bifocals and in addition, they are available for myopia control and corneal refractive therapy. Just like any other lenses, GPs also have their drawbacks. GP lenses can slip off center of eye more easily than other types. Debris can sometimes get under the lenses causing many unwanted problems. And they also require regular office visits for follow-up care.
• Hybrid contact lenses are designed to provide wearing comfort that rivals soft or silicone hydrogel lenses, combined with the crystal-clear optics of gas permeable lenses. Hybrid lenses have a rigid gas permeable central zone, surrounded by a “skirt” of hydrogel or silicone hydrogel material.Hybrid lenses are great for patients with astigmatism, as the GP lens is able to mask most, if not all, of the corneal astigmatism. Despite these features, hybrid contact lenses are more difficult to fit and are more expensive to replace than soft and silicone hydrogel lenses. Also, hybrid lenses do not correct for lenticular astigmatism. Hybrid lens wearers will need proper training with insertion, removal and lens care. This new modality is easy for some and difficult for others.
• PMMA lenses are made from a transparent rigid plastic material called polymethyl methacrylate (PMMA). PMMA has excellent optical properties and was developed as a lightweight and shatter-resistant alternative to glass for many applications. But it is impermeable to oxygen and other gasses, and the cornea needs a significant supply of oxygen to stay healthy. Since oxygen cannot pass through a PMMA contact lens, the only way for this vital element to reach the cornea was for tears to wash underneath the lens with each blink. In order for this blink-induced, tear-pumping action to occur, PMMA lenses had to be made relatively small in size. Also, there had to be a significant gap between the edge of the lens and the surface of the cornea. These design characteristics made many people very aware of PMMA lenses on their eyes or caused discomfort that made wearing the lenses impossible. In some cases, these features also caused problems with PMMA lenses popping off the eye, especially during sports.
Types Of Contact Lenses According To Wear Time
There are two types of lenses classified by wearing time:
• Daily wear contact lenses are intended to be worn for an entire day, generally around eighteen hours, and are designed to be comfortable for hours on end. Daily wear contacts can be reusable or disposable, meaning they are discarded after several uses. The advantage of daily contact lenses is a very short adaptation period, they are comfortable and more difficult to dislodge. Additionally, they are available in lenses that do not need to be cleaned and thus great for active lifestyles. The downside of these lenses is that they do not correct all vision problems, they require regular office visits for follow-up care and these lenses wear out and must be replaced in a timely fashion.
• Extended wear contacts can be comfortably worn both night and day. Part of the struggle in initial extended wear contacts involved the lack of oxygen flow and the discomfort that can come from long term wear, including dry, itchy eyes. Modern technologies have made the adjustment easier with more breathable plastics and technology permitting lenses to move on the eye to prevent dirt and debris from becoming trapped under the lens. Extended wear contacts might be convenient but there may be some health risks involved. Positive sides of these lenses are that they can usually be worn up to seven days without removal and some lenses are approved for up to 30 days. However, there are always negative sides. These lenses do not correct all vision problems and require regular office visits for follow-up care. They could increase risk of complication. Extended wear contacts require regular monitoring and professional care.
Types Of Contact Lenses According To Replacement Schedule
Daily disposable contact lenses are lenses that should be discarded after a single day of wear. Daily disposable contact lenses provide several advantages, the main of which is convenience. Since contact care, cleaning and disinfection are not required with daily disposables they provide a measure of ease of wear. They reduce the risk of protein deposits, eye infections caused by the lenses or the solutions used to clean them – including fungal keratitis and eye ulcers. Since they are replaced daily, the time for deposits like proteins and allergens to build up on the lenses is reduced. Moreover, many of these lenses offer some protection from UV rays, which is particularly relevant for patients who play sports. On the other hand, daily disposable lenses might not be available for certain uncommon prescriptions, and certain specialty soft contact lenses, such as color contacts and theatrical contact lenses, are not available in daily disposable form. Most of the problems with these lenses arise from non-compliance with the prescribed wear schedule.
• Disposable contact lenses should be discarded every two weeks, or sooner. Thankfully, various cleaning solutions and disinfectants were created to extend the life of a lens. Traditional contact lenses were, and still are, too expensive to discard on a regular basis. Advances in manufacturing processes have made contact lenses cheaper, enabling wearers to discard their lenses more often. Disposable soft contact lenses are convenient, healthy, and affordable, and when cared for properly, they carry little to no risk. Plus, they can correct most vision problems. Although these lenses are made to be worn for days at a time, they should still be cleaned before their scheduled replacement date to avoid complications. Sleeping with any type of contact lens increases one’s risk of infection. The best way to avoid this is to not sleep with your contact lenses in every night.
• Frequent replacement contact lenses should be discard monthly or quarterly. Some monthly contacts can be worn for up to seven days straight without taking them out of your eyes. Wearing contact lenses not approved for sleeping can pose risks to your eyes because the lenses deprive your eyes of oxygen. These risks include cornea infections and corneal neovascularization, where blood vessels begin intruding into the whites of your eyes. During the day, having your eyes open brings in oxygen to your cornea. Monthly and other replaceable contact lenses are typically silicone hydrogel lenses that have higher gas permeability: they allow five or more times the oxygen through the lens compared to standard contacts.
• Traditional (reusable) contact lenses should be discarded every six months or longer. The key advantages of these lenses are that they often represent best value in price for regular wearers. They are available in the widest range of strengths and fittings to suit most eyes. These lenses provide good comfort and they tend to be more resistant to drying out than daily disposable lenses. The disadvantages of reusable contact lenses are that they require regular cleaning and care and that you have to make them last longer.
Types Of Contact Lens Designs
Bifocal/Multifocal contact lenses are designed to provide clear vision at all distances for people who have refractive errors and also are experiencing the normal age-related decline in near vision called presbyopia. Bifocal and multifocal contact lenses come in both soft materials and GP materials. They can also be found as hybrid contact lenses. Some can be worn on a disposable basis. Some of the lens manufacturers offer multifocal contact lenses made of silicone hydrogel material. These lenses allow significantly more oxygen to reach the cornea than conventional soft lenses for greater wearing comfort and are available for both daily wear and extended wear. Bifocal contacts lenses have two prescriptions in the same lens. Multifocal contact lenses have a range of powers in each lens. Some of the advantages of bifocals are good vision at all distances, not carrying around two pairs of glasses, and not having to search for glasses simply to read. This means no heavy glasses and being able to enjoy activities such as sports more easily. The new generation of soft bifocal and multifocal contact lenses have provided acceptable results for many present and future lens wearers, so people who once thought they could never benefit from contact lenses now have a second chance. Unfortunately, there are also some disadvantages of bifocal/multifocal lenses which include difficulty adjusting to the new viewing experience of these contact lenses, nighttime glare, shadows and starbursts during the initial adjustment to the new lenses. And higher cost of these premium contact lenses due to the extra measurements and fittings required, and complex design.
• Orthokeratology or (ortho-k) is the fitting of specially designed gas permeable contact lenses that you wear overnight. While you are asleep, the lenses gently reshape the front surface of your eye (cornea) so you can see clearly the following day after you remove the lenses when you wake up. They are prescribed for two purposes - to correct refractive errors (primarily nearsightedness, but also astigmatism and hyperopia) and to slow the progression of childhood myopia. Only highly “breathable” GP lenses that have been approved for overnight wear should be used for orthokeratology. The advantage of ortho-k is that it can usually reduce myopia within the first two weeks. Vision of successful wearers can usually be maintained throughout the day after lens removal. Night time wearing modality can bring convenience to those who dislike wearing glasses or contact lenses during the day or participate actively in sports. Also, problems, such as dry eye, arising from normal day time contact lens wear can be avoided. Disadvantages of ortho-k lens wearing is that wearers should strictly follow the instructions given by the optometrist, non-compliance could result in corneal infection and possible vision loss. These complications can be kept to a minimum if proper instructions and lens usage are followed. Wearers have to spend more time initially for ortho-k lens fitting and follow-ups. In the absence of adverse events, regular aftercare visits (3-6 monthly) are still essential to ensure the health of the eyes. Ortho-k cannot cure myopia. The myopia reduction effect will wear off gradually after stopping of lens wear.
• Toric lenses are specially designed soft contact lenses that correct astigmatism. Most toric contacts for astigmatism are indeed soft lenses, but there are toric contact lenses made of rigid gas permeable contact lens materials, too. These lenses have different powers in different meridians of the lens to correct the varying amount of nearsightedness or farsightedness in different meridians of the eye that characterizes astigmatism. The advantages of a toric contact lens include easy adaptability, as comfort is usually quite good even initially. Other advantages of soft toric lenses include extended wear option –allowing up to thirty days of continuous (overnight) wear. Their biggest disadvantage is the potential for fluctuating vision. No matter how good the lens design and fit, all soft toric lenses rotate and move a bit in the eye. The more astigmatism a person has, the more likely he/she will be sensitive to the lens movement.
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