Specialty Lenses (includes: disposables, frequent replacement, astigmatism correcting, colored and tinted, multifocal and monovision)
There are two primary types of contact lenses: hydrophilic (water-loving) soft lenses and rigid, or hard type materials. Most contact lenses are manufactured from a polymer based on methacrylate, cellulose and silicone based plastics.
Soft lenses are hydrophilic (water-loving) hydrogel polymers which contain 36%-74% water. Other plastics and co-polymers may be added to alter the physical characteristics of the lens. The diameter varies from 10.5mm to 15.5mm w ith most stock lenses having an effective diameter of 13.5mm-14.8mm. The inside curve is designed to approximate the shape of the cornea. Center thickness can be from .03 mm-.20 mm.
Soft lenses are available in almost any optical correction, including myopia, hyperopia, astigmatism and bifocal. They can be colored with either transparent hues or opaque patterns to change apparent eye color or to mask cornea or iris malformations. They are available for daily, weekly and bi-weekly disposable use, one to three months planned replacement use and standard annual replacement lenses (which are now essentially obsolete.)
Hard lenses used to be manufactured from a material called PMMA, also a methacrylate plastic. They have generally been replaced by gas-permeable materials which contain silicon and fluorine. These lenses are called RGP (rigid, gas permeable) or simply GP. As with soft lenses, they are available in almost all possible optical corrections. RGP lenses have diameters between 8.0mm-10mm and inside ³base curves² which match the curvature of the cornea and there are a number of unique ³aspheric² designs. Center and edge thickness vary as does specific gravity, which effects weight and lens positioning.
There are also a number of lenses which combine soft and RGP materials. In the last couple of years manufacturers have come up with RGP lenses with a modified hydrophilic surface. Plasma treatments can also increase the wettability and deposit resistance of the lens. Synergeyes has an RGP center and soft periphery. These are unique lenses with characteristics that help with certain fitting issues.
Both RGP and soft lenses have certain advantages and disadvantages. As RGP lenses are dense, solid materials, they generally, but not always, provide sharper vision than soft lenses. They contain little or no water and do not require high levels of tear production to maintain water and therefore do not rely on the production of tears to maintain their structural integrity. RGP lenses are easier to keep clean and last much longer than soft lenses. They are generally less costly.
Soft lenses require a shorter adaptation period and are well suited for occasional use. Compared to RGP lenses, they are not as susceptible to comfort problems when used in dusty or windy conditions. They are better suited for water sports and people play ing sports requiring rapid eye movements or that may involve contact with the face or eye. Of course in such instances, one should be using eye protection whether or not they wear any kind of contact lens!
From a purely optics -based consideration, contact lenses have significant advantages over spectacle lenses and are an excellent means to optically correct the refractive error of the human vision system. The contact lenses significantly reduce optical distortions due to their position in contact with the cornea, as compared to spectacle lenses mounted some 12- 14mm in front of the eye The optical center of the lens is always aligned with the optical center of the cornea. . The image projected onto the r retina is normal in size, peripheral aberrations are essentially eliminated and chromatic distortions are significantly reduced. To take advantage of these potential benefits requires that the contact lens be fitted correctly to provide clear, high contras t visual acuity and be able to maintain normal corneal physiology and health.
Disposable lenses are designed for frequent replacement daily or once each week or two. They are available for extended wear and daily wear and some are tinted. They may help people with allergies or lens deposit buildup problems wear their lenses more comfortably, as the lenses are replaced before problems develop.
Planned Replacement Lenses are similar to disposable lenses except that the wearer replaces them less frequently. Essentially, these lenses are more durable than the closely related disposable lenses. There are also more options as far as astigmatism correcting, colored contact lenses and materials for special needs.
Astigmatism correcting toric lenses are specially designed to correct for the optical error of astigmatism. Most soft lenses have the additional astigmatic optics ground onto the front surf ace while the lens was in the dry state. Then the lens must be stabilized so that it does not rotate on the cornea with each blink. This is accomplished in a number of ways. The back surface of the lens may be ground so that it is unequally curved (toric base curves) to match the unequal curves of the cornea. If the cornea is spherical (equally curved), this design fails to stabilize the rotation. So another technique is utilized: weighting the lens. In this case, one edge of the lens is thicker. Sometime es the lens is ground where the center optic zone is spherical and the remainder is elliptical. Yet another design uses thin zones and thin edges to minimize interaction with the lids to help stabilize the lens rotation. Some manufactures combine a number of these designs to create the final astigmatic soft contact lens.
Clearly, not all designs work for all situations, but there is often at least one that will indeed provide good optical correction for the wearer.
Rigid Gas Permeable lenses correct astigmatic error in one of two ways: the first is quite similar to the soft lens designs and is called toric base (also known as back toric) and front surface toric. Sometimes it is necessary t o combine these and then the lens is called bi-toric. The same optical theory discussed above applies here as well. But sometimes a standard RGP lens automatically corrects the astigmatic error. If the astigmatism is caused by a toric cornea, placing a spherical lens over the non-spherical cornea masks the astigmatism. The tears flowing under the lens create a ’Äútear lens’Äù which is equal and opposite to the toric shape of the cornea. This is one of reasons why RGP lenses sometimes provide better overall optics than soft lens.
Colored Lenses for Eye Color
Change and Colored Lenses for Cosmetic Masking are most
often soft contact lenses. They are available in transparent colors
for enhancement and changing of lighter colored eyes and opaque
colors for changing brown and darker eyes. Opaques have an ³iris
pattern² printed onto the front surface of the lens while
transparent colors allow a blending of the natural iris color with
the lens tint. Masking lenses are used to cover irregular and
disfigured corneas and in theatrical presentations. The front surface
could contain a pattern to simulate a normal iris or something
unique, for examples: cat-like pupils, iridescent and mirrored
effects and most anything the artist can paint!
Multi- and Bi-Focal Contact Lenses. When the need for near vision correction due to presbyopia occurs, contact lens wearers have an number of options: RGP and soft multifocal designs and monovision. Bifocal means that there are two discrete focal powers, one for distance and the other for near. Multifocal indicates a lens that provides more than two focal distances.
There are a number of designs and many variations on a theme. Rigid lenses can be "translating types": the optical axis passing through the middle of the lens for distance optics and a place closer to the edge for near optics. The lens moves up as you look down, causing the line of sight to pass through or "translate to" the near segment. This is very similar to using spectacle bifocals. Some of these lenses have a flat-top near segment, some are crescent shaped, some divide the lens into two halves. The near segment power is achieved by adjusting the front surface curvature and/or using an embedded segment of a different refractive index. The lens is stabilized to "ride" to the correct positions by varying the edge thickness to create a ballast, or weight, and/or by truncating (slicing off) the bottom edge of the lens, and/or using the lower eyelid to support the lens.
Another design, available in soft and rigid material, is called "back or multi aspheric". The lens produces a constantly varying power from the center radially toward the edge. Some designs position the distance optics in the center of the lens and others place the near power zone in the center. Optically, the varying power creates a multifocal system, unlike the translating bifocal lenses. This is similar to PAL spectacle lenses, also known as "progressive" or "no line" bifocals.
A third design utilizes zones or bands for different focal points, for example: far, midrange and near. These zones are often concentric rings of varying diameters. A fourth design uses diffraction bands to produce a holographic, superimposed image for two or more focal points which your brain learns to interpret and choose as appropriate to the task. These type of lenses are called "simultaneous vision", as multiple images are created and which require a "sorting out" by the visual processing centers in the brain.
But the plot thickens, as there are designs that combine the aspects of some of the above. In fact, one recently arrived (USA, at least) rigid gas permeable lens uses a unique combination of translating and simultaneous vision optics. European lens fitters and wearers think this is an excellent lens for certain applications.
Which brings me to the next, important issue: Some of the multifocal/bifocal lens designs are better suited to some unique prescriptions, anatomical issues (like pupil size, eyelid tension) and vision correction needs. And not everyone "fits" into one particular design or material. Sometimes we might even use one design on one eye and another for the other. Whatever works!
Monovision is a fitting technique in which the contact lens in
the dominant eye is used for distance viewing and the other,
non-dominant eye is set for a near focal distance.
When near vision correction is needed along with a distance correction, contact lens wearers have an number of options: they can use near vision spectacles over their contacts, use mutifocal contact lenses, or utilize the monovision system.
Monovision works well for people who do not use both eyes simultaneously and who therefore lack stereoscopic binocular vision. In this situation it is possible to alternately ³see² with one eye at a time. For those with normal stereo vision, monovision could cause a potentially significant loss of depth perception. (See the EyeCare Report: Binocular Vision for details.) Some people can ³retrain² their brains to give up stereoscopic vision by alternately suppressing the information fro m one eye at a time. The success rate in converting strongly binocular people to monovision is poor. Even if adaptation is achieved, there are some potential problems that may be encountered.
There is some controversy among eye doctors as to whether forcing the brain to make this adjustment is really such a good idea. The loss of stereoscopic depth perception could put the wearer at risk of misjudging distances. Many practitioners recommend against the use of monovision while operating a motor vehicle. Another concern is the appropriateness of altering brain function for a cosmetic benefit, especially when considering that there are alternatives that provide good visual acuity while preserving binocular vision.
The real risks may be overstated: someone who is heavily reliant on stereoscopic binocular vision will simply give up trying to adapt to monovision and those who easily adapt most likely did not have very good stereoscopic vision at the start.
There is also a similar ³modified monovision² technique which to some degree addresses the problem of loss of depth perception. Instead of a dedicated near vision contact lens in the non-dominant eye, a multifocal lens is used. This preserves st ereopsis and depth perception at distance to a significant degree in most cases. It is not always appropriate for those requiring good stereo vision at near, for example people involved in electronic assembly or manual typesetting.
Multifocal contact lenses are the best alternative for many ³emerging presbyopes², those who are just beginning to need help with near vision. For current RGP lens wearers, the transition if quite easy and very often successful. Today, there is no reason for long time contact lens wearers to have to give up their contacts because of presyopia!
In the U.S. contact lenses are marketed by prescription only and for daily wear or extended wear (sleep-in) based on oxygen and permeability and transmissibility. A lens can be considered for extended wear if the gas permeability exceed s a certain threshold. A lens ma be approved for one week, two weeks or 30 days of continuous wear. (USA labeling) Not everyone can safely or comfortably use lenses in this manner. Most practitioners now consider that only the newest generation of silicone hydrogel lenses are suitable for continuous wear.
There are a number of variables which determine if an individual can successfully (e.g.: without signficant risk to eye health) use their lenses on an extended wear basis. The cornea is an avascular tissue. This means there is no blood supply: oxygen is absorbed directly from the atmosphere and, along with other nutrients, from what is dissolved in the tears. It appears that the need for oxygen varies from person to person. When the eyes are closed, oxygen is provided from the capillaries on the inside surface of th e eyelids.
Although a minimum threshold for oxygen permeability guides the practitioner to choose lenses suited for extended wear, it is the individual¹s physiologic need that ultimately determines the safety and efficacy of sleeping in contact lenses. In fact, current thinking is that for most people three or four days is the maximum continuous wearing time that allows for reasonably normal corneal physiology. There are, of course, exceptions. Although studies of proved the safety and efficacy of some of the silicone hydrogel lenses for as long as 30 continuous days, a some people may not be able to safely tolerate long wearing times.
The cornea replaces its epithelium on a regular basis. Part of this process includes the sloughing off of cells so that new ones can migrate to the surface. Placing a lens between the eyelid and the cornea limits that effect. The longer the continuous presence of the lens, the more cells become trapped. Studies have shown most people's corneas respond to this situation by reducing cellular turnover rates so this may not be an issue. However, for some it is and at a certain point, this becomes problematic. A similar situation occurs with oxygen deprivation. At a certain point, corneal physiology becomes compromised by the lack of available oxygen and defects in the structure of the tissue becomes obvious. The oxygen deprivation situation has been addressed with the advent of silicone hydrogel lenses, but the use of a damaged or dirty lens can reduce the oxygen flow to below safe thresholds.
In the short term, these corneal changes are easily reversible with no long term consequences. But allow the negative impacts to continue, and the damage can become significant and effect vision and eye health. Regular replacement, proper care and use of contact lenses, and regular follow up examinations including microscopic evaluation of the cornea are vitally important to assure your eye health. Squeezing out a few extra days of use or using dirty or damaged lenses may risk your future vision.
Problems occurring from the use of contact lenses are most always caused by misuse or improper care of the lenses. Contact lenses do not cause eye infections; bacteria and viruses do. Contact lenses do not contaminate an eye; dirty fingers or non- disinfected lenses are often the cause. All contact lenses (except daily disposables and sometimes weekly extended wear) require cleaning with a surfactant (detergent) cleaning product and storage in a bacteria-killing disinfecting solution. Soft lenses require rinsing in saline solution, hard lenses may be rinsed off with tap water, assuming it is free of contamination. (Not a given, these days). Some lenses require, in addition to the surfactant cleaner, a special protein dissolving agent, called enzymatic cleaner. Enzymatic cleaners are products which contain either papaya extract, pancreatin from pigs, or subtillisin, a chemical protein dissolving agent.
A number of companies sell all-in-one solutions. Practical experience indicates that do-everything solutions don¹t do everything very well but are certainly better than doing nothing at all! Most MPS products say ’Äúno rub’Äù. But rubbing the lens in the product can significantly boost cleaning power as well as improve disinfection time. Some people are sensitive to chemicals or preservatives in these solutions. There are a number of products designed to solve this problem. Disinfecting solutions utilizing hydrogen peroxide, cleaning solutions with alcohol and saline solutions that are just pure salt and water are examples. Hard lens wearers have fewer preservative-free product options.
There are a number of possible complications which could occur from wearing contact lenses. Generally, these occur as the result of improper fitting, inappropriate lens care and handling (e.g. not disinfecting the lenses between uses) or the use of old, worn out lenses. Even so, these complications are fairly uncommon and easily remedied.
Corneal abrasion: The most common complication is a scratched cornea, also called an abrasion. This may occur from a particle of abrasive material (for example sand or some airborne debris) getting under the lens. This is far m ore common with RGP than with soft lenses. Symptoms vary from slightly annoying to moderately painful. Treatment consists of first removing the source of the irritation (sometimes requiring lens removal) by tearing or irrigation with water or saline solut ion. Often no further action is necessary. The cornea heals very quickly. If the abrasion is deeper (and likely more painful) or discomfort persists for more than a few hours, an eye doctor should be consulted. Sometimes antibiotic drops to prevent infect ion and patching of the eye to speed recover may be advised. Lack of treatment may result in keratitis (see below). A similar condition may result from wearing an RGP lens with an edge defect or a soft lens with an edge tear.
Conjunctivitis: Lack of disinfection of contact lenses and handling lenses with unwashed hands are the primary causes of conjunctivitis in contact lens wearers. This is an infection of the thin membrane which lines the sclera a nd the inside of the eyelids. Symptoms include red eyes and often some fluid discharge. The eyelids may be ³stuck together² upon awakening. Some irritation, usually a burning sensation, may be present. The infection is caused by the transfer of a bacteria or virus to the eye. Treatment is antibiotic drops or ointment if caused by bacteria. With some rare exceptions (herpes simplex, for example), there is no treatment of virus-caused infections. (They are self-limiting and the tissue usually retu rns to normal heath in three days to three weeks.) Bacterial infections can also spontaneously resolve without treatment. Warm compresses are often an effective therapy. Any infection which persists for more than three days or worsens with increasing disc harge or is accompanied by pain requires medical attention. If properly and promptly treated, there is very little likelihood of permanent consequences.
Keratitis: Infection of the cornea is called keratitis. There are significant and potentially vision threatening complications if this infection is not properly managed. Symptoms include pain, sometimes rather significant, and sensitivity to light. There is often a conjunctivitis present at the same time. Lens wear must be discontinued and medical treatment with antibiotics and sometimes steroids is indicated. If not treated, keratitis may result in permanent scarring of the cornea. This infection is most common with soft lenses that are not properly disinfected. It can also be caused by lenses stored or rinsed with contaminated water or solutions.
Cornea edema (swelling), neovascularization (new blood vessel growth) and infiltrates (cellular defects): Primarily problems relating to insufficient oxygen to the cornea and corneal cellular irregularities, improperly used extended wear lenses are the most likely cause of these conditions. If detected in time, the cornea will most likely heal without complications. There are often no symptoms. When there are symptoms, they include: hazy vision, especially upon awakening, halos around lights and pain upon removal of the lenses. Allowing the condition to continue could result in permanent scarring of the cornea and infection. Treatment is to discontinue contact lens wear until the problem resolves. Prevention is the best treatment. Regular follow-up examinations can detect oxygen deprivation and microscopic cornea changes before they become problematic. Replacing contact lenses as recommended and limiting overnight use as necessary to maintain normal eye health is very important.
In conclusion, almost any optical correction is available in a
contact lens and there are materials and designs to satisfy most
everyone¹s needs. Contact lenses require attention to care and
fitting and wearers must be responsible for a certain degree of
maintenance and follow up care. When used as directed, contact lenses
have a proven track record of safety.