Copyright 1995 Larry Bickford. All Rights Reserved.


The EyeCare Connection

abstracts and answers to commonly asked questions

Contact Lenses


Types of Contact Lenses

There are two primary types of contact lenses: hydrophilic (water-loving) soft lenses and rigid, or hard type materials.

Soft lenses are made of plastics which contain 36%-74% water. The diameter varies from 10.5mm to 15.5mm. 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 ³disposable ² use, one to three months ³frequent replacement ² use and annual replacement lenses.

Hard lenses used to be manufactured from a material called PMMA. They have generally been replaced by gas-permeable materials and are called RGP (rigid, gas permeable). 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.

There are also a number of lenses which combine soft and RGP materials. The NovaLens is an RGP lens with a modified hydrophilic surface. SoftPerm has an RGP center and soft periphery. These are unique lenses with characteristics that help with certain f itting situations.


Use of Contact Lenses

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!

Contact lenses are an excellent means to optically correct the refractive error of the human vision system. They significantly reduce optical distortions due to their position in contact with the cornea, as compared to spectacle lenses. Peripheral aberrat ions and distortions are eliminated or greatly reduced. To take advantage of these potential benefits requires that the contact lens be fitted correctly to provide clear, high-contrast visual acuity and be able to maintain normal corneal physiology and he alth.


Specialty Lenses

Disposable lenses are designed for 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.

Frequent Replacement Lenses are similar to disposable lenses except that there is longer interval bewtween relacement. Essentially, these lens are more durable than the closely related disposable lenses. < P> Astigmatism correcting ³toric² lenses are specially designed to correct for the optical error of astigmatism. Most soft lenses have the additional astigmatic optics ground on to the front surface wh en the lens was in the dry state. Then lens must be stabilized so that it does not rotate on the cornea with each blink. This is accomplished in a number of ways. (See the EyeCare Report: Contact Lenses for complete info.) Not all designs work for all sit uations, but there is often at least one that will indeed provide good optical correction for the wearer.

Rigid Gas Permeable lenses can also correct astigmatic error. Sometimes this occurs as an expected part of the fitting and sometimes they require the astigmatic optics to be ground onto the lens quite like soft lenses. (See the EyeCare Report: Contact Lenses for complete info.)

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 co ver irregular and disfigured corneas and in theatrical presentations.

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. Bifo cal 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.

These special lenses are available in RGP and soft lens materials. (See the EyeCare Report: Contact Lenses for complete info.) There are many configurations from which to choose and they are manufactured from a varie ty of materials. Determining which type of multifocal lens has the greatest chance of success requires practice and skill. There¹s usually at least one design that will satisfy the vision requirement of the wearer.

Monovision

Monovision is a fitting technique where 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. The concept works well for people who do not use both eyes simultaneously and who t herefore 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 ³re-train² their brains to give up stereoscopic vision by alternately suppressing the information from one eye at a time. There is some controv ersy 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 mono vision 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.

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.



Extended Wear (Sleep-in) Lenses

In the U.S. contact lenses are marketed by prescription only and for daily wear or extended wear (sleep-in) based on oxygen and carbon dioxide gas permeability and transmissibility. A lens can be considered for extended wear if the gas permeability exceed s a certain threshold. No lens is ³approved² for more than seven consecutive days of use and not everyone can achieve that level of wearing time. Current thinking is that for most people three or four days is the maximum continuous wearing time that allows for reasonably normal corneal physiology.

Care and Handling

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 disposable and sometimes weekly extended wear types) require cleaning with a surfactant (detergent) cleaning product and storage in a bacteria-killing disinfecting solution. Soft len ses require rinsing in saline solution, hard lenses may be rinsed off with tap water, assuming it is free of contamination. (Not a universal given, these days). Some lenses require, in addition to the surfactant cleaner, a special protein dissolving agen t, called enzymatic cleaner.

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! Some people are sensitive to chemicals or preservatives in these solutions. Preservative-free products are available.

Complications


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. A complete description of signs, symptoms and treatments are included in the EyeCare Report: Contact Lenses. Examples of eye hea lth problems resulting from problematic contact lens wear include:

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 more common with RGP than soft lenses. There is a varying degree of pain or discomfort and a feeling of ³something in there². May result from wearing an RGP lens with an edge defect or a soft lens with an edge tear. Often does not but may requi re medical treatment.

Conjunctivitis: This is an infection, usually bacterial, sometimes viral, of the thin membrane which lines the sclera and the inside of the eyelids. Lack of disinfection of contact lenses and handling lenses with unwashed hands are the primary causes of conjunctivitis in contact lens wearers. Symptoms include red eyes and often some discharge. The eyelids may be ³stuck together² upon awakening. Some irritation, usually a burning sensation, may be present. Warm compress es may be an effectine trerapy. May be self limiting or require medical treatment.

Keratitis: Infection of the cornea. 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. This infection is most common with soft lenses that are not properly disinfected, stored or rinsed with contaminated water or solutions. Requires medical treatment, sometimes as an emergency.

Cornea edema (swelling), neovascularization (new blood vessel growth) and infiltrates (cellular defects): Related to insufficient oxygen to the cornea. Improperly used extended wear lenses are the most likely cause. If detecte d 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. Prevention is the best treatment. Regular follow-up examinations can detect oxygen deprivation and microscopic cornea changes before they become problematic. Replacing contact len ses as recommended and limiting over-night use as necessary to maintain normal eye health is very important.

Although this abstract is pretty complete, there's more detailed information contained in the EyeCare Report: Contact Lenses, especially in the topic areas: astigmatism lenses: how they work, multifocal lenses, compl ications: treatments and cautions. There is a fee for EyeCare Reports. This link provides additional information, not the report.

Return to the topic list at top of the page.