Vision and EyeCare FAQ
Section 3: Contact Lenses

Web site reference: Contact Lens Institute
This site presents their contact lens and optical lens products. It features, each month, basic information on Eye Condition (Astigmatism, Diabetes and Your Eyes, Dry Eye Syndrome, Presbyopia, ...) in a Questions/Answers way



3.1 Difference between soft and hard contact lenses

Soft lenses are manufactured from a plastic hydrogel polymer, HydroxyEthylMethacrylate (HEMA) which has a varying water content (38% - ~70%). Lens size is between 13.00 and 14.50mm. Centre thickness from ~30um.

Hard contact lenses are manufactured from a rigid material, PolyMethylMethacrylate (PMMA). This material can be combined with other plastics to increase the oxygen permeability. Lens size is between
8.0mm and 10.00mm. Centre thickness from ~100um.



3.2 How to Read a Contact Lens Description:

A contact lens prescription differs from a spectacle prescription primarily by the addition of lens parameters. It should be noted that the power of a contact lens will not necessarily be the same as that of a spectacle prescription due to the optics of a lens being closer to the eye. The technical term is "effectivity" of the lens.

The other information that is often given with a contact lens prescription will be the diameter of the lenses, basecurve, possibly additional curves lens material, design or manufacturer name.


For example,

OD: 8.6/14.0/-1.00DS
OS: 8.6/14.0/-1.50DS

The data is
BaseCurve: 8.6mm
Diameter: 14.0mm
Power: -1.00D and -1.50



3.3 Types of Soft Contact Lenses

Soft contact lenses vary depending on either the refractive correction that they are prescibed for or the design and type of material.

Major types include :



3.3.1 Spherical

Single prescription power 360 degrees around the lens.



3.3.2 Toric

Contains both a spherical and cylinder component to correct prescriptions which have astigmatism. Lenses may be thicker in one meridian or have modified thickness profiles to enable the lens to maintain correct orientation on the eye.




3.3.3 Disposable

Spherical or toric contact lenses which are designed to be worn for a certain time period, eg, weekly, two- weekly, monthly. Lenses are generally sold in a "blister" combination pack, eg 3 months supply with intention that lenses are "disposed" of at the end of the time period.



3.3.4 Extended Wear

Lens manufactured from a high-water content material and/or with a very thin centre thickness to enable maximum oxygen transmission. Lenses can be worn overnight, or for a number of days without removal (see comments on complications for further information).




3.3.5 Bifocal Lenses

There are a variety of designs in bifocals, essentially all trying to provide a transition or reading zone for use at near. The different designs include; + aspheric multifocal - + simultaneous vision concentric - has either a central near or distance zone with surrounding zone of opposite type to centre (eg, centre near,distance surround) + diffraction/holographic - based on diffraction grating principles




3.4 Types of Rigid Contact Lenses

Rigid contact lenses also vary depending on the material and the design of the shape of the contact lens. Rigid lenses provide a "new" front surface to the eye and help in the elimination of astigmatism because the tear film fills the gap between the lens and the astigmatic cornea. For information on lens conditions see the Scotlens laboratory web site: This site maintains some nice research photographs of RGP lenses with conditions such as - Drying - Surface Wetting - Deposits - Mucous Deposits Major Material types include;




3.4.1 PMMA

Original material used in the construction of "hard" contact lenses.



3.4.2 RGP

Combination of PMMA and other polymers to increase the oxygen permeability and allow longer wearing time. Lenses are also often larger in diameter than PMMA to increase the comfort of wearing the lens.




3.4.3 Spherical

same as for soft contact lenses.



3.4.4 Toric

Can be either toric periphery, which is used for fitting reasons, or bi-toric used to correct residual astigmatism.




3.4.5 Bifocal

The different types include; + multifocal aspheric - light from all distances focus without interruption by lines or zones of the lens + concentric - different zones of the lens for distance and/or near + zone based - parallel bands of focal zones for various distances + segmented - three basic types (a) lens divided into two roughly equal parts like an executive bifocal (b) lens divided into two unequal parts with near part like a segment of bofocal spectacles, either flat-top or crescent shaped (c) internally fused segment of different refractive index.




3.5 Solutions required for CL maintenance

In general all contact lenses, whether soft of rigid type, will require a cleaning, disinfecting and rinsing solution. Enzymatic agents (protein remover) may also be required to reduce build up of protein on the lens surface.




3.6 Common CL Wearer Questions



3.6.1 Why do I have to clean my Contact Lenses ?

Cleaning removes surface debris and bacteria that may adhere to the contact lenses.



3.6.2 Why is there a limit to the length of wearing time ?

The cornea the "clear part of the eye", is avascular or without a blood supply. It is avascular otherwise it wouldn't transmit light without distortion. As a result of this living tissue being avascular it is necessary to obtain oxygen from the atmosphere. The wearing of a contact lens interrupts the flow of oxygen to the cornea and due to changes in the metabolic pump of the corneal cells the tissue thickens, called oedema. Contact lenses, as described in Sections 3.3 and 3.4, are manufactured from material that allows maximum oxygen transmission. But this is still not exactly the same as the 20.4% therefore the lens wearing time must be controlled to reduce oxygen deprivation to the cornea.




3.6.3 Can I lose the lens behind my eye ?

Short answer is no. The conjunctiva, the tissue that covers the white part of the eye (the sclera) forms a cul-de-sac between from the edge of the cornea to the eyelid margin. Sometimes a contact lens, especially a soft contact lens may roll up and become difficult to find. An eyecare practitioner will be able to locate the lens and remove it.




3.6.4 How do I tell if I've lost the lens from my eye?

Cover the other eye to determine if vision is still clear from the eye where you suspect that you've lost the lens. If vision is blurred then more than likely the lens is either dislodged from the cornea or has fallen from the eye.




3.6.5 How can I tell if I've put them in the wrong eye ?

Check the vision of each eye by covering alternate eyes with your hand. If the vision is unclear then try swapping the lenses and then recheck the vision.




3.6.6 How to tell if the lenses are inside out ?

Soft lenses will appear as a smooth dish shape when placed on the end of a finger and when the lens is the right way around.




3.6.7 How do I know when to dispose of my disposable lenses ?

The lenses will not automatically self-destruct at the end of the wearing period :-) The lenses should be discarded when the lens wearing time has elapsed, as advised by your eyecare professional. This wearing time/period has been chosen to minimise complications with contact lens wear so it should be adhered to !




3.6.8 Can I store my lenses in tap water ?

Short answer - No !. There are a number of "nasty" micro- organisms, especially acanthomeba (sp) which likes feeding on corneal tissue. Storing lenses in water also defeats the purpose of using a disinfecting solution as there is no disinfection occurring. If anything you are exposing your lenses to more potentially dangerous microorganisms.




3.6.9 Can I store my lenses dry ?

Soft lenses - definitely not !. Soft lenses stored dry will turn into "corn-flakes" and only go "snap-crackle and pop" when you touch the lens. Hard lenses should also be stored in a soaking/disinfecting solution to reduce lens contamination.




3.6.10 How long does it take to adapt to new lenses ?

Soft lenses are generally worn for 2-4hrs on the first day and the wearing time is increased by about 2hrs per day, up until 8hrs of daily wear is achieved. The rate at which the wearing time is increased and the maximum number of hours that the lenses can be worn will depend on the recommendations of your eyecare practitioner. Rigid lenses are generally worn for 2-4hrs on the first day with an increase of 1-2hrs each day until 8 hours of daily wear is obtained. Again the rate at which the wearing time is increased and maximum number of hours of wearing time will depend on the person and the recommendations of your eyecare practitioner.




3.6.11 Why do I have to use protein cleaner ?

Contact lenses are exposed to a complex environment whilst on the eye. The tearfilm contains a number of different proteins which potentially adhere to the surface of the contact lens. The buildup of protein reduces the wettability of the surface of the contact lens which causes a "smeary" or "fogginess" to vision. A protein cleaner, often an enzymatic agent, will help in reducing the potential for protein to adhere to the contact lens surface. One of the benefits of disposable contact lenses is that protein cleaners are not necessary because the lenses are disposed of before the is a large build up of proteinateous material.




3.6.12 Can I use eyedrops with my contact lenses ?

In general eyedrops shouldn't be used with contact lenses because the lens can absorb the eye drop and result in a concentrated buildup of the solution. There are special, "in-eye" lubricants that many manufacturers/pharmaceutical companies produce for use with contact lenses. Check with your eyecare professional if any doubts about the solution.




3.6.13 How do I get around dryness with contact lenses ?

Try using an "in-eye" lubricant. There can be some dryness if you work in an air-conditioned environment. If the problem persist consult a professional.



3.7 Risks and Benefits of Contact Lenses

Benefits: - no need to wear glasses - no spectacle scotoma - ie "blind-spot" due to frame edge - overcome problems of spectacle magnification, especially when large difference in spectacle prescription between the two eyes. Risks: - corneal odema - corneal ulcers - contact lens induced conjunctivitis




Index:


© Grant Sayer , email: grants@research.canon.oz.au
 

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