Much fuller explanation.
People experimented with early forms of contact lenses over 100 years ago
but thankfully enormous amounts of research has been done since then and we now have a huge array of types and materials to use to try to get over different types of problems.

There are two main categories of lenses in use to day,
hard and soft, although these are both continually evolving.

With a hard contact lens the lens is a rigid structure
that normally covers the central nine millimetres of the eye, there are special lenses called haptic lenses that cover the whole of the white of the eye but they are seldom used now except in certain medical conditions.

Soft lenses as their name suggests
are made of a much more flexible material and tend to ‘drape’ themselves over the whole of the coloured part of the eye much as if you had placed a small piece of cling film on the eye.

 

How Contact Lenses Work.

To understand how lenses work
and why one type might be more suitable than another it is important to understand the basics of how the eye is made.

The power of the eye is governed to a great extent
by the curve on the front of the eye (the cornea).
If it is too flat the power will not be strong enough (long sighted), if it is too steep it will be too strong (short sighted) and sometimes there is astigmatism which is a combination of them both and is normally due to there being a combination of relatively steep and flatter curves present. The amount of difference between the two curves is the astigmatism. We aim to ‘reshape’ the front of the eye with an artificial surface when we use a contact lens.

The cornea is an unusual part of the body
in that it doesn’t have any blood vessels feeding it, relying instead on the fluid at the back of it for nutrition and taking oxygen from the tears in front. When a contact lens is placed in front of the cornea the delivery of oxygen can be disrupted to too large a degree, correct choice of lens material and design stops this happening.

When hard lenses were first fitted they were made of
a type of plastic called PMMA and relied solely on the exchange of tears under the lens to give an oxygen supply to the cornea. Over the last twenty five years lens materials have been manufactured that allow oxygen to pass through and these lenses are called gas permeable lenses.

The hard versions of these lenses are
known as rigid gas permeable (RGP) lenses.
They are designed similarly to the PMMA lens and give oxygen to the cornea both by exchange of tears under the lens and oxygen transmission through the lens. They have effectively replaced the older PMMA variety but still have the disadvantage of taking some time to get used to, due to the fact that the edge of the lens sticks up from the cornea to ensure tear exchange.
This ‘edge lift’ is felt as quite an irritant by the lids initially but normally the eyelid adapts, however it does make it easier for pieces of dust etc. to get underneath the lens and irritate.
This form of lens is often better at correcting astigmatism due to its rigid nature.

All soft lenses are gas permeable
and they are made of different materials with an element of water held within. The water content percentage can vary from the low thirties to mid eighties. For many years it has been the water content that has been the vehicle for transporting the oxygen through the lens, generally the higher the water content the better the material’s oxygen transmission. Over the last couple of years a new material has reached the market that only has a low water content but a phenomenal ability to pass oxygen through its main chemical structure.

When a soft lens is placed on the eye
it moulds itself to the curve on the front of the eye. If we want the lens to correct short sight we start with a thin middle to the lens and make the lens thicker towards the edge, this effectively flattens out the curve that was too steep. The greater the amount of power to be altered the greater the padding out. For a long sighted eye the situation is reversed with a thicker centre to the lens and a thinner edge.

If there is astigmatism we find that a soft lens will seldom correct this fully.
If we choose the correct padding to correct the flatter of the two corneal curves as the lens drapes and moulds to the eye the flatter curve will be well compensated for but the steeper curve will still cause too great a fall away on the new front surface, effectively the astigmatism has not been corrected.

To compensate for the astigmatism
we need to use a ‘toric lens’ that has a main base curve that corrects the flatter corneal curve and supplementary padding on part of the lens to make up where the astigmatism still falls away. If we make a lens like this we then have to stop it rotating in the eye as we blink and make sure it stays at the correct twist to coincide the extra padding on the lens with the corneal fall away, if this happens the astigmatism and any long or short sightedness is corrected.

These toric lenses are more difficult to manufacture
and fit than a standard lens and are often not as successful as standard lenses. A rigid or hard lens tends not to have this problem as it holds its spherical front curve and pads out itself behind the lens with a slightly thicker tear component where required.

 

How long will a lens last?

A hard lens will often last for a few years
and it is probably more common for the lens to be lost or broken before it is ‘worn out’, although some gas permeable materials become contaminated by secretions in the tears earlier.

Soft lenses will not last anything like as long
and there are differences between opticians as to how long a lens should be used for. One of the greatest problems associated with contact lenses is infections. Using any contact lens increases the risk of infection compared to not wearing lenses at all but my experience is certainly that the older a lens is the more likely it is to not be as clean and the greater the incidence of problems. For this reason I strongly promote regular replacement of soft lenses.

A tale I tell patients about types of lenses
is to imagine that you are at a meeting in the boardroom of a contact lens company. The MD presides at the end of the table and asks how things are coming along. The research department says, “We’ve got this wonderful new material that passes oxygen through fantastically”. “And we can make it really thin” chirps in the manufacturing department. “Wonderful, let’s make them” says the MD. “Hold on” says the marketing department, “if we use that material it will soil up too quickly and if it’s that thin it will keep ripping when you clean it. We’ll lose our good name with all the complaints”. “Those are good points marketing, I tell you what why don’t you all get together and try to reach the best compromise” says the MD as he leaves the room.
That is how lenses were once made.

Now imagine the same meeting some time later
but where the manufacturing department says the same but adds ‘and we’ve found a way to make it so that the costs are reasonable enough to replace regularly’ – heaven, welcome to the world of regular replacement lenses! I am so sure that regularly replacing lenses is the best way to wear lenses that I do not fit a lens and leave it until it needs replacing as by the time that decision has been taken there are generally problems already being caused.

 

Even with regular replacement lenses there are choices.

We offer a scheme with regular replacement of lenses and also a daily replacement lens.
I consider the main advantage of daily lenses to be that every time you come to insert your lens you have a guaranteed sterile lens, this minimises the risk of infection as much as it is possible to do so.

Even though every patient I see tells me that they will always clean their lens properly every night and rinse their case everyday I know that in the real world short cuts start being used, familiarity breeds contempt and at some point problems will, not may, occur. Daily lenses stop this worry.

Another advantage is
that at the end of the month monthly lenses still generally feel fine but when the new month’s lens goes in it feels better, that’s what happens every day with dailies.

Patients tell me that the convenience factor is wonderful,
no more trying to clean a lens when all you want to do is go to bed and no carrying bottles of solution around when you stay away.

Daily lenses may be a little dearer than the older style of supply.
Dailies are a bit dearer (at present we charge £30 per month for full time wear including all examinations) but ask yourself – Are your eyes worth it?
Monthly replacement lenses start at £20 per month including solutions and all examinations.

Lenses are available that are intended to be worn overnight for up to a month at a time.
Over the years there have been periods where similar wearing schedules have been suggested with different lenses but with the benefit of hindsight almost all opticians feel that they were not a good way of wearing.

The new generation of lenses are wonderful in their performance
but I still do not feel happy at recommending them. I feel that there is still an appreciable increase in the risk of infections and I prefer that my patients are ‘involved’ with their lenses each day. I want them to think, before they put a lens in, are my eyes comfy today. If someone feels such a desperate need not to have to deal with a contact lens daily I have suggested considering laser surgery which I feel is no riskier than overnight wear.

I am always happy to consider each case on its own merits but the above thoughts hopefully convey an idea about my attitude to contact lens fitting and wear.