Are You Still Wearing Hard Lenses
PMMA (Polymethylmethacrylate) Lenses were invented in 1948, (that’s before colour television!). At the time they were a revolution. They were the first successful contact lenses that could be worn all day. Their only draw back was they did not allow oxygen though to the cornea. To prevent problems the lenses were made very small and relied heavily on the tear film to flow under the lens to supply oxygen to the cornea. However they give excellent vision due to their rigid nature and are very durable.
So how does the eye work?
The cornea is the transparent window in front of the coloured part of the eye known as the iris. The light passes through the cornea, through the pupil & focused by the lens just like a camera.
The cornea needs to be transparent to allow light to pass through & give clear vision, just like the lens on a camera. Therefore the cornea does not contain any blood vessels & gets its oxygen supply from the air.
When a contact lens is worn this inhibits the supply of oxygen to the cornea.
Developments in Technolgy
Over the last 70 years there have been many developments in the contact lens world. In 1979 Gas Permeable lenses (RGP) were introduced by adding silicone to the PMMA material this allowed the flow of oxygen through to the cornea.
Lenses could be made larger giving clearer and more stable vision. The negative being that the lenses were softer so not as durable as PMMA. They must be cared for with a more delicate hand and replaced more regularly, yearly as a minimum. RGP lenses are still common as they give excellent vision although their use has reduced dramatically over the last 20 years.
Following the invention of RGP lenses soft lenses joined the market. This has lead to PMMA becoming almost obsolete. Soft lenses are immediately comfortable, breath freely and are available in a wide range of powers and modalities such as daily disposable, 2 weekly and monthly. Handling is a bit more tricky but can be over come with practice. 90% of all new wearers are now fitted with soft lenses.
Why Do You Need To Change Your Contact Lenses?
Wearing PMMA lenses for many years can cause changes to the eye which can now be prevented by refitting you with more modern alternatives.
Changes can include:
Ptosis – dropping of the eye lid caused by weakening of the upper eyelid muscles from wearing contact lenses for many years.
Corneal Hypoxia – this means your cornea is showing signs it is being starved of Oxygen and can put your eyesight at risk. This can also lead to reduced corneal sensitivity and means you can no longer feel when something is wrong such as an abrasion, ulcer or start of an infection. Once oxygen is restored some of this feeling will return although it may be uncomfortable at first it is important to repair any nerve damage and restore feeling.
This means your cornea may have changed shape or thinned in places due to many years of wearing hard contact lenses.
If your eyes are dry it is unlikely the lens is working well. PMMA lenses rely heavily on the tear film to act as a cushion for the lenses to float on. Without this cushion the lenses can rub against the front surface causing small scratches which you may not even feel due to poor sensitivity from many years of contact lens wear. The tear film also supplies oxygen to the cornea as your lenses do not breath, and without oxygen your cornea and vision are at risk.
What Do you Do Now?
Book an appointment to see the Contact Lens Optician to discuss your options.
Your Contact Lens Optician will ask if it is possible to leave your lenses out overnight before your appointment. This is to allow oxygen to be restored to the cornea and the eye can to begin to return to its natural shape before new measurements can be taken. The longer you can leave your lenses out before new measurements can be taken the more accurate the result will be. Please bare in mind we will not get true eye measurements unless the lenses are left out for at least 1 month.
New lenses will then be ordered to trial, these will be either soft or RGP depending on your needs and prescription.
A further appointment will be made for you to collect and try your new lenses. Ideally if your old lenses can be left out again, as a minimum, the night before your new fitting, the new lenses will fit better as your cornea will be a more natural shape.
As the oxygen is restored and normal sensitivity returns you may become temporarily more aware of your lenses. Your prescription may also change as your cornea returns to its more natural shape. We advise you do not purchase spectacles until your optician says your prescription is stable.
Follow up appointments will then be advised, as necessary, by your contact lens optician to check your progress and make adjustments to your contact lenses as your eyes change.