Gas Permeable Lenses: All You Need To Know
When lenses were first introduced as a cooler alternate to eye glasses they were made of acrylic glass. This is poly methyl methacrylate (PMMA), a type of hard plastic. While PMMA gained popularity in many different applications owing to its high impact resistance and its lightweight, it could not prove to the best fit for use as contact lenses.
The problem with hard contact lenses:
They were impermeable to oxygen, a healthy supply of which is needed to maintain good condition of the cornea. Lack of oxygen to the cornea can lead to hypoxia related problems. This meant that the oxygen for the cornea needed to come from another source. This source was the natural lacrimation system of the eye. The cornea could fulfill its oxygen requirement from the tears that were produced by glands in the lid and spread by blinking. However, for this to work efficiently the lenses had to be small in size and its edges could not be in close contact with the cornea. This structure of the lens meant that it was very prone to falling out of the eye. There were also complaints of discomfort and the ever present sensation of wearing lenses.
The introduction of soft contact lenses
The solution to all this came in the 1970’s when soft contact lenses were introduced. They were made from silicone, a material whose versatility meant that these hydrogel lenses had a lot of benefits:
- Silicone is permeable to oxygen. This allowed the cornea to receive its oxygen supply from the air while the user was wearing lenses.
- Silicon is flexible. This property, along with the thinness and softness of the lens made them very comfortable to wear for the user.
- These lenses cover the entire cornea and a small part of the sclera. Their huge size and edge adherence to cornea meant that they could note easily pop out of the eye. This made them ideal for sports and other physical activities.
What are Gas Permeable Lenses?
Introduced later in the seventies, they were an innovation of the earlier introduced soft contact lenses. Known by many names, oxygen permeable lenses or RGP lenses, they are made of silicon containing compounds. This means that like its predecessor soft contact lenses, they allow oxygen to pass through. They are relatively small in size, smaller than the traditional hard contact lenses, covering 75% of the cornea. They are also rigid, unlike soft contact lenses.
The benefits of gas permeable lenses:
These lenses since they are made of silicon allow oxygen to pass through. This oxygen is important for the health of the cornea as mentioned earlier. So why would you prefer these specific lenses, if soft contact lenses do the exact same thing? The answer is simple. RGP lenses are ‘better’ at oxygen permeability:
- Their silicon compounds make them more permeable to oxygen than many soft gel lenses, except silicone hydrogel soft lenses which are on par with rigid gas permeable lenses.
- Their smaller size means that a greater area of the cornea is uncovered and hence allowed to be in contact with the oxygen in the air.
- Their rigidity means that they conform to their shape, not fitting around the cornea. So whenever the user blinks, the RGP lenses move. This allows for oxygen rich tears to wash over the surface of the cornea. This is not possible with soft lenses which are pliable and relatively unmovable by blinking.
Better visual acuity:
Since these lenses are rigid and tend to keep their shape, they do a better job at correcting errors in visual acuity and provide a rich visual experience. This is because your vision is sharper with oxygen permeable lenses. Another added benefit is their ability to correct astigmatism better than the other lenses.
Rough handling of the soft contact lenses causes tears and rips, which make them unsuitable for use. This is not the case with RGP lenses. Their rigid shape means that they are more protected against physical damage and hence last longer. Organic materials like lipids and proteins cannot adhere to RGP lenses. Thus if these lenses are handled with care, they will last for a long period.
The pitfalls of gas permeable lenses:
Period of Adaptation:
If RGP lenses offer so many advantages then why do more people not use them? The biggest reason is that they need to be worn for some time before they are truly comfortable for the user. If you stop using them, you would have to re-adapt when you wear them again.
Since they are smaller than most lenses, it means that high impact collisions and force to the head might make them fall out of the eye.
These lenses are expensive to replace as they are custom made for the user’s eye. This can be cumbersome if you lose them or if there is a change in your prescription.
Dust and Dirt:
Since these lenses float on your tears and do not conform to your eye, it allows for dust and dirt to get under your lens. While this mostly is inconvenient and causes discomfort, in the worst case scenario it can lead to corneal abrasions.
Uses of GP lenses:
GP lenses are best for use in individuals whose astigmatism cannot be fully corrected by soft contact lenses. They were also great for people who suffer from presbyopia or keratoconus. Individuals who require lenses after refractive surgery can also use them. They were also used to reshape the cornea during sleep as part of ortho-k therapy, for which they are specially designed.
Hybrid Lenses: where do they come in?
They are the bridge between soft contact lenses and oxygen lenses. With a central portion of RGP lens, they provide sharp vision and clarity. The peripheral silicon hydrogel soft lens makes it fit well offering great comfort to the user. Currently SynergEyes is the sole producer of FDA-approved hybrid lenses available in the USA.