Introduction
PMMA contact lenses are hard lenses made from polymethyl methacrylate (PMMA). Although PMMA contact lenses offer excellent optics, they are rarely prescribed today because they do not allow oxygen to pass through. Their inability to allow in oxygen makes them hard to wear and adapt. Gas permeable contact lenses were designed in response to that shortcoming and have significantly replaced PMMA contact lenses.
A continuous supply of oxygen will keep the cornea healthy. Because PMMA contact lenses do not allow oxygen and other gases to reach the cornea, the eye must find a mechanism to cope. That’s why every time one blinks, tears wash underneath the lenses. These tears send oxygen to the section of the cornea covered by the lenses. That’s why PMMA contact lenses are small, with a significant gap between the lenses’ edge and the surface of the cornea. These properties allow someone wearing PMMA contact lenses to be aware of their presence in the eye.
Material
PMMA is a transparent rigid thermoplastic material which also goes by its trademark names Perspex, Plexiglas and Lucite. PMMA is also called acrylic or acrylic glass. It is lightweight and resistant to shatter. PMMA material is similar to polycarbonate due to its impact-resistant properties. Its use extends beyond contact lenses to airplane windows, submarine periscopes, acrylic nails, LCD screens, medical devices, furniture and security barriers. PMMA is available in a variety of colors. It also allows for the internal transmission of light.
Advantages
PMMA contact lenses have several advantages such as:
Unlike polycarbonate, the PMMA material does not contain bisphenol-A, which is a harmful substance. PMMA contact lenses are made through annealing where the material is heated and cooled in a continuous process. The result is contact lenses that are free of chemicals, inert and safe to use.
PMMA material is readily available and not expensive. The lenses are less costly because there are many PMMA suppliers, with few differences between competing brands.
PMMA material is scratch-resistant.
PMMA contact lenses are durable. They can be renewed when minor surface scratches are polished. The lifespan of these lenses is five to seven years.
The kind of plastic used does not mold to the shape of the eye.
PMMA contact lenses offer excellent vision because they can be customized to an individual’s needs. This is because once these lenses have been made, they can be reworked and modified.
Cleaning and keeping the lenses in hygienic conditions is manageable. They require minimal cleaning, soaking and wetting solutions.
Because PMMA contact lenses come in various colors, it is possible to have them tinted. Tinting has the advantage of protecting the wearer against light sensitivity. Besides, tinting makes the lenses easier to find and gives the wearer a better physical appearance.
Disadvantages
PMMA contact lenses have several drawbacks which include:
They are uncomfortable to wear since they are impermeable (don’t allow oxygen to pass through). There have been several attempts to improve permeability. One was fenestration – the drilling of small holes in the lenses. Another attempt was adding hydrogel to the material. However, these attempts have failed to make PMMA lenses permeable. Many people may not be able to produce enough tears for comfortable wear. Dry eyes and swelling limit the number of hours one can wear the lenses. Most people will wear them for 8-12 hours making them ‘daily’ wear lenses.
They can easily get dislodged or fall off, especially during activities like sports because they do not fit closer to the eye.
They are designed to be small since they lack permeability. Consequently, when fitted, they do not cover much of the cornea. PMMA contact lenses are also thin and light. This means they may not offer excellent vision for people with corneal astigmatism.
Due to their lack of permeability, PMMA contact lenses are not appropriate for everyone. Those with better tear-production mechanisms are the only ones who can wear these lenses. Another group of individuals who can wear PMMA lenses is those with single vision refractive errors like nearsightedness and farsightedness. Those with a high need for oxygen in the cornea require difficult multi-correction and will not find suitability in PMMA lenses. If one is unable to follow the daily care routine, PMMA contact lenses may not be ideal.
Contact lenses made of PMMA material are challenging to adapt to and may even take several weeks. Many people eventually discard the lenses.
Individuals who wear PMMA contact lenses face the risk of a condition called overwear syndrome. Contact lens overwear syndrome occurs when a wearer over-extends their contact lens’ wearing time. It can occur as a result of the lens being worn while sleeping. The result is less oxygen reaches the eye. This can lead to eye pain, decreased vision and sensitivity to light.
