They are Not Hard !
Most folks are very familiar with vision correction from glasses, soft contact lenses and even surgical procedures like LASIK and PRK. Few patients truly understand the incredible advantages of Rigid Gas Permeable (RGP or just GP) lenses sometimes referred to as “Hard Lenses” or their bigger brother, Scleral Lenses. I’d like to highlight some benefits of this vision correction modality. Lets look at the limitations of common forms of vision correction. Hold on though, we’ll get a little technical as it is important to understand the ‘hows and whys’.
The vast majority of eyeglass lenses correct low-order aberrations referred to as spherical (nearsightedness or myopia and farsightedness or hyperopia) and astigmatic (when one symmetrical part of the person’s field of view such as 12:00 to 6:00 is more near-sighted or far-sighted than the meridian 90 degrees away (in this case 3:00 to 9:00). This may sound odd, but it is very common and typically the surface of a person’s cornea is where this is manifest. Correcting these low-order optical aberrations with eyeglasses or soft contact lenses works well for most people. However, persons who have irregular astigmatism or distortions in their vision generally will not do well vision-wise with glasses. This optical distortions may be from injuries to the eye (cornea especially), surgeries gone wrong such as RK (Radial Keratotomy), diseases which upset the normal configuration of the cornea such as herpetic lesions (shingles), hereditary corneal dystrophy or perhaps even chemical injuries to the eye. There as such things as eyeglass lenses custom-made to correct some optical irregularities, however they are very expensive, few places produce these and the lenses only perform when the patient is looking through the optical center of the lenses in ‘straight ahead’ vision.
Soft Contact Lenses
Just like eyeglasses, they generally only correct more common optical irregularities. Unlike eyeglasses, they move with the eye and can provide better optics especially for higher prescriptions. Newer soft lenses benefit from excellent material advances and can be five times or more higher in oxygen permeability than soft contact lenses from just 15 years ago. However, the soft contact lens essentially ‘drapes’ over the corneal surface. Any hills, peaks or valleys in the person’s irregular cornea are telegraphed through the soft lens and show up on the front surface of the lens. Back in optics classes, I learned how the first interface surface between air and a refracting medium (such as a lens) is very critical in image formation. Light does not focus when the surface is irregular such as a soft lens fitting on a non-smooth cornea.
Gas Permeable / GP / Scleral Lenses
This is where some magic happens in terms of correcting optical irregularities. Gas permeable lenses are now made of materials which breathe very similarly to soft lenses. The beauty of these type of lenses is that the do not flex like a soft lens – which amazingly is a huge advantage in optics. The very smooth surface of a gas permeable lens easily beats that of a soft lens. So we have a stable, smooth lens that does not flex adding to great optics in step one. There is a second step in the ‘refracting process’ of an GP/Gas Permeable/Scleral Lens – that is the tear lens. The GP lens rests on a dynamic layer of tears. If you can imagine the nice, smooth surfaces (front and back) of the GP lens resting on a layer of tears which coat the cornea. These tears actually fill-in any irregularities such as peaks and valleys of the cornea which may have been formed by injury, disease or surgery processes. The very cool part is that the GP lens can rotate and move on this tear layer like a raft on the ocean yet the optics are stable because the tear film dynamically moves between the back side of the GP lens and the front of the cornea – instantly correcting all kinds of optical defects on the surface of the cornea. GP lenses are typically 9 millimeters across, a typical cornea is 12 millimeters across.
Sometimes the smaller GP lens is a liability because it moves too much or doesn’t center on certain corneas because of injury, surgery, etc. This is where a Scleral Lens enters. These lenses are a bit larger, often 14 millimeters or larger. The beauty here is that the lens now rests on the white of the eye (sclera) and is stable. The lens surface is still super smooth and the back side of the lens is cushioned by a layer of tears or saline solution. Unlike a soft lens which dehydrates during the day, the GP and Scleral lenses have not water content within the lens matrix yet are still permeable to gases such as oxygen. Also, unlike a soft lens, Gas Permeable lenses can have really neat optics placed on the front surface of the lens such as a reading prescription, extra correction for astigmatism, etc.
Alpine EyeCare Center is the perfect place to evaluated for all types of contact lens fittings. Let our experience and advanced equipment help you see better.
I look forward to personally caring for you!