vitalogy44 said:
Oxygen permeability (Dk/T) is not as good a measure as oxygen transmissibility (Dk/L) and there is a huge difference between PMMA and other contact lenses. Your statement about lens thickness in high + powers giving no oxygen to the cornea is incorrect. A PMMA lens will have Dk/L of 0, but Silsoft from B&L is an aphakic lens available to +20DS with a Dk/L of 71, so even though lens thickness must be taken into account, there is no way that it is the only factor.
PMMA might be an optically superior material, but it a terrible choice when corneal health is concerned, especially in a high plus lens.........
"Polymegathism: The cornea is only about 500 to 600 microns in thickness (25,400 microns equals 1 inch), the innermost layer being only 1 cell layer thick and known as the endothelium. This single layer of cells is responsible for maintaining the proper amount of water in the cornea. These cells are unique in that we are born with a certain number and they are never replaced. As we age, the number of cells decreases, so the remaining cells must take on irregular shapes to fill in any gaps in the endothelium. The first contact lens designs in the '50s and '60s were made out of a plastic known as PMMA and only allowed minimal amounts of oxygen to pass through to the cornea. Lack of oxygen promotes corneal edema and endothelial cell death thus PMMA is no longer recommended for long term use. This is another reason for the importance placed on the ability of a contact lens to permit oxygen to pass through onto the cornea. A high oxygen transmission contact lens allows the cornea to maintain proper long term health."
One of the reasons this occurs is that after long term use the hard/RGP lens wearer has lost most, if not all of his/her corneal sensitivity and is obliovious to any symptoms. Also, with endothelial swelling you will have a certain amount of stromal swelling giving corneal warpage, etc. Plus with decreased dexterity in the elderly there is a higher probability of a corneal abrasion with these lenses.
Just because a patient says vision is good and lenses are comfortable--there is no reason to compromise corneal health especially in the elderly who tend to be more susceptible to infection, etc.
When you fit these patient (w/PMMA)--how often did you follow-up or chack their lenses?
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