Could some please explain how these work in terms of Daily, weekly, monthly and pernament. What happens if you wear past the designated time?
What are contact lens' Advantages and dis advantages
Could some please explain how these work in terms of Daily, weekly, monthly and pernament. What happens if you wear past the designated time?
What are contact lens' Advantages and dis advantages
This answer is partially true and partially BS from manufacturers. The theory is that every lens forms deposits from day one. Differing materials have differing attractions for deposits, polymers break down, etc. Hense the disposable lens. There are those who think even HGP lenses become coated and become "less oxygen permeable."
There is also a growing cultural change that says : "Patient's won't follow care protocals, practioners are too busy (read: lazy~greedy) to instruct patient in proper care, and no one wants to fool with follow-ups. Hense the disposable lens, that the practioner can spend little effort to fit and follow, charge an extra $ 80.00 for a $35.00 eye exam. The dispoable lens manufacturer can make some money repeatedly. The FDA is happy because every one gets the same "standard of care" (no matter how mediocre) all the lenses are the same.
As to what will happen to the eye? Maybe nothing, maybe a lot, most likely the patient will sooner or later have GPC.
Chip
You ask an interesting question (and one that has gotten notable manufacturers such as B&L and Vistakon in hot water in recent years).
There are numerous factors which have driven the contact lens market towards shorter and shorter replacement schedules:
1.) Contact lens manufacturing has evolved from Lathe-cut to spun cast to cast molded. Each evolution has reduced the production cost of soft contact lenses and has increased the quantity of lenses that a manufacturer can produce (yes, lenses are still lathe cut and spun cast- but cast molding is the cheapest way to make tons of lenses).
2.) Since lenses are so cheap to produce (and the retail price of the lens has dropped accordingly), the manufacturer needs to sell more lenses in order to support their profit margins. Therefore, we have contact lens manufacturers promoting monthly, then bi-weekly, then daily replacement of lenses.
3.) The advantage to the consumer is increased convenience. There is very little point in cleaning a lens that is going to be replaced in a week or two, and losing a lens isn't so tragic when it can be replaced for $3... There is also a health benefit because shorter replacement schedules do not allow protein to accumulate on the lens (protein deposits inhibit oxygen flow through the lens and provide nourishment for bacteria which can eventually invade the eye through an abrasion in the epithelial layer of the cornea). One undeniable problem has been the consumer's tendancy to grossly overwear two-week lenses. The problem is not so much the fact that they wear the lenses long term (most disposable lenses would probably last fairly well if properly cleaned... although they do tend to be thinner and therefore more fragile than planned replacement and conventional lenses), but in the fact that these lenses are often worn continuously for weeks and weeks without cleaning or removal!
The "trick" is in determining how long a lens may be safely worn. Vistakon (and in years past, B&L) recently lost a class-action suit which asserted that the "daily replacement" lenses were essentially no less suited for two-week wear than the lenses sold under the "two week" recommended replacement schedule. The problem Vistakon faced involved the fact that the lenses were made from the same material (etafilcon-A) and had basically the same design. In the 90's B&L was taken to task for selling the same lens design and material under three different names (and wearing schedules).
No matter the material or design, the truth is that any lens is apt to be "over-worn" depending on the individual eye they interact with. Someone with a lot of protein in their tears (or someone with chronic dry eye, or someone with GPC) may not be able to wear the same lens for as long a period as someone without limiting factors.
The consequences of overwear are both acute and chronic. Hypermia (swelling of the cornea) always occurs during overnight wear, and giant papillary conjunctivitis (bumps on the palpebral conjunctiva) can occur with overwear. Perhaps most sinister, corneal ulceration can ultimately occur when bacteria is trapped against the cornea due to poor tear exchange. Although the consequences are unclear, the endothelium apparently undergoes contact lens wear related changes over years of wear.
Interestingly enough, two new materials (lotrafilcon-A and balafilcon-A, by Ciba and B&L respectively) have recently arrived on the market which may allow for longer safe and comfortable wearing schedules. Why? Because they are both silicone-containing hydrogels. Silicone transmits oxygen more efficiently than water, so the lenses feature low water content and high Dk values. It will be interesting to see what happens if the cost to produce these lenses falls (i.e., how are they going to assert that the lenses should be disposed of in shorter and shorter periods when it becomes necessary to produce more lenses to maintain profits?). My guess is that the wholesale cost of these lenses will not go down- even though the cost to produce them will.
Contact lenses aren't really "my thing," but I hope this goes a little way to answering your question (or at least what I perceived your question to be).
Pete Hanlin, ABOM
Vice President Professional Services
Essilor of America
http://linkedin.com/in/pete-hanlin-72a3a74
Bump. Great post Pete
Wow, very informative! Good job!
Good post Pete but spin cast was first, then lathe cut (if we confine this to soft contacts) then molded. If you include rigid lenses, blown (glass) was first then ground, then lathe cut, moulded and spin cast.
Chip
"...Hypermia (swelling of the cornea) always occurs during overnight wear, and giant papillary conjunctivitis (bumps on the palpebral conjunctiva) can occur with overwear..."
I'm not sure that the etiology of GPC is exactly as you describe. I think it is an oversimplification to say it is simply over wear.
1. Edge design
2. Bio film
3. Protein deposition on the surface of the lens.
You can get sufficient bio film or protein content on the first 8 hours of wear to create the conditions for GPC.
For us uneducated types, hyperedema is generally just called edema.
In fact in 50 years of fitting contact lenses I have never seen the word hyperedema. Have seen my share of edema though.
Chip
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