Why only two silicon hydrogel lenses approved for thirty day wear?

Discussion in 'Optometry Archives' started by MS, Sep 21, 2009.

  1. MS

    MS Guest

    The first two silicon hydrogel lenses on the market, Night & Day (Ciba) and
    Purevision (B&L) were approved for thirty day wear.

    Since then there are several more silicon hydrogel lenses out, including
    others by Ciba, but none AFAIK, other than those two, are approved for
    thirty day wear. Most for not more than six or seven days, even with a
    thirty day replacement schedule. (Please correct me if I'm wrong.

    N&D is still the lens with the highest DK/T. (Surprising too, that that
    record has not been matched or beaten yet.) However, Purevision is probably
    the lowest DK/T Si-Hy lens. So, it cannot be explained solely by DK/T, why
    others have not been approved for thirty days. There are several lenses with
    higher DK/T than Purevision, but are not approved for thirty day wear, at
    least here in the USA. (It would be interesting to hear though, if some of
    them have been approved for that length of wear in other countries. Anyone
    know?)

    What explains this? Is it just that the companies don't want to go the
    expense of the testing that might be required, to get thirty day approval?

    I would think it would be advantageous for a company, to get thirty day
    approval for a lens, even though most eye doctors do not recommend actually
    wearing them for thirty days non-stop. (I'm sure some people do though.) For
    instance, I take mine out and clean and disinfect them, leave them off
    overnight, etc. about once a week, just to be on the safe side. (Although
    sometimes I have gone longer than that, even two weeks.) Yet, even if not
    too large a percentage of wearers actually wear lenses for thirty days
    straight, one might think that, if a lens has been approved for thirty days
    wear, four times longer than one week wear lenses, that they (30 day lenses)
    might be safer for any EW, to have been approved for such a long time. Is
    there any logic to such an assumption?

    But then again, why would Purevision have been approved for thirty days
    wear, when other newer materials, with higher dk/t, for example Biofinity,
    only for six or seven days?

    Are there other factors besides dk/t, that could account for it? Is
    Purevision better at resisting deposits than Biofinity and others (besides
    N&D)?

    Or is it just that the companies did not want to bother with the time and
    expense involved of the testing required to get 30 day approval in the USA?
    Have some of them been approved for 30 day wear elsewhere (perhaps with a
    less expensive and time-consuming testing requirement), but not in the USA?

    Just curious. Your input would be appreciated.
     
    MS, Sep 21, 2009
    #1
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  2. MS

    Mike Ruskai Guest

    I have no idea what goes into the approval process, but I can make some
    guesses assuming the process is actually rational.

    I've tried both N&D and PureVision. Both are very stiff and feel rather like
    smooth plastic. It's hardly a scientific conclusion, but they seem less
    likely to hold onto deposits because of that. They also have rough edges,
    making them very uncomfortable, but that's another matter.

    I now wear primarily Biofinity, with Oasys for switching off. Both are
    considerably more flexible and feel less like smooth plastic. They both also
    pick up deposits too much to consider wearing for 30 days. I know Biofinity
    is approved for 30 day wear in some countries, but I don't see how anyone
    could leave them in that long. I typically have them in for less than a week,
    and often will pop them out for a quick rub and rinse to improve comfort and
    clarity.

    Again, that's only my experience and conjecture.
     
    Mike Ruskai, Sep 21, 2009
    #2
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  3. MS

    MS Guest

    Thanks, Mike

    Anyone else? Any of the eye docs here?

    By the way, I haven't found either Purevision or the old N&Ds to be
    uncomfortable.

    I tried one of the new N&Ds--same material as the old, but they have been
    supposedly treated some way to make them more comfortable. That lens was
    actually quite uncomfortable. My eye doc said that a lot of people have had
    trouble with the new N&Ds. I think they messed something up, with their
    "improvements".

     
    MS, Sep 22, 2009
    #3
  4. MS

    MS Guest

    Any other input on this question?


    Thanks in advance.
     
    MS, Sep 23, 2009
    #4
  5. MS

    MS Guest

    In other words, you think that there is not necessarily something
    different about ND and PV which make them safer for EW than the other
    silicon hydrogel lenses, but it is just that the companies have made a
    decision not to market and seek approval for the other lens brands,
    for 30 day wear?
     
    MS, Sep 23, 2009
    #5
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