Ciba N&D requiring different power to other lenses

Discussion in 'Optometry Archives' started by Clive, Oct 17, 2004.

  1. Clive

    Clive Guest

    I have just read
    (http://www.siliconehydrogels.com/editorials/previous_editorials_kathryn5.asp)
    and this states:

    Other Optical Considerations with Silicone Hydrogel Lens Wear
    There have been a number of anecdotal reports of patients requiring higher
    powers with Focus Night & DayT SH lenses than conventional lenses. The
    apparent requirement for additional power is related to the aspheric design
    of the Focus Night & DayT lenses and a resultant relative decrease in
    spherical aberration compared to other spherical lens designs. This is most
    noticeable in the higher minus or plus designs. For example if a patient is
    wearing a -9.00 D spherical lens as their current lens, that lens is -9.00
    in the centre but will have several dioptres of extra minus spherical
    aberration across the optical zone. This effectively makes the average power
    across the optic zone somewhat higher than -9.00 D. When the aspheric design
    Focus Night & DayT lens -9.00 D is placed on the eye, there is less
    spherical aberration and thus the average power is somewhat lower than its
    spherical counterpoint (but in actual fact closer to labeled power). There
    have been reports of -0.50 to -1.00 D "extra" power being required in some
    patients.


    How does this make sense? It says that a regular -9.00D lens has several
    dioptres of extra minus spherical aberration across the optical zone which
    makes the average power across the optic zone somewhat higher than -9.00 D,
    and that the Ciba lens is closer to labeled power. If the contact lens power
    is initially calculated from a spectacle Rx taking into account the back
    vertex surely the initial calculated power should be closer to the Ciba
    power (as this is closer to the labeled power). i.e it is the other lens
    that introduces an error.

    Having said that the ciba lens calculator on the ciba website does provide
    strange results. In some cases it predicts a higher power as indicated
    above - at other times a lower power - I cannot work out the logic for this.

    i.e Spectacle -5.00/-4.00 - Focus Monthly -4.75/-4.00 Focus
    -4.75/-4.00
    i.e Spectacle -5.50/-6.00 - Focus Monthly -5.25/-5.50 Focus
    -5.25/-6.00
    i.e Spectacle -7.00/-8.00 - Focus Montly -6.50/-7.25 Focus
    -6.50/-7.00

    If we introduce a cyl correction the results are stranger still

    -5.75/-6.00 with 0 cyl = -6.00 N&D OS & OD. or -5.50 OS & OD Focus Monthly.

    You have to add -0.75 of cyl to the -5.75 Rx before power changes to -5.75
    however you have to increase cyl to over -2.50 before the power of the Ciba
    N&D changes.

    You have to add just -0.25 of cyl to the -6.00 Rx before power changes
    to -5.75 however you have to increase cyl by -1.75 before the power of the
    Ciba N&D lens changes. A Px should be less aware of cyl as the sphere
    decreases.

    Do other find that the lens power suggested by Ciba work when tested by
    overrefraction? or in general are you finding that the powers required are
    the same as for other soft lenses?

    Can anyone explain why Ciba calculates higher powers in some instanecs - and
    lower in others?
     
    Clive, Oct 17, 2004
    #1
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  2. Clive

    LarryDoc Guest

    This has been discussed here repeatedly. Your assumptions are simply
    untrue for the most part and the answer is quite simple---much more
    simple than the path you are trying to follow.

    First, the labeled power is the labeled power, for any lens, within
    tolerance. It is not the final effective power of the lens.

    Second, your issues are talking about .25D differences. That is
    certainly not strange or unusual when dealing with higher powers. Even
    at .5 or .75, when dealing with aspheric designs and entering in a cyl
    component is not that big of a deal.

    Third, the fact that the lens is aspheric is only a part of the story.
    The part that you are missing is perhaps the key. That is, the lens,
    being stiffer, tends to vault the cornea apex causing a plus powered
    tear layer lens under the contact lens. The degree of plus generated is
    anywhere from zero to .62 D and is variable depending upon the
    eccentricity of the cornea vs the back curve of the lens----but we are
    still dealing where the same general amount of power.

    Finally, the whole thing is moot point. The final lens power is that
    which is determined by measuring it with the lens in place on the eye.
    The "calculator" method is merely a starting point.

    --LB

    --
    Dr. Larry Bickford, O.D.
    Family Practice Eye Health & Vision Care

    The Eyecare Connection
    http://www.eyecarecontacts.com
    larrydoc at eye-care-contacts dot com (remove -)
     
    LarryDoc, Oct 18, 2004
    #2
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  3. Clive

    Simon Guest

    First, the labeled power is the labeled power, for any lens, within
    The Ciba lens may be closer to the labelled power than other lenses and in
    comparison between contact lenses the Ciba may require a higher power lens
    for the same result. However if a contact lens has a overall higher power
    than labeled (as is suggested) then this may indicated a lower power lens
    than expect would be used compared to the spectacle Rx. i.e on the basis of
    the information given the Ciba lens is closer to the theoretical valve (but
    see below)

    There are differences even in the ciba way of calculating things. The online
    predictors shown these higher strength powers for Night & Day and Dailies,
    whilst the product fitting sheet for the daillies used the standard
    spherical equivalent method. Night and Day however may require a differnt
    strength to Dallies due to fit.
    Agreed. A lens is only avalible in 0.25D steps (and usually 0.5D at higher
    strength) so the differences we are talking are not significant. However if
    you are over 40 it may be better to err towards less minus, Otherwise, it
    probably doesnt make much difference.
    Fit may affect the effective power of the lens (but see below)

    At the end of the day we are talking differernces in the "predictors" of
    less than 0.5D and probably 0.25D. Overrefraction will quickly establish the
    correct lens power.
     
    Simon, Oct 18, 2004
    #3
  4. Clive

    LarryDoc Guest

    The Ciba lens may be closer to the labelled power than other lenses and in
    comparison between contact lenses the Ciba may require a higher power lens
    for the same result.[/QUOTE]

    Not the reason, as I detailed, and, as I said, that statement is simply
    untrue.

    Which is exactly what I wrote. So.......then what IS your point? Why
    repeat the same stuff.

    -LB

    --
    Dr. Larry Bickford, O.D.
    Family Practice Eye Health & Vision Care

    The Eyecare Connection
    http://www.eyecarecontacts.com
    larrydoc at eye-care-contacts dot com (remove -)
     
    LarryDoc, Oct 18, 2004
    #4
  5. Clive

    CliveM Guest

    The Ciba lens may be closer to the labelled power than other lenses and
    I am not convinced that the possible plus tear lens is the reason. This is
    not the stated reason at the siliconehydrogels website, and the ciba
    calculator gives the same higher powers for all of the ciba aspheric lenses
    (choice AB, Night&Day and Dallies). This therefore appears to be caused by
    lens design rather an varience of fit.


    All fittings should be overrefracted, but I am sureprised at the number of
    opticians who just use the calculated lens strength
     
    CliveM, Oct 20, 2004
    #5
  6. Clive

    LarryDoc Guest

    Obviously you cannot be convinced.

    Well, you're wrong. Accept it. I fit these all the time and it is quite
    easy to see the lens vault the central cornea with the slit lamp
    microscope. The plus tear lens is not a mystery, it is fact. Observable.
    Repeatable. Measurable.
    So they're not doing the best job that they could. Too bad. That does
    not make your assumptions correct.

    --LB
     
    LarryDoc, Oct 20, 2004
    #6
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