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Thread: Prism thinning progressives

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    Prism thinning progressives

    I have a patient who is seeing double when she tries to read with her progressives. When I checked them on the lensometer I am getting 2.50 Vertical prsim in one eye and 1.75 vertical in the other. Shouldn't they be the same in each eye ? This is not prism I prescribed but rather the lab usually will put prism in each eye for thining purpsoses.

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    Please post the RX this would help in understanding the problem. At what point on the lens are you checking for prism.

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    Quote Originally Posted by Lensman11 View Post
    Please post the RX this would help in understanding the problem. At what point on the lens are you checking for prism.
    The rx is -2.00-1.00 x 90 OD -2.50 -1.25x 120 OS +2.25 add OU.

    I checked these on an automated lensometer in the proper manner.
    When I check another pair that the patient wears successfully the vertical prism is the same for each eye. The pair that the patient sees double with has a three quarter diopter difference in the prism between the eyes. At this point I sent it back to
    the lab for analysis. I did not prescribe any prism. I understand that progressives use “prism thinning” which results in an equal amount of vertical prism in each lens.

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    If you checked for prism imbalance at the prp their should be no imbalance. If you checked at the reading level you will get about 3/4 diop imbalance. If their old pair was the same Rx they would have the same issue at the reading level. This amount of imbalance is small and hardly ever would cause any double vision. Let us know what the lab says union reinspection.

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    While minus powers can produce vertical imbalance there is no need for prism thinning that I am aware of.

    Plus powers only and I believe from past posts of others the rule of thumb is about 2/3 of the add power as read from the PRP (prism reference point).

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    Quote Originally Posted by Uncle Fester View Post
    While minus powers can produce vertical imbalance there is no need for prism thinning that I am aware of.

    Plus powers only and I believe from past posts of others the rule of thumb is about 2/3 of the add power as read from the PRP (prism reference point).
    Minus lenses can be prism thinned and do benefit in certain situations. I moderate to high minus power with a low fitting height can be thinned with base down and a high fitting height with base up.


    It doesn't make sense that this lens would be getting prism thinning, let alone over 2 diopters. My money is on checking the prism not at the PRP like everyone else has suggested. Then again if the patient is seeing double, there could just be an error from the lab. It would be a pretty weird error to have unwanted prism added to both lenses in different amounts though. Seems like the stars would really have to align for that to happen.

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    Some labs prism thinning minus lenses as it will make the edges equal thickness. This particular Rx is actually slightly plus in the reading power in the 90 meridian so it would benefit from prism thinning. Using 2/3 the add power is not the best way to determine the the amount of prism. It should be based on the add power height and shape of the frame. Lab computer systems should calculate the prism very easily using those variables.

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    All great, informed responses.

    If you are an OD, I will remind you of another ODs perspective:
    1. symptomology: "double" means a lot of different things.

    2. remediation: get out some loose prism and eliminate (what seems to be) vertical diplopia. How much, and which direction? Does this match up?

    3. examination: are you sure this patient doesn't have an organic vertical deviation that is "just-so" exacerbated by this apparently very minimal unwanted prism? And/or, what are vertical vergences, here? If "brittle", then that says the patient is very intolerant.

    4. it's very easy to put the patient in a trial frame with the add and see if there's diplopia in downgaze with that power. There's 1 or more diopters of "anisometropia"-induced prism-imbalance on downgaze. (Lensman said this, too.)

    5. Let's do math! She will get about 1^ BD OS on downgaze from the power difference @ 90. Then you say there is unequal prism at the PRP(!)(I'm not convinced you did that, BTW) of ~0.75^ (but you didn't state the direction/eye!). Is that supposed unequal prism-thinning antagonistic to the power-induced prism imbalance, or offsetting? And, again, what is the patient's vertical deviation on downgaze WITHOUT power imbalance*. Does all this prism go against any vertical heterophoria? There are three variables, here.

    *just have her take off her glasses and do a maddox-rod type test.
    Last edited by drk; 11-03-2021 at 01:09 PM.

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    Quote Originally Posted by Kwill212 View Post
    Minus lenses can be prism thinned and do benefit in certain situations. I moderate to high minus power with a low fitting height can be thinned with base down and a high fitting height with base up.

    Do labs prism thin minus routinely or only if requested?

    I can't recall seeing an invoice that prism thinned a minus but again I am not until now looking for it.

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    Quote Originally Posted by Uncle Fester View Post
    Do labs prism thin minus routinely or only if requested?

    I can't recall seeing an invoice that prism thinned a minus but again I am not until now looking for it.
    I think most don't do it by default and it does need to be a rather unique fitting and power to make it worth doing.

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    -2.00 with a plus + 3.00 add needs thinning. One eye plus one eye minus needs thinning.

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    Quote Originally Posted by Lensman11 View Post
    -2.00 with a plus + 3.00 add needs thinning. One eye plus one eye minus needs thinning.
    Where's the cutoff? By far +2.50 is the most common and final add most Doc's rx's give me.

    I assume it applies in proportion to how much of a fit height, frame thickness and B frame dimension we are dealing with. Yes?

    I see I'm going to have questions for my labs after this.

    Thanks for the replies!

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