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Thread: Consequences of wrong PDs

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    Consequences of wrong PDs

    Do you think patients who have lenses made based on PD measurements made independently of the frames they'll be wearing end up with PDs that are significantly off?

    More and more people are wearing frames without pads and I've seen those sit quite askew. I saw one person who has one eyebrow covered by the frame and another not because the frame is so askew.

    Do you see these kind of wrong PDs possibly doing any serious permanent damage? What do you think the effects can be?

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    Are you referring to seg or oc heights perhaps?

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    I'm thinking of SVs and I was thinking about PD primarily, but I suppose height could be an issue, too.

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    Well, if I'm understanding your question right, I don't see how the patient is getting an incorrect PD if the frame is out of adjustment, especially not with a plastic frame. If one side is higher the patient will be looking out a different spot in the lens vertically, but the horizontal line is the same. In single vision the issue would be more relevant if you're talking about a higher RX.

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    I agree with GAgirl. Since the trend where I live is all about the big zyl frames now, especially with a higher RX, I always specify OC. The higher the RX with pd's off can cause induced prism. "A simple equation can be used to calculate the prism induced by decentration. Prentice’s rule states that prism in diopters (Δ) is equal to the decentration distance (c) in centimeters multiplied by the lens power (D).Prentice’s Rule
    Δ = cD"



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    Quote Originally Posted by GAgirl View Post
    Well, if I'm understanding your question right, I don't see how the patient is getting an incorrect PD if the frame is out of adjustment, especially not with a plastic frame. If one side is higher the patient will be looking out a different spot in the lens vertically, but the horizontal line is the same. In single vision the issue would be more relevant if you're talking about a higher RX.
    What I mean with regards to the horizontal line is that if the center of the bridge of the frames doesn't coincide with the center of the bridge of the nose you could get decentration even if you measured monocal PD.

    So, the glasses are made according to perfectly accurate PD measurement, but the patient has a knobby nose and the glasses don't sit properly, they are shifted to a side, so you get decenterment. (this is, of course, assuming you didn't mark pupil position on the lenses)

    This link talks about it more:

    https://www.2020mag.com/article/your-judgement-35839

    I'm concerned about whether such errors might cause, beside the temporary problems, some more permanent eye alignment problems. Does anyone know of any studies indicating that possibility or have you seen it in practice.

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    Generally these types of issues with decentration/oc heights will cause a person to get used to prism. On initial fit they would feel pulling, maybe have headaches, and tired eyes. Long term they will continue to have issues adjusting to new glasses, even if done correctly. I have one very fashion forward patient who has 5 pairs of glasses, 2 from me (oc and pd's properly taken), 2 from online, that she can only wear a little while without experiencing some discomfort (pd is out and no oc heights done), 1 pair she can't wear at all (purchased at B&M, but contact RX used). There is no permanent damage according to Dr, but patient definitely feels the difference between the 3 pairs. She is a - 4.75 -0.75/-5.25 -0.50 . I have other patients who have been wearing the wrong PD for years, that I can't switch back to their correct pd because they are so used to seeing things wrong.

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    Darn Gremlins
    Last edited by JuliaG; 05-09-2019 at 03:04 PM.

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    Ok, now I have a better understanding of what you are saying. I agree, with an odd shaped nose, or a bump on one side that can cause the frame to sit differently and can cause problems and I have had patients like that. In those cases I try to steer the patient to a metal frame, or a plastic with nosepads that I can adjust. I show them the flat metal frames, that have a similar "chunky" look that the plastics do, but give me better control of the fit/adjustments.

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    These big zyl frames drive me nuts. There is no way most people are picking these frames out with the help of an optician. There is absolutely no way it is reasonable to decenter a lens for a patient that has an OC at the very top of the lens.

    Click image for larger version. 

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    Perfect example directly above. It drives me absolutely insane!! Especially when the person has 10 D of sphere and 5 D of cylinder and they want to use a frame like that.

    Okay, that's my one rant I allow myself for the day.

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    Quote Originally Posted by Lelarep View Post
    These big zyl frames drive me nuts. There is no way most people are picking these frames out with the help of an optician. There is absolutely no way it is reasonable to decenter a lens for a patient that has an OC at the very top of the lens.

    Click image for larger version. 

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ID:	14322

    Perfect example directly above. It drives me absolutely insane!! Especially when the person has 10 D of sphere and 5 D of cylinder and they want to use a frame like that.

    Okay, that's my one rant I allow myself for the day.
    hahaha

    So, do you (and other folks) think getting the vertical measurement wrong to some significant degree could cause some more serious/permanent problems? Say you put the centers on the lenses in some reasonable mid position, meanwhile she's looking out the top of the lens.

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    Quote Originally Posted by Lelarep View Post
    These big zyl frames drive me nuts. There is no way most people are picking these frames out with the help of an optician. There is absolutely no way it is reasonable to decenter a lens for a patient that has an OC at the very top of the lens.

    Click image for larger version. 

Name:	b3901b77b9c7aff5ba9da6bed2060c00.jpg 
Views:	18 
Size:	68.2 KB 
ID:	14322

    Perfect example directly above. It drives me absolutely insane!! Especially when the person has 10 D of sphere and 5 D of cylinder and they want to use a frame like that.

    Okay, that's my one rant I allow myself for the day.
    Totally with you in this. We are again back in the 80’s, except, PRPs were seldom notated back then. Love when opticians sub high index materials, then complain about thickness.
    I bend light. That is what I do.

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    Quote Originally Posted by Alfred001 View Post
    hahaha

    So, do you (and other folks) think getting the vertical measurement wrong to some significant degree could cause some more serious/permanent problems? Say you put the centers on the lenses in some reasonable mid position, meanwhile she's looking out the top of the lens.
    This question, I believe, was already answered by JuliaG above - it will cause the pt to get used to prism. It's a brain thing, not something that will necessarily cause irreversible damage.

    May I ask why you're asking? It almost sounds to me like you are wanting to purchase glasses online and want to make sure you don't "hurt" yourself by doing it wrong...

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    Quote Originally Posted by Lelarep View Post
    These big zyl frames drive me nuts. There is no way most people are picking these frames out with the help of an optician. There is absolutely no way it is reasonable to decenter a lens for a patient that has an OC at the very top of the lens.

    Click image for larger version. 

Name:	b3901b77b9c7aff5ba9da6bed2060c00.jpg 
Views:	18 
Size:	68.2 KB 
ID:	14322

    Perfect example directly above. It drives me absolutely insane!! Especially when the person has 10 D of sphere and 5 D of cylinder and they want to use a frame like that.

    Okay, that's my one rant I allow myself for the day.
    but frames were like this back in the 80's. How did we pull it off then? my beef with this is that blanks seem to have shrunk and we have cut out issues now. also how on earth did we get plus lenses into pornstar aviator frames so thin back then?

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    Quote Originally Posted by Oscar View Post
    but frames were like this back in the 80's. How did we pull it off then? my beef with this is that blanks seem to have shrunk and we have cut out issues now. also how on earth did we get plus lenses into pornstar aviator frames so thin back then?
    People were less picky about thickness back then. No such thing as 1.7, nor 1.74. Mostly glass and plastic, some Poly. Opticians sold what was available, the patient wore what was available and dealt with their bricks on their noses.
    I bend light. That is what I do.

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    Quote Originally Posted by Alfred001 View Post
    Do you see these kind of wrong PDs possibly doing any serious permanent damage?
    Not in healthy, adult eyes.

    What do you think the effects can be?
    Unwanted base-down prism causes a floor or other flat surface to seem concave, and the patient feels like he or she is standing in the bottom of a depression. Unwanted base-up prism makes the floor look convex, so the patient experiences a sensation of walking downhill. And, unwanted base-in or base-out causes horizontal objects, such as a table top, to look too high on one end and too low on the other, with the too-high side toward the base of the prism.

    Furthermore, unwanted prism may cause asthenopia, as well as mispositioning the aspheric pole and fitting cross, resulting in power error, marginal astigmatism, and chromatic aberration.

    Hope this helps,

    Robert Martellaro
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    Science is a way of trying not to fool yourself. - Richard P. Feynman

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    Quote Originally Posted by Oscar View Post
    but frames were like this back in the 80's. How did we pull it off then? my beef with this is that blanks seem to have shrunk and we have cut out issues now. also how on earth did we get plus lenses into pornstar aviator frames so thin back then?
    I remember the first wave of poly stock lenses I saw in 1991 all came in 80mm blanks. They were gigantic.
    I'm Andrew Hamm and I approve this message.

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