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Thread: Monovision Bifocals

  1. #1
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    Confused Monovision Bifocals

    Hello,

    One of the doctors I work for is into pediatric optometry. He has started fitting amblyopes with a PAL in the suppressed eye stimulate focusing. Now the question..

    We do not want to create unwanted prism imbalance for these amblyopes. We know that prism is measured in PALs between the trademarks and we have "turned off the prism thinning". If we put the OC for the other eye in line with this, it is still not where the patient looks through the lens. With some of these patients having significant differences in power between their two eyes, this is creating some imbalance to begin with.

    How can we match theses lens up as to not create imbalance as the patient looks through them at distance?? Near point imbalance is another story, but we will work on the distance first.

    If any more info or clarification would be of help, I will be happy to post it.

    Thanks for any input on this quest.

    GlassEye

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    None of the pediatric ophthalmologists that I know will allow the use of anything other than an executive in these cases.

    Part of the reasoning is that if the vision is impared or only partially developed the compromised vision pals allow is throwing gasoline on the fire.

    Chip

  3. #3
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    This can be done for an amblyopic patient who probably also has an accommodative disfunction which is very common. Also the doc may not be giving the full plus for the distance because it interferes with the distance vision but the patient needs the extra plus for the near vision. This does work. A progressive certainly will look much better than a pair of glasses with only one lined bifocal. Remember we want the kid to actually wear the glasses.

    Chip, no disrespect to your MD friend but most (not all) know very little about binocularity or vision therapy and even less about optics. If you have to use a lined bifocal I would prefer a large flat top. In a childs small frame it will give you almost as much coverage as an executive bifocal. It will look better, has better optics, will be lighter, thinner and will have less image jump.

  4. #4
    Master OptiBoarder Jeff Trail's Avatar
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    Stopper said:
    If you have to use a lined bifocal I would prefer a large flat top. In a childs small frame it will give you almost as much coverage as an executive bifocal. It will look better, has better optics, will be lighter, thinner and will have less image jump.

    I'll let you and Chip battle over the other part of your post :)
    But Chip is correct here.. induced image displacement is HIGHER in a FT D seg compared to an E-Line in this case do to the location of the near OC in the add... Another plus is that the seg in an E-Line is inset while in a ft it is not.. and using a really large FT like a 45 is the worst for kids, they chip them to pieces..
    Design wise optically the E-Line is the better choice here...not as much by cosmetics but the way the lens effects the vision ..where in a FT if you have the distant OC 4 above than you are 9 mm above the near oc and offset by 1.5 to 2 mm depending on the lens and that can make a difference in the amount of near prism depending on the power.. while an E-Line is on center and right at the seg line.. just something to consider :-)

    Glasseye

    As for the original post, you can use certain surface designs to help you out, when tinkering with the problems you are wanting to tinker with a TRUE aspherical PAL is far better than using a semi-aspherical design...you have far less induced displacement of OC's do to surface design...
    Also you can still use prism thinning to tinker and manipulate the OC's if need be.. you just use a different set of numbers and instead of letting the computer figure your prism thinning you plug in the numbers that you desire.. and it is a little more brain flexing on your part but it can be done even with compound prism in one eye.. you need to take advantage of the design in this case.. and having the Doctor refract and give you prism by AXIS instead of giving you a compound makes it far easier to do..since at times we tend to over look the compound number with what we actually end up as a total resultant prism amount..
    By giving you the prism in axis i.e. 4x150 instead of say 3.5 out and 1 up (btw I'm making these numbers up so you math freaks don't pester me:)) ..as an eample than you can run it through your surface program and figure out how much you can cut or add to both sides and be able to still use prism thinning.. as an example I wanted 1^ of prism BI I could technically tinkering with it into combining my prism thinning with the BI and using thinning in the other eye so we would not have induced vertical imbalance in the 90 degree merridian :)

    Since this is out of the scope of most of the software, you kind of have to figure out the right numbers to plug into the program to fool it into doing what you want...

    I would though change over to a true aspherical surface here for these jobs, I would also work with the OD or MD to get them to give you a prism amount in axis form to make it far easier to figure out... than toss out the "prism thinning formula" and use your own numbers.. I usually run in the .75 to 1^ instead of the recommended 2/3 of the add power
    It can be done and you can add the corrective prism with the prism thinning and use surface design and it should help you out a little more ...

    I think I may have explained it so it "makes sense" .. if not post away.. and if you have any examples feel free to post them and we can break it all down into a "sample problem".. sometimes working it that way gives a clearer picture.. than you can compare what you did and the result to with the way we figure it out and compare the end result :)

    Jeff "just love the complicated" Trail

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    One eye sees, the other feels OptiBoard Silver Supporter
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    We do not want to create unwanted prism imbalance for these amblyopes. We know that prism is measured in PALs between the trademarks and we have "turned off the prism thinning". If we put the OC for the other eye in line with this, it is still not where the patient looks through the lens.
    They should be looking through the fitting cross, which is usually 4mm above the mrp/oc. If the panto is eight degrees, there will be no VI in the distance if the single vision lens is placed as you describe... vertical oc level with PAL mrp (assuming that the eyes are level). If there is VI at near then a bicentric (slab-off) would be indicated, or a separate pair for periods of extensive near task.

    Robert

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    Bad address email on file dfisher's Avatar
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    Assuming, or hoping, you are ordering these lenses uncut (if not then a long talk with the lab manager is in order), then measure the amount of prism that shows up in the pal at the prism mark--that little dot between the engravings usually. Then either decenter the lens vertically to induce prism or have that amount of prism ground into the single vision lens.

    In cases of this type more concern is given to horizontal prism than vertical due to the convergence issues as long as the vertical prism is within tolerance

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    Jeff my error on the image jump and oc location. I see why you will be optiboarder of the year. But I still wouldn't give a Kid a pair of glasses with an executive bifocal in one eye and a SV lenses in the other. Even if this the ideal optical solution the kid still has to wear it in the real world. I guess we will have to agree to disagree. Thanks for the lesson.

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    Master OptiBoarder Jeff Trail's Avatar
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    Stopper,

    I actually agree with you if I had my "choice" as well, I prefer to use a blended RD bifocal if a PAL is out of the picture, mainly it is a cosmetic thing for the kid :) ... although since a lot of the time we are dealing with fairly low add powers and more so in degree's of prism I have done it about every way I can think of..
    I know when an OD or MD starts working in this area it sure is handy to have experienced staff and either a good optical guy (if in-house lab) or a good lab (communication) sure is handy..
    I did not mention another answer as well that might come in handy and I have done it and that is to actually tinker with the 180 line and move the decentration point inwards or outwards as needed.. did that trick a time or two as well over the years..
    Personally I didn't find mixing SV/PAL as helpful as using a PAL OU..even if it is in the .75 add (which is available) and since the VAST majority of people (even kids) can tolerate that addition in accommodation it helped with the process.. It still is a kind of "shoot by the seat of your pants" thing and each time we have done anything it was never the same way twice :-)
    I think sometimes we let the "numbers" have more meaning than they have actual physical results.. like using a .75 add in a non presbyopic person but attempting to arrest the problem we are discussing here with a multitude of things (magnification, prism etc., etc.) sometimes we need to give a little to gain a lot, and being preadolescent you have a lot of room to tinker :) I'm simply amazed at times with RX's these kids were functioning with in an uncorrected way till the got a refraction..mainly I have a personal interest, I have a niece who is 4 where she has some muscle disorder and amblyopia and was suppressing and I noticed it by watching her putting puzzles together when she was visiting.. she had a fairly good sized blind spot centrally and was not fusing.. to look at her it was really hard to pick up.. other than she had a slight esophoria.. but she was putting the puzzles together and the middle would be missing and she would get up and walk away.."all done".. amazing when I had her examined she wears a +6'sh OU and 3^ of prism.. it was like the world opened up for her :) She just never knew any other way... so my hat is off to ANY OD or MD wanting to tinker in this part of the field.. that one kid who pops them on and lets out a "wow" and a big ole smile..NOW THAT"S OPTICS!! :)

    Oh well.. keep it up Glasseye.. it does take some trial and error and a lot of learning curve, but it is well worth it in my book..very rewarding on a personal level as well..

    Jeff" I love low vision stuff" Trail

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    Stick out tongue

    dfisher has the right idea for solving your problem, but I think he stated it backwards. You want to measure the prism in the PAL at the point the patient is LOOKING THROUGH for distance, then put equil prism in the other eye either by decentration or by ordering it ground in. Of course you will still have problems at near, but this will solve your distance problem. If you clearly state your request to the manufacturing lab they should be able to figure the required prism for you. Hope this helps.

    shutterbug

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    Master OptiBoarder Jeff Trail's Avatar
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    Shutterbug,

    This might be one of those times where doing it that way might not be the best option, you are trying to correct an eye that is suppressing (for whatever reason, amblyopia etc., etc.), now that eye would be getting a PAL, so the best corrected acuity will not be 20/20 (chances are) but in the SV side most likely you would have corrected 20/20... if you had to move that OC to far to neutralize the induced imablance due to mixing designs you would have an effect on the acuity of the good eye... plus depending on the power mix you might also be creating a bigger problem, knowing the properties of optics where you go with the motion in a plus lens and against in a minus.. say you had to push that OC down 6 mm in a minus power with cyl. you would be creating some major changes in both the amount of cyl. as well as axis... my theory has always been trying to keep that good eye with the best correction and doing the tinkering with the suppressed side, and if it falls as it usually does as you gain in that suppressed you get better muscle control and or strength and you can cut the tinker a little more with each lens change... I don't want to make a good eye worse to try to help an effected eye..it slows the gains down more and more and it does not make the eye you are wanting to correct work as hard... comfort wise it is a little more painful but optically your gains are better...
    If you were going to do it, matching OC's I would hope that they are going to be using prism thinning.. and even more so using a true aspherical design on the PAL side not a semi-aspherical..

    Jeff "low vision is fun, but sometimes frustrating" Trail

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    Stick out tongue

    Jeff - so you are recommending putting the required prism in the PAL to match the SV lens. OK, that works for me. I hadn't considered all of the possible higher Rxs that might be involved.
    As long as we get the imbalance at distance corrected we'll accomplish the goal. :0)

    shutterbug

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    Master OptiBoarder Jeff Trail's Avatar
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    SB

    This is one of the main reasons when an OD calls and we talk about this type of corrections I'll always try and talk them into using hard edge bifocals especially prefer the E-line, is it the best choice cosmeticly and cost wise? Nope... but it sure is easier to accomplish a better end result, which hopefully that is the reason they are doing it to begin with ... Most PAL's with it's abberations and contour plots make it the worst choice to work with when trying to do this type of thing and every little problem such as off axis power, abberation is multiplied by the nature of the PAL design.. why throw gas on a brush fire that you are trying to control :)
    You pop an E-Line in there and AR coat it and if you can get that amblyopic eye be it exo or eso and they usually like have it not wonder or cross over and people tend to tease more for that than getting that lens.. :) ..gee kids are mean, was we ever like that?

    Jeff "sometimes we have to do what is best, not what pays the most" Trail

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