It would seem to me when an ECP presribes any type of horizontal prism, and more especially in the stronger rx,s, that you as the provider of the glasses, would need to know at what PD, the ECp used at the time of refraction, as what he used VS: what you measure, might be two different measurements and affect the outcome. THOUGHTS
Presumably the ECP used the proper PD in the phoropter or trial frame. Have you ever had a problem that you could trace to that issue?
I think current ANSI tolerances handle this grey area just fine.
B
Last edited by Barry Santini; 03-09-2011 at 10:04 AM.
I wear 3 diopter B.In O.U. with combining prism O.D. 2.50 Dn and O.S, 2.5 Up, and have never had a problem with the opticians mono PD. measurement. Even with a single vision rx with prism I take a mono PD. With Progressive I will also take the near mono and report that to the lab if there is a convergence issue. My Mono Distance PD is 36/34. I have a hi plus rx as well so measurements are very important.
I suspect that most of my clients are not getting phoria testing unless there's a CC of diplopia.Originally Posted by fjpod
I'm seeing Rxs sans prism when the current pair has significant vertical and horizontal prism, and I am seeing prism Rxs that don't match the eyeglasses when I'm told that they should.
If the powers are fudged, it will be more of a problem with vertical and base out prism. Do it right and there won't be any issues.
But doesn't Z80 deal primarily with fabrication tolerances? WRT to the intended Rx, it seems rather faith based to me- one hopes that both the refractionist and dispensing optician are doing this by the book, and are communicating with each other when doubts exist.
Harry,
In general and IMO, if the Rx comes from fjpod, or essentially any experienced optometrist, you should be good to go, and everything else should be scrutinized with a fine-tooth comb.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
Robert:
To your point, are we expecting that the refractionist did not adjust the pupilarry setting of the phoropter to closer than 2.5mm (ANSI horizontal)?
B
I believe ANSI is only about fabricating and not testing conditions.
But even if then refraction is "spot on" with the phorpter position, ANSI allows up to 2.5mm devation binocularly in the finshed eyewear.
So it makes sense to me that the same precision is reasonable in the exam.
???
Discussion...
B
Last edited by Barry Santini; 03-09-2011 at 03:37 PM.
I expect everyone to be as accurate as the situation requires. I don't know how accurately the diagnostic lenses can be positioned, but I'd recommend spending a lot more time positioning a -10.00 then a -1.00, especially if you are prescribing prism.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
Just a question that occurred to me: What difference does vertex difference make with prism? Does anyone have a formula for vertex compensation for prism? I can understand how having the wrong PD at refraction would would effect the outcome if the Rx had signifcant power other than prism. But I have never thought of compensating the prism.
And yes I'm sure someone making binoculars, like Zeiss or other reputable companies must have all sorts of figures for this.
Chip
Let's play a thought experiment:
What if a strong sphered client had a latent phoria, and so even a properly-geometrically *horizontally -positioned phoropter/trial frame induced some unwanted prism, in any manner...how would you know?
Further, with markedly-dissimilar pupil positions, what about the vertical is these non-adjustable, in-the-vertical-vector instruments?
The ANSI tolerance is, I believe, a compendium for all the factors that affect fabrication to a reasonable production standard. I beleive the exam protocol has no such updated vettingh and analysis.
Good Discussion!
B
None for prescribed prism.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
You opticans... always so worried about precision. Here's the cold hard truth; the examination process is anything but precise, furthermore we usually adjust our Rx's (power and prism) to some quantity that does not match what we refract, or find in binocular testing. This is called "prescribing," vs. "refracting."
For example, patient has complaints of headaches while reading, I find 10pd exophoria at 20ft, 18pd exophoria at near with reduced compensating vergences. I'm not going to Rx 10-18pd BI (no way in hell!), I'm not going to calculate the ideal based on Sheard's criteria or whatever either. Now ideally the definitive treatment for convergence insufficiency is orthoptics/vision therapy, but no-one wants to do that, so maybe I'll give her a little BI prism.
How much? It's an art, I pull a number out of nowhere where I'm pretty certain I can reduce symptoms without creating a spectacle remake for someone. :)
BTW, it's impossible to perfectly position a patient behind a phoropter, just doesn't happen, and guess what; its usually (99.9%) doesn't matter.
Bravo!....
....Oedema. Well stated!
A ophthalmologist friend said that the best amount of prism, that he liked to prescribe, was the minimal amount that had the maximum benefit.
This is not a criticism or compliment of the above. But unequal prism reqirements at near and distance is one of the best uses for a Franklin bifocal. Not to mention being a whole bunch cheaper and faster than something from a specialty lab.
It's art and science for us also, as we strive to effectively translate such remedies into the field without creating additional chair time for someone.:D
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
Prism in and of itself is a cheat, if we were concerned with accuracy the unit of measure would be the centrad. The fact that the accepted measure for deviation is the prism is because only the extremes would be effected by the lack of accuracy and even then the Rx would be prescribed in the equivalent prism diopters.
I agree that prism isn't given it's proper place on the prescription by the majority, if the patient were to bring in their current eyewear and the doctor were to follow protocal and measure current Rx, compare to their findings like most will, then the issue of properly prescribing prism would be moot. I think this is more a case of the onus being on the patient to bring in their current eyewear to a comprehensive examination and to be a bit more vocal in their eyecare.
I also think that opticians should alternate cover when measureing PD's which doesn't happen so the ball gets dropped on the supply side as well. I think trial frames are a great resource to opticians to catch these anomolies and a good relationship with the prescriber can help to avoid future incidents by informing the prescriber of any findings.
I think the conditions where a prism should be prescribed are so polarizing that often times the prescriber can rely on a simple question asked to a patient to rule out the need for spending additional chair time with the patient measureing for prism. Of course their are always going to exists offices that do everything by the book but with reimbursements for an eye exam at the paltry levels they are the time spent isn't justifiable.
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Agreed. The back vertex power of a lens needed to focus a point source of light is dependent on where it is placed. Prism is a displacement of a focused image. but I'm sure others can state this better than me.
+1 We alter what we find based on many things including habitual Rx, severity of symptoms (if any), gait, occupation, hobbies, uncorrected VA versus best corrected VA. While the visual system can be very adaptable, it also doesn't like to be disrupted very much.
I think you kind of missed the point of what i was really asking here.
What i mean is, if the epc is off say a millimeter in each eye, IE, patient is a 62 pd, but the phoropter is at 60mm, patient is a -5.00 OU, every millimeter off creates a half diopter of prism, now that there is 1 full diopter of prism present, that should not be there, will that throw off any needed prism the ECP may deem necessary.
PLEASE NO MORE QUOTES OF ANSI STANDARDS
Disagree. Cover test is not going to give you much info as to whether the PD is correct. No matter what you're going to elicit movement on cover test with most people, and this doesn't mean the PD is wrong or that they need prism. You can only come to that conclusion by evaluating the patients phoria (cover test), compensating vergences, and taking a history.
Even if we've prescribed prism for someone like in my example, you'll still elicit a significant amount of movement on cover test. Measure twice, or three times if you must, but without all the other info cover test isn't going to tell you anything useful.
No, 1pd is soooooooooooooooo insignificant. We're not only testing phoria and vergences through the phoropter, we're also checking in in free space with cover test and "near point of convergence." Usually, 99% of the time, I don't do any binocular testing through the phoropter, I do a cover test and NPC, if thats abnormal or there is a specific binocular vision complaint or i suspect something funny then I move to more specific phoropter based testing. Doing binocular testing through the phoropter on every single patient is overkill, and a monumental waste of time.
Completely agree with the binocular testing without the phoropter. If anything is off, though, I turn to a maddox rod and dont touch the phoropter. With the maddox rod, you can pick up hyper deviations and cyclotorsion much better.
As Oedema has stated, we rarely prescribe the prism we find in the phoropter. (and for that matter, we often don't prescribe the exact focal power of the lens we find in the phoropter either) It's not that simple Occasionally, we will measure it just to see the extent of it, or to see what capacity the patient has to recover from prism...but there are better ways to qualitatively assess this in free space. The amount of prism given depends more on the patients symptoms and their habitual Rx.
Last edited by fjpod; 03-10-2011 at 05:32 AM.
But why? Take that same example you stated of a -5.00 OU. ANSI says you can fabricate and deliver this job with a net prism imbalance of 1.25D horizontally. If this person does NOT need any prism, or better, actually requires/favors prism in the opposite dirction of the fabrication deviation, the same question of Rx efficacy arises.
This just points out the fact that the RX, as currently configured, is insufficiently robust enough to mate to current ANSI standards, and that this is the area the requires, IMHO, more changes.
Think like this: I don't want my GPS to simply tell me "I've arrived." I also need the context of the surrounding local in order to determine when I'm at my *desitination.*
Same for the Rx. Sphere, Cyl and axis simply don't tell me enough to do my best for each client.
I *do* understand your question. But I think its part of a a bigger picture.
BTW, *not* being a trained refractionist, I was unaware of the points made by the doctors who participate here. Thank you for your submissions. I'm still learning.
B
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