Technical trivia questions of the day:

[DC Optix]

First one to answer correctly wins a smile and a "attaway"!

Patient comes in wearing:
OD: +0.50 +0.75 x 6 6^BI (6 prism base in)
OS: +0.25 +0.75 x 155
Add: +2.50 OU

The doctor has you place a 7D Fresnel prism on the right lens, base in, and at a 45 degree angle up at the nasal. He instructs the patient to try, and if suitable, come back and have the new lens made with the prism ground in.

Question #1: If the patient accepts the correction, what is the total prism that you grind into the lens?

Question #2: If the doctor gives you the same Fresnel instructions, except to place it on the left lens, then what is the total prism you would grind into the lens?

Good luck!

[MakeOptics]

Question is too vague.

[optical24/7]

Or;

1. 12 D @ 24 degrees (same as Phi's answer)

2. With the same instructions as the OD, the OS would be 7 D at 135 degrees (up and in, otherwise I think Phi nailed it.)

[Robert Martellaro]

First one to answer correctly wins a smile and a "attaway"!

Patient comes in wearing:
OD: +0.50 +0.75 x 6 6^BI (6 prism base in)
OS: +0.25 +0.75 x 155
Add: +2.50 OU

The doctor has you place a 7D Fresnel prism on the right lens, base in, and at a 45 degree angle up at the nasal. He instructs the patient to try, and if suitable, come back and have the new lens made with the prism ground in.

Question #1: If the patient accepts the correction, what is the total prism that you grind into the lens?

Question #2: If the doctor gives you the same Fresnel instructions, except to place it on the left lens, then what is the total prism you would grind into the lens? Good luck!

Tell the prescriber that if they using 180 notation, they should note the prism as base up or down (not in and out). Base direction is not needed if they are using 360 notation.

For question #1, I would add that the prism should be split, which gives...

Prism

OD 2.5 BU 5.5 BI
OS 2.5 BD 5.5 BI

The second question needs clarification. If the prescriber means the equivalent of...

Prism

OD 7 @ 45

placed in the left eye, then...

Prism

OS 5 BD 5 BI or OS 7 @ 225

[Darryl Meister]

I would add that the prism should be split

With the exception of the occasional case of non-concomitant strabismus.

Best regards,
Darryl

[Darryl Meister]

The doctor has you place a 7D Fresnel prism on the right lens, base in, and at a 45 degree angle up at the nasal.

Just a bit of useless historical trivia for math geeks...

If I recall correctly, all of the questions on the original ABO Master exam relied on angles like this, in which the trigonometric functions could be easily figured without a scientific calculator using the properties of special right triangles (45-45-90 and 30-60-90).

For this question, for instance, you would only need to divide the prism by the square-root of 2 to get the horizontal and vertical prism components, instead of having to calculate the sine and cosine of 45 degrees, which would otherwise require a table or a special calculator.

Best regards,
Darryl

[Robert Martellaro]

With the exception of the occasional case of non-concomitant strabismus.

I used to call when all of the prism was written for one eye, but they pretty much told me to stop bugging them (over 39 years and many prism Rxs). I believe the onus is on the prescriber- if they don't want it split for optics and cosmetics, they need to write that (do not split prism) in the special instructions. Still, it's certainly prudent to be aware of this, and to keep one eye open, especially with new prescribers.

[Darryl Meister]

Well stated, Robert.

Best regards,
Darryl

[MakeOptics]

Probably shouldnt post this, but if the question was read and understood the posters should realize that splitting prism in this scenario is wrong, unless the prescriber was called. This scenario sets up a situation where the lens has been trialed with a fresnel, that takes precidence over theory. Also trialing prism generally indicates a potential sensitivity. I would definately call if spliting.

I can also see where I can be wrong on the OS, if I interpret 45 instead of the nasal desrciption, nice insight optical24/7.

The trivia question needs work. Amazing how such a simple question on the surface can become so complicated.

[optical24/7]

That was my thinking Phi, he said "right lens, base in, and at a 45 degree angle up at the nasal." He didn't say at 45 degrees. If I sub left for right in that sentence I would say at 135.

I agree, doc needs a call on this one.

[Robert Martellaro]

Question #1

Establishes the degree and direction of the prism (Rt 5^ BU 11^ BI) that "the patient accepts". If there was no question #2, and because the patient was already wearing prism in one eye only, it behooves any optician to call for authorization to split the prism.

However...

Question #2

Implicitly authorizes splitting the prism. (6^ BI Rt and 5^ BI Lt is essentially the definition of splitting the prism). Assuming that everything else is equal (Rx and VA), it should be split 50/50. There really is no clarification needed for the direction of the vertical prism because that has been established in Question #1.

[MakeOptics]

I redact my entire answer, the question is too vague to be answered properly and I don't feel like arguing with other competent opticians over the meaning of the question.

It is obvious by the posts everyone knew exactly how to calculate the answer, but everyone is still waiting on the proper interpretation of the question to see the original posters answer.

Robert, I did not read question 1 and 2 were linear scenarios, my assumption was that they were two separate scenarios.

[Robert Martellaro]

Robert, I did not read question 1 and 2 were linear scenarios, my assumption was that they were two separate scenarios.

There's no doubt that there can be more than one "correct" answer when there's this level of ambiguity. I wonder if this was a test question? Maybe it was a good question because it had more than one answer?

BTW, there's no need to argue- civil discussion will suffice.

Thanks DC Optix for keeping us thinking. You owe us an immediate explanation concerning the source of the scenario you posted, educational or clinical.

[Darryl Meister]

I think that debates like this spark the most productive conversations and provide the greatest sources of education on OptiBoard.

Best regards,
Darryl

[oopsmyeye]

Answer for questions 1 and 2: refer the patient to a good OD rather than MD. Whenever a patient comes in with a + cyl Rx I'll have our doc check the numbers, more frequently than not they do need to be tweaked a bit... Ophthalmologists don't care as much about the refraction as optometrists (from my experience, I know there are some good MD's out there that can do a good refraction).

I also second the question... why wasn't the prism split between lenses?

Lastly, if the glasses were recently made with that much prism in one eye and needing another 7D prism change in ANY direction... refer them to the closest ER for an immediate CAT scan to look for a brain tumor. I actually had the misfortune of losing a patient last year to a brain tumor that was discovered because my doctor and I realized she was having such a huge problem like this. She came in because she was seeing double and I made glasses with ~3D prism, she had them 2 days after ordering. 2 days later she called to complain it was getting worse and I trial framed her with another 4D prism and sent her to the ER. She only lived another 50 days after that but they say it would've been only a week or 2 if we didn't send her in so quickly.

Aside from that... I'm a fan of enucleation. I only have one eye and I'll never have to deal with seeing double (but I'll also never get to see 3D movies).

[tonyg]

1- 7^ @45

2- 7^ @225

[MakeOptics]

Robert, I did not read question 1 and 2 were linear scenarios, my assumption was that they were two separate scenarios.

There's no doubt that there can be more than one "correct" answer when there's this level of ambiguity. I wonder if this was a test question? Maybe it was a good question because it had more than one answer?

BTW, there's no need to argue- civil discussion will suffice.

Thanks DC Optix for keeping us thinking. You owe us an immediate explanation concerning the source of the scenario you posted, educational or clinical.

By argue I dont mean it in a negative sense. Maybe debate would have been a better explanation. I dont feel that a question that truly should be a straight forward mathmatical answer should be so ambiguous as to have 3 seperate interpretations afterall math is the universal language.

[DC Optix]

Sorry, everyone, for throwing this out there and then disappearing! Ran into a few emergencies here and couldn't log back on until today.

So, let me "out" myself right off the bat ...

The reason I asked this question is because question # 1 was exactly what was presented to me by one of our physicians. I asked it here in a "fun, trivia kind of way" because I needed help coming up with the answer! So I greatly appreciate everyone's discussion here. Darryl, I agree, these debates always seem to bring out some of the best educational situations.

To clarify, they were, in fact, two separate questions, independent from one another. The physician's tech presented the scenario in #1 to me, so I put the Fresnel on the right lens and immediately started trying to calculate the amount of prism that would be ordered through the lab. Then she came back and informed me she had made a mistake and it should have been on the left lens. So I put one on the left, took the other off of the right, and started to calculate what it would be on the left. I realized pretty quickly that I needed help, and here I am!

I was having a hard time because I've never had a doctor request for a Fresnel to be angled in any way. It's always been either up or down, in or out. To clarify, it was not 7D @ 45 degrees, it was 7D @ a 45 degree angle up at the nasal. So, literally, what he wanted was for me to take a 7D Fresnel prism, align it so that it was base in, and then turn it up at the nasal 45 degrees. Does that make sense?

It was not my intention to be vauge, so I apologize. Phi, you're right, it's amazing how something that seems like a straight forward question on the surface can become so complicated!

So, would any one of the master-minds here be willing to "show their work" so I can get a handle on this! I know I can use one of those resultant prism charts and work my way backwards from it, but I'd really like to be able to do this myself. I was able to at one point, but haven't had to do this in quite some time

Thanks, in advance, for all your help!

DC

[optical24/7]

Prism X axis, cosine = horz. prism amount

Prism X axis, sine = vert. prism amount

7 X 45 cos = 4.95 or 5D prism in.
7 X 45 sin = 4.95 or 5D prism up.

One more example;

5.8 D up and out @ axis 31 OS lens

5.8 X 31 cos = 4.97 or 5D out
5.8 X 31 sin = 2.99 or 3D up

[Robert Martellaro]

The physician's tech presented the scenario in #1 to me, so I put the Fresnel on the right lens and immediately started trying to calculate the amount of prism that would be ordered through the lab. Then she came back and informed me she had made a mistake and it should have been on the left lens.

We need to know exactly what should have been incorporated into the OS eye Rx- the equivalent of OD 7Δ @ 45, or, when she was working the prism in front of the client's eye (probably over the eyeglasses), it was the left eye instead of the right eye. The difference being the direction of the vertical prism. There can be no miscommunication...the difference would be a grievous error!

To clarify, it was not 7Δ @ 45 degrees, it was 7Δ @ a 45 degree angle up at the nasal. So, literally, what he wanted was for me to take a 7D Fresnel prism, align it so that it was base in, and then turn it up at the nasal 45 degrees. Does that make sense?

For the right eye, it's the same thing, if you're using 360 notation (commentary- do not use 180 notation). Take a trial frame and put a 7Δ lens in the right, and set the axis at 45 degrees (or up 45 degrees at the nasal). That's writen as 7Δ @ 45.

Thanks, in advance, for all your help!

You're welcome.

[DC Optix]

Prism X axis, cosine = horz. prism amount

Prism X axis, sine = vert. prism amount

7 X 45 cos = 4.95 or 5D prism in.
7 X 45 sin = 4.95 or 5D prism up.

One more example;

5.8 D up and out @ axis 31 OS lens

5.8 X 31 cos = 4.97 or 5D out
5.8 X 31 sin = 2.99 or 3D up

Part of my confusion here is that the axis isn't given. The axis is not 45, you just place the prism on the lens at a 45 degree angle (up at the nasal). Those are different, are they not?

[optical24/7]

In your initial question, the angle is given. "..at a 45 degree angle up at the nasal." That (at least to me) means up and in which would be @45 for a OD lens or @135 on an OS lens.

The formulas I've given would tell you the amount of prism to order (in, out, up or down) if only the amount of prism and the angle (axis) is known.

To reverse this ( knowing the amount of vertical and horizontal prism requested) and find the amount of prism and at what angle (axis), you would use your resultant prism formula;
P² = H ² +V ² √

P² is the amount of resultant prism
“H” is the horizontal prism amount
“V” is the vertical amount
√ is square
To find the angle (axis) you would divide the vertical amount by the horizontal amount then Tan‾¹.

Example;

O.S. 5 out and 3 up

3 ÷ 5 = Tan‾¹

Your calculator should say 30.963…

So the final Rx checked on the O.S. Should be 5.8 diopters of prism @ axis 31. Keep in mind the direction of the requested prism in relation to which lens you are neutralizing. If the above prism amounts were on a right lens, an axis of 31 would be up and in. ( wrong direction!)

When you Tan‾¹ your prism you are only getting the angle from your 0-180 line. If your prism direction is down you would need to add (to 180) or subtract (from 360) that angle (using 360 notation) depending if it’s a right or left lens. Clear as mud?

[MakeOptics]

Very nice answers from the posters above. I'd like to second, the 360 degree notation recommendation if and only if degree's must be used. In this case with a fresnel it is required. Any other time just used base directions in the office setting and let the lab tech or lab optician work with the prism in 360 notation. Calculators are useful, http://www.opticampus.com they were created to be used so use them.

[Robert Martellaro]

Part of my confusion here is that the axis isn't given. The axis is not 45, you just place the prism on the lens at a 45 degree angle (up at the nasal). Those are different, are they not?

It's not accurate enough for orbital mechanics (way to go NASA!!), but if you have a good eye, probably good enough for some situations, although I suspect this isn't how it's taught in optometry school.

Thanks optical24/7 for taking the time to show examples of how this is done. If you are a tad lazy like me, one might just use D. Meister's tools at...

http://www.opticampus.com/tools/compounding.php

http://www.opticampus.com/tools/resolving.php

[DC Optix]

We need to know exactly what should have been incorporated into the OS eye Rx- the equivalent of OD 7Δ @ 45, or, when she was working the prism in front of the client's eye (probably over the eyeglasses), it was the left eye instead of the right eye. The difference being the direction of the vertical prism. There can be no miscommunication...the difference would be a grievous error!

The tech meant that the Fresnel should've been placed, using the same instructions, over the patient's left lens instead of the right. She just messed up and told me the wrong lens. I just carried the question, concerning the right lens, over here because it was a situation I had never encountered. Thanks for all your help on this! Sorry if I haven't been clear.