Slab off...

[TLG]

Hey Everyone, give me your opinion (or preferably expertise) on this -

Rx
OD: +1.00 +1.00 X 110
OS: -1.00 +3.50 X 25
Add: +2.50 OU

Doc has prescribed a slaboff OS. Now, I know the slab generally goes to the lens with the most minus or least plus but I'm not sure if I need to transpose first. I suggested to the doctor that once this rx is transposed to minus cylinder the slab would then go to the OD. He asked me to 'check with the lab' but hey - they're closed and you're looking to share your opinions and smarts here - so let me hear what you think. Thanks!!

[lensgrinder]

Put it in minus cylinder since you will not be grinding it in true plus cylinder.

OD+1.88 X 90
OS -0.37 X 90

[gemstone]

Lensgrinder, I believe you must have inadvertently calculated the power in the 180. There should be no slab off here. Even if the reading spot is 10 MM below the OC the prism created would be only .76. For a slab it is recommended to be an imbalance of 2 diopters or more. With my apologies to the prescribing doctor, this does not need a slab. The patient should be happy to receive this news. The add makes no difference. Plus cyl makes no difference in determining slab or not. Slabs are based on the power in the 90 meridian period.

[HarryChiling]

good catch gemstone.

the power in the 090 would be:

OD: +1.12
OS: +1.87

Eighther the difference between the axis could be used in the equation:

Sph + Cyl * sin(difference in axis)²
or you could use
Sph + Cyl * cos(axis)²

[Da Hammer]

I agree completely, I would not do slab off here either, even if you don't do the optical calculations, just eyeballing it in my head the difference isn't great enough to RX it. However I don't know how good your relationship is with your doc to reccomend that.

[TLG]

Although I appreciate your opinions on whether or not a slaboff is indicated (I totally agree it shouldn't be necessary), I have been instructed by my doc that the patient NEEDS it and that I need to order it. So let's just say we're gonna order it anyway - am I correct that it should be on the right instead of the left?

[MarcE]

I think I would respectfully show the resultant calculations as Harry has shown to the OD. You are right that the OD prescribed the slab off for the wrong lens. That said, maybe he/she isn't envisioning this lens set as it will be. And maybe also is thinking it needs slab off based on the sphere only. I believe that a rational OD will see that slab off is not needed if they see the resultant calculations at 90 degrees. Or maybe the patient had a specific complaint that the OD feels that slab off will fix.
Also, when you order the slab, the lab will call you back and tell you that it isn't needed.

[Uncle Fester]

How long have you worked with this Doc and are you willing to quit your job on principle? Ka-ching is ringing in my ears

[TLG]

How long have you worked with this Doc and are you willing to quit your job on principle? Ka-ching is ringing in my ears

You ARE joking, right? You think I wanna quit my job because my doc (MD, not OD) wants to try anything possible to improve his patient's vision? The guy is post-cataract and this is the 3rd time he has been back for a repeat refraction. The doctor is convinced a slaboff will help him. What do I care if he's right or wrong? It's his money, his patient, his practice. Being wrong isn't a reflection on me so I don't know what 'principle' I should be considering quitting on. And please - explain to me your 'ka-ching' you have ringing in your ears. I really don't get your drift at all...

[HarryChiling]

OD = Slab - Off (if you must)
OS = Reverse Slab - Off (if you must)

Slab = most minus
Reverse = most plus

I would assume since the script is written in (+) cyl that your doctor is an OMD(not always the case), is this correct?

[HarryChiling]

The ka-ching is becuase slab-offs are expensive and the lab will hessitently do this job after seeing the powers. I don't think he meant it with malace towards you. try different seg styles to try and reduce the minimal amount of prism difference, it may work and save the patient a bunch of ka-ching.

[TLG]

Thanks Harry. Understood. Man, you are a nightowl - didn't expect to hear from you 'til morning :)

[Fezz]

Man, you are a nightowl - didn't expect to hear from you 'til morning :)

You got to watch out. Lots of freaks on here at night. Good thing I just got up for my midnight tinkle and warm milk.......;)

[TLG]

You got to watch out. Lots of freaks on here at night. Good thing I just got up for my midnight tinkle and warm milk.......;)

Hope the online chatter didn't wake you up...

[HarryChiling]

If you want to use dissimilar segs you could use the following formula:

Dec = Prism / Add

Dec = 0.75 / 2.50

Dec = 0.3 cm = 3mm

Now you just need two different segs with the seg OC's having a 3mm difference:

OD: FT-22 OC on seg line
OS: FT-25 OC 3mm below

Of course this is using the figures for 10mm that was given in a previous example, if you gave the B demensions and seg eight I could give you a more fitting example, but if it was 10mm below this option of a 22 and 25 would be a great and almost unnoticable lens choice to acomplish the same thing as a slab off would without having to spend hundreds extra for the patient and to help you save money on the lab bill.

[optical24/7]

It's been years since I gound slaboffs, but I remember even a 1 degree slab gave us problems. Can labs even do the .50 or .75 this Rx calls for? Also, do they even offer .50 or .75 in reverse? (I'm at home, my lens books at work)

[HarryChiling]

I believe 1.50 is the lowest reverse, good catch.

[lensgrinder]

Lensgrinder, I believe you must have inadvertently calculated the power in the 180. There should be no slab off here.

Oops! Thanks for catching that, I transposed and did not change my axis.:hammer:

[Fezz]

I believe 1.50 is the lowest reverse, good catch.

Harry is correct!

http://www.youngeroptics.com/products/core/cslab.shtml


:cheers::cheers::cheers::cheers::cheers:

[Diane]

Oops! Thanks for catching that, I transposed and did not change my axis.:hammer:

That's why I simply love the lens cross. I teach it constantly for a number of reasons. Prentices rule is only one.

It makes everything visual even to the newbies.

Diane

[HarryChiling]

That's why I simply love the lens cross. I teach it constantly for a number of reasons. Prentices rule is only one.

It makes everything visual even to the newbies.

Agree, then further down the line when more advanced topics are discussed ,the third demension (z-axis, cylinder as a sin wave) the cross can be expanded on visually.

[Uncle Fester]

TLG- I went to bed right after the post and like Harry said my post was not intended to be malicious. I've worked long enough to know some rx's are written for profit purposes only. That said I also know some patients will benefit from a psychosomatic effect like the guy who's rx was plano -0.125x180 OU. "What a difference" was his reaction (let the engineer jokes start:)).

[TLG]

...my post was not intended to be malicious.

Uncle Fester, my apologies that I took offense. Sometimes the printed word hides the essence of the real meaning and I obviously misconstrued your post. Thank you for your input. Best wishes - :cheers:

[EyeFitWell]

More to the general topic at hand, and my props for a very good point, by the way!
I am still in my studies and don't feel totally confident in these things. However, I would think that the easiest way to figure this is out (regardless of the notation used on the Rx) is to first determine the power in the 90th meridian for both lenses (if add is the same). Then, based on if it's minus or plus, you know which direction the Base is in when reading below the OC. Lastly, you know that Slab Off is BU and Reverse Slab Off is BD, so knowing the base directions of the imbalance, you know which lens to order it for.
Please correct me if you don't agree! ;-)

[HarryChiling]

I am still in my studies and don't feel totally confident in these things.............Please correct me if you don't agree! ;-)

I don't agree, I think you got a firm grasp on it. :D