Calling Jeff, Darris, Pete, Al, Dave, Chad, Chip...etc.

[karen]

Ok guys, here is my harebrained girl question of the day. Ya know how sometimes you think you have an optical concept pretty well set in your mind? You know how it works, or so you think, and why it works but then someone else asks you to explain it and you realize you really have no grasp on the concept whatsoever??? Well. this happened to me yesterday when an account asked me about slab offs. I will keep my limited knowledge to myself and ask you experts to clarify for me these things.
1) why do we slab off? (is there a certain power difference that it is recommended to start doing that ?)
2)what is the difference between slab and reverse slab?
3) is there anything else I should know about slab offs that you haven't already said??

I am slightly embarassed to even ask but I figure admitting I don't really know what is up and actually learning is better than continuing in ignorance.

thanks in advance for your expertise!! :)

[This message has been edited by karen (edited 09-27-2000).]

[chip anderson]

Will try this: You must conceder that all lenses are prisms. Minus lenses are two prisms base out. Plus lenses are prisms base in.

A prism bends light toward it's base. Now when one turns the eye down to read (or in any other direction for that matter) one gets prism displacement. If the two lenses are of significant power differences (I was taught 4 dio. or more. Others say even less) One perceives one image to be higher than the other. i.e. double image. By grinding prism (same curve as is already on the surface but a different (slabed off) angle. The images can be made to be perceived to be in the same place in the reading position. Unfortunately, there is not a who lot one can do for other positions of gaze though.

[Jeff Trail]

Originally posted by karen:
1) why do we slab off? (is there a certain power difference that it is recommended to start doing that ?)

You generally slab because of a power imbalance and how that "imbalance" effects you when you converge and read..looking through the distance your eyes fuse images through the OC, but since a seg has a separate oc the amount of power difference on the 90 degree meridian causes you to "see" images that do not "fuse" ..you slab a lens to raise the oc in one eye in the seg to match the other..tada no more double vision in the reading :)
Really there is not a "set rule". It really depends on the person being refracted and the type of imbalance.. I have see a natural imbalance with a 5 degree and they did not need a slab.. but have slabbed as low as 1.25 degree's.. go figure.. who would ever think we all are "different".. generally it's in the OD's hands on slabbing.

2)what is the difference between slab and reverse slab?

if you want to really "dazzle" them with your optical repertoire. the when you "grind" a slab it actually is called a bicentric and reverse is just what it says..it's reverse to grinding..you grind the back surface and a reverse slab is on the front..in a bicentric the slab prism is base up and in a slab it is base down..so you would slab opposite sides,(od or os) depending on which type of slab :) (least plus most minus rule or vice versa depending on the type of slab)

3) is there anything else I should know about slab offs that you haven't already said??

There are things you can do besides slabbing, such as mixing seg types, ft one side rd seg on the other..has to do with the location of the seg oc. .. If the imbalance is smaller you can always grind the distant oc down on the seg line there by pushing the imbalance out of visual range as well.

BTW Karen, there never is "dumb" question ..only silent dumb people :) everyone I ever knew in optics wasn't born with some end all know all amount of knowledge (even though I have met a few that would tell you they were).. and it's a good thing you aren't in my lab.. they learned very quickly when I'm explaining something and they have that "head nod" thing going and I'll stop and ask them to explain it :).. needless to say the older guys love seeing the new guys get nailed.. but going along with it and shaking your head does not mean you know what's up.. atleast next time you can explain it to them.. oh and be sure to wave your arms around a lot!! I usually demonstrate it by holding both arms out straight ahead.. then as you lower your arms and move them inward (converge) stagger them, then you tell them about cutting a slab (base up prism) it moves the arms back to ending at the same spot..seems childish but it sure is easy to grasp :)

Jeff "someone else want to fill in the blanks I missed?" Trail

[Rich R]

Hello Karen
1. A slab off is prism in the lower part of a lens to balance the prism that comes from lenses having different powers at the 90 degree meridian, everyone has their own opinion on how much difference there should be before doing a slab. If they're not using one and the power has not changed much I would not do the slab, if they had surgery as a result there is 3 or more diopter difference it's a good idea to do the slab off
2. Slab offs have for many years been only base up prism in the most minus or least plus lens, now there are also reverse slab which is molded lens already with the required amount of prism only it is base down prism, so to balance the reading area it now goes in the opposite eye, or most plus or least minus.
Sounds confusing especially to explain to someone else.
Good Luck with it.
Rich R

[This message has been edited by Rich R (edited 09-27-2000).]

[stephanie]

Hi Karen, I am by no means an expert here, but....I will share something that has helped me keep them straight TREMENDOUSLY!!! When I was getting ready to take the abo I went to a class and the ABOM taught us what he called the BUM theory. What is stands for is Base Up in Minus. That is always going to be your conventional slab off which is the lens with the most minus. Reverse is the lens with the most plus. Everytime now I get one I look at my Rx and say ok BUM(base up in minus) and I know exactly which one I need to use. I know it's not very technical but it does work. Ok guys did I do ok?!!!
I sometimes get intimidated with this kind of stuff!!
Have a great day!!!
Steph

[Dave Nelson]

Hi Karen. Anisemetropia is often associated with reduced visual acuity in one eye. Amblyopes, for example, often have the kind of prescription you would think would need a slab-off, but may have best corrected visual acuity in one eye of 20/200, making a slab-off unecessary. Where acuity is better, some people have "suppression skills" that is, they learn to suppress one image, (usually the least sharp one) making a slab, again, if not unecessary, then questionable. Amblyopia itself is usually caused by image suppression in early childhood. Jeff said he has seen imbalances of up to 5 diopters go without a slab-off. This would certainly be a case of either reduced acuity in one eye, superb suppression, or, more likely, a combination of both. Amblyopes are hard to spot: the rx rarely indicates best corrected acuity, and patients rarely offer the information,and assume it has no bearing. (I once had an Ophthalmologist write a prescription for a prosthetic eye.) Ask the patient if he/she sees equally well with both eyes, and if you have doubts, call the doc and ask for BCVAs. All the best. Dave

[Rich R]

Hey Stephanie, I like that BUM theory, easy to remember, it'll help me.
Rich R

[Pete Hanlin]

Hi Karen,
Everyone has done a great job explaining slab-off... To visually demonstrate it to yourself, however, go to a trial lens set and get about a 2 diopter prism lens. Now, make the little etched lines go up and down and place the lens over one eye... Try to read, and you will see what a person who has prismatic imbalance at near sees.

How much prism is your patient looking through? Just calculate the amount of prism at the reading portion of the lens (using Prentice's Rule, go from pupil height to about 4mm below the seg line and multiply by the Rx in the lens at the 90th meridian). The difference between the lenses at that spot is the imbalance.

Pete

[Texas Ranger]

Karen,et. al, lots of good comment; I'd like to hear from Bob"mr.slab-off"Rihl. Usually you see the possibility of needing a slab off with pts that have a developing cataract or a post implant pt with a significant rx change. these situations usually have the compounded problem of induce aneisikonia, an image size difference from eye to eye, where the brain's vision center can interpret binocular images as more than one image due to size and prism shift. these rx's present other optical challenges beyond just slab off. if you move the images to the same plane with the slab, but they're 10% different in size, it's looks sort of holographic to the pt. and in slab off situations, be aware that when the pt looks UP they'll get the same prism displacement as they do when they look down. they, and you need to be aware of that.hope this helps.

[karen]

Thank you everybody for all your help- I think I actually get it now. One last question...Is it the lens with the higher power that you slab???

[mikeh]

Everyone has given great advice Karen. Here's my two cents worth. Don't do a slab off unless you really need to. Over the years I have seen many instances where I would have bet my last dollar that the patient was going to need a slab off. But the wanted quickly and I figured if I had to, I'd remake them. Very seldom did the need arise.

On the other hand I have seen new opticians, fresh with scientific optical knowledge, making slab offs for patients with Rxs that you would think needed it by looking at their prescription, but in fact they were wearing an rx very similar with no problems. Slab offs can be pretty hideous affairs appearance-wise, so be sure it's necessary first.

[Rich R]

Hello Karen,
As for your last question on slab offs.
If you're doing a conventional slab off, it's the highest minus if they are both minus lenses, however if they are both plus powers it would be in the lower power lens, and visa versa for the reverse slab.
Rich R

[Alan W]

Hi, Karen
If I may offer still another opinion . . .
I use the rule of "most minus or least plus".
As for when to do slab off I follow three different rules I call the Opticians Triage.

1, Assess. What is the difference in power through the segs between the two lenses along the 90 degree meridian? Use that information to calculate induced vertical imbalance as the patient looks through the segs at a point about 5 mm or more below the seg line.
2, If the imbalance exceeds ANSI standards, call the doctor and ask if the patient can maintain fusion looking through the amount of prism calculated. If the doctor says the patient can . . don't do the slab off. But, LISTEN TO THE PATIENT from previous experience with bifocals. Are their symptoms? Headaches, double vision, short attention span while reading? Report that to the doctor. If the doctor says no slab off . . . no slab off. If the doctor has no clue (he'll never admit it!)advise the patient that slab off is an option based on the symptoms and the doctors opinion (which may be optional). If the doc says yes do it.
3, If the patient says do it . . do it. ANSII says no more than 0.5 prism doctors imbalance, and the patient may exercise INFORMED CONSENT!
Your objective is to provide clear, comfortable, single, simultaneous, binocular vision. But, temper that with the "don't rock the boat" theory. Respond to a known need or prescription.

[Dave Nelson]

Sorry, Alan, I have to disagree. Asking the Dr. if the patient maintains fusion with the amount of imbalance calculated is'nt relevent. Most people can maintain fusion with a fair amount of vertical prism, but not for very long: maintaining stereopsis for 10 seconds is a lot different from maintaining it while reading a novel for four hours. Further, even when maintaining it, eyestrain can result, leading, as you mentioned, to headaches, shortened reading times, ect. I find bi-centrics, if anything, under-utilized in our profession. Respectfully, Dave Nelson

[Alan W]

Hi Dave,
This is not an issue of dissagreeing. I don't dissagree with you at all. From the clinical standpoint, I will continue as I have for 35 years to consult with the refractionist to see what his findings are. From the real world standpoint we can all assume that at some point in time the patient may experience discomfort or as much as breaking fusion (to the order of vertical diplopia). I insist that any practitioner be aware of patient symptoms and visual history.
Asking the doctor as a partner in the eyecare process is still wise thinking, whether you agree or not. It serves two purposes. One definitely includes establishing rapport and confidence. I'm sure that is an agreeable point for us both. The other is to create a wake up call to the doctor. Heaven only knows I have had several dozen wake up when they realize that their prescription is under scruitiny. I just don't argue with them. I let the patients failed prescription speak for itself. And, yes, I agree that the patient can still experience symptoms even if the doctor says there are no grounds for slab off. That's a subthreshold issue and calls for a wholistic approach which, as I said, should include taking the patient on as a partner by getting some kind of a profile on previous visual experience. I used to "sock it to em" with the works, Rx analysis, convergence prism, slab off, etc. etc. I also had some "super techy" jobs come back and bite me in the "tush"! The bottom line is how good we each are at what we do. It's a matter of technique. And, I'm sure you are good at what you do. Still, the goal is: CCSSBV, clear, comfortable, single, simultaneous, binocular vision. I'm sure we share that objective passionately.