slab off

[beth]

1. Can someone give a simple definition of slab off

2. IS it always used on the eye with the most minus?

[chip anderson]

Slab off is bi-centric grinding. Essentially the same curve ground with two different centration points on the same side of the lens. Object: To offset prism induced when looking down through two lenses of widely differing powers so that the near image will not be displaced. If it is displaced very much the patient will see two different images at near in different places although he is trying to fixate on the same point.

[Jeff Trail]

Taking it a little further and adding the to chip'sters answer :) ... no you do not "always slab the most minus" .. it depends on what type of slab you are using.. a regular slab or a reverse slab (front side).. in the case of using a reverse you do just that.. reverse the side you would slab if you were "grinding" it. (the most plus)
Another way to look at a slab and the "why" is you have an imbalance of power.. we can move the OC anywhere we like in the distant portion of the lens, but you have an OC in the segment that is not movable per'se , so even though we have equaled the OC in the distance in you have no vertical imbalance when you converge to the OC in the segment then you will have an imbalance of power effecting how you percieve the image..result? you see double :) so you grind prism in one lens in the reading area only to move the optical centers of power so the are at the same point. Tada no more imbalance due to the seg.
Another trick is you can run the optical centers for the distant down onto the seg line or even below it and it pushes the vertical imbalance further down, you can also mix "seg design" because not all seg OC's are in the same place. Say mixing a FT on one side and an RD on the other...
Maybe someone can even add further to the thread.. anyone?..

Jeff "hates to grind slabs with a passion" Trail

[EyeCon]

Beth,
The process used to effect base up prism in the reading field of a lens while inducing no prism in the distance portion of the lens. Called a slab-off, it always results in prism base up. It is ground on the lens that has the weakest plus power, or the strongest minus power, in the 90 degree meridan.

[karen]

ok boys, to take Beth's question one step further...

Is there a set rule about where slab off begins? How much of a power disparity does there need to be? And if y'all are going to tell me "it depends" please tell me what is depends on :)

[Jeff Trail]

Eyecon,

It is base up if it's a ground slab, but in a reverse slab it's most plus least minus and it's base down prism :)Can't forget mentioning those "reverse slab" rules as well.

Karen,

A good rule of thumb is start watching if you have a 1.5^ or greater imbalance.. that is if the are post op. If they have a natural imbalance of power I have seen upto a 4^ difference and still they did not require a slab. I would think if it has spread (the RX power) naturally over time then they have learned to accommodate the imbalance, but post op, they are a different story :)
Now if they do fall under under 2^ I can usually grind the OC down into the seg line and "push" the imbalance far enough down that they do not see double. This is also a good time to know what the BCA is you are dealing with to help you determine the best step. If the other eye is coming up to the point it can be done at times the MD may prefer to give them just a plano or at times I have just frosted the old lens...
As for the where?, always at the seg line and in PAL's usually right above the reading power point works best, mainly it's easiest to take a rule or tape and do a little tinkering to get the PAL's correct, usually it falls right in the middle to lower side of the intermediate. NOT on the 180 line..
Other then those there are really no set rules, people are all different and it seems to fall under a "if they need it do it" if they can tolerate it then do not..it is a good time to get to talk to the MD if it is post op, since at times they decide to just go the plano route for the pre-op side if the cataract has reached a certain acuity and they more or less decide to fog it.
Slabbing is a funny business.. and one other thing do NOT let people tell you if they are wearing a slab and you decide to give them a reverse slab on the opposite eye it will not work..that's pish posh, you can change someone, from the most minus to the most plus and they won't miss a beat.
Maybe someone else has a few rules of "thumb" to go by.. we'll wait and see :)