# Thread: Limits of a slab-off

1. ## Limits of a slab-off

Robert, you're gonna love this one:

I have an aniseimetrope whose vertical imbalance is over 9D/cm.

I've read that a slab off can effectively address 5D. The lab is assuring me that a slab-off can handle 11D.

Px has historically been reading with just one eye, but likes the idea of binocular vision if possible.

Am I just spinning my wheels with a slab-off? Even with monocular images on the same axis, will the size difference kill fusion anyway? What's the typical tolerance there?

Mathematical explanations most welcome!

2. Check with slabsplus.com in Florida. They are slab-off specialists and can do slab-off as low as 0.5^. Who knows how high they can go.

3. I'm no Robert, but historically for me, if a pt. has been mono for long they're hard wired to it. How about more specifics? I take it they have a 9 D power imbalance. Ouch! Good luck!

4. I think you're right, George. I'd love to hear there's some hope of dodging aniseikonia, but I don't think I'm going to get it. Wherever the line is, I'm sure I've passed it with this patient.

So given that working assumption, this is my first patient with Duane's Syndrome. Px is -9.25 OD, essentially plano in the OS. Has been wearing traditionally surfaced Physios with little complaint. Establishing PDs was tricky, but I'm going with 28.0/28.5

To accomplish a forward gaze, the patient has to tilt her head about 10 degrees to the right, which suggests I should shift her channel left.

With the purchase of 3 new frames we have 3 as-worn vertex distances of 12, 11, & 9. The wrap on these frames is 10, 5, & 7, respectively.

Factoring in frame wraps into each trig calculation of a flat face form--adding the wrap to 90 in the OD, subracting it from 90 in the OS,
(For example, frame 2 with a vertex of 11, calculating the displacement caused by 10 degrees would put the adjoining angle connected by the original vertex distance at 95 degrees on the OD, but 85 degrees on the OS)

--we have resultant PDs of 25.8/30.6; 26.0/30.4; 26.4/30.1

We are dealing with skewed vertex distances, but since the patient had the same head tilt at the exam, we don't need to factor that in for additional lens power compensation beyond the usual.

Hypothetically, if I were trying to "POW" this order, I'd input monocular wrap values of 0/20; -5/15; & -3/17.

Someone feel free to correct me, but I expect this patient "breaks" the digital designs anyway--so I'm not going there. Sticking with the old fashioned Physio. (Unless Pete wants to take a stab that contemporary Variluxes can handle the divergent wrap values and really deliver this patient binocularity?)

Did I miss anything?

5. I had an adult Duane's Syndrome client just recently but for the life of me I can't remember her name and how I handled it. Is this the Rx they've had throughout their adult years, or has there been some recent event that caused the anisometropia?

Does the client complain of diplopia at near, due to vertical prism imbalance, with the present eyeglasses?

As George said, I would assume that although fusion is unlikely, the brain may be getting useful information from each eye; they may have learned to live with or ignore the double image, rather than suppress completely. Ask your client about this, and about their vision in general. Try to get them talk about how they feel about 'how they see' with their present eyeglasses.

Hope this helps,

Robert Martellaro

6. Thanks Robert, this patient historically 'winks' to read. How much of the diplopia is from vertical imbalance and how much from anisiekonia, I'm not sure and didn't think to investigate at the fitting. As for her distance vision, I suspect you're right: both eyes are actively working, if not fusing. Acuities are quite good. I was initially curious to see if a slab-off could get her 'close enough,' but I'm just not sure the attempt is justifiable considering the cost and daunting odds. (I'm not sure of the history of anisemetropia, other than it's long-standing--doesn't slow her down at all.)

7. ... considering the cost...

Just what is the lab costs for slab-off in your part of Texas? Other parts of USA? Just curious.

8. Originally Posted by Hayde
Thanks Robert, this patient historically 'winks' to read. How much of the diplopia is from vertical imbalance and how much from anisiekonia, I'm not sure and didn't think to investigate at the fitting. As for her distance vision, I suspect you're right: both eyes are actively working, if not fusing. Acuities are quite good. I was initially curious to see if a slab-off could get her 'close enough,' but I'm just not sure the attempt is justifiable considering the cost and daunting odds. (I'm not sure of the history of anisemetropia, other than it's long-standing--doesn't slow her down at all.)
Hayde,

Your welcome, although I haven't done much more than ask questions and write "out loud" (it helps me to see/hear more clearly).

Squinting? Only at near? Somehow, folks manage that way. Some possible solutions that might help...a Fresnel prism at near only, SVNO lenses, SVDO in non-dominant eye, or contact lenses. OTOH, except for CLs, trying to correct the VI at near might make it worse. No kidding. Use a trial prism lens to be sure.

Regardless, this person is presently reading monocularly, and to make that work with a PAL, we need to know where the eye is positioned at near due to the head turn, and maybe due to lack of convergence. The best way to do this is probably with the "Mirror Method". Measure with the habitual eyeglasses in situ, especially if the dominant eye is the high minus lens.

http://www.optiboard.com/forums/show...g-progressives

Once you've established the near IPD, find a lens that can provide that value (I think Seiko is the only PAL presently that can take a user selected inset), and then consider a SVDO lens for the fellow eye, as needed.

Originally Posted by tmorse
... considering the cost...

Just what is the lab costs for slab-off in your part of Texas? Other parts of USA? Just curious.
Tmorse,

I charge \$190 for a custom segmented slab. I've never slabbed a PAL. The strongest slab I've ordered was 6 PD.

Hope this helps,

Robert Martellaro

9. Originally Posted by Robert Martellaro
Hayde,

Your welcome, although I haven't done much more than ask questions and write "out loud" (it helps me to see/hear more clearly).

Squinting? Only at near? Somehow, folks manage that way. Some possible solutions that might help...a Fresnel prism at near only, SVNO lenses, SVDO in non-dominant eye, or contact lenses. OTOH, except for CLs, trying to correct the VI at near might make it worse. No kidding. Use a trial prism lens to be sure.

Regardless, this person is presently reading monocularly, and to make that work with a PAL, we need to know where the eye is positioned at near due to the head turn, and maybe due to lack of convergence. The best way to do this is probably with the "Mirror Method". Measure with the habitual eyeglasses in situ, especially if the dominant eye is the high minus lens.

http://www.optiboard.com/forums/show...g-progressives

Once you've established the near IPD, find a lens that can provide that value (I think Seiko is the only PAL presently that can take a user selected inset), and then consider a SVDO lens for the fellow eye, as needed.

Tmorse,

I charge \$190 for a custom segmented slab. I've never slabbed a PAL. The strongest slab I've ordered was 6 PD.

Hope this helps,

Robert Martellaro

Robert, I have slabbed a few progressives but would not recommend it due to very poor acceptance.

10. Originally Posted by rdcoach5
Robert, I have slabbed a few progressives but would not recommend it due to very poor acceptance.
Right. Have you seen a free-form/software generated slab? Get a sample before you order one.

Best regards,

Robert Martellaro

11. Originally Posted by Robert Martellaro
Right. Have you seen a free-form/software generated slab? Get a sample before you order one.

Best regards,

Robert Martellaro
They are awful IMO. Just a big blurry sloppy line. I have fit probably a handful of people in progressives with a slab-off. Old school sharp slab lines only. I request the slab line at the 180 line, haven't had any rejects yet... I think for most people it is a vanity thing of refusing to wear FTs and will accept a less visible slab line even if vision is slightly less usable.

Recently, thanks to an older thread on this forum, I have fit a few people with dissimilar corridor lengths with success instead of a slab.

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