Hi I have a question from one of the docs and want to give the most accurate answer looking for some guidance.
We have a patient with about 2.5D of anisometropia. Plano in one eye +2.50 in the other. Usually would go with slab off prism. Is slab off generally used with PAL's? Is it recommended?
Thanks Optiboarders!
OK I just turned to page 502 of the Blue Bible (System for Ophthalmic Dispensing by Clifford Brooks) and guess what. You can use slab off with a PAL. I thought they were just an option for bifocals, but live and learn.
Welcome to OB.Originally Posted by dcook
Depends on their health and visual activities. I don't slab PALs, instead using separates optimized for near, when needed. I always discuss this with the client, listing the symptoms and the solutions. Consider using aspheric (flatter BC) PAL designs and fit close.
One of my mentors was Dick Cook from Benson Optical. Maybe your first name isn't Dick, but whenever I see your name in the future, I'll think of him (that's a good thing).
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
We get orders for slab-off progressives, oh, roughly one a month. I don't know that I'd necessarily recommend them, but they certainly can be made. What I've always thought looked strange are the SV slab-offs.....
Oh, and @psuedonym, you can slab-off trifocals as well.... strange when you only slab-off the near and not the intermediate too.
There are rules. Knowing those are easy. There are exceptions to the rules. Knowing those are easy. Knowing when to use them is slightly less easy. There are exceptions to the exceptions. Knowing those is a little more tricky, and know when to use those is even more so. Our industry is FULL of all of the above.
I'll agree with Robert.
Also what's the add?
First time progressive?
Same pupil heights?
Best acuity each eye?
I usually sell this with a long detailed explanation and the honest answer to the subjective question- "Will it work?"
"I don't know!"
Ryser's Rule is in play here big time.
We see a fair amount of slab Rxs. Our lab now does the slab off on their digital generators. Depending on the amount of prism, you may not even see the line, so it works quite well for us. We usually don't do slabs unless there is a 3D difference, though.
I did one last year for a pt with 2D of aniso. We used a Varilux Comfort in poly. It came back beautiful and it worked for the patient.
Applying slab-offs to progressive lenses isn't uncommon, particularly since these lenses are more susceptible to vertical imbalance at near due to the lower position of the reading area compared to traditional, lined multifocals. You should keep a few pointers in mind when applying a slab-off to a progressive lens though:
1. Recommend an anti-reflection coating. An AR coating will make the slab line considerably less conspicuous, which is important for a premium lens often sold on the basis of cosmetics. Otherwise, the slab line may defeat the purpose of a "no-line" bifocal for some patients.
2. Have the slab line placed low enough not to interfere with distance vision but high enough not to interfere with intermediate or near vision. You may want to experiment with the patient using Scotch tape or a marker on the demo lens to determine the optimal placement of the slab line. Place the slab line a few millimeters below the prism reference point if possible. This will allow you to verify the prism of the distance portion of the lens normally. It will also ensure that the slab line is not interfering with distance vision.
3. The total vertical imbalance at the reading level will be approximately proportional to the distance between the prism reference point and near checking point of the lens and the difference in power between the vertical meridians of the right and left lenses. Because the patient will still have some degree of vertical fusional reserves, however, you seldom need to prescribe the full amount of vertical prism imbalance. I wouldn't use any more vertical prism than necessary to ensure comfortable single binocular vision.
4. The slab-off prism will be base up prism applied to the lens with the least plus power or most minus power in the vertical meridian. You can verify the approximate amount of slab-off prism using a lens clock by comparing a reading taken in the distance portion of the back surface to a reading taken across the slab line (with the center pin on the line).
Best regards,
Darryl
Darryl J. Meister, ABOM
I always recomend trying it without first. At which of many points through the progression are you going to match the prism. I think using a progressive slab would be very confusing. Remember that a progressive is sposed to be useable for all distances. Lab guys will tell you that there are many people out there reading with one eye. Not just monofit contact wearers. You can always add it later if you need to. Refit! Patients will be happier.
My SIL has worn a progressive with a slab off for years with no problems.
Hi Guys,
I've been a bit quiet recently... nice to be back.
Over the years we've done dozens of these in every material possible and they work 99% of the time.... and we've found it only worthwhile when there is at least 2.50D differential.
We had an interesting case a couple of days ago:
First time PAL
Right: -3.00sph
Left: -7.00sph
Add +2.50
The lady says "I don't want one lens with a line and the other without"
So we're trying her without slab and if there's problems, we'll increase the s/o prism on the Left and put that same differential as a slab-off on the Right....... she will then have 2 lenses with lines.
I wonder if anyone has done this?
I frequently see pts with 3+ diopters difference between the eyes and they are wearing pals without any slab off and I ask them how their vision is and they look at me like I'm crazy. I thought they would be uncomfortable with this type of rx?
My first thought is a lot of these patients are using them monocularly and don't realize it.
I'll usually spend some time with an ani/antisometrope explaining the issue and if they choose to go with a multifocal to try it first with no slab. If it works great if not then I'll re do and charge for the slab only and we now know they'll probably need it in the future.
I will tend to actively discourage progressives but that's me and the office environment I work in where costly re do's get laid at my feet.
I have a patient with an rx very close - Just curious - what was the final outcome?
Thanks,
Kelley
You don't need a slab-off if you manage base curve index and thickness to manage the dynamic aniseikonia check out shawlens.com
I would add a question of whether anyone has noticed models of freeform PALs that have better results with slab off (perhaps their designs are customized well to handle anisometropia)? e.g. What would people recommend for say someone plano in one eye (post unilateral cataract surgery) and -7 in the other (for when they aren't wearing a contact lens which is obviously the preferred approach)? I'd guess the best result would be splitting the difference with reverse slab off on one and slab off on the other?
I have found that a lot of Anesokonic patients can merge images.
In your case your patient may be able to pull both together without going slab, or modify the BC of of the low power lens, or apply a small amount of prism only OS. Slab works, but its these are less expensive and your patient is borderline.
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