Has anyone checked this so-called 'ophthalmologist's??? educationOriginally Posted by Jim Mush
credentials???
Has anyone checked this so-called 'ophthalmologist's??? education
credentials???
Get a life. Troll......
My guess is an ophtho or optometry student, most likely the latter because opththos know sqaut about optics.Has anyone checked this so-called 'ophthalmologist's??? education
credentials???
However, if it wasn't for bad manners, neither one of you would have any manners at all.
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.
I Love To Do Slab Offs !! The Harder The Better. I Slab All Materials Except Glass. From Progresives To Flat Tops. A Progressive Lens Is Actually Easier To Craft. My Only Limitation Is The Ammount Of Prism I Can Grind On My Generator Is 10 Degrees. I Haven't Seen One That Requires More Than That . It Would Take A Lot Of Imbalance To Go That Far.
The visual system is actually capable of adapting to quite a bit.
Vertical fusion reserves range from 2 to 4 prism diopters, and research has demonstrated that some spectacle wearers can adapt to as much as 3 prism diopters of vertical imbalance. Further, it is not uncommon for persons to develop an "adaptive vergence," known as gaze-specific adaptation, to the differential prismatic effects produced by asymmetric lenses. And all this assumes that the wearer exhibits binocularity at near in the first place, and isn't habitually suppressing one eye.
If the patient is currently symptomatic (exhibiting double vision, asthenopia, etcetera), demonstrates phorias at near through the correction, or has acquired the anisometropia abruptly or later in life, then he or she is more likely to benefit from a slab-off (with either a bifocal or progressive). And progressives will generally induce more prismatic imbalance at near than traditional flat-top bifocals, simply because the distance to the near zone is longer.
That said, I've never seen any evidence to suggest that anisometropes have difficulty adapting to slab-offs. And, in at least one study, anisometropes found that lenses with a slab-off were either comparable to or preferable to lenses without one. Consequently, if you want to err on the side of caution when in doubt, just use a slab-off.
However, I wouldn't recommend using the full prism correction, which is probably overkill in most cases. And if the patient has chosen a progressive lens, which are often purchased for their cosmetic benefit, you would definitely want to recommend an antireflection coating to offset the visibility of the slab line.
Darryl J. Meister, ABOM
We will often recommend trying an Rx without the slab, if the patient has never worn a slab before and the imbalance is less than 3D on the simple basis of cost (if that is an issue), since patients can often adapt in that range.
In this Rx you have 4 diopters of prism at 10 mm below OC, however since we're talking about a progressive, as Darryl alluded to, it is important to keep in mind the reading area will be lower than 10mm and you could likely be dealing with 6 diopters or more of prism imbalance at near, depending on your corridor length.
-Keith
This is exactly why if you MUST use a progressive in such a case, you MUST use a slab, unless the patient is non-binocular. It's also the reason that for most of these types (aniso more than 3 D.) contacts are the first treatment of choice (and most vision plans will cover them in full as being "medically necessary"), regular lined bifocals are next best choice, and progressives are the WORST choice. And for what it's worth, I never met an ophthalmologist who understood ophthalmic optics, except one who was first an optometrist.
There are a number of options presented in this thread, some touted rather agressively, but I see no reference to two other options: 2 pairs of single vision, and dissimilar segments. Now I'm not advocating either in this particular case, but rather presenting all options. Strongly advocating any one method over all others is futile. There are just to many factors to consider before initiating treatment options. The acuities, the degree of disparity. the time of onset, the expected duration of the condition, the hobby and vocational needs, the quality of life, the cost, the waiting time the practicality, ect ect. Maybe single vision readers will keep someone working until their bicentics arrive. Maybe dissimilar segs will solve the problem entirely for one person. Maybe we need to apply the KISS principle. (keep it simple,stupid.) At any rate, ophthalmic pros should have a good fundamental understanding of prism and eikonic disparity, and sound knowlege of a number of different options, and remain flexible when presented with these types of problems. Jumping up and insisting there is only one way to proceed by only looking at the spectacle rx is simplistic, limiting, and not exercising professional judgement.
Dave,
You said it.
Moreover, add "listening closely" to the above... the OP said PAL slabs might not be available in his area, not too hard to believe when you consider that about 25% of the world's population doesn't have safe drinking water.
Regards,
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.
2 pair is obviously always an option, but was not part of the original post. Dissimilar segs might have been mentioned, but I'm not sure what dissimliar segs would do for the 4 ^ vertical prism/cm of vertical off center viewing. Maybe an example of what you're talking about would be helpful.
You can get a minor reduction by using the wider flat top on the more minus or less plus side, but the wierdness of it to me far exceeds the wierdness of a slab off. I don't see how different corridor lengths on pals would work at all. The limited area of near viewing with pals would be further narrowed by such an idea.
Hi William. Harry did indeed refer to dissimilar segments, but referred to using two different flat-tops to eliminate prism imbalance. In fact, one of the segments is usually a round seg, either 22mm round or an ultex and one a flat-top. The method is an old means of correcting a relatively small amount of vertical prism imbalance. It isn't used a lot these days, but can still be a very cost-effective way to bring vertical imbalances to within tolerable levels. It can, and should be another tool in the toolbox for opticians or optometrists. One applies Prentice's rule to the segments only to calculate the amount of compensating prism: thus the method is fairly simple to use. A flat-top has very little prismatic effect or "image jump," due to its design, while a round seg has considerably more due to the distance from the segment edge to its oc. For example, with an add of 2.50, using a ftop-28 on one side, and a 22 round seg on the other. The oc on a 22 round seg is 11 mm, or 1.1 cm from the seg top, giving 1.1 x 2.50= 2.75 bd. Since the oc on the ftop is 5 mm below the seg top, the wearer will be reading 6mm below that. .6 x 2.50= 1.5 bu, giving 1.25 bd on the eye wearing the round seg: not much prism for what may amount to an objectionable cosmetic appearance, but may, again, reduce the total from say 3.00 of imbalance to 1.75, easily tolerable for most people. Some don't care about the cosmetics, and it remains an inexpensive means to bring an imbalance to tolerable levels, either on a permanent or temporary basis.
Interesting. Your example means the round seg ht is set about 5 mm higher than the flat top. To me this means that the patient will have a 5 mm window where he is looking through a round bifocal and the other eye is still looking through the distance part of the flat top. Not only that, but he will have two very different amounts locations AND shapes of diplopic "jump" between the two eyes. I can't imagine anyone getting used to that. If OTOH you adjust the specs so the lines are more or less coincident, you will eliminate up to half the prismatic benefit. Slabs are so inconspicuous and trouble free that I just can't understand their avoidance. They do cost, but not much more than going out for a nice dinner with your honey.
William, The segments are set at the same hight. The 5mm discrepancy is because the oc on a flat-top is not right at the seg line, its normally about 5 mm below it. The prismatic benefit is derived from the disparity in the distance from the segment top to the oc on each type of segment, and from the very same image jump you indicate would be intolerable. Image jump and prism are one and the same. The amount of prism can be increased by using an ultex type bifocal, but would be quite impractical if that were the case. High segment placement is also an advantage when using dissimilar segments, since less imbalance is created in the first place. In closing, I certainly agree other methods are more practical in most circumstances, particularly the slab off, but dissimilar segments are still a viable option in some limited circumstances.
If the segs are set at the same height, as in your example, the eye reads 6 mm below the seg top. That would have to also apply to the round seg, or at a location where the prism is much less than you indicated, and certainly less than that at the seg line. Prism is not the same as jump. Jump is the amount of prism at a seg line. The prism amount 6 mm down from that seg line is quite different, significantly less. You calculated this correctly for the flat top, but not for the round seg.
What Dave was saying is that the vertical prism produced by the flat-top at, say, 6 mm below the ledge (or 1 below the seg OC) is different from the vertical prism produced by the round seg at 6 mm the edge (or 5 mm above the seg OC). Since there is a 6 mm difference between the vertical positions of the two segment optical centers, a vertical prismatic effect equal to 0.6 x Add is induced anywhere in the segments when they are fitted at the same height.If the segs are set at the same height, as in your example, the eye reads 6 mm below the seg top.
Darryl J. Meister, ABOM
Agreed.
i recently fitted a patient with considerably more anisometropia than this, i think he was +1.00 ish in one eye and -5.00 ish in the other, with a slab off progressive. It took a fair bit of effort to get it right and we had to drop the slab line from where the lab initially placed it by about 2mm as it was too high and causing double images in his distance vision but when he collected the glasses this saturday he was amazed at how much better they were than his previous flat tops.
Point to remember.....always specify where you want the slab off line to be otherwise the lab will just slap it on the prism reference point.
Bezza,
Where did the slab line originate on the first pair? Where did you put it on the second. Was it in line witht the fitting cross, horizontal/180 line, or below? I had assumed that it always went with the prism reference/180 horizontal reference point. Thanks!
Last edited by Fezz; 10-31-2006 at 10:18 AM. Reason: clarification.
In the first pair the slab line was along the prism reference point, which is the default position, but being a lens design with the fitting cross 2mm above the PRP this was a little too high for the patient to tolerate and so I telephoned the supplier and asked if i could specify where I wanted the slab line placed, they said thatd be fine but that typically the optimum position was at the PRP, so I went ahead and moved it 2mm lower anyway and it worked a treat.
Thanks Bezza!
:cheers:
[quote=Darryl Meister;160855]The visual system is actually capable of adapting to quite a bit.
Vertical fusion reserves range from 2 to 4 prism diopters, and research has demonstrated that some spectacle wearers can adapt to as much as 3 prism diopters of vertical imbalance.
Although we all have a vertical fusional reserve, leading authorities suggest solving for Vertical Imbalance when power difference between OD & OS is one (1) dioptre of more. This might explain why we have a maximum vertical prismatic imbalace tolerance of 0.50^ between two (2) eyes as found in any list of acceptable optical tolerances.:cheers:
Honestly, I've never really seen any evidence to suggest that 1.0 PD of vertical imbalance will routinely result in symptomatic wearers, and I think this might be overkill. For that matter, many labs won't even try a 1.0 slab-off, and your other vertical imbalance options are limited. And there are certainly thousands of wearers out there right now with more vertical prism imbalance than this at near. Traditionally, the "working limit" has been around 2.0 PD in the US, though many clinicians seem to ignore vertical imbalance altogether anymore.Although we all have a vertical fusional reserve, leading authorities suggest solving for Vertical Imbalance when power difference between OD & OS is one (1) dioptre of more.
Keep in mind that the vertical prism tolerances used in optical standards are based in no small part on the process capabilities of a typical laboratory, and are not necessarily limited by the acceptable visual "tolerances" of the actual wearer. Besides, if I recall correctly, the vertical prism tolerances proposed by many countries (except the US) for the new ISO mounted pairs standard would actually exceed 1.0 PD for lenses in excess of +/-5.00 D.This might explain why we have a maximum vertical prismatic imbalace tolerance of 0.50^ between two (2) eyes as found in any list of acceptable optical tolerances
Darryl J. Meister, ABOM
People can tolerate horizontal imbalances far greater than vertical ones, but the allowable tolerances for unwanted horizontal tolerance are very small. It isn't a matter of how much people can tolerate, but how much they should have to tolerate. Deciding when to correct a vertical imbalance is still debated on this board, but there is no real right answer. It depends largely on the acuities, and ocular dominance: a parameter often overlooked by spectacle opticians, but considered by contact lens fitters who practice with bifocal and monovision. I submit we need to give far more consideration to ocular dominance when deciding on our approach to vertical prism imbalances. Note this is entirely different from acuities, but may play as important a role. even assuming equal and normal acuities, the degree of dominance may be an essential parameter in determining the amount of vertical imbalace which may be tolerated by any given individual. Essentially, the more pronounced the dominance, the more quickly and readily suppression can be "activated" or learned. There are many people who are strongly right eye dominant, some who are mildly dominant, (yes I mean their eyes) and some who are somewhat equal, just as people who are ambidextrous with their hands. (I'd give my right arm to be ambidextrous.;) ) Their are a numbr of tests one can give to determine ocular dominance, but the majority of people who are right handed are also right eye dominant. Have the person, with both eyes open, line a pencil up with a distant object while focussing on the pencil. Then have them close their left eye. If the pencil is still lined up, the person is right eye dominant, if the pencil has moved from the distant object, the person is left eye dominant. There are many other factors to consider when determining an action plan for a vertical prism imbalance, as already posted on this thread by myself and others. Never overlook acuities and dominance as parameters to consider as well.
Last edited by Dave Nelson; 11-08-2006 at 03:48 PM. Reason: rewording for clarity
If the visual system of the patient is capable of binocular fusion and stereopsis, and the vertical imbalance induced by the lenses will result in suppression of either eye, I'd certainly recommend correcting it...
Darryl J. Meister, ABOM
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