PAL / PD issue

[Spexvet]

I've run into a situation twice this year that I don't recall encountering before, or I didn't recognize the problem.:hammer:

The patient has trouble with their near vision when using their PALs. After analyzing the glasses and the patient, I finally hit on the issue. Their Near PD is 5 mm less than their Distance PD! In both instances, I've put them into flat tops, because I don't know of a PAL that has anything other than a 2.5mm inset.

Is there any other solution?

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[CME4SPECS]

I thought they were all 2.5mm inset. It use to be recommended that you take the near mono pd and add 2.5mm to get the distance pd. I haven't seen that for a while, but, it sure was recommended at one time.

[drk]

This is a confusing issue for me!

How can people have variable near pd's? It seems that everyone with a 60 dpd that is looking at 40cm would have the same near pd! The eyes have to align at 40 cm, regardless. It's geometrically impossible to draw two triangles with the same base (p.d.) and height (reading distance) and to have more than one angle of rotation (ergo linear change in corneal reflex or near pd measurement.)

It's well-known that a higher PD will have a higher difference between distance and near measurements. That's math. But equal pd's should mean equal near pd's, given equal convergence. Even phoric patients would be subject to this. How big was this p.d. that provided a 5mm difference? Should be a 66 or so!

The only other factors I can think of that would contribute to reduced binocularity at near (assuming that this is what it truly is...how specific were you with troubleshooting?) would be the variability in prism that higher Rx's provide, but that should be factored into the lens blank based on the base curve...e.g. a +6.00 base with a progressive can assume that the Rx range that will be cut on it is +2.00 to -4.00 (whatever) and that should contribute an additional prismatic effect of 0.3 cm x +2.00 = 0.6^ BO to 0.3 cm x -4.00=1.2 BI. Thus, on a 6-base, induced prism should be about 1/2^BO (the worst kind) or 1^ BI (which is actually beneficial, unless the patient is highly myopic and esophoric.). For higher plus base curves or flatter base curves, the inset should be manufacturer-adjusted accordingly, more for higher plus base curves, and less for higher minus base curves . (Isn't this true?) What kind of lens did you use? Was it a "good" one?

I've run into similar problems, but I've decided to rule out "excessive convergence and attributed it to visual pickiness. My best recommendation to you (other than bailing into a D-seg) would be to provide a larger near zone in these cases, a la Sola Max. You alternatively could try to center the near zones by measuring near pd and order the distance pd as near pd + 2.5 mm (which I think is the standard binocular inset, but at which base curve?).
I am considering whether the combination of a higher plus exophore with a high p.d. (>>60 mm) and a higher minus esophore with a higher p.d. could have trouble with the induced prismatic effect with their progressive insets.
In other words, the larger p.d people could need some screening to be sure that they are not high Rx's, and then that they do not have significant near phoria, and then that it's not in the "wrong" direction. (Not unlike vertical phorias with anisometropia!) Was the distance Rx high minus or high plus?

I'd really like to hear from you on this.

[shellrob]

I agree with DRK, shcange to a larger near vision lens. Like he said the Solamax or that new Liberty lens. I have never heard of what you'e explaining. Not to say it doesn't or won't happen, I don't think that's the issue with your patient.

[Robert Martellaro]

The only exceptions would be the variability in prism that higher Rx's provide, but that should be factored into the lens blank based on the base curve...e.g. a +6.00 base with a progressive can assume that the Rx range that will be cut on it is +2.00 to -4.00 (whatever) and that should contribute an additional prismatic effect of 0.3 cm x +2.00 = 0.6^ BO to 0.3 cm x -4.00=1.2 BI. Thus, on a 6-base, induced prism should be about 1/2^BO (the worst kind) or 1^ BI (which is actually beneficial). For higher plus base curves or flatter base curves, the inset should be manufacturer-adjusted accordingly. (Isn't this true?) What kind of lens did you use?

Drk,

I believe that would be true with the premium PALs. In the above example the +2.00 eye will turn towards the apex by .3mm per diopter of prism equal to .6^ x .3mm or .18mm. The -4.00 eye with 1.2^ BI will turn out .3mm x 1.2^ = .36mm. Not much, but I would guess that +7 BC would have more inset, probably 3mm and a +3 BC would have less inset, maybe 2.00. Higer powers will induce greater amounts of prism so that +.50 BC might be as little as 1.5mm, different enough to make taking the near PD and adding 2.5mm a not to good idea, unless you know for sure the exact inset for every BC for every design.

Regards,

[Spexvet]

Case 1:

In 2000, older man was successful with Poly Adaptar. Last October, I put him in Poly Panamics.
OD +2.75 -1.00 X 70 +2.50 add
OS +2.25 -0.75 X 75 +2.50 add

He came back in Feb, complaining about reading for long periods, mainly magaziines.

I:
redotted lenses, and looked at them on him.
checked PD and ht. measurements against the order.
Checked the Rx.
Rechecked his monocular distance PD.
I had him hold reading material, covered one eye, had him move reading material to position where it is clear.
Then covered other eye and had him move material to where it was clear.

That's when I noticed that each eye was clear for reading more temporally. I took his near PD and found that his PDs were 63/56.5.
I made him a pair of computer bifocals (INT over NEAR in FT-28) and he has been very happy since.

Case 2:
Woman, first time multifocal wearer.
OD +0.25 -2.50 X 013 +1.50 add
OS -0.50 -2.00 X 163 +1.50 add

Fit with Ellipse. Same trouble shooting process. PD = 56/51

Hope that will help.

[Spexvet]

I also got a new pupilometer in December.

[Jason Carruthers]

Case 1:

In 2000, older man was successful with Poly Adaptar. Last October, I put him in Poly Panamics.
OD +2.75 -1.00 X 70 +2.50 add
OS +2.25 -0.75 X 75 +2.50 add

He came back in Feb, complaining about reading for long periods, mainly magaziines.

I:
redotted lenses, and looked at them on him.
checked PD and ht. measurements against the order.
Checked the Rx.
Rechecked his monocular distance PD.
I had him hold reading material, covered one eye, had him move reading material to position where it is clear.
Then covered other eye and had him move material to where it was clear.

That's when I noticed that each eye was clear for reading more temporally. I took his near PD and found that his PDs were 63/56.5.
I made him a pair of computer bifocals (INT over NEAR in FT-28) and he has been very happy since.

Case 2:
Woman, first time multifocal wearer.
OD +0.25 -2.50 X 013 +1.50 add
OS -0.50 -2.00 X 163 +1.50 add

Fit with Ellipse. Same trouble shooting process. PD = 56/51

Hope that will help.

In Case 2, the lens power at 180 is so low, I would simply widen the distance PD. In Case 1, it may not work.

[Aarlan]

This is a confusing issue for me!

How can people have variable near pd's? It seems that everyone with a 60 dpd that is looking at 40cm would have the same near pd! .

Theoretically the 60 dpd individuals should have a similar npd, however the difference in inset for a 65 dpd versus a 55 dpd patient are going to be markedly different. I was also under the impression that virtually all Progressives were manufactured with a 2.5 mm inset, therefore manufactured inset may not be an optimal match for the patient's actual inset. Sometimes it is beneficial to alter the PD we order in the glasses based on the relationship between the patient's inset and that of the progressive design.

AA

[Spexvet]

I've run into a situation twice this year that I don't recall encountering before, or I didn't recognize the problem.:hammer:

The patient has trouble with their near vision when using their PALs. After analyzing the glasses and the patient, I finally hit on the issue. Their Near PD is 5 mm less than their Distance PD! In both instances, I've put them into flat tops, because I don't know of a PAL that has anything other than a 2.5mm inset.

Is there any other solution?

I left out some key words in my original post - sorry :hammer:

[Darryl Meister]

Your more sophisticated progressive lenses will generally have a variable near inset. The inset is generally tweaked based upon 1) the median power of each base curve in order to account for prism-induced convergence and 2) the intended working distance. Any add powers above +2.50 D, for instance, would require a shorter working distance than 40 cm.

Best regards,
Darryl

[karen]

Darryl, I was under the impression that the newer PALs (i.e SolaOne and the lot) were variable inset based on power and PD which should address the problem, right???? Would the SolaOne in the 1.67 with the freeform stuff be a better option in that sort of case? Not that she would need that type of material so much but the design?

[Aarlan]

But none are tweaked based upon the variation in the patients actual pd/inset. A similar Rx/add combination will have the same inset regardless of whether the patient's DPD is 66 or 56. It stands to reason that the dispensing professional may have to manipulate the lens measurements to adequately accomodate each individual's specific measurements.

AA

[drk]

You alternatively could try to center the near zones by measuring near pd and order the distance pd as near pd + 2.5 mm (which I think is the standard binocular inset, but at which base curve?).

Robert,
Agree completely. Good point. Do you have any insight into this?

[drk]

Your more sophisticated progressive lenses will generally have a variable near inset. The inset is generally tweaked based upon 1) the median power of each base curve in order to account for prism-induced convergence and 2) the intended working distance. Any add powers above +2.50 D, for instance, would require a shorter working distance than 40 cm.

Best regards,
Darryl

Cool. Thanks, Darryl. What do you think is happening here?

[drk]

But none are tweaked based upon the variation in the patients actual pd/inset. A similar Rx/add combination will have the same inset regardless of whether the patient's DPD is 66 or 56. It stands to reason that the dispensing professional may have to manipulate the lens measurements to adequately accomodate each individual's specific measurements.

AA

Very good point, Aarlan! Variable inset BY POWER (base curve) is one thing, but variable inset by p.d is unknowable to lens manufacturers!

Your point infers that smaller pd people will have the near zone artificially inset too much, and larger pd people not enough.

To summarize, if the dpd is 64 up (slightly arbitrary) we can expect a complaint of having to look temporally (monocularly) or general near blur. I've experienced this, but the guy's pd was only about 62/59.5.

If the dpd is 56 or lower, then they would have to look nasally, or c/o generalized near blur.

But what's the correct solution, Aarlen? Specify true near pd and an artificial distance pd, assuming you know what the variable inset is per base curve per progressive design? That's too cumbersome or impossible, as Robert M. suggests. Plus, what are the drawbacks of having the distance incorrectly decentered? Unwanted prism at distance can be a major bear, especially BI! It would probably work for low powers, where a millimeter or two isn't important, like Shellrob said.

I think the best policy is to not worry about it, but if a person non-adapts, reselect a progressive that stands the best chance of working, a la Solamax, Piccolo, VIP, Compact, or even Comfort (all per Sheedy) and avoid the somewhat narrower near zones, a la Natural, Top, Definity, ECP, Image, GP Wide, even Panamic, assuming that the distance vision is not paramount to the wearer.

If the distance vision is paramount, then maybe flat top is the way to go.
Of course, one could still attempt to calculate the artificial distance pd after researching the inset provided by that design at that base curve, and the amount of induced distance portion prism (and probably consider the person's phorias) and see if it works in any given design. Man, that's complex, though!

[drk]

Case 1:

That's when I noticed that each eye was clear for reading more temporally. I took his near PD and found that his PDs were 63/56.5.
I made him a pair of computer bifocals (INT over NEAR in FT-28) and he has been very happy since.

Case 2:
PD = 56/51

I still stubbornly maintain that it is impossible to have the pds you describe here. I'd have to do an empirical graph, since it's not straightforward to derive near pd, but I'd say, roughly, that a 63 dpd gives no less than a 59 npd, and that the 56 dpd would give about a 54 npd! I guess you're crazy:D

According to my theory,
Case 1 should be closer to the case where he didn't get enough near inset, and would have to look temporally, as you describe.

Case 2 should be the opposite, though, where she got too much inset, standard, for her liking, not less.

Maybe this theory is missing something?

[Robert Martellaro]

Spexvet,

If there is binocular vision all you need to do is get the fitting cross on the pupil. The near zone will take care of itself. Monocular clients might need to have the lens rotated to cancel or minimize the inset.

Personally I would never let a PAL client try to read with one eye unless they had monocular vision. Both eyes, always, no exceptions. If you need to confirm the near pd I would use the "mirror trick" that I read about in this forum. Might be a good idea just to see how far out of calibration your pupilometer is.

The issues here were probably related to the PAL design change and any Rx change, not the near PD, IMHP.

Your other client is probably thinking "Hey, I can read just as well without the glasses, without having to look through that "small area at the bottom of the lens. The diistance peripheral vision is blurred also!" Tough to compete against that, but that's the way it is with myopic emerging presbyopes. An atoric design would work better for her, and I'd recommend the Multigressiv or Autograph, both haviing much better distance peripheral vision due to the PAL desgn, with the atoricity further decreasing off-axis blurring due to oblique astigmatism.

Regards

[Jason Carruthers]

Ophthalmic dispensing is as much about counselling as it is measurement. People can get used to just about anything as long as you set the right expectations. How many times have you investigated a complaint, only to find the old glasses were made with a PD that was 5 mm off?

[CME4SPECS]

The mirror trick?? Could you expand on that a little please?

[Spexvet]

But what's the correct solution, Aarlen? Specify true near pd and an artificial distance pd, assuming you know what the variable inset is per base curve per progressive design?

Wouldn't that leave the corridor unusable, also?

[Spexvet]

Might be a good idea just to see how far out of calibration your pupilometer is.

I'll check it and let you know.

The issues here were probably related to the PAL design change and any Rx change, not the near PD, IMHP.

In My Humble P....? ;)

[Robert Martellaro]

Originally Posted by Robert Martellaro
The issues here were probably related to the PAL design change and any Rx change, not the near PD, IMHP.

In My Humble P....?

'pinion? I checked my keyboard and the O is right where it should be, next to the P.

Ever notice that the minus sign is next to the plus sign? That can't be a good thing.

[opticalgenius]

Do you know that Varilux Panamic will vary the near decentration depending on the RX more than the standard 2mm or so. Out of all of the Varilux product, this PAL design will accomidate a large shift in decentration the best.

[Darryl Meister]

Darryl, I was under the impression that the newer PALs (i.e SolaOne and the lot) were variable inset based on power and PD which should address the problem

Unfortunately, it's not really possible to adjust the inset of a semi-finished lens blank for differences in PD since they can change across the same base curve and add power.

Would the SolaOne in the 1.67 with the freeform stuff be a better option in that sort of case?

I'm only aware of one free-form lens that adjusts the inset, though we only sell it in Europe right now (SOLAOne Ego+).

Cool. Thanks, Darryl. What do you think is happening here?

Obviously his patients are experiencing an allergic reaction to Essilor lenses. ;) But, seriously, you're right about the Near PD. He should require an uncorrected near PD of around 60 mm, unless he has some oculomotor dysfunction at near (and assuming the CRP wasn't set to 25 cm or something).

In 2000, older man was successful with Poly Adaptar. Last October, I put him in Poly Panamics... He came back in Feb, complaining about reading for long periods... I made him a pair of computer bifocals (INT over NEAR in FT-28) and he has been very happy since.

I'm a bit surprised you didn't just put him back into Adaptars though?

different enough to make taking the near PD and adding 2.5mm a not to good idea, unless you know for sure the exact inset for every BC for every design.

Additionally, the corridor of most progressive lens designs is relatively narrow, and aligning the near zone will significantly reduce the binocular field of view for far-intermediate vision (some unwanted horizontal prism will be induced in distance as well, though this may or may not be a concern).

Best regards,
Darryl