Redo of progressive. Wrong P.D.

[MIOPE]

On the average dispensing room you will find to measure PD a pupilometer and/or a ruler.

Each one has a preferred method for measuring PD and surely everyone has had a case where their preferred method has failed and the progressive lens is poorly centered (for far distance or for near) and the lab manufactured the eyeglass exactly as it was ordered. Also assume that the far PD was correctly measured.
If using a pupilometer, we measure the monocular PD (one eye at a time) and we obtain, for example, R=32 L=33 but
also do a second measurement without occluding any eye of the patient and obtain R=34 L=33.
How would you proceed to determine the PD to order to the lab?

Another problem that is seen occasionally is that the inset does not match the amount of convergence of the patient.
How do you determine the amount of patient convergence in a dispensing room? Setting the pupilometer to 33 or 40 cm to
measure the near PD?

[pknsbeans]

You could end up with results like those you listed if the customer has a right exo deviation (strabismus). I would use one eye at a time method in that case.

[Hayde]

Agreed, a differential of 3mm or more (where the hand measurement is greater) is the patient outsmarting the pupilometer and converging despite device setting to infinity. Less than 3 means there's some thought required, including an overlooked CI like pknb mentioned. (Ptosis can conceal some doozies until you start measuring some wildly diverging numbers!)

Other possibilities include how the patient held the pupilometer--confirming disparities with a visual glance might quickly show you which read was more accurate. A third measurement to break ties can help, hand measurement through a transparent ruler so you can see the hashes line up yourself. Some days, you just create a weighted average to best anchor ANSI deviations to mitigate worst-case measurement error range.

Regarding convergence, your thread is talking PALs. If you're selling good PALs, you're selling better equations of finessing convergence variances than anything I could tell you. How many PALs come back to you with reading issues attributable to the PAL not serving an unusual reading PD even if distance PD was accurately measured and accurately made? Which PALs were they?

I'm going to assume 40cm is where the doc set reading, unless the Rx notes otherwise. If the patient tips me off it should be different, then the doc will be getting a call from me...so life is much smoother for all 3 of us if y'all catch this first.

(And please, if you're patient is not seeing binocularly, please note it on the Rx so we know too!) :)

Good thread, MIOPE!

[Tallboy]

Exotropic or essotropic patients will be difficult. Always measure previous pair PDs and then of course any major RX shift will need to be included in your decision. I would split the difference if it was a first time I made glasses and had no previous pair - and that may be a dumb thing to do but I've done it and its worked.

[Hayde]

Exotropic or essotropic patients will be difficult. Always measure previous pair PDs and then of course any major RX shift will need to be included in your decision. I would split the difference if it was a first time I made glasses and had no previous pair - and that may be a dumb thing to do but I've done it and its worked.

+1, if it's dumb then I'm dumb too. If the Px lived with and acclimated to previous ECP's oversight, then 'stair-stepping' their PD back to reality 1.5mm per lens replacement cycle can be merciful and wise.

[Speed]

Mesure the near pd and add 2.5 to each eye for distance.

[lensmanmd]

+1, if it's dumb then I'm dumb too. If the Px lived with and acclimated to previous ECP's oversight, then 'stair-stepping' their PD back to reality 1.5mm per lens replacement cycle can be merciful and wise.

+1...totally agree.

[lensmanmd]

Measure the near pd and add 2.5 to each eye for distance.

With all due respect, I disagree. This is like using a screwdriver to hammer nails, or using a shotgun for target practice. Inset varies on add and corridor length.

TB and Hayde have it right.

[Kwill212]

With all due respect, I disagree. This is like using a screwdriver to hammer nails, or using a shotgun for target practice. Inset varies on add and corridor length.

TB and Hayde have it right.

Agreed. Inset also varies with with vertex and distance power.

[Lelarep]

Mesure the near pd and add 2.5 to each eye for distance.

I didn't think anyone even thought this was reasonable any longer...

[MIOPE]

This is a real case that happened in one of our stores. I did not attend this patient however I could see all the information collected. The monocular PD was used and the symptoms that the patient referred pointed to an incorrect PD.
We redo the job and use the PD measured without occluding any eye of the patient and obtain R=34 L=33. In this case that solved the problem.
Should mention that the PD was checked with two different pupilometers to eliminate the possibility that it was an error caused by a de-calibrated pupilometer.

What I learned with this case is that seems to me a good idea ( if using a pupilometer) to measure the monocular PD and also the PD without occluding any eye of the patient and if the diference is maybe greater than 1mm a problem like the one in this case should be suspected.

[lensmanmd]

This is a real case that happened in one of our stores. I did not attend this patient however I could see all the information collected. The monocular PD was used and the symptoms that the patient referred pointed to an incorrect PD.
We redo the job and use the PD measured without occluding any eye of the patient and obtain R=34 L=33. In this case that solved the problem.
Should mention that the PD was checked with two different pupilometers to eliminate the possibility that it was an error caused by a de-calibrated pupilometer.

What I learned with this case is that seems to me a good idea ( if using a pupilometer) to measure the monocular PD and also the PD without occluding any eye of the patient and if the diference is maybe greater than 1mm a problem like the one in this case should be suspected.

When you consider most of our patients use binocular vision, this makes perfect sense. I'm happy that you found the solution for your patient.

[Robert Martellaro]

Agreed. Inset also varies with with vertex and distance power.

Work/object distance, high or low value for the far IPD, add the center of rotation to the back vertex distance (stop distance).

When you consider most of our patients use binocular vision, this makes perfect sense.

Yup, but cover if the eye turns.

Miope,

For PALs and/or lenses with high values of dioptric power, consider measuring in situ, with a preadjusted frame, placing plus signs on the lenses, bisecting the corneal reflex. Values are read using a PAL centration chart.

Watch for parallax errors (rule of sixteenths), for every 1mm difference between the PD of the fitter and the subject, the fitter will over-estimate the subject's PD by 1/16 mm if the fitters PD is larger than that of the subject.

Watch for head turns.

Hope this helps,

Robert Martellaro

[Hayde]

I factor in lens power to determine reading PDs [for FT lenses,] but aside from the machine measurements (which I distrust), I'm unaware of any way of estimating any given patient's stop distance. I typically rely on the 27mm "average" default. Does anyone have any rules of thumb how to make that variable more responsive to individual patients?

[Robert Martellaro]

I factor in lens power to determine reading PDs [for FT lenses,] but aside from the machine measurements (which I distrust), I'm unaware of any way of estimating any given patient's stop distance. I typically rely on the 27mm "average" default. Does anyone have any rules of thumb how to make that variable more responsive to individual patients?

Only an A-Scan knows for sure, but usually less for hyperopes, more for myopes. Most of the premium semifinished PAL designs use more or less inset depending on the base curve, ditto for most free-form generated lenses.

I'm curious about your segment decentration strategy by distance power; I decenter myopes strictly by trigonometry, with no concern for prism induced. My thinking is that they've adapted to the base in prism at near, likely for decades, and why make it harder to converge? I'm not looking for looking for trouble, no way no how.

OTOH, hyperopes get base out at near, usually much later in life when they have weaker fusional reserves, with possibly other health and visual disorders. To be sure, increasing the decentration 1mm or 2mm per eye will reduce some of the induced BO prism, sometimes it's best to supply separate SVNO, where the optics can be tailored for the wearer's eyes and Rx, and use the primary multifocals for general purpose wear.

Best regards,

Robert Martellaro

[Hayde]

Only an A-Scan knows for sure, but usually less for hyperopes, more for myopes. Most of the premium semifinished PAL designs use more or less inset depending on the base curve, ditto for most free-form generated lenses.

I'm curious about your segment decentration strategy by distance power; I decenter myopes strictly by trigonometry, with no concern for prism induced. My thinking is that they've adapted to the base in prism at near, likely for decades, and why make it harder to converge? I'm not looking for looking for trouble, no way no how.

OTOH, hyperopes get base out at near, usually much later in life when they have weaker fusional reserves, with possibly other health and visual disorders. To be sure, increasing the decentration 1mm or 2mm per eye will reduce some of the induced BO prism, sometimes it's best to supply separate SVNO, where the optics can be tailored for the wearer's eyes and Rx, and use the primary multifocals for general purpose wear.

Best regards,

Robert Martellaro

(I just realized I owe the OP an apology for hijacking his PAL thread into FT considerations. I'll try to satisfy Robert's curiosity and then promptly zip it.)

Thanks Robert. I'd love to find some lit that gives us a hint of physiological correlations, or if the curves diverge for sexual dimorphism. Until then, the Rx is a right good inclusion when the powers climb up.

It's reading power, not distance. Nothing creative, just NPD=DPD-{DPD/[1+W(1/s-F/1000)]}

You make a good point about acclimation. I suppose one could make as good a case to acclimation to "Drop 3" and I really don't have to worry about NPD math at all. (My contracted software has been "updated" for such an assumption for NPD autofill....) I just can't bring myself to do it.

(I could be wrong, but...) I suspect my lab favors the 'drop 3' to the degree that they inset my DPDs just enough to keep ANSI satisfied on both ends. If so, I can live with that. I'm still getting more precision out of them than had I left the inputs at default.

Regarding torturing a patient already used to the prism, I think that's a great reservation. However, (knock on wood) I have yet to have a single patient report such an issue. Granted for most powers we're really talking angels on the head of a pin--but the high power ones whom you point out could be at risk for BI symptoms seem uniformly happy too. Go figure.

[Pete Hanlin]

Some interesting reading on this subject can be found at the following link: http://www.opt.indiana.edu/v665/CD/C...on/CH4/CH4.HTM

A few years ago, I met with an Optometrist who had developed a novel way of measuring the kappa angle and factoring it into the measurement of PD. It was one of the more interesting conversations I've had at Vision Expo.

Personally, I've found an accurate pupilometer (you have to check them, they do get out of calibration)- compared to and confirmed with dotting the lenses- works pretty darn well for the vast majority of patients. Old school for sure, but it always worked for me.

[Robert Martellaro]

Some interesting reading on this subject can be found at the following link: http://www.opt.indiana.edu/v665/CD/C...on/CH4/CH4.HTM

I've read this, and understand some of it. As to the history of how angle Lambda and Kappa can be the same, see...

http://pabloartal.blogspot.com/2008/...gle-kappa.html

A few years ago, I met with an Optometrist who had developed a novel way of measuring the kappa angle and factoring it into the measurement of PD. It was one of the more interesting conversations I've had at Vision Expo.

Is this the same subjective self-testing system that Barry has referred to in the past?

Personally, I've found an accurate pupilometer (you have to check them, they do get out of calibration)- compared to and confirmed with dotting the lenses- works pretty darn well for the vast majority of patients. Old school for sure, but it always worked for me.

I'm blessed with having all the time I need with my clients, using ink lines that occlude/bisect the corneal reflection on a preadjusted frame. If I had to go faster, the Rxs were more routine, and I had no clients with head turns, I'd use the best calibrated pupillometer that money could buy.

DIY folks should read this: Accuracy and repeatability of self-measurement of interpupillary distance.

Best regards,

Robert Martellaro