Dissimilar seg heights and variable corridor designs

[DanLiv]

I have a few progressive patients with significantly dissimilar seg heights (3.0+ mm). I have resisted putting them into my favored variable seg digital designs assuming that would obviously create equally dissimilar corridor lengths. I wouldn't imagine one eye dropping 15mm to the reading area and the other having to drop 18mm to get the same reading would be very good. I have been keeping them in fixed designs or fudging the segs on variable designs I edge myself (e.g. if the actual heights are 18 and 21, I might order uncuts both at 19 or 20 so the corridors are equal but still edge at 18 and 21).

I feel in the technology should be there to take care of any fit situation, but I'm not 100% confident it is. Am I right to be resisting ordering dissimilar segs for variable designs, or does the software accommodate for such dissimilarities and ingeniously calculate a complete binocular vision solution?

[standarduck]

I would be very worried about having dissimilar corridor lengths.

I haven't had any issues with using the same corridor lengths with patients with assymetric fitting heights. Unless the difference were huge, I wouldn't normally think too hard about corridor length. Fitting a long corridor in one eye and a short in the other sounds like it would be a recipe for disaster.

Chris

[DanLiv]

Yes that's what concerns me. The issue is then with certain manufacturers I am limited in design choice as just about everything coming out is variable corridor. On uncuts it's not a big deal, but if the lab is doing it complete I'm not do confident they will be able to grasp the same concept and carry through on my specs correctly. I use primarily Varilux, so my fixed corridor digital design choices are limited to DRx and S Design, and I don't get fit values with those either. Sure I can go Auto 2 or 3 fixed, but is that my only recourse? I'm just wondering if maybe the designers have anticipated these scenarios and have programmed the software to make compensations when dissimilar segs are specified. Seems these days I should not have to try and outsmart the software to get it right.

[Robert Martellaro]

I have a few progressive patients with significantly dissimilar seg heights (3.0+ mm). I have resisted putting them into my favored variable seg digital designs assuming that would obviously create equally dissimilar corridor lengths. I wouldn't imagine one eye dropping 15mm to the reading area and the other having to drop 18mm to get the same reading would be very good. I have been keeping them in fixed designs or fudging the segs on variable designs I edge myself (e.g. if the actual heights are 18 and 21, I might order uncuts both at 19 or 20 so the corridors are equal but still edge at 18 and 21).

I feel in the technology should be there to take care of any fit situation, but I'm not 100% confident it is. Am I right to be resisting ordering dissimilar segs for variable designs, or does the software accommodate for such dissimilarities and ingeniously calculate a complete binocular vision solution?

We should use fixed designs, or at a minimum, make sure the fitting height (usable height including frame shape) isn't less than 19mm for either eye, keeping the corridor's power profile the same for both eyes, for most PAL designs (check with the manufacturer).

[sharpstick777]

I find that patients with dissimilar needs are compensating in someway already, usually through dominance, but also posture. When I fit dissimilar segs I keep a close eye on how they hold their head, read etc. Ask the OD which eye is dominant and fit to that eye first. So if the patient has a 3 mm difference I would usually split that, with fixed segs, depending on how they see or compensate now, for example and fit one .5 lower and 1mm higher. It takes more time at the dispensing table testing effective widths with a reading card and .25 flippers.

[Robert Martellaro]

When I fit dissimilar segs I keep a close eye on how they hold their head, read etc. Ask the OD which eye is dominant and fit to that eye first. So if the patient has a 3 mm difference I would usually split that, with fixed segs, depending on how they see or compensate now, for example and fit one .5 lower and 1mm higher.

Compromising the fitting height disparity might be okay with segmented multifocals, if that's what you mean. Possibly better cosmetics for those who might be sensitive to that, but you have to watch that you don't tax their vertical fusional reserves when the distance Rx is moderate to high power (we can adjust the distance vertical OCs to compensate of course).

For PALs though, we want the fitting point placed center pupil for both eyes, so that we don't compromise the wearer's near binocular vision by forcing their eyes to experience unequal add powers.

[MakeOptics]

Compromising the fitting height disparity might be okay with segmented multifocals, if that's what you mean. Possibly better cosmetics for those who might be sensitive to that, but you have to watch that you don't tax their vertical fusional reserves when the distance Rx is moderate to high power (we can adjust the distance vertical OCs to compensate of course).

For PALs though, we want the fitting point placed center pupil for both eyes, so that we don't compromise the wearer's near binocular vision by forcing their eyes to experience unequal add powers.

I agree with Robert, no compromise in the near. A PAL has a limited near field as it is and any deviation tends to compound when viewing binocularly. My preferred method of fit is to measure near PD's then use a design with a fixed inset (2.5mm) and add it to that near PD when laying out the lens for edging. This ensures that horizontally the lens design is as centered in their view as possible with all fudge or compromise in the distance portion of the lens which allows for a wider peripheral or horizontal view. Now in this particular case the vertical section of the segments will not coincide if not fit properly. Use a design that allows flexibility in the corridor and adjust the corridor 1 for 1 for every mm you fit the fitting cross dissimilar. So a 18/20 seg should get a 14/16 corridor minimum, again this allows the patient to view binocularly through the same spot in the lens for reading. You may notice that neck pain and head tilts will disappear, your client maybe compensating for the disparity but that doesn't always mean they are happy about it. Good luck Daniel great blog BTW.

[DanLiv]

Thanks for all the pro advice! Glad to have most of what I was thinking confirmed.

Thanks for the compliment on the blog! I've been pretty lazy with it of late. Time to get back at it.

[Nobody77]

I have a few progressive patients with significantly dissimilar seg heights (3.0+ mm). I have resisted putting them into my favored variable seg digital designs assuming that would obviously create equally dissimilar corridor lengths. I wouldn't imagine one eye dropping 15mm to the reading area and the other having to drop 18mm to get the same reading would be very good. I have been keeping them in fixed designs or fudging the segs on variable designs I edge myself (e.g. if the actual heights are 18 and 21, I might order uncuts both at 19 or 20 so the corridors are equal but still edge at 18 and 21).

I feel in the technology should be there to take care of any fit situation, but I'm not 100% confident it is. Am I right to be resisting ordering dissimilar segs for variable designs, or does the software accommodate for such dissimilarities and ingeniously calculate a complete binocular vision solution?

Daniel,

what brand of lenses do you sell? Just to understand... Cause you ordering something, will not result in the manufacturer adjusting corridor lenght "in the majority of cases" (there is now exceptions). So if you order 19/18 height, the corridor lenght for "close to all lenses" will be the same R/L. If you yet cut them according to what you measured, the person will be fine.

But just consider this...

Take into account a facial asymmetry on one side and a wrong fitted frame on the other. Using a digital measurement device may give you strange height values in both cases (even if person is symmetric, but the frame fits wrong/is not adjusted beforehand). Now imagine you have a classic eye asymmetry, with the person being "quite OK symmetric looking". If you fit same height (for what ever reason) person will see thru not corresponding points in R/L lens, resulting in one eye seeing better than the other, so that is not really recommendable.

Now if you look at a client like this and check for phorias, you may end up with some quite extra vertical prisms just because of the eye assymetry - just consider, don't get off on me :D

Now imagine another situation, imagine a person with a higher Anisometropic prescription. Let's say +3.00 one eye and +5.50 the other (could be even less, but lets just imagine this). When looking into the distance, this person will probably not have "great issues". Yet when they start reading, the unwanted vertical prismatic effect comes into play. Due to this effect, the client is basically forced to look thru the lenses at different heights of the corridor. (longer the corridor, higher the vertical prismatic effect the client has to fight, the easier tired they get and the more problems they may experience while reading)

Now with symmetric corridor designs, this results in one eye looking thru the "sweet spot" in the lens (the proper spot) and the other slightly below or above the corresponding sweet spot on the other lens. Result? One image is more clear than the other, which means, more blurry eye starts to accommodate (but since eyes can't accommodate independently, the eye that previously was clear is not so clear anymore - Retinal Rivalry).

To fix this issue, you could create lenses (and one company does does have such a lens *cought* Hoya *cough* :D ) , which according to your measurements (prescription, centration, fitting etc.) will have their corridor lenghts along with R/L lens designs adjusted to the anisometropic prescription etc. Now imagine if you order something and edge something different then... You in fact would be already providing the wrong input for the calculation itself with your order.

Hope this doesn't sound in any way offensive :D It sure would not be my intention :) As I said "Just consider" :)

I think you know what I am trying to point out here :)

Cheers :)