Why is this working?

[Trip]

During the last few months I have had two antimetropic Rx’s that were originally unsuccessfully fitted with progressive lenses. The complaints were both centered on discomfort at near. One had been wearing progressives before, and the other was unknown if a new progressive wearer or not. Both cases had seen moderate Rx changes that had increased the amount of antimetropia.

The Rx for the first case:
R) -0.75 -1.00 X036 PD 31.9 seg hgt 22 2.50 add
L) +1.75 -1.25 X095 PD 32.5 seg hgt 22 2.50 add

After a review with the optician my first recommendation was to redo with a slab off. There was more than 3 diopters of power difference between the lenses and we would expect to see more than 3.75 diopters of prism imbalance at near. However the optician wanted me to come back with another option first before reverting to the slab. The pt. had not needed a slab before (about 0.75 less power difference in the 90) and it was felt the pt. would be put off by the cosmetic detraction of the slab.
With the slab option removed I was not certain that I could offer anything that would improve the pt.’s discomfort. We reviewed the pt.‘s previous pair which had the BC’s balanced and a split seg hgt, the R fit 1mm lower than the left. We agreed to balance the BC’s in the redo pair, however I did not think the 1 diopter change in front curve was going to help much with the issues at near. After a day or so for review I reached out to consult with the optician and suggested we run the R eye in a short corridor and the L eye to remain in a normal corridor length. This was done, dispensed and somewhat to my surprise relieved the pt. of their discomfort.

The Rx for the second case:
R) -1.25 -1.25 X094 PD 30.0 seg hgt 22 2.00 add
L) +1.25 -1.25 X077 PD 30.0 seg hgt 22 2.00 add

This came out of a separate office somewhat later and originally came to me to address the magnification differences between the two lenses. This was thought to be the primary driver for the non-adapt to the original progressives. However after some discussion with the optician it suggested that the pt.’s issues were more problematic at near vs. distance. I offered the same solution (along with aniso of the BC’s), a short corridor on the R and normal corridor on the L. This also was a successful dispense.

Both orders were re-run with full back side progressives but without any nod to POW.

The question then:
Without relieving the prismatic imbalance at near why is this working?
- Is binocularity still possible in this configuration?
- Is suppression and some form of monovision taking effect?

[HarryChiling]

During the last few months I have had two antimetropic Rx’s that were originally unsuccessfully fitted with progressive lenses. The complaints were both centered on discomfort at near. One had been wearing progressives before, and the other was unknown if a new progressive wearer or not. Both cases had seen moderate Rx changes that had increased the amount of antimetropia.

The Rx for the first case:
R) -0.75 -1.00 X036 PD 31.9 seg hgt 22 2.50 add
L) +1.75 -1.25 X095 PD 32.5 seg hgt 22 2.50 add

After a review with the optician my first recommendation was to redo with a slab off. There was more than 3 diopters of power difference between the lenses and we would expect to see more than 3.75 diopters of prism imbalance at near. However the optician wanted me to come back with another option first before reverting to the slab. The pt. had not needed a slab before (about 0.75 less power difference in the 90) and it was felt the pt. would be put off by the cosmetic detraction of the slab.
With the slab option removed I was not certain that I could offer anything that would improve the pt.’s discomfort. We reviewed the pt.‘s previous pair which had the BC’s balanced and a split seg hgt, the R fit 1mm lower than the left. We agreed to balance the BC’s in the redo pair, however I did not think the 1 diopter change in front curve was going to help much with the issues at near. After a day or so for review I reached out to consult with the optician and suggested we run the R eye in a short corridor and the L eye to remain in a normal corridor length. This was done, dispensed and somewhat to my surprise relieved the pt. of their discomfort.

The Rx for the second case:
R) -1.25 -1.25 X094 PD 30.0 seg hgt 22 2.00 add
L) +1.25 -1.25 X077 PD 30.0 seg hgt 22 2.00 add

This came out of a separate office somewhat later and originally came to me to address the magnification differences between the two lenses. This was thought to be the primary driver for the non-adapt to the original progressives. However after some discussion with the optician it suggested that the pt.’s issues were more problematic at near vs. distance. I offered the same solution (along with aniso of the BC’s), a short corridor on the R and normal corridor on the L. This also was a successful dispense.

Both orders were re-run with full back side progressives but without any nod to POW.

The question then:
Without relieving the prismatic imbalance at near why is this working?
- Is binocularity still possible in this configuration?
- Is suppression and some form of monovision taking effect?

It's like using a round seg and a FT28 to offset some of the prismatic difference. With the short corridor lens the power ramps up faster causing the prism to ramp up faster. You may have the patient viewing through the short corridor at 100% of the add power and the traditional corridor he's at 85% of the add power and the brain is OK with that. I like to use more comfortable in a scenario like this because the vision provided is probably more comfortable then NO glasses or the older ones so it is perceived as a success. My guess is that things could be made sharper and clearer with a slab or other nuances but your cosmetic trade offs are going to detract from the experience and negate any gains. Great job with your new super powers.

[leeopt]

It may not always work... prism thining of PAL's...someone on here could explain. Just reading on my lunch break.

[Trip]

It's like using a round seg and a FT28 to offset some of the prismatic difference. With the short corridor lens the power ramps up faster causing the prism to ramp up faster.

Yes, one of the things that I had also considered with the difference in corridor length. Considering a generic 3mm difference in where the full add power takes effect I thought there may be a modest prism offset tied to the add power. However I was not able to fully verify through standard lensometer measurement. I picked up about a 0.25 diopter on the last pair ran with the 2.00 add. This result being a bit less than half of what I was expecting and too small to appreciable negate the existing VI.

Thanks for the response.

[HarryChiling]

Yes, one of the things that I had also considered with the difference in corridor length. Considering a generic 3mm difference in where the full add power takes effect I thought there may be a modest prism offset tied to the add power. However I was not able to fully verify through standard lensometer measurement. I picked up about a 0.25 diopter on the last pair ran with the 2.00 add. This result being a bit less than half of what I was expecting and too small to appreciable negate the existing VI.

Thanks for the response.

I'll do you one better check out the attached spreadsheet, I use minkwitz therom to determine power down the corridor and pluged in your first patients variables. In reality the power increases quadratically in most progressive lens designs but minkwitz provides a good linear fit. You'll notice by reducing the corridor in one lens, in the case you provided; the lens exhibits less prismatic difference down the corridor. Once the power stabalizes you loose the benefits of the differential fit and the difference jumps back to a normalized fit, however chances are the patient will adapt to the position of the short corridor view over viewing further down to the regular corridor where the effect diminishes. In essence you stop the clock so Cinderella never has to leave the ball.

In theory you are weighting the decentration variable in the prentices formula.

You can play with the numbers in the spreadsheet and the graph will adjust along with the numbers. If you are interested in the work look underneath the graph. ; )

[HarryChiling]

New sheet comparing a short corridor, regular corridor and uneven corridors. Look at the depression created on the graph, that's your ah-ha moment. Increase the difference and watch the depression, which correlates to reduced prismatic difference.

I used to fit this way a lot but got so much flak I just stopped telling people I did it and eventually stopped doing it. If a patient goes somewhere else most opticians will try and convince them something is wrong, because they don't know what they don't know. Hopefully this spread sheet will illustrate the idea if needed.

[Trip]

I'll do you one better check out the attached spreadsheet

One better uh ….. , “chuckle” ….. appears to be a tad more than that.

Played with the last file a bit … thanks for sharing, … for me a wonderful illustration of the prism differences between the progression corridors.