FT28 1.6 Seg Issues?

[Andrew Weiss]

We have a patient with a relatively high plus (+6.25 R, +7.50 L) for whom we made a pair of computer FT28s (add is +1.00). We used 1.6 high-index lenses. He got a large-ish frame, giving him a seg height of 21.5. He tells us that he has a slim ribbon of focus through the seg, giving him maybe 5-6 lines of type, and below that it blurs out. Any ideas what's going on? I suspect it has something to do with the power and resulting prism, but I'm not sure. Thanks for the help.

[Robert Martellaro]

Andrew,

I would check the seg area for waves and distortions, then I would check the base curve_a flatter than recommend BC will create off axis blurring. Clock the back curve_plano or convex is not acceptable. This might be aggravated by a low abbe number for the lens material in question. Check the vertical OC position_should be 3mm to 5mm below the pupil. Check for excessive panto and adjust as needed.

Hope this helps

Robert

[jediron1]

Sounds like you got a bad lens. I would do every thing Robert described. It could be prism with that much plus and where the oc is in a st.top 28. But from what you said I m betting that it's a bad lens, distoration and all.

[Andrew Weiss]

Thanks guys. The patient is due to come in Monday, and I'll let you know what happens.

This is a bit of a tangent, but have you noticed that you have to place the OC over the pupil vertically with high-index lens materials? I've noticed improved vision in SV lenses with higher Rxs in poly and 1.6, even with traditional spherical design. If this is true, does this pose an interesting dilemma in fitting a high-index bifocal on someone with an Rx like this who may prefer having his seg lower? Oddly, most of our patients seem to prefer having the seg 5mm below the lower lid -- a phenomenon I'd never encountered before working where I am now.

[grace angel]

I've noticed that OC ht placement can even affect whether or not a lens will cut out properly. For an example we had a surgeon with -900ou and after calculating decentration based on monocular pds and specifying OC ht placement at 7.5mm above the datum line of the frame, the lab sent us two different size lens blanks and claimed they were surfaced. We could see the injection molding marks in the sides of the blanks and they still insisted they were surfaced. The frame was a Lux and i called the company to verify the ED which was 65mm for that frame. One of the blanks was a 65mm and the other a 60mm. The 65 mm blank would still not cut out due to the vertical optical center placement being so high. I had calculated the ED and specified it to the lab at 68mm. Needless to say we had to send the lenses back and reorder them from another lab in 70mm blank sizes so they will cut out properly to the decentration calculated and OC ht specified for this client! You must underline or boldly tell the lab where you want the OC placement in reference to the datum line of the frame cause if you don't most of the time it will end up smack above (1or2mm) the flat top seg in these larger frame styles with deep B's sometimes causing prism issues. As technology gets better and better it will be of upmost importance that monocular pds for near and distance and monocular seg hts as well as OC placement be considered in all RX's!!!

[Andrew Weiss]

We use a lab that doesn't mind having us specify where to put the OC. So we're in luck on this.

At this point I'm routinely taking monocular PDs and monocular OCs on all plus Rxs over 3.00 and all minus Rxs over 5.00, and monocular seg heights on all line bi/trifocals and progressives. Glad to hear someone else finds it makes a difference in patient comfort.