Bi-concave bifocal advisable?

[Andrew Weiss]

We had a patient come in for an exam last night wearing bi-concave single vision CR39 lenses. She also has reading glasses (didn't have those with her), and in conjunction with our lead OD, who examined her, she decided she is ready for a bifocal. Her Rx is -11.25 right and -12.50 left. She's originally from England, and the glasses she is wearing were made by an optician in London. Her new Rx is the same in distance and calls for a +2.00 add.

Her current lenses have a base of -2.00. My inclination was to put her in a bi-concave bifocal, seeing as she is used to that kind of front surface. The only lens blank available is for a round-seg, maximum seg diameter of 28mm. I advised the patient and wrote up the job. As we were closing up, the OD and I had a conversation, and he told me I was being far too cautious and "over-thinking" the situation, and advised we go with a FT28 with a plano base, considering that the FT is a superior lens.

As I was driving home, I also wondered why I'd gone with a bifocal to begin with. After all, all she has to do is move her glasses down her nose a smidge and she'll get her reading Rx, right?

So what do you think? Am I overthinking this? Am I being far too cautious in wanting to keep this patient in a bi-concave lens? What would you do?

Thanks for the input. :idea:

[Andrew Weiss]

Hey, folks, doesn't anyone have any ideas about this? I'm waiting for some responses before I talk with the patient again.

Thanks.

[CME4SPECS]

Are you sure you can even get a concave BC in FT 28? I've never heard of such a creature! Go with a plano BC Ft 28, I'm sure it'll be fine. If not go back to the 2 pairs.

[QDO1]

biconcave seems a bit extreeme for this RX. i would be thinking 1.67 c-seg from rodenstock, or 1.74 progressive from Nikon/essilor etc.

[SpecialT]

I personally think you are overthinking things a little, but you may not be wanting the opinion of an O.D. Change in base curve does matter but can be adapted to fairly easily as long as both lenses are about the same. I think your options open up a little when you go with a convex or flat front surface. I just don't see the benefits to the biconcave surface.

[Andrew Weiss]

Thanks SpecialT and QDO for the input. I appreciate it alot. I have to admit I don't see the advantage of a bi-concave surface in an Rx like this, but then again, I didn't make the lenses she's wearing. My concern was only whether it would be a relatively easy adjustment moving from a concave base to a flat base. I was probably being too conservative in recommending the bi-concave bifocal. You've helped my thinking process along here.

At this point I'd say I'll go with a FT28 (QDO: We can't get the Rodenstock CT28 1.67 in the US ), and in a 1.6 considering that she's wearing CR39 at the moment.

BTW, CME4SPECS, I didn't find a FT bi-concave, only a TK (round-seg) made by Air-O-Lite and only in CR39. Not a great choice if she can adapt to a flat base.

Thanks again, folks.

[chip anderson]

I think Essilor now has a 167 ST28.

[Andrew Weiss]

Thanks, Chip, I'll check that out. And thanks also for your input on this. The idea of a bi-concave Franklin was interesting -- back to the old days. ;)

[Robert Martellaro]

Andrew,

X-CEL makes a 1.55 index with a -3.25 BC.

To get a ballpark best form base curve take half of the sphere power and add +6.00. That would give a +.50 BC for the right and plano on the left.

Using a minus base curve will decrease the edge thickness but increase the oblique astigmatism. This is more of a concern with multifocals because the gaze is substantially off-axis at near. With SV lenses she could just tuck in the chin or raise the object higher (or both) to keep the near gaze on-axis, inducing less astigmatism.

Regards,