Bi-Concave vs. Myodisk

[Pete Hanlin]

This morning, I had a very pleasant German woman visit our office. Her Rx is approximately -23.00 OU. Her problem was double vision that becomes progressively more pronounced as she moves closer to objects (especially after wearing the glasses for awhile).

I've seen this patient before, and I know she has realistic expectations of her eyewear. Checking her p.d., if find it to be 67.5 (which is consistent with the new eyewear). Her previous eyewear, however, had a p.d. of 62. I figure she had quite a bit of base out prism before, and she misses it (since base out would help with reading and the eyes have a greater reserve for convergence anyway). Putting 5D of b.o. prism seems to help a lot, so that's what we're going to try (by means of using a p.d. of 64).

Anyway, all this to ask my real question. What are the advantages of biconcave vs. a myodisk for this patient? I would imagine there would be optical disadvantages to having a negative base curve. The biconcave design seems to work well for her, I'm just curious (her previous lenses were myodisks, which also worked just fine).

Pete

[David Wilson]

Hi Pete,
With regard to the issue of the curves, if we follow the recomendations of the Tscherning ellipses you'll find that the shallower Ostwalt arm would recommend a back surface of about -23D, for distance vision, for your -23D Rx (depending on the material and assuming spherical curves, rather than aspherical). So your myodisc would fit this case. The steeper Wollaston arm of the ellipse would suggest a back surface of
-27D for distance vision, giving a concave front surface. So either of your lens forms will eliminate oblique astigmatism. At such a high power the two arms of the ellipse almost coincide. In low powers this is not the case, with the shallower Ostwalt arm being preferred by most manufacturers. That is until Sola, rather cleverly, released a lens (Enigma) which I assume, is based on the steeper Wollaston curve, allowing them to create that fashionable wrap-around style without compromising the optics (at least, without exacerbating oblique astigmatism).
Having said all that, this poor lady certainly has a lot to put up with and I wouldn't blame her for being fussy.
Regards
David

[Darryl Meister]

Hi Pete,

A couple of things for you to consider...

Baes out prism, in general, is easier for the eyes to adapt to since the eyes have greater positional fusional vergence reserves. However, base in prism would probably help more at near, since it reduces the amount of convergence the eyes need to exert. (Changes in horizontal prism at near may have some effect on accommodation.)

I am assuming that you are referring to full-field lens designs when you say "bi-concave," since myodisc designs are generally bi-concave designs, as well. In particular, the cylinder power has to be ground upon the front surface with a myodisc, since the aperture would otherwise be oval in shape -- not circular. As David has pointed out, there are certainly some optical consequences to switching between designs with different base curves. However, the biggest consideration between these two designs is mostly mechanical. Myodiscs are considerably thinner and lighter than full-field designs, like bi-concaves, but have a reduced field of view compared to full-field designs and "look funny" when viewed from the front since they have an aperture. A nice alternative is the Younger "blended myodisc" design, which gives the thickness reduction of a myodisc with the cosmetics of a full-field design. (Myodisc is a bit of a misnomer for this product, since it is really a blended minus-lenticular design -- with either a concave or a convex carrier -- not a myodisc.)

Best regards,
Darryl

[chip anderson]

I seem to remember from the ancient dark regions of my mind that a Myodisc is ground on the back to reduce thickness and the optics are no different in the field (optics) than those of a minus lens. A minus lenticular is ground on the front for the same reason, same result. Myodisc has flatter curve on it's periperal posterior outside the optical "bowl" Minus lenticular has a steeper curve ground on the peripheral anterior surface for the same reason, same optical non~effect. Bi~concave lenses are as the name implies: concave on both sides to keep the posterior from being so steep as to be peripherally un-usable. This does have an effect on the optics and somewhat widens the field of view.

Chip (Who forgot a lot of this because it didn't make much difference to a contact lens mechanic, but this is what I vaguely recall) Anderson

[Jackie L]

I have had great results fitting the blended myodisk to extremely nearsighted people. Not the most technical post, but experience speaks.

[Alan W]

I am inclined to use the blended myodisk / minus lenticular configuration because it is more manageble from the edge thickness standpoint. While the field may have some limitations, most often in the 42mm range, with frame styles smaller these days, it seems less of a problem. I find it easier to control vertex distance, also. Of course, in every circumstance, knowing what the refracting vertex was and the final fitting vertex is to be is essential.

[Darryl Meister]

Generally, the aperture (or "bowl") of a myodisc and minus-lenticular is ground upon the back surface. The bowl contains the strongest surface power, so this makes sense optically. It would also look odd having the bowl face observers.

Myodiscs generally have two near-plano surfaces, like a hockey puck. A high-minus bowl is ground into the back surface, creating a lens with basically a plano carrier curve. Minus-lenticulars, on the other hand, generally have convex carriers -- creating basically a plus lens with the high-minus bowl ground into the convex surface. (The blended Younger product is available with either a convex or concave carrier curve, though.)

Anyone who has ever fit a Younger blended myodisc or minus-lenticular has probably realized that decentering the lens creates the opposite problem that decentering a full-field minus lens does. Decentering a full-field minus design decreases nasal edge thickness, while decentering a minus-lenticular design actually increases nasal edge thickness. This can be a hassle with adjustable nose pad/guard arms. I've suggested grinding the carrier curves of these designs with base out prism to circumvent this problem.

Best regards,
Darryl