|Ophthonix’s iZon SL Poly Lenses are offered in single vision and PALs.|
Ensure that free-form lenses provide the best vision for your patients by correctly inputting all fitting measurements like position-of-wear.
Suppose you are renovating your home and you’ve hired a carpenter to do the pricey crown molding work. You know the best result will depend upon the most accurate measurements (measure thrice, cut once), along with the most skillful cutting, angling, and fitting of all the pieces together. Just a few inaccurate shortcuts can make the entire job look amateurish. It may be marginally acceptable, but it’s not what you paid for, and the molding installation did not do justice to the high-end molding product you purchased.
Ophthalmic lenses correspond to this analogy. You can use the best materials, the newest designs, and the latest surfacing methods, but if you don’t take detailed and accurate fitting measurements, you won’t give patients the full benefit of today’s premium free-form lenses. With these lenses offering 1/100th diopter accuracy and compensations for things like vertex distance and frame tilt, it’s the little things in free-form lenses that count these days.
THE POW FACTOR
Position-of-wear (POW) is a term that is becoming popular with premium free-form lenses. These are the measurements taken that add to the lenses’ precision and have the potential to give the wearer superior optical and visual results. POW measurements take into account the wearer’s head posture, pantoscopic tilt of the frame, vertex distance, frame face-form (wrap), and frame size and shape. Each one contributes a little to the end result, but when combined, they can add up to some major vision benefits.
Some of the lenses that fall into this category include Zeiss Individual™ (Carl Zeiss Vision Inc.), Autograph II® with As-Worn Technology™ (Shamir Insight, Inc.), Identity (ProFit Optix), Supercede (Seiko Optical Pro-ducts of America, Inc.), and the iZon® single vision and progressive lenses (Ophthonix, Inc.). Varilux Ipseo IV™ customized progressive lenses (Essilor of America, Inc.) also fall into this POW category.
When discussing frame and lens tilt, the first thing that comes to my mind is Martin’s Formula (dig
|POW measurements add to the precision found in Seiko’s Supercede lenses.|
deep into your memory, you knew this once!), which you can use to calculate the power errors induced by inaccurately tilting a lens in front of an eye. There are two types of tilt a frame can exhibit: pantoscopic and face-form. Both can add unwanted power errors to a prescription.
Some opticians and optometrists know it as “wrap,” but the proper name for this effect is face-form tilt. Face-form tilt is the horizontal angle the lenses make with the patient’s face plane, which is a result of how much wrapping effect the patient’s frame has incorporated into it. Neutral face-form means the frame front is straight across when viewed from above. Positive face-form occurs when the frame is curved around the face, as in a wrap design. Negative face-form is the exact opposite—the endpieces are bent in front of the bridge (not ophthalmically useful or cosmetically appealing).
Here’s an example of how this works. With an Rx of +1.50D, you give it 15º of positive face-form wrap. If you were concerned about the little things, you would actually order +1.47 -0.10 * 90. That would not be practical when using conventionally surfaced lenses, but it can be done with digitally surfaced lenses. Another example is using single vision +9.00D lenses and incorporating the same 15º of face-form as in the first example. This time, the Rx you should order would be +8.80 -0.59 * 90. Similar results occur with minus lenses.
Most eyecare professionals (ECPs) use the term pantoscopic tilt for vertically tilted lenses, although there are actually three versions of vertical tilt: pantoscopic, the lower rim is titled in toward the face; orthoscopic, which has no vertical tilt; and retroscopic, where the lower rim is tilted further away from the face than the upper rim. According to Martin’s Formula, for every 2º of incorrect pantoscopic tilt, the optical center should be lowered by 1mm. But similar to face-form tilt, there are power errors at work here.
Let’s go back and look at those single vision +1.50D lenses. With properly adjusted lenses, you would simply order a +1.50D. With 15º of excessive pantoscopic tilt, you would order +1.47 -0.10 * 180. And what about that +9.00D lens? Try a +8.80 -0.59 *180 after the tilt compensation. Again, you are not dealing with huge power errors here, but add the pantoscopic and face-form errors to the other lens errors and things are starting to add up.
|Shamir Autograph II with As-Worn Technology is an example of a lens that falls into the POW category.|
Opticians used to compensate for vertex distance routinely when they supplied high plus powered post-cataract eyeglasses to correct aphakia, but with the advent of intraocular lens technology, measuring for frame fitting vertex has become somewhat of a lost art. Well, dust off that distometer. Small differences in low Rxs are not much of an issue, but as the power increases, so does the margin for error. That +9.00D lens refracted at 13mm might be fit at 9mm. If so, the power you should be ordering is +9.37D. Again, similar results occur with minus powers. Add this error to the ones above and you can see these “little things” are really adding up.
There are several methods for calculating these measurements. You can try with an old fashioned PD ruler and protractor, but frankly, they’re not very accurate. Shamir offers the Panorameter Kit to enable ECPs to measure vertex distance, pantoscopic tilt, and panoramic angle, and Carl Zeiss Vision has a similar kit. These devices are an improvement over the old PD stick and protractor. Even so, there are also electronic options that precisely handle these measurements. For example, Carl Zeiss Vision’s i.Terminal™ measures these values to within 0.1mm of accuracy.
Free-form technology is radically changing the lens industry and will probably be the way most lenses are designed and processed in the near future. You might have noticed that many ordering sites, like VisionWeb, Eyefinity, and Davis Vision, are asking for POW measurements for selected premium free-form lenses.
The information can be added, or not. If you don’t add the POW measurements, the computer will supply average values for those parameters. However, if you hone your skills, you can supply them accurately. This will result in correcting the little things that can add up to so much more for a patient’s vision. After all, it’s the “little things” that really count with free-form lenses.
Sharon Leonard is a licensed optician and contact lens practitioner in the Syracuse, NY area.
WHERE TO FIND IT
Carl Zeiss Vision Inc.
800-358-8258 • zeiss.com/lenses
Essilor of America, Inc.
800-542-5668 • essilorusa.com
866-996-7849 • profitoptix.com
877-FOR-IZON • izonlens.com
Seiko Optical Products of America, Inc.
800-235-5367 • seikoeyewear.com
Shamir Insight, Inc.
877-514-8330 • shamirlens.com