Navigating the many lens materials now available is made much easier when you use the common denominator of standard plastic to chart your course. This article offers a quick and easy method to locate the best material-of-choice for every patient. A two-digit multiplier is used to guide you when filling patient Rxs within the +10 to -10 diopter range. This simple and clinically practical technique will help you achieve patient-pleasing eyewear designs every time.
As a consultant for a large lab, the eyewear problems that I encounter have usually been screened by highly experienced prescribers and dispensers. Issues such as regret about the frame selected, cost and problems that can be resolved by simple re-measurement or realignment have typically been addressed before the questions reach me.
Several years ago I realized the single most common “not-happy-with-new-eyewear” problem that reaches my desk is related to an inappropriate choice of lens material. In fact, this led me to suggest an easy-to-use benchmarking to help practitioners navigate through the many indices that are available today. For a detailed discussion of this, please refer to “A Rosetta Stone for Eyecare, Decoding the Issues of Index,” which appeared in the March, 2007 issue of 20/20. Below we delve into how using a Curve Variation Factor (CVF) can help you decide whether increasing the index is a good choice for your patients.
Why Change the Index
The most common beliefs for moving a patient to a higher-index material include the lenses will be thinner, the lenses will be lighter, the lenses will be less curved and the lenses will perform better optically. The thinner, lighter and less curvature reasons are based on physics and fact. The myth that higher-index lenses perform better optically probably persists because of their relative newness to the market,their status as a “premium product” and their higher cost. The most patient-troubling aspects of higher index are related to high reflectance and low Abbe values (i.e. greater chromatic aberration). Fortunately the portion of the reduced optical performance caused by higher reflectance is much reduced by AR, so practitioners who use AR encounter relatively fewer problems with increased index.
Reasons for avoiding higher-index materials can include: More problems with night driving, more veiling glare during daylight hours, reduced lens performance related to low Abbe values and higher cost (especially when there is little or no patient-perceptible improvement in either appearance or performance).
When the power of a lens is not the same for all colors of light, the lens has chromatic aberration. The Abbe value tells how much chromatic aberration is present. A high Abbe value means less chromatic aberration and a low Abbe value means more is present. Chromatic aberration causes images formed by white light (multi-wavelength light) to be more (i.e. lower Abbe) or less (i.e. higher Abbe) out of focus. When the patient is looking through the optical center of the lens, you can think of the red end of the spectrum and of the blue end of the spectrum as being slightly “underfocused” or “overfocused” relative to the yellow-green bling when the line of sight passes through the optical center of the lens because the slightly out-of-focus wavelengths are centered on the yellow-green, in-focus image. When prism is introduced the situation becomes more troublesome because the various color components are shifting differing amounts to the side or up or down, causing color fringes to become visible.
Patients, especially those in the intermediate prescription ranges who are accustomed to using peripheral areas of their lenses, tend to be more susceptible to problems of low Abbe values. For these patients especially, try to use the smallest possible eye size and avoid large amounts of decentration if possible when you must use a higher-index material. Patients with prism prescriptions are also more troubled by low Abbe values because the multi-wavelength images are shifted further along the base-apex line of the prism. There is no practical way to eliminate chromatic aberration in spectacle lenses, but using AR makes patients more tolerant of the problem.
The issue of increasing the index is a more slippery slope. If the eyesize is about the same and if the prescription is not greatly changed, the patient can compare the old lenses with the new. The question is, will the new lenses be perceptibly thinner and lighter than the old ones. Even more important, is the difference of a magnitude the patient will appreciate and is that difference a fair value considering the additional cost?
The simple multipliers (CVFs) presented in the “Rosetta Stone for Eyecare” are your key to finding your way to the best material choice for your patient. The multipliers for the most common indices are given in Fig. 1. For clinical purposes you can simply multiply the power in the strongest meridian of the patient’s Rx by the multiplier for the material you are considering. You then easily compare the power in the strongest meridian in standard plastic with the result of your multiplication and use your clinical judgment to decide if the change to a higher index would be meaningful for your patient.
Example 1: Your patient’s prescription is -8.00 -2.00 x 090 in both eyes. The power in the strongest meridian in this prescription is -10.00. You have managed to find a frame that is cosmetically and functionally acceptable. Its size is 48-18, so the frame PD is 66mm. Your patient’s PD is 65mm so you will only have 0.5mm decentration in each eye. The mounting line falls at the base of the patient’s pupils in the straight ahead, level gaze position. With minimal decentration and an MRP placement just halfway from the top to the bottom of the lens you have an eyewear design that is advantageous for this prescription. The question now is whether you should opt for reduced optical performance of a mid- or high- index lens to further reduce thickness, curvature and weight of the finished lenses. You must also select the amount to increase the index.
Since you are very familiar with standard plastic you have a pretty good idea of how thick a 48mm lens would be in a 1.49 material. If you consider poly for this patient you would multiply the -10.00 by .85. The result, -8.50, is the power of a standard plastic lens that would have the same volume and thickness as your patient’s -10.00. Better yet, poly can be surfaced 1.0mm thinner all the way across, meaning that your edge thickness would be 1.0mm thinner than a -8.50 in standard plastic. If you consider 1.60, the multiplier is .83, and the resultant lens would be the equivalent of a -8.30 in standard plastic. If you go to 1.70 the multiplier is .71 so the resultant would be close to the curvature and volume of a -7.00 in a standard plastic.
For most patients you would not want to have the 7.8mm edge thicknesses of 1.49 material, but would poly, at about a 5.7mm edge thickness, be enough of a thickness reduction? A 1.70 index lens would give about a 4.9mm edge, but of course the higher the index, the greater the cost and the greater the reduction in optical performance. The edge thicknesses above assume a 2.0 CT for standard plastic and a 1.0 CT for poly, 1.60 and 1.70 materials.
Example 2: If the patient’s strongest meridian is only -5.00 instead of -10.00, going to a poly (.85 x -5.00) would give a resultant of -4.25. Even with the additional reduction of 1mm due to poly’s break resistance, do you think your patient would perceive and appreciate the equivalent of a 0.75 drop in power?
In the case of a larger eyesize the reduction in thickness yielded by going to a higher-index material is naturally greater. On the other hand the chromatic aberration problems related to a low Abbe value is more troubling to patients who are eye-turners rather than head-turners (i.e. usually those who have lower rather than higher Rxs) and the problems of a low Abbe are more apparent to patients who can turn their line-of-sight further from the distance MRP (e.g. when ≥ 2mm of decentration is needed in each lens).
Patient tolerance is hard to quantify, but every successful dispenser develops a sixth sense about it. In the exam room highly sensitive (i.e. low tolerance) patients may be acutely aware of 0.12 diopters of blur, or a shift of a very few degrees in a cylinder of as little as 0.50 diopter. They may be excruciatingly exact in trying to perform even the simplest of subjective tests. Sometimes this is called the “Princess and the Pea” phenomenon. There are surely combinations of physiological and psychological reasons that cause some patients to have low tolerance, while others can tolerate or adapt to a surprisingly wide range of power, magnification, transmittance and weight combinations.
If careful frame selection eliminates thickness and weight sufficiently, you should use that to your patient’s advantage and avoid increasing the index of their lenses. Aspheric lenses may be an alternative to a large increase in index for a patient who wants thinner lenses since using an aspheric design sometimes allows a more moderate increase in index. Increasing the index should be a court-of-last resort decision once you have explored other alternatives. Transforming lens prescriptions into finished eyewear requires science, art and judgment. Today’s excellent higher index materials offer advantages to patients above +4.50 to +5.00 Rxs, especially with large lens sizes, but they should be used with careful consideration if you want the eyewear you design to be truly patient-pleasing.
—Palmer R. Cook, OD, is director of professional education for Diversified Ophthalmics in Cincinnati.