L&T: RxPertise

Mar
2007

A Rosetta Stone For Eyecare

Decoding The Issues of Index

BY PALMER R. COOK, OD

Our patients expect to see well, look good and be comfortable in their new eyewear. They also expect, quite reasonably, to perceive a value equal to or greater than the cost of the eyewear. If you are a full-service provider, you have a responsibility to meet these four expectations. But sometimes things don’t go as planned and the disappointed patient may turn up on your doorstep. If you think about it, that’s probably the best of a bad situation. It gives you a chance to resolve the problem and set things right.

Unfortunately, many patients try to tolerate marginally satisfactory aspects of their eyewear experience. These patients are prone to tell friends, neighbors, co-workers and family members of their less than ideal experience. They are also likely to take their prescription “elsewhere to have it filled” the next time.

I regularly field questions from doctors and dispensers about patients who are distressed in one way or another about their new eyewear, and I have come to think of this as “eyewear distress syndrome.” This is not the same as “eyewear buyer’s remorse,” but sometimes the two are often almost inextricably intertwined. A major cause for this distress lies in the difficulty of selecting the lens material best suited for the prescription and the patient. Of all the eyewear design decisions you make one of the most confounding and frustrating is selecting the best lens material.

When we only needed to select either glass (clear, scratch-resistant, but twice-the-weight) or standard 1.498-index plastic (half-the- weight, more shatter resistant, but prone to scratch easily), the presentation to the patient was relatively easy and straightforward. Today there is a growing array of lens materials available and deciding which will best meet the needs of your patient can be a daunting and confusing task. After reading Professor Mo Jalie’s discussion of curve variation factors in Ophthalmic Lenses & Dispensing, I realized that this concept could be converted from a standard of glass to one based on plastic. Doing this conversion to standard index plastic (n = 1.498) makes sense because it’s the material with which U.S. dispensers have greatest familiarity. This conversion yields an easy-to-use “decoder,” truly a kind of Rosetta Stone that allows you to better apply your optical knowledge. The examples and “how to” discussion that follows will enable you to begin to make the best lens material decisions. As with any tool, this Rosetta Stone for Eyecare is not a stand-alone, single answer for every lens material decision. It provides a benchmark that can be used with your experience and professional judgment to achieve the best eyewear for each of your patients.

Although there are many root causes for “eyewear distress syndrome,” a preponderance of these calls involve an inappropriate choice of lens material. Practitioners who contact me are generally experienced, skilled and knowledgeable. They usually identify and deal with problems such as “unhappy with the frame I picked,” “issues of cost” and predictable “disappointment in best correctable vision” (e.g. macular degeneration or advancing cataracts) before they call. This makes my perspective on “eyewear distress syndrome” somewhat skewed, but be assured that a reconsideration of lens materials should be part of resolving eyewear distress problems. Playing the part of an optical Sherlock Holmes, a role every practitioner must assume from time to time, is always made easier by carefully considering the patient’s primary complaint about the eyewear. Some common complaints and how they relate to the issues of lens material include:

“I don’t like how I look in my new eyewear.” If the material is chosen with a refractive index that is too low, there is usually either too much lens thickness or the eyewear has been made inordinately small to minimize the low index/thickness and curvature issues. Of course, using a smaller eye size reduces the edge thickness of minus lenses and allows plus lenses to be ground thinner, but this reduction in size has its practical limits.

If the refractive index is too high, the cosmetic issue is primarily reflectance. As index increases, lenses reflect more light. Light reflecting from both the front and back surfaces toward anyone looking at the patient creates a veiling glare that obscures the most expressive part of the face—the eyes. For standard plastic (1.498), 3.97 percent of light falling on the front of the lens will be reflected. If the index is increased to 1.59 (poly), the reflectance is 5.15 percent, an increase of 29.7 percent. Using anti-reflective lenses is the obvious and easy answer for this problem. Regardless of the material chosen, reflectance and related veiling glare will be greatly reduced if lenses with high-quality, anti-reflective (AR) properties are used.

“I don’t see well in my new eyewear.” Not seeing well with new eyewear can be the result of power or axis errors, inappropriate base curves, incorrect placement of the MRPs (i.e. major reference point—the point that gives the correct prescription), internal or surface imperfections or even an incorrect prescription. When the cause for not seeing well is an inappropriate choice of lens material, the difficulties are usually related to increased reflectance, chromatic aberration or the combination of the two. The increase in reflectance that causes glare from the front surface of the patient’s lenses also causes ghost images when night driving, annoying reflections from the backs of the lenses and less clarity due to veiling glare from light reflected within the lenses. The night driving difficulties result from light that either passes through, or is reflected from, four interfaces of differing index. Clinicians should realize that the ghost image from a non-antireflective 1.58 lens is ultimately approximately 65 percent brighter than the ghost image from a non-AR, standard index lens. Is it any wonder that patients complain? Again, this difficulty is best dealt with by using high-quality AR lenses.

When lens power varies depending on the color (wavelength) of the light passing through it, chromatic aberration results. As patients look through the optical centers of their lenses, chromatic aberration is usually unnoticed. When the lines of sight pass through any point in the lens other than the optical center, the induced prism displaces the images formed by the red, orange, yellow, green and blue portions of the spectrum to a greater or lesser extent depending on the wavelength you consider and significant degradation of the retinal image can occur.

When looking one centimeter away from the center of a -5.00 lens, the patient experiences 5 prism diopters of image displacement if there is no chromatic aberration in the lens (i.e. all wavelengths are bent equally by the lens material). Chromatic aberration displaces images when the line of sight passes through the periphery of the lens according to Prentice’s Law. (Prentice’s Law states that the prismatic effect in prism diopters at any point in a lens is equal to the lens power times the distance in centimeters from the optical center (e.g. 1 centimeter from the center of a 1 diopter lens will cause a prismatic deviation of 1 prism diopter). If the power of the lens is greater for blue light and lesser for red light, then the prismatic effect for these wavelengths varies and the image is degraded by an “image spread” or “rainbow fringe” that can be seen best along image contours which are more or less perpendicular to the base-apex line of the induced prism. The width of the “rainbow fringe” naturally becomes greater as the line of sight strays further from the optical center of the lens.

Patient tolerance for this aberration varies. An Abbe value is assigned to lens materials to indicate how much chromatic aberration they will yield. (The Abbe value is equal to: (nD –1) / (nF – nC) where nD is the index of the material for YellowGreen, nF is the index for Blue and nC is the index for red.) A lens with an Abbe around 30 will give about twice as much image spread as one with an Abbe of about 60, and a lens with an Abbe of about 45 will give about 25 percent more image spread than one with an Abbe of about 60.



Patient complaints related to chromatic aberration can include blurring, color fringes or may simply be of a vague nature. Patients with prism corrections and those who tend to turn their eyes away from the optical center for viewing are more prone to be troubled by materials with low Abbe values. Be wary of using frames with large lens sizes, especially ones that require significant decentration of the Rx, because this gives your patient more opportunity to be troubled by chromatic aberration. Using AR lenses will not reduce chromatic aberration, but patients with AR lenses tend to be more tolerant of these problems.

“I’m not comfortable in my new eyewear.” If issues of visual discomfort are ignored, discomfort complaints are mostly related to incorrect frame alignment or result from the lenses being too heavy. Using lenses with a higher index (more light bending muscle) decreases lens volume because these lenses don’t have to be curved as much to bend light. If the volume of one lens is less than another, the weight of the first lens will naturally be lighter, provided the materials used for both lenses have the same specific gravity.

 Both specific gravity and index vary from one material to another, so knowing which material will give the least lens weight is not easily determined, especially in a clinical setting. Selecting a lens material of a higher index may yield a finished lens of less volume, but the differences may not be as great as you (and your patient) expect. A high-index plastic lens (1.66) in a -2.00 power will have roughly the same volume as a -1.50 standard plastic lens. Therefore if your patient needs a -2.00, and if you switch him to a 1.66 material, he will end up with a lens that is about the same volume and curvature as if he were given a -1.50 standard plastic lens but with a power of -2.00. Most patients would have difficulty detecting the thickness and weight difference between a -2.00 and a -1.50 standard plastic lens, but they will not have any problem recognizing that the cost of the higher index lens cost is greater. Problems of perceived value and buyer’s remorse can arise from situations of this sort. Not only that, decreased optical performance of the higher-index material can add fuel to the flames of perceived value and buyer’s remorse problems.

“My new eyewear just isn’t right.” Non-specific, vague and many-faceted complaints (e.g. these hurt my ears, I can’t see with them, they don’t look good on me, etc.) can indicate buyer’s remorse rooted in low perceived value. These patients may feel they did not receive benefits in fair proportion to the eyewear cost. Using more expensive lens materials is best accepted if the results are positive, “patient perceptible” and with few or no induced optical problems.

Fortunately there is a simple way to predict what lens volume and thickness will result when a different index is selected. Not only can volume and thickness be easily predicted, the relative weights of the lens for various materials can be estimated with good clinical accuracy for lens powers between + 10 diopters. To do this, a two-digit multiplier is used to give a comparable standard plastic lens equivalent. For example: all experienced clinicians in the U.S. have a good idea of how thick a -8.00 lens will be in standard plastic if they know the size of the lens. The real question is, what material should be selected if you want your patient to have a lens with the same volume and curvatures as a lower power standard plastic lens in the same size? And, compared to a lower power lens in standard plastic, what will the new lens weigh?

 

ROSETTA STONE FOR EYECARE
The Rosetta Stone, an antiquity discovered in Rosetta (now el-Rashid), Egypt in 1799, allowed us to decode then unreadable hieroglyphics by comparing a hieroglyphic text with a known language. There is a multiplier known as the curve variation factor (CVF), which reveals the power of a lens in one material that would have the same volume and curvature as a lens of another power in another material. Clinicians can easily use this multiplier as a Rosetta Stone for selecting the best lens material. For example, if your patient requires -10.00 lenses, you know that although a standard plastic lens would cost less, it would also be unnecessarily thick. You can certainly reassure your patient that, through the use of some other material, you can reduce both the thickness and possibly the weight of their lenses. You can also easily quote the added cost for this improvement, but it has been difficult to visualize and communicate how much improvement will result. The CVF enables you to tell your patient that going to polycarbonate, for example, will yield a lens with the needed -10.00 power but with approximately the curvature and thickness of a -8.50 standard plastic lens and with a weight that is at least 9 percent lighter than the -8.50 in standard plastic. Alternatively, if you consider using a 1.66 material, the CVF allows you to predict that the finished -10.00 lens will have the curvatures and volume approximately the same as a -7.60 standard plastic lens, and it will have a weight 2 percent heavier than the -7.60 standard plastic lens. By using the CVF, you can balance the increased cost, reflectance and Abbe (chromatic aberration) issues of higher-index material against the lower cost, thickness and optical advantages offered by standard plastic lenses. This CVF multiplier has great potential for helping you give the very best lens design advice to every one of your patients.

HOW IT WORKS FOR CURVES AND THICKNESS
Because of the widespread use of standard plastic lenses, nearly every clinician is conceptually grounded in their use. In other words, you have a pretty good idea of how a lens of about any power is going to look in standard plastic, so the approximations presented are relative to standard plastic. Also, the CVFs are rounded for simplicity of use and practicality. The formula is CVF = (1 – nstandard)/(1 – nnew). In this formula, nstandard represents whatever material you wish to use as your standard (in the case of this article 1.49 was used), and nnew is the index of whatever material you are considering using. (For a more thorough discussion of Curve Variation Factors refer to Professor Mo Jalie’s excellent book, Ophthalmic Lenses & Dispensing.) The formula is clinically accurate to up to about + 10.00 diopters. Any clinician can use this CVF formula to develop his or her own CVF chart and carry it to as many decimal places as they wish. It is also possible to use the formula to determine the CVF for new materials of indexes that are not currently available.

HOW IT WORKS FOR WEIGHT
To predict the weight of the finished, higherindex lens, you must first determine the equivalent standard plastic lens. The density of the higher-index material can be used to predict the final relative weight the lens will have. For example, if a 1.66 material (CVF =.76) with a specific gravity of 1.35 is being considered, then the finished -10.00 lens will have the curves and volume equal to a standard plastic lens with a -7.60 power. The weight of the finished 1.60 lens will be about 2 percent heavier than the -7.60 standard plastic lens because the ratio of the density of the 1.66 (i.e. 1.35) to the density of standard plastic (i.e. 1.32) is 1.02.

USING CVF IN YOUR OFFICE
Patients want perceptible differences when they purchase new eyewear and they want any trade-offs in decreased optical performance to be compensated with significantly thinner and lighter lenses. It is your professional judgments0 that must lead to the best recommendations. A -4.00 Rx that is fabricated in 1.60 material will have about the same thickness and flatter curves as a -3.25 standard plastic lens—a difference that may only be discernable by an expert. The light reflected from the front surface of the 1.60 material will be 34 percent brighter than the light reflected from the standard plastic. Ghost images during night driving will be 175 percent brighter with the 1.60 material and there will more chromatic aberration because of the decrease in Abbe from 58 to 43.

An easy way to demonstrate how changing index will affect finished lens thickness and curvature is to have your lab make up a set of plus and minus standard plastic lenses in some suitable diameter. For a patient with a -10.00 Rx, you can show them a -10.00 standard plastic lens (at perhaps 55mm diameter) and a -8.50 standard plastic lens of the same diameter if you want to show how going to polycarbonate will affect the curves and thickness of their lenses. If they have a -5.00 Rx and if you are considering poly, you can show them a - 5.00 and a -4.25 in standard plastic to demonstrate the same relative comparison. Because a minus lens made of polycarbonate or PPG’s Trivex material is usually surfaced .8mm thinner than other materials, the weight of a disc 50mm in diameter and .8mm in thickness can be deducted from the weight of a 50mm round lens in a minus power if you want to compensate for the thinner centers. This reduces the weight by 1.88 grams for poly or 1.74 grams lighter for Trivex. Plus lens are usually surfaced .2 to .3mm thinner for these materials, the weight savings due to thinning is somewhat less in plus powers. Since the surface areas of most lenses today are significantly less than that of a 50mm round lens this thickness and volume difference has been ignored for the purpose of simplifying this presentation.

A common error is to try to make the thinnest, flattest lenses possible for every patient regardless of their prescription. This often leads to patient frustration, adaptation problems and disappointment, especially in the mid-range and lower powers. This is because ghost images are especially troublesome in these powers. Also, chromatic aberration that was negligible with a high Abbe value may cause image degradation when areas away from the center of the lens are used. It is important to recognize that patients with high prescriptions tend to view through the centers of their lenses, but in mid-range and lower powers, off-center viewing is more common.

THE GOOD NEWS
The use of today’s top-quality AR products greatly reduces the optical issues of the increased reflectance of all higherindex materials. In addition, although chromatic aberration is an attribute of each lens material and cannot be reduced, patients are better able to tolerate its effects when the reflectance problems are reduced. It is important to keep in mind that AR lenses improve the performance of all lens materials, but the need for them becomes imperative when higherindex materials are used.

A WORD ABOUT ASPHERICS
The use of aspheric base curves has long been known to improve the performance of lenses in the higher plus powers. However, the use of an aspheric base curve will neither significantly reduce nor eliminate chromatic aberration, which tends to increase as index increases. Because reflectance increases in direct proportion as index increases, the use of an aspheric base curve will not improve most reflectance related problems. Using an aspheric lens design can be a good alternative to increasing index, especially in the plus powers, if you want thinner and flatter lenses.

FRAME SELECTION CAN BE CRITICAL
Using frames with wider bridges and smaller eyesizes to keep the frame PD as close as possible to the anatomical PD is another way of allowing lower index materials to be used. Frames with wider eyewires can also decrease lens size and may help hide the edge thickness of minus lenses. Turnback temples can help give the eyewear the appearance of needed overall width while keeping the A measurement of the eyewire and the frame PD smaller. The bottom line is to use all the frame design techniques you can first and only then increase the index if you must.  LT


Palmer R. Cook, OD is director of professional education for Diversified Ophthalmics in Cincinnati.

 

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