Photograph by NED MATURA
By Palmer R. Cook, OD
As the stroke of midnight approached, Cinderella fled, leaving behind her glass slipper. The prince then searches the kingdom for the one foot that perfectly matches that dainty slipper. Apparently it never occurred to the prince that if the slipper/foot match were perfect, the slipper wouldn’t have fallen off in the first place. So the premise of his search was flawed. Apparently he should have been looking for a slipper/foot match that was a bit shy of ideal.
When trying to get an ideal lens/frame match, you are dealing with factors that are fixed (Givens), and others that you can control (Variables). An understanding of both Givens and Variables requires careful observation and knowledge of the extent and limits of ophthalmic technology.
GIVENS
In the film industry, eyewear narrower than the facial width is used to give actors a nerdy look, and eyewear wider than the facial width gives child actors a helpless look. In just about any other case, eyewear is fitted that is about as wide as the patient’s face. That’s a given that applies to eyewear purchasers every day. The patient’s distance pd and lens prescription are also givens. Similarly, anatomical features such as unequal monocular-pds, off-center noses, ear placement, and brow and cheek anatomy are all givens with which dispensers must deal. Other important givens include the patient’s visual demands and expectations.
VARIABLES
The 1963 Outer Limits science fiction series opened with an authoritative narration that included, “We will control the horizontal. We will control the vertical.” Indeed, when designing eyewear you must similarly control the horizontal (width), the vertical (depth) and a whole lot more. If you think that your control is limited to the frame size you order and to the way you bend the frame, your thinking is too limited. The pantoscopic angle, the pantoscopic tilt, the vertex distance (to some extent), the location of the mounting line relative to the pupillary center, the MRP height, the seg height, the wrap, the optical properties of the lenses, as well as the near pd and anterior/posterior positioning of the lens should be controlled by you.
START WITH THE LENSES
Although most patients will want to start with frame selection, it makes a great deal more sense to begin with preliminary decisions about the lenses. For most lay people, lenses all look alike. “Lenses are thick or thin, and they have lines or they don’t” is a common level of lens sophistication. Your level of knowledge is much higher, and by putting the lens discussion first, it will be easier for you to guide the whole “frame/lens matching” process. Asking new patients, “What didn’t you like about your old lenses?” can be very revealing: “They never were right,” “They seem to be very thick,” “They were hard to get used too,” and “I wish they weren’t so heavy.” The answers to these and similar questions can be loaded with clues.
CLUES
“They never were right,” is a statement of general dissatisfaction that usually deserves gentle probing (e.g., “In what way?”). A misaligned frame can be the cause for prolonged difficulties and complaints. Placement of the MRPs or Fitting Crosses inappropriately can also cause such complaints. A third cause could be the use of a too-high index. Simply changing from standard plastic to poly increases the veiling of the retinal image by about 60 percent, and a low Abbe value (compared to earlier lenses) reduces the size of the “sweet spot” for long viewing distances.
Myopes often complain about thickness because of the lateral edges. If your first thought is to jump to a higher index to reduce thickness, think again. Although increasing the index gives thinner edges, it also decreases lens performance for both myopes and hyperopes. As a rule, only increase index to reduce thickness as a last resort. Going to an aspheric design is helpful especially in plus. For all patients, compare the frame monocular pds to the patient’s monocular pd’s. Generally, frame monocular pds should be no more than 3 mm greater than the patient’s monocular pds. Reducing the lens size and rounding the lens shape can also be helpful (Fig. 1). The unsightly facial inset of high minus powers is also reduced when the lens is rounded and made smaller.
If the patient recalls a difficult adaptation, look for warped lenses and MRPs or Fitting Crosses that are not well placed. If the problem relates to PALs, the patient’s near pds may not be aligned well with the Near Reference Points (NRPs) of the lenses. This problem is often related to the pantoscopic tilt of the eyewear (Fig. 2).
If weight is the issue, you can reduce the lens volume, select a lens with lower density, improve the bridge fit, or use a combination of all three approaches. Lens volume will be reduced if a digital aspheric design is used instead of a corrected curve design. Digital aspheric/atoric designs can also increase the size of the sweet spot in cases of higher cylinders (Fig. 3). Using a smaller eye size also reduces the lens volume. But reducing the A measurement may not be very effective if the B measurement is increased.
Trilogy has the lowest density of all ophthalmic lens materials (1.11). Lenses made of this material are lighter than lenses of the same shape, thickness and decentration in any other material from about +12D to about -12D. In materials other than standard plastic, Trivex lenses have a somewhat greater thickness. The weight differences are smaller in the higher powers, and the thickness differences are smaller in the lower powers.
THE PRESCRIPTION
Carefully exam the Rx and measure the patient’s monocular-pds before beginning the frame selection. If the patient’s nose is off-center, the bridge must be compensated using wedged or flat build-up pads or adding adjustable pads. In these cases, the monocular pds must be re-measured after the frame is fitted. Fortunately, most frames can accommodate powers under about 3 diopters if the needed size is available. For higher powers, the lens/frame match must be done with caution. Also, if your preliminary lens discussion and your assessment of the patient’s visual needs suggest a particular material is needed, be aware of frames types that would be inadvisable. For example: A minimum edge thickness of about 2 mm is needed for grooved mountings. It makes little sense to use so high an index that the lens thickness must be increased to accommodate the groove. It would be better to use a lower index (for better optics) that would give the needed edge thickness. Drill mounts also are “material sensitive,” and some materials just won’t work well with drill mounts or grooved designs.
If a break-resistant material is needed, your best choices are Trivex and poly. Poly tends to craze and crack around the holes in drill mounts. Poly can give reasonable performance in lower powers if AR is used to reduce veiling of the retinal image. This veiling is due to internally reflected light.
Poly has a 0.12∆ image spread from blue to red at 1 cm from the optical center (a viewing angle of around 20 degrees) of 3.60 diopters (either + or -). Trivex only reaches the same image spread at 5.40 diopters. This difference is due to the higher Abbe value of Trivex. The advantage of using a higher Abbe value is more appreciated by patients who do critical viewing at longer distances.
The power in the horizontal and vertical meridians gives you warning of potential prism problems, and edge thickness problems with minus lenses and center thickness problems with plus lenses. Because eye movements are mostly in the horizontal meridian, knowing the power in that meridian helps you decide if a low Abbe value could be a problem. When powers in the vertical meridians differ by a diopter or more, frames with deep vertical measurements are usually best avoided.*
For low power lenses, the shape of the eyewire opening and the overall shape of the front can be chosen to complement the patient’s facial anatomy. For medium and stronger lens powers, choices become more restricted. In these powers, decentration has more of an impact on the weight and appearance of the completed eyewear. For powers of about 5.00 diopters (+ or –) or greater, the closer you come to both minimal decentration and a round or oval shape, the better the final result will be. A good rule is to limit decentration to 1 to 3 millimeters whenever you can.
In high minus powers, excessive decentration thickens the lateral edge of the lens. In plus, excessive decentration thickens the nasal edge of the lens and makes the magnification of the patient’s eyes greater. This magnification is greater when the vertex distance is long. Too much decentration makes the effect of using a lens material with a low Abbe value worse. The problem is greater if the eyesize is large and/or if the prescription includes prism.
FRAMES AND MOUNTINGS
For low power lenses, there are few limitations outside of cosmetic considerations and patient preference. For mid-range and higher powers, the frame choice becomes critical. During frame selection you have a responsibility to guide your patient to frames with potential for a great outcome. Labs receive orders every day for lens/frame combinations that cannot yield patient-pleasing results. Most of these orders arise from allowing patients to make frame selections that are beyond their understanding. Manufacturers need to give better information about their products, and practice owners and managers should provide staff training on how to best use those products.
Oddly enough, a successful outcome may be as much related to the frame as to the lens design. Patients sometimes interpret a poor bridge fit as a “The lenses are too heavy” problem. Simply selecting a frame with a better fitting bridge may resolve the “weight” problem, so take a close look for sore spots or pressure marks that have been caused by the old eyewear (Fig. 4).
Among the most important frame considerations for all lens powers: Will it be about as wide as the widest part of the patient’s face? Can the pantoscopic tilt be adjusted? Is the location of the temple attachment appropriate for the patient’s anatomy and Rx (Fig. 5), and is the frame well constructed and cosmetically acceptable? Is the B measurement adequate for the Rx, and is the bridge fit acceptable?
MID-RANGE AND HIGHER PRESCRIPTIONS
For mid-range and higher Rxs, there are additional frame considerations: Will the decentration be small? Is the wrap minimal? Because wrap rotates each lens around a vertical axis, there will be power changes associated with excessive wrap. Individualized (or “as worn”) digital lenses should be used when the wrap exceeds 8 or 10 degrees, depending on the lens power.** Labs need vertex distance, pantoscopic tilt and wrap values when individualized lenses are ordered. It is important not to use default values when ordering individualized lenses. The advice of your lab should be followed when ordering higher powers for wrap frames. Ask your lab to compensate monocular pds if the wrap causes an increase of 0.5 mm or more in either or both eyes.
As the MRP or Fitting Cross (FC) is raised above the Mounting Line, the center and upper edge thickness of a plus lens will increase, and for minus lenses the thickness of the lower edge of the lens will increase. If the MRP or FC is too low, however, the patient will experience optical and comfort problems with the eyewear.
Using a zyl frame helps disguise the edge thickness of minus lenses. Using a frame that has turnback temples or that is designed for the same effect in zyl minimizes the center thickness of plus lenses and the edge thickness of minus lenses. Drill mounts do nothing to hide the edge thickness of minus lenses, and nylon cord/grooved mountings have the same issue; in addition, the thinnest edge of plus lenses must have a thickness of about 2.2 mm to accept the groove, and that will increase the overall thickness of the lens a millimeter or more.
Takeaways
- Always look at and ask about the previous eyewear.
- Recognize the challenges presented by the prescription, the anatomy and the patient's expectations.
- Your choice of materials for drill mounts and lenses that will be grooved must be cautiously made.
- For high prescriptions or if the patient's ears are at unequal levels, avoid frames with a fixed pantoscopic angle. For a discussion of the optical importance of pantoscopic tilt, see System for Ophthalmic Dispensing, Brooks and Borish, 3rd Ed., pp. 63-69.
- Increase the lens index only when there is no other way to make the lenses sufficiently thin and light. A common error is to raise the index when prism is prescribed while ignoring other ways of reducing thickness.
- Chromatic aberration is closely related to prism so a material with the highest practical Abbe value generally should be selected for prism prescriptions.
- Use the correct centration chart whenever you are fitting PALs.
- When in doubt about the frame/lens match, contact your lab for a consult before ordering.
The eyesize of the frame and its effective diameter allows you to control the thickness, and therefore the weight and overall magnification/minification and weight of the finished lenses (Fig. 6).
The eye size plus the DBL is the frame pd. Dividing that by 2, gives the frame’s monocular pd. A 50□20 frame has a 70 frame pd and monocular pds of 35.
Excessive decentration is a common cause for both poor lens performance, and finished eyewear that is cosmetically and mechanically unacceptable. By keeping decentration to a minimum, especially for higher powers, to 1 to 3 mm is a good general rule. To limit the decentration in either lens to 1 to 3 millimeters, the patient’s monocular pds should be no more than 34 and no less than 32. Begin frame selection with a cosmetically acceptable frame meeting the size, shape and mechanical requirements you have already estimated, and you are well on your way to a successful and efficient matching of frame/lenses and patient.
Finding the best Lens/Frame match is more complicated than finding a Dainty Foot/Glass Slipper match. But the satisfaction of making that eyewear match gives an unmatchable professional satisfaction to everyone dedicated to helping people have the best vision possible.
Contributing editor Palmer R. Cook, OD, is director of professional education at Diversified Ophthalmics in Cincinnati, Ohio.
*To find the power in the 090 meridian, use a hand calculator to find the cosine of the axis of any cyl that is in the lens (use minus cyl form) and square it. Then multiply by that value by the cyl power and combine it with the sphere power. E.g., -1.00 -2.00 x 120. The cosine of 120 is -0.5, squared it becomes 0.25. Multiplying by -2.00 equals -0.50. Combining -0.50 with -1.00 (the sphere power), yields a total power in the vertical meridian of –1.50. If the sine of 120 is used, squared and combined with the sphere power, the result will be the power in the horizontal meridian (-2.50).
**Your lab is one of your most valuable resources. If you have a questionable frame/lens match, contact your lab for a consult. If your lab spots such a match and calls, please don’t permit your staff to shrug the matter off with a, “Just run it as ordered” comment. This will save your practice time, money and needless embarrassment.