By Palmer R. Cook, OD
Middle children often feel left out, invisible and neglected. Is there a similar issue in eyecare, and are you part of that problem? Because of the rapid refractive changes during the growth years, a lot of attention is given to early detection and regular care for the pediatric set. Likewise, the sudden awareness of presbyopia when accommodation fails causes concern among those in their mid-40s and beyond. Eyecare is often focused on the needs of these two groups of needy folks, but there is a middle group that you may be overlooking.
The visual demands for pre-presbyopic adults can be intense. About 80 percent of our younger adults (ages 18 to 44 years) have significant refractive errors. These people generally do not require multifocal corrections, but many have a need for improved optics to help them with prolonged visual tasks at all distances. They are the breadwinners, the people who are developing their careers. They work long hours, often under significant pressures at work and at home. They have need for the best in vision care too.
THE NEGLECTED MIDDLE CHILD
Why are the benefits of today’s greatest advances in ophthalmic technology so seldom offered to patients who are not yet presbyopic? Has the need to address presbyopia caused us to neglect the “middle children” of our population? There has been a great deal of attention in the ophthalmic professions and in the optical industry to provide improved lens technology for presbyopic patients and to a somewhat lesser extent for children. But if you are still routinely prescribing corrected curve lenses to your non-presbyopic patients including children, it’s time to consider what digital technology can do for them.
CORRECTED CURVE LENSES
Yesteryear’s corrected curve lenses have front curves provided in about 2 to 3 diopter base curve steps by the lens manufacturer (See Fig. 1). The aberrations these lenses address are marginal astigmatism, power error or a combination of the two. Each manufacturer picks the aberration(s) and produces and markets according to their own choices. It remains a system sort of like selecting dueling pistols, taking 10 paces and firing marketing claims at will. If your patient’s prescription falls near the middle of the recommended power range for one or another manufacturer’s design, that is probably your best available choice in corrected curve lenses. But if the prescription fell near the division between one base curve and the next (steeper or flatter), the results could be less than optimal.
Cylinders always were and remain a problem for the corrected curve system. If for example, your patient’s Rx was -3.00 -3.25 x 090, the chart (Fig. 1) indicates a +3.50 base curve should be used. The power in the 090 meridian is -3.00, so far so good, but the power in the horizontal meridian is -6.25. According to the manufacturer’s chart, a power of -6.50 is best served with +1.50 base curve. This creates a “Sweet Spot” that is a vertical oval (Fig. 2). Unfortunately, since most of our lateral eye movements are in the 180 meridian, this patient might be better served with a +1.50 BC or a free-form or individualized design.
THE “SWEET SPOT”Every eyecare provider is aware that the best lens performance centers around the Major Reference Point (MRP) of the lens, which is the optical center unless prism is prescribed. Great care is or should be taken to position that point in every lens so that it is slightly below the line-of-sight when the patient is in the primary position of gaze.
The Sweet Spot is useful as a concept. It can be thought of as the area in the lens through which the patient’s line-of-sight can pass without the patient being aware of either strain or blur. In Fig. 1, the boundaries of both right and left Sweet Spots are shown with wide diffuse borders. When lenses are newly dispensed, the Sweet Spots can be thought of as being rather small, but once the patient becomes adapted to the properties of the new lenses, the Sweet Spots effectively enlarge.
As patients move their eyes so that the lines-of-sight move into the lens periphery outside of the Sweet Spots, they find that clarity and comfort is diminished. The cause for this includes aberrations such as power error, marginal astigmatism and chromatic aberration.* The boundary of the Sweet Spot is made up of the points away from the MRP at which the patient perceives a decrease in the lens’ performance. The Sweet Spot’s area varies with pupil size, viewing distance and many other factors including the patient’s perceptual tolerance and perhaps his expectations. All ophthalmic lens technologies seek to give patients the largest possible Sweet Spot.
IMPROVED TECHNOLOGY: FREE-FORM
We all know of the improved technology of digital lenses. For free-form lenses, digital technology is used to customize each lens so that its curvatures are corrected to reduce peripheral aberrations (usually marginal astigmatism) at some angle of view (usually about 35 degrees) from straight ahead. Better yet, digital technology allows all meridians to have optimized curvatures, unlike corrected curve technology that allows only one meridian to have a somewhat or partially optimized curve whenever the lens has cylinder power.
EVEN BETTER: INDIVIDUALIZED
For Individualized (or “As Worn”) lenses, digital technology allows lens designers to specify the curvatures needed to eliminate the power errors induced by wrap or pantoscopic tilt when the line-of-sight passes through the optical center. When the doctor prescribes any power, that power is based on the line-of-sight being normal (perpendicular) to the test lens surfaces. If the refracting vertex distance varied from the vertex distance assumed by the design, Individualized computer programs do not make any adjustment. If the fitting vertex is 16 mm, and you refract at 16 mm, the computer will likely adjust for whatever vertex the designer used. The best choice is to refract at the vertex distance used for the design you are planning to fit.
In actual practice if a -7.00 lens is fitted at a 9-degree pantoscopic tilt, the effective power is -7.05 -0.18 x 180. The power that must be supplied to correct this error would be about -6.82 -0.18 x 090 for a 1.60 material. This is the power indicated by your lensmeter when the light rays are perpendicular to the lens’ surfaces, but when worn with a 9-degree pantoscopic tilt, the patient will have the -7.00 that the doctor prescribed.
Better yet, Individualized lenses give improved peripheral performance by using optimized curvatures for all meridians. These curves are produced according to the way the lens is worn—compensating—for the measured wrap, vertex** and pantoscopic tilt. When fitting Individualized lenses, it is important to take actual measurements. Using default measurements can rob your patient of important benefits. Moderate errors in measurements will nearly always give better results than using default measurements.
As patients become presbyopic, most eyecare practitioners recommend the higher performing technology that digital technology offers. But what about those who are wearing single vision lenses? We know that free-form designs are a significant improvement over the old corrected curve system. That system approximates best base curve for the spherical power of the written prescription in one major meridian. Free-form gives the optimal base curves for all meridians. As a result, these designs give a larger Sweet Spot in all directions.
Individualized designs perform even better because they tailor the central power to give the desired Rx by compensating for vertex, wrap and pantoscopic tilt, and the periphery is designed to address peripheral aberrations like free-form designs, but based on correctly compensated powers. Why should our middle group of patients be waiting until they become presbyopic to enjoy the enhanced performance offered by single vision free-form and Individualized lenses?
WHAT TO LOOK FOR
Patient sensitivity to blur and distortion can vary widely, and this makes it difficult to establish guidelines regarding the value of switching all patients to a free-form or an Individualized design. The Sweet Spots in Figs. 2 and 3 have broad, ill-defined borders because the size of the Sweet Spot is never completely static. In addition, there is a bigger question: At what power is your patient likely to begin perceiving an improvement in lens performance? The answer is probably that more sensitive patients (e.g., those who could be refracted in 0.12D increments) might perceive improvements if free-form and Individualized designs were used starting at lens powers of about plus or minus 3.00D in the strongest meridian and with cylinders of more than about 2.50D.
It would be immensely helpful to prescribers if manufacturers released meaningful data about their designs. Such data should include which peripheral aberrations are addressed, what refracting distances were used in calculating the designs, and what amount of power error and marginal astigmatism would be present at some industry-accepted standard viewing angle for each design, material and at selected powers (e.g., plus or minus 3, 6 and 9 diopters).
Savvy ECPs are already realizing that if they compare the compensated prescription powers received by the lab with the prescribed powers, they will know how much error was avoided in sphere and cylinder power, as well as how much prism and axis error were also avoided. For example: if the ordered power was -5.00DS and the compensated Rx to be read from the lensmeter was -5.10 -0.12 x 081, the failure to use an Individualized Design would have induced an error of 0.10 diopters of sphere power, and it would have introduced 0.12 diopters of unwanted cylinder powers. This error would be present when the patient views through the optical center of the lens. Small power errors such as these, coupled with the aberrations inherent in all ophthalmic lenses can add up to a lot of patient dissatisfaction.
WHEN PRESBYOPIA LOOMS
For patients in their late 30s, the choice is not so difficult. Switching these patients to a free-form or Individualized single vision lens may make adaptation later to a free-form or Individualized PAL easier. If the patient wants the best, the choice should be an Individualized design because that is as close as today’s technology can come to giving an aberration-free periphery with compensation for the power differences caused by the position of wear.
By making the switch to the improved performance of free-form or individualized technology before an add is needed simplifies adaptation, and the patient can experience improved lens performance with easier adaptation. When an add must be fitted, the adaptation process is then less complex. This sort of preplanning makes anticipated changes an easier matter. This is part of the professional care that patients deserve.
Manufacturers now offer free-form and Individualized single vision lenses that will give improved performance for your “Middle Child” patients. The question is: How can you judge which patients will most appreciate the improved technology? Experienced ECPs can generally estimate a patient’s visual sensitivity during testing. Of course, patients with very low powers (e.g., -0.75 spheres) will not get much improvement in lens performance with free-form or individualized lenses, but those in high-powered lenses will tend to be very quick in appreciating the improvement in peripheral clarity and comfort. The middle ground will be those with moderately powered prescriptions. Clinical judgment and experience will be important in selecting those who will benefit from the free-form and Individualized designs.
Try this simple test to help you identify the single vision patients who would appreciate better peripheral vision. Ask your patient (with his current eyewear on) to view a target straight ahead, and then ask them to look at a target to the side. Do this for both distance and near targets. If the patient simply turns his eyes to view side targets, he probably is not much troubled with peripheral aberrations. But if he instinctively turns his head to get his lines-of-sight more centered in the lenses, then he may be troubled by the peripheral aberrations. A little practice and careful observation will help you identify the pre-presbyopic patients who are likely to appreciate the better technology that no one had ever thought to offer them previously. ■
Terminology
Free-form—Free-form lens design utilizes digital generating technology to produce customized aspheric curvatures that can reduce certain peripheral aberrations. As with other aspheric lenses, free-form lenses tend to have flatter curves and thinner edges, but these curvatures are specific for the exact power of the prescription in all meridians. Aspheric lenses can either be produced from molds, or they can be digitally produced but this will not necessarily qualify them as free-form. All free-form lenses must be digitally produced, but not all digital lenses are free-form.
Individualized—Individualized (sometime called “As Worn”) lenses must be digitally produced. Their periphery utilizes aspheric curves that can reduce certain peripheral aberrations in the same way as free-form lenses. Individualized lenses utilize measurements such as the vertex distance, pantoscopic tilt and wrap to address the power changes that occur when a prescribed lens is fitted at a different angle and vertex distance than the angle and vertex distance at which the refracting tests were performed. All Individualized lenses must be digitally produced, but not all digital lenses are Individualized.
Marginal Astigmatism—Marginal astigmatism is a lens aberration consisting of unwanted cylinder that is experienced when the line-of-sight shifts from the optical center to the periphery of the lens.
Power Error—Power Error is a lens aberration consisting of an unwanted change in lens power that occurs when the line-of-sight shifts from the optical center to the periphery of the lens. The power shift is toward more plus or less minus.
Corrected Curve Lenses—A corrected curve lens uses spherical curves on either the back or front of the lens to correct either marginal astigmatism, power error or an average of the two. Corrected curve lenses are more steeply curved and thicker than aspheric, free-form or Individualized lenses. Corrected curve lenses can be produced on digital equipment, but typically this is not done.
Overall Magnification—Overall magnification is a condition in which a lens has the same magnification in all lens meridians. A circle that is magnified by an overall magnifying lens will simply appear to be a larger circle.
Meridional Magnification—Meridional magnification is a condition in which a lens has differing magnification in the various lens meridians. A circle that is meridionially magnified will appear to be an oval.
Adaptation—In reference to wearing prescription lenses, adaptation is the period and the changes during which the visual system adjusts from blurred and/or uncomfortable vision to clear, comfortable vision. Most adaptation problems are caused by differences in magnification (meridional and/or overall), or in differences in the positioning of the retinal images by prismatic effects, or in changes in aberrations compared to the formerly worn lenses.
*Other lens aberrations such as distortion, which cannot be controlled by base curve changes, and spherical aberration and coma, which are generally limited by the small size of the pupil, and chromatic aberration, which can only be limited by using higher Abbe value materials are generally not addressed by aspheric lens designs. A few advanced lens designs address coma, but this is not a problem for patients with about 5 mm or smaller pupils.
**When the computers that control the digital lens generators correct for your patient’s vertex distance, the assumption is made that the refractive testing was done at a certain “designer selected” distance. Especially for higher power prescriptions (perhaps 4 diopters and above), it is a good idea to refract at the design distance used for the design you are planning to prescribe.
Contributing editor Palmer R. Cook, OD, is director of professional education at Diversified Ophthalmics in Cincinnati, Ohio.
