Focal Points

Bifocals, Trifocals, and Progressive-Addition Lenses

H. Jay Wisnicki, M.D.

Introduction

As we enter the new millennium, the number of Americans of presbyopic age (40+) is estimated to be greater than at anytime in history. The interests of this new generation are quite different from those of their parents and grandparents. They have more active lifestyles, and place more emphasis on fitness and appearance. The optical needs of the modern presbyope are quite diverse. Successful multifocal dispensing requires that the practitioner know the patient's occupational and recreational needs in addition to the refractive and accommodative states of his or her eyes. Awareness of the various lens types available is crucial to the optimal treatment of the patient. Bifocal, trifocal, and progressive- addition lenses share the common design of correction for distance and near vision in a single lens. The objective of this module is to update the practitioner on the currently available spectacle options for presbyopia.

Multifocal Lens Design and Characteristics

Benjamin Franklin invented the first bifocal lens in 1785. He cut lenses from two pairs of spectacles in half and mounted them in one frame. Several bifocal lens designs were devised in the nineteenth century, including the Solid Upcurve (a one-piece bifocal), Perfection, Cemented, and Kryptok lenses (see Figure 1a). Another lens manufactured in the early 1900s was the Ultex lens. The additional plus power required for the reading add was created by grinding a different radius of curvature on the back of the reading portion of the lens.

 

Figure 1. Types of multifocal lenses. 1a: Early bifocals. 1b: Single-segment (common) bifocals. 1c: Double-segment bifocals. 1d: Minus-add bifocals. 1e: Trifocals. 1f: A quadrifocal. (Illustration by Christine Gralapp)

The first fused bifocal was produced in 1908. A spherical segment made of flint glass, a different material than the major distance lens, was fused with a countersunk area of the distance lens using high temperatures. A decade later, the fusing process was improved by using a segment of two different types of glass. The top portion of the segment was made of the same material as the major distance lens. This enabled different-shaped near portions of the lens. Fused-glass bifocals are still in use today. In the 1950s, the Executive bifocal was developed, in which the front surface of the lens was ground with a smaller radius of curvature in the bottom than the top, causing a line across the lens. Trifocals were first introduced in the 1930s, as a modification of the processes for bifocals.

Choices in multifocal lenses today include segment style (shape and size), position, material, and number of segments. These specifications also indirectly specify other items, such as refractive index and fused or one-piece designs. Factors that should be considered when deciding between these choices are the patient's occupational and recreational needs, expense, cosmesis, and optical performance, including aberration.

Bifocal Lens Design

Most bifocals today are single-segment bifocals (see Figure 1b). The most commonly used bifocal design is the flat-top bifocal. It comes in four common sizes, 22, 25, 28, and 35, referring to the horizontal width in millimeters, respectively, of the flat top of the segment. The flat-top 28 is the most commonly prescribed. Smaller sizes are generally used for smaller frames. The flat-top 35 can be used when a larger near-viewing area is desired. Executive-style bifocals, described earlier, offer the advantage of a large reading area. Disadvantages of Executive-style bifocals include greater weight and less cosmetic appeal. Larger flat-top segments, as high as 40 or even 45 mm, offer a compromise for these factors. A ribbon-segment bifocal is a modification of the flat-top with the lower portion cut off, giving the patient a distance viewing area below the segment (for patients who may want to see stairs or play golf).

Double-segment bifocals have two equal but separate reading areas within a single lens (see Figure 1c). They are useful for people with special occupational needs such as painters, carpenters, electricians, or others who need to perform near work above and below primary gaze. They are available as Double-D (double flat-top), double-ribbon, or double-Executive bifocals.

Very large close viewing areas can be obtained with minus-add bifocals, in which the segment is actually the distance field and is rotated to the top of the lens (see Figure 1d). The add is negative, thus enabling most of the lens area to be used for close work. This may also be accomplished with an Executive-style bifocal with a very high segment top.

Blended bifocals are a type of invisible multifocal. This is a round-segment bifocal that has been made less visible by blending the edge of the bifocal segment over a small zone. These blended bifocals can be cosmetically more appealing to patients.

A recently introduced segment to the market is the Optx 20/20 soft reading lens manufactured by Neoptx, Inc. These are soft, reusable, plastic reading lenses available in multiple spherical add powers that adhere to a base lens. They are marketed as an inexpensive alternative for sunglasses but can be used on any lens. They can also be cut or trimmed to a specific shape and size if desired.

Trifocal and Quadrifocal Lens Design

A typical initial presbyopic add is +1.00–+1.50. People in the early stages of presbyopia usually manage by holding reading material farther away, removing their glasses (for low myopes), or sliding their glasses down their nose (for high myopes, decreasing effective lens power). People with hyperopia often delay getting near glasses the most, as they may have never had visual blur to bring them to an ophthalmologist.

When presbyopia advances, requiring an add of +2.00 or more, an intermediate range occurs in which vision is blurred through either the distance or near portion of the lens. Trifocal lenses were the initial attempt to solve this problem, by providing three different powers for distance, intermediate, and near vision. The segment is divided, with the intermediate-distance power generally half the near power. Trifocals can be prescribed for special purposes with intermediate segments of greater or lesser powers. Although several other types of trifocals are available, most are flat-top or Executive-style segments (see Figure 1e), in which the distance portion remains the same and the segment is divided for the intermediate add. A combination trifocal lens incorporates both the Executive-style and flat-top bifocal segments in the same lens, resulting in a trifocal with a larger intermediate viewing area.

Quadrifocals are special-use lenses that combine the concepts of trifocals and double-segment bifocals (see Figure 1f). In addition to a trifocal lens, in these lenses another near segment is placed above eye level to fill a special need.

Bifocal and Trifocal Lens Performance

Lens performance is based on power, position, optical centers, and the patient's line of sight through the different portions of a multifocal lens. Power is the one parameter that all practitioners are familiar with and is written on all prescriptions. We all understand how a +1.00 add performs differently than a +2.50 add. Segment position also affects performance, because the different powers of a multifocal lens must be placed so that the patient's line of sight is appropriate for the add and occupational need. Segment optical centers are inwardly displaced, a practice known as segment inset . Because the near pupillary distance is 3–4 mm less than the distance pupillary distance, segment inset avoids induced horizontal prism and centers the field of view of the segments.

Figure 2. Optical effects of bifocal segments. 2a: Image jump. Left: The object that the eye sees in the inferior field when looking straight ahead suddenly jumps upward when the eye looks down at it. Right: If the optical center of the segment is at its top, no image jump occurs. 2b: Image displacement. (Courtesy of David Guyton, M.D.)

Other optical effects of segments include image jump and image displacement. Image jump is the movement experienced by the patient when they look down across the bifocal segment. It is caused by the prismatic power that occurs at the top of the segment (see Figure 2a). From Prentice's rule, the prismatic power of a lens (D) is equal to the power in diopters (D) multiplied by the distance in centimeters (h) from the optical center (D = h x D). A base-down prismatic effect causes the appearance of the image to jump upwards. This produces the "jack-in-the-box" phenomenon described by the patient.

 

 

 

Figure 3. Bifocal, trifocal, and progressive-addition lens designs compared. 3a: In a bifocal lens, the distance-vision sphere is above the near-vision sphere and they are linked by a single "step" that is seen as a single line. 3b: In a trifocal lens, an intermediate-vision sphere is added between the distance- and near-vision spheres, producing two visible lines. 3c: In a PAL, an uninterrupted series of curves link distance, intermediate, and near vision parts of the lens with no visible separations.

Executive-style bifocals have no image jump, because the optical centers of the distance and near segments coincide, resulting in no prismatic effect at the top of the segment. Flat-top bifocals have minimal image jump, because the optical center of the segment is near the top of the segment. Round-top segments have maximal image jump, because the optical center of the segment is farthest from the top of the segment, resulting in the largest induced prism when the patient looks from the distance portion of the lens to the segment.

Image displacement is the total vertical prismatic power, combining both the distance lens and the near segment, between the two eyes in the reading position (see Figure 2b). With single-vision lenses, looking downward results in base-up prism in plus lenses and base-down prism in minus lenses. As long as the vertical meridional powers of the lens are the same or almost the same, there is no or almost no net vertical prism. Round-top segments cause base-down prism. Flat-top segments cause base-up prism. To minimize image displacement, round-top segments are preferred on plus lenses and flat-top segments are preferred on minus lenses. Image displacement is more bothersome than image jump for most patients, so give precedence to minimizing displacement when choosing the type of add.

Segment widths and shapes are subject to patient preference and any special needs. Segment height on the lens should be designed so that the top of the segment is 6 mm below the pupil. This is generally at the level of the lower lid margin. Patients who spend more time doing near work should be fitted higher.

Progressive-Addition Lens Design and Performance

Progressive-addition lenses (PALs) are a type of invisible bifocal in which the power increases gradually from the distance portion to the near portion of the lens. The original Varilux® progressive lens was developed in Europe, and introduced there in 1959. The first PAL available in the United States was the Omnifocal, marketed in 1965. The Varilux was introduced in the U.S. in 1967.  The PAL continues the concept of trifocals to the maximum, giving the wearer every power between distance and near (see Figure 3 for a comparison between bifocal, trifocal, and PAL design). The radius of curvature of the progressive surface constantly decreases from the distance to the near portion of the lens. There is a large top distance area, a narrow progressive transitional corridor, and a medium-sized reading area. Because there is no separate segment, no image jump occurs. Therefore, PALs offer the advantage of clear viewing at all focal distances (see Figure 4). They also offer the best cosmetic appearance of spectacle lens to presbyopes, with a completely invisible segment.

 

 

 

Figure 4. Fields of clear vision with bifocals, trifocals, and progressive-addition lenses. 4a: Bifocal with a 2.50 D add. 4b:  Trifocal with a 2.50 D add. 4c: PAL with a 2.50 D add.

The main disadvantage of PALs is the less usable lens area on each side of the progressive corridor and reading area. This area of the lens has induced cylinder from aspheric curvature. The slower the rate of change of power from distance to near, the less this undesirable cylinder. The longer the progressive corridor, the less the cylinder, but the greater the eye depression necessary to reach maximum near power. There are several other disadvantages of PALs. Patients may need a longer adaptation period. More head movement is required than with multifocals. Longer transition zones require farther excursions into downgaze than with conventional bifocals. Newer designs have improved both head and eye postures (see Figure 5) and movements to be closer to the postures and movements of the nonpresbyope.

 

Figure 6. Comparison of hard-design and soft-design PALs. These illustrations compare the power progression and peripheral aberration of these two PAL designs.

The two basic types of progressive lenses are hard design and soft design (see Figure 6). Hard designs give larger distance and near zones, but a shorter, quicker, and narrower progressive corridor. There is significant distortion from cylinder, causing blurred vision in the other adjacent portions of the lens. The soft design has smaller distance and near reading areas, but a wider and longer progressive corridor. The adjacent areas of the lens are larger, but have less distortion and visual blur.

Isocylinder and isosphere contour plots (see Figure 7) show the amounts of cylinder (causing soft focus or image blur) in the less usable portions of the lens. Sharp focus occurs over smaller areas of the lens, but the visual blur is less and better tolerated at the other peripheral portions. In general, the larger the add, the harder the design and the smaller the add, the softer the design of the lens.

 

Figure 7. Contour plots of different PAL designs. 7a: Isocylinder contour plots. 7b: Isosphere contour plots.

The corridors of PALs are usually not vertical, but are slightly oblique, pointing nasally. This is related to the fact that the near pupillary distance is slightly less than the distance pupillary distance. The corridor is oblique because the progression of lens power must follow convergence for near-in downgaze. Poorer lens designs use a symmetrically rotated, obliquely tilted corridor (see Figure 8a), with the same lens used for the right and left eyes (as mirror images). Although this is less expensive, horizontal movements of the eyes as the patient looks down the lens toward closer ranges result in less accurate focus. Better, but more expensive, asymmetric aspheric designs are separate for the right and left eyes (see Figure 8b), in which there is more symmetric and accurate focus in the two eyes with small shifts to right and left gaze as the patient looks down the lens towards nearer ranges. Newer designs have also permitted less horizontal eye movement, closer to the nonpresbyope in intermediate ranges.

 

 

Figure 8. Comparison of binocular vision in symmetric and asymmetric PAL designs. 8a: Poor binocular vision with symmetric-rotated design. 8b: Good binocular vision with asymmetric right/left design. (Illustration by Christine Gralapp)

The design of the PAL has gone through several generations over the years. These changes have included softer designs, increasing area of asphericity, and multidesigns in which the hard or soft design of the lens changes with the amount of add specified (see Figure 9). Multidesigns offer the advantage of better acceptance by the early presbyope, who usually prefers a soft design with a smaller add, and the advanced presbyope, who usually prefers a hard design with a larger add. Asphericity is used in PALs to allow increased power down the lens while limiting soft focus or optical aberrations in the periphery.

As with minus-add and double-segment bifocals, there are special types of PALs for patients who spend most of their time at near-to-intermediate work or need near-work tasks above eye level. Another special type of progressive lens is a flat-top bifocal style with the segment as a progressive instead of near-only focus. This compromise between a flat-top trifocal and a progressive lens offers a more usable distance area free of distortion, while having progressive focal distances in the small segment. It is uncommonly used and has limited application.

Adaptation can be an obstacle to the success of a patient starting with PALs. Because the optically desirable areas on PALs are smaller than those on single-vision or multifocal lenses, patients need to learn to move their heads rather than their eyes for clearest vision. This process can take a minimum of 7–14 days (a month is not uncommon), and is generally quicker with soft designs. Patient education is a key factor for success. Early presbyopes or first-time, non-single-vision lens wearers are the most likely to adapt readily. Patients who request invisible bifocals for cosmetic reasons are quite motivated and are usually very successful and satisfied with PALs. Patients satisfied with their current multiple single-vision eyeglasses, bifocals, or multifocals should not simply be switched to progressives, as they are less likely to tolerate the imperfections and compromises of PALs. Patients who are poor candidates are established bifocal wearers with high additions or wide segments looking for a wide field of vision. Editors, engineers, drafters, and patients with occupations that require wide fields should be considered on an individual basis.

 

Figure 9. Multidesign PAL. This figure shows how the lens designs of hard and soft monodesign and multidesign PALs  change as the amount of add changes. 

Success with PALs is also dependent on accurate fitting and dispensing. PALs have less tolerance for fitting errors than virtually any other type of spectacle lens, due to the smaller areas of focus in the distance, near, and progressive corridors. Dispensers should be careful to always seat themselves at eye level with the patient. The selected frame is then adjusted to the patient's face before taking measurements. Monocular pupillary distances at distance and near should be measured to accurately place the lens position for each eye. The center of the pupil is marked on the demonstration lens. The vertical position from this point, a line horizontally tangent to the lowest portion of the frame, is then measured. This distance, measured separately for each eye, is the monocular fitting height. The frame selected should be a minimum of 22–24 mm beneath the pupil. The frame should have a total vertical measurement of at least 38 mm to accommodate this requirement. Adjustable bridges should be used, but any proper fitting frame with the eyes fairly centered should work well. The vertex distance should be from 12–14 mm and the pantoscopic tilt from 12–15. Ophthalmologists should work with several selected opticians in their area who are experienced with multifocal and PAL fitting and dispensing. Manufacturers can also educate dispensers on the specific fitting requirements of their lenses.

Multifocal Lens Materials

Patients requiring multifocal correction have many choices in the proper selection of lens material and frame. Lens materials are  made of different substrates, and so have different indexes of refraction and different effects on light rays entering the lens at the critical angle. The higher the index of refraction, the thinner the lens and the greater the angle of refraction. Common lens materials include CR-39 plastic (refractive index of 1.49), Hi-Index plastic (1.56 to 1.66), Crown glass (1.52), Hilite glass (1.80), Hi-Index glass (1.90), and polycarbonate (1.59).

CR-39 lenses are about half the weight of glass, are fairly resistant to breakage, and qualify as safety lenses when they are produced with a center thickness of 3.0 mm. Compared to glass, however, plastic is thicker, scratches more easily, and filters out less ultraviolet (UV) light. Although polycarbonate has a high refractive index, it does not have the quality optics of CR-39. It has the highest impact resistance but is also the softest material and the most prone to scratching. For this reason, scratch-resistant coatings are often applied at manufacture. Polycarbonate has a higher rate of UV absorption than CR-39.

Multifocals and PALs are also available in a number of special materials. Photochromatic lenses, or lenses made of materials that change color and transmission qualities with the amount of ambient light, are available either in glass (Photogray, Photobrown) or plastic (Transitions). Polarized materials are also available now for PALs. Refer to a multifocal or PAL availability chart to determine whether a particular design comes in the material of interest. Lenses made of high–refractive index materials are particularly useful for high power (either high-plus or high-minus) prescriptions. These materials have the advantage of significantly reducing lens thickness, weight, and size and thereby increasing the comfort and appearance of highly ametropic eyeglasses. Disadvantages include increased optical abberation, especially chromatic (which can be minimized by antireflective coatings) and increased cost.

Special Considerations in Prescribing Multifocal Lenses

A primary concern of the optician is matching the various available bifocal, trifocal, and PAL designs to the occupational and recreational needs of the patient. Special needs that prescribers need to be aware of include lens-induced prism with anisometropia, and ocular motility abnormalities.

Lens-Induced Prism with Anisometropia

Spectacle prescriptions with different powers in the vertical meridian, from sphere or cylinder differences between the two eyes, can produce an unequal vertical prism between the two lenses. Double vision while reading or a measured vertical deviation by cover test in downgaze indicates the need for this adjustment.

This image displacement can be addressed by prescribing slab-off prism. Using bicentric grinding, the laboratory can compensate for these vertical imbalances. The process essentially subtracts base-down or adds base-up prism to the lower portion of one lens, correcting the vertical prism in downgaze. The slab-off is placed in the most minus or least plus lens. While the prescriber can specify the amount of slab-off prism in the prescription, the laboratory can also calculate this amount from the lens prescription and specification.

Ocular Motility Abnormalities

Patients with eye muscle imbalances, either horizontal, vertical, or oblique, can have difficulty using multifocals or PALs. Eye muscle deviations often have incomitance (different deviations in different fields of gaze, specifically primary gaze and downgaze when reading through a segment), or distance–near disparity (different deviations in distance versus near vision). Because it is difficult to place different amounts of prism in different parts of a lens (for both primary and downgaze), these patients generally do best with separate single-vision glasses for distance and near. Fresnel prisms that are cut out to cover either the distance or near portion of a bifocal can be used, although the disadvantages of these prisms (blurred vision, prism falling off, dirt accumulation, etc) should be considered.

Some pediatric ophthalmologists do use PALs in the treatment of accommodative esotropia, as they have both optical and cosmetic advantages. However, following the deviation and the degree to which the PAL controls the deviation for the pediatric patient can be more difficult.

Aspheric Lenses for Higher Sphere Powers

Aspheric lenses are other special computer-designed lenses that can be used for higher sphere powers. Aspheric design flattens the lens considerably, allowing a thinner profile and edge design with superior optics. Using sophisticated programs and multiple base curves on a single lens, they increase peripheral vision and cut astigmatism of oblique incidence in high prescriptions.

Conclusion

Multifocal and especially progressive-addition lenses have evolved considerably over the past several decades. Designs and lens materials continue to improve. As our population ages, presbyopic patients will be requesting the many choices available to them. It would be helpful for ophthalmologists to have a working knowledge of the options of new lens technologies for our patients.

H. Jay Wisnicki, M.D., is Chairman, Department of Ophthalmology, Beth Israel Medical Center, New York City, NY; and Associate Professor of Ophthalmology, Visual Sciences, and Pediatrics, Albert Einstein College of Medicine, New York City, NY.

Special thanks to Gary M. Mandel, ABOC, and Dell Anne Van Vleck, COA, for assisting in the research needed to prepare this module.

Figures 3-9 are courtesy of Essilor International. Varilux is a registered trademark of Essilor International.

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