Prescribing Multifocal Lenses
JAMES E. SHEEDY
Table Of Contents
SELECTING THE MULTIFOCAL FOR THE PATIENT
DETERMINING THE ADD
FITTING BIFOCAL LENSES
PROGRESSIVE ADDITION LENSES
OCCUPATIONAL PROGRESSIVE LENSES
ANISOMETROPIA—VERTICAL IMBALANCE AT NEAR
FUTURE OF MULTIFOCAL LENSES
|Multifocal spectacle lenses are primarily used in the treatment of presbyopia. Secondary
usages include treatment of esophoria and pre-presbyopic
accommodative dysfunctions such as reduced accommodative amplitude
or accommodative infacility.|
Until the early 1980s almost all multifocals were segmented bifocal and trifocal lenses. Since that time there has been steady growth in usage of progressive addition lenses (PALs) and approximately 50% of multifocals currently dispensed in the United States are of progressive design.
There are significant optical differences between and within the segmented and progressive multifocal lenses that affect the vision that is provided to the patient. The primary objectives of this chapter are to present and discuss those optical differences as they apply to meeting the viewing needs of individual patients and to the fitting, adjustment and adaptation to the lenses.
|Benjamin Franklin is commonly credited with inventing bifocals in 1784. He
simply cut his round distance and near lenses in half and bound them
together in a frame to create a bifocal. Although it appears Franklin
invented bifocals independently, S. Pierce (1760) and A. Smith (1783) also apparently independently invented them.1 As might be expected, the two-piece lenses had poor structural
integrity in the frame and debris accumulated at their junction. Schnaitmann (1837) produced and patented the first one-piece
bifocal by grinding the top of a reading lens to produce the distance
power in the upper portion of the lens. An unfortunate consequence, however, was
poor optics resulting from a large amount of base-down
prism in the top portion of the lens.|
B.M. Hanna (1884) developed and patented the cemented bifocal, which consisted of a thin round reading lens cemented to the back surface of a distance lens with Canadian balsam. This lens had better optics than previous designs but still had the difficulties associated with two-piece lenses. Hanna (1886) also introduced the Perfection Grooved Bifocal, which was an improved two-piece lens similar to the Franklin bifocal. It had a half moon curved reading portion and also a bevel/groove interface between the two lenses that improved stability.
The modern multifocal era began with introduction of Kryptok, the first fused bifocal. It was invented in 1908 by John L. Borsch, a Philadelphia ophthalmologist and was a 22-mm round bifocal segment. The bifocal segment was made of flint glass (index 1.654, Abbe value 35) fused with heat (more than 1000°F) into a distance lens fabricated of crown glass (index 1.523). The Kryptok became a popular bifocal because it did not suffer from the instability of the cemented type, the segment was less conspicuous, and it eventually became the least expensive to produce. A one-piece bifocal (Ultex) was constructed in 1910 by Connor, an Indianapolis optician. This design involved grinding a round bifocal segment onto the distance portion of a single piece of glass. The result was a lens having a construction similar to that of the cemented bifocal but without any of the previously mentioned disadvantages. In 1915, the flat-top bifocal of fused construction was patented by Courmettes, a French citizen and resident of New York City. This was one of the segment styles introduced and manufactured by Univis Lens Company of Dayton, Ohio, in 1926. These newer fused bifocals used a barium glass for the segment (index 1.632, Abbe value 56) primarily because it has a better Abbe value and hence lower chromatic aberration compared to flint glass. The one-piece Franklin-style bifocal was introduced by American Optical Company in the early 1950s under the trade name Executive. This lens is based on the same principle as the first bifocal invented by Benjamin Franklin, with the optical centers of the distance and near portions placed adjacent to each other. Trifocals were introduced in 1826 by Hawkins of London. The first trifocal patents were taken out by Aves in England in 1907 and by Boness in America in 1911.
Throughout the history of multifocal lens development there has been an effort to devise a lens with an invisible segment. As early as 1916, Stead Optical Company made and patented a one-piece bifocal in which the boundary between the distance and near portions was rounded or blended. The most common method to accomplish this is with a progressive addition lens in which the power changes gradually from distance to near. The first PAL design was patented by Owen Aves in 1907. However, few advances occurred until 1951 when the Varilux 1 was developed by Maitenaz in France.2 It was not until 1962 that Omnifocal became the first PAL available in the United States. The Varilux 1 lens was introduced in the United States in 1965, followed by the Varilux 2 in 1973. Since that time numerous PALs have been introduced to market.
|SELECTING THE MULTIFOCAL FOR THE PATIENT|
|The many optical differences between the various multifocal designs create different visual environments for the patient. The differences between categories (bifocals versus PALs) and within categories are large and can significantly affect patient performance, comfort and acceptance. The patient's occupational and recreational pursuits should be identified and analyzed to determine unique viewing distances or viewing angles. Previous experience with multifocal lenses is instrumental in determining the new correction. Satisfaction with the current multifocal design nearly always predicts repeat success, however, it does not necessarily mean that the patient's vision cannot be improved with a different design. Many patients who currently wear bifocals appreciate the better vision and cosmetics of a PAL, or a patient successfully wearing one PAL design can often appreciate a different PAL design that suits their visual needs better.|
|DETERMINING THE ADD|
|Nearly all current multifocal lenses are constructed with the multifocal
optics on the front surface of the lens. The curvature of the back surface
is fabricated to provide the distance prescription of the patient. The
power of the multifocal lens is “in addition” to the
distance refractive power required for the patient and is specified
in the optical prescription as the amount of plus power in the “add.”|
Most clinicians perform routine near-vision testing and determine the power of the add at a near-viewing distance of 40 cm or 16 inches. Each clinician establishes for him/herself a successful method for determining the appropriate add for the patient.
The most common methods of determining the 40 cm add are:
Any of the above methods can be used to determine the add power for 40 cm or other viewing distance. The viewing distance is incorporated into the equation for the first method, and the other methods can be used with the target placed at the particular distance required by the patient. The fourth method (i.e., placing an object at the viewing distance and determining the add power that provides the best range of clear vision), can be particularly successful for unique viewing distances.
|As the name implies, bifocals provide two power zones. The primary lens
contains the distance power and the add segment contains the near power. All
bifocals provide a large distance viewing zone with homogenous
power and most also provide a large near-viewing zone with homogenous
power. However, the two viewing zones have a sharp demarcation. Cosmetically
the line of demarcation is quite noticeable and can be
particularly disturbing to patients who are sensitive to showing their
age. Visually the line of demarcation represents a largely unusable portion
of the lens. If the eye rotates so that the line of sight is near
the line, then the pupil receives light from both viewing zones. This
results in diplopia because of the prismatic effects of the segment
and each image results from a different refractive power. The optical
center of the add is not at the line (except in a Franklin-type
segment, Fig. 1D), hence there is a base-down prismatic effect caused by the
add at the top of the segment. This prismatic effect is called image
jump. The magnitude of the image jump (in prism diopters) is
calculated by multiplying the power of the add (in diopters) by
the distance from the top of the segment to the optical center of
the add (in centimeters). The locations of the optical centers
of the adds are indicated by the dots in Figure 1A to 1F and discussed further below. The magnitude of the image jump usually ranges
from 0.5 to 4 prism diopters. Because of the double images and conflicting
focus near the top of the segment, bifocal wearers habitually
avoid fixating near the line and use saccadic eye movements to alternate
between the distance and near viewing zones. Bifocal wearers experience
a large zone of unusable visual space immediately below the primary
gaze position, this is the primary visual compromise of bifocal
The shapes of the bifocal segments in the market today are essentially the same as those reviewed above from a historical perspective. All bifocals were initially developed in glass, today glass has a small market presence and most bifocals are made from plastic, polycarbonate, trivex or other resin materials.
By far the most commonly prescribed bifocal is the flat-top (FT) or D segment, directly derived from the Univis “D” segment developed in 1926. (There were also segment shapes designated “A–C” that were not as successful.) In glass products, then as today, the segment is formed by fusing a segment of barium glass (index of 1.623) to a crown glass (index 1.523) carrier lens as shown in Figure 2. The countersink curvature is calculated to provide the intended add amount and to enable the front curvature of the segment to match the curvature of the carrier lens. This results in a smooth front surface and the bifocal junction is indistinguishable by touch. Note in Figure 2 that the countersink is actually circular and the flat top is created by making the top part of the segment out of crown glass which seamlessly blends with the carrier lens. All nonglass bifocals are of one-piece construction, meaning that the entire lens is made from the same material and that the power difference between the distance and near portions is created with changes in curvature. The segment is formed (molded) as part of the lens and the power addition is created with a different curvature (Fig. 2). Because the surface has a discontinuity it is readily identified by touch, this is, the top of the bifocal segment forms a small ledge.
Available bifocal types are shown in Figure 1. The round segment (Figure 1A) has evolved from the Kryptok bifocal and classically has a diameter of 22 mm although other diameters are available. A chief advantage of round segments is that they are the least noticeable cosmetically, especially if a light tint (such as pink) is used. Disadvantages of the round segment are that image jump is fairly large because the distance from the top of the segment to the optical center of the segment is fairly large (i.e., half of the segment diameter). Also, because the top of the segment is curved, the top portion of the segment provides little width of near vision and is not very useful. This effectively enlarges the zone of unusable vision at the top of the segment. The 15 mm round segment is useful for patients engaged in activities such as golf or outdoor labor where a segment is considered bothersome yet the patient still requires some near vision. One successful solution for golfers is to use a monocular 15-mm round segment located superiorly or temporally on the same eye as the handedness of the golfer. Glasses with 15 mm round segments are usually for special usage and the patient will require a separate pair for other daily activities.
FT bifocal segments (Fig. 1B), sometimes still referred to as “D”-type segments, lessen the problems of the round segment at the top of the segment. By eliminating the top of the circle, useful near-field width is attained immediately below the top of the segment line. Also, because the optical center of the segment for FT bifocals is 5 mm below the top line, the magnitude of the image jump is less than for round segments. FT bifocals have become the standard bifocal because of visual advantages, even though they are more noticeable cosmetically than round segments. FT28 is the most common width, however, other widths can be used dependent upon the visual needs of the patient.
The Ultex-type bifocal (Fig. 1C) is characterized by very large amounts of image jump (the optical center is 19 to 20 mm below the segment top) and by limited distance zone in the lower portions of the lens. Because of these characteristics, the lens is not well-suited for general use. However, it is a very good lens for hyperopic patients who perform considerable near work such as at a desk. This is because the hyperopic patient obtains base-up prism with depressed gaze, hence re-quiring excessive depressed gaze to view typical near materials. Base-down prism from the Ultex bifocal counteracts the prism from the plus distance lens and reduces the amount of required gaze depression for near materials. The Ultex bifocal should be considered for this specialty use, but the large amount of image jump at the segment top and the minimal distance zone in the lower lens are detriments to prescribing the lens for general use.
The Executive bifocal (Fig. 1D), sometimes still referred to as a Franklin bifocal, has the advantages of an extremely wide near-viewing zone and no image jump at the top of the segment because the optical center of the add is on the segment line. Despite these advantages, the lens is limited in scope because of the lack of any distance zone in the lower portions of the lens. This is very bothersome for ambulation and general daily wear. The lens is best suited to meet extensive near-viewing needs such as at a desk. However, many practitioners prefer to use a FT35 or FT45 instead for these purposes.
The blended bifocal (Fig. 1E) is a variant of the round bifocal. The boundary between the distance and near zones is blended to produce an apparent seamless lens. Cosmetic appearance is the sole reason to use this lens because the blend area (2 to 3 mm wide) increases the area of unusable vision and decreases the diameter of the usable near vision zone. The curve top bifocal (Fig. 1F) is intermediary to the round and FT bifocals and also possesses intermediary properties. Although it can be a successful general usage lens, it has not enjoyed broad usage probably because of the larger visual advantages of FT bifocals.
First-time bifocal wearers will go through an adaptation period. Some of the most common problems involve the altered location of objects through the bifocal segment, creating problems with stairs, curbs, and similar situations. First-time wearers should be counseled about these initial problems.
|FITTING BIFOCAL LENSES|
|For good visual performance, it is important that the bifocal lenses be
properly placed within the frame and the frame properly fitted to the
face. Before making measurements of lens location, the frame should be
selected and fitted to the patient's face and incorporating the appropriate
pantoscopic tilt. Eyeglass frames with nose pad arms are preferred
because they allow postfitting adjustment of the bifocal height.|
The segment height is specified as the vertical distance from the top edge of the segment to the level of the lowest portion of the lens. The person making the measurement should be located directly in front of the patient and at the same eye level. For most patients the best location of the top of the segment is approximately 1 mm below the lower limbal margin. Round segments should be fitted approximately 1 mm higher than FT segments. Consideration should be given to the location of the previous bifocals (if the patient has previously worn them) and the patient visual needs and preferences. Observe the segment height location in the current bifocals: if the location seems reasonable and the patient is satisfied then duplicate the location in the new spectacles.
Sometimes the current bifocal height is considerably lower or higher (less commonly) than ideal, yet the patient expresses no complaint. Observe patient behavior with the current bifocals to determine if awkward posture is required to use the segment and/or probe with questioning. Change the height only with caution, but do so (probably only partially) if it will result in better vision and patient satisfaction. The patient's particular viewing requirements should also be considered in determining the height of the bifocal. For patients with minimal near-viewing requirements the height should be lowered whereas patients with extensive near-viewing requirements should be fitted higher, perhaps even to the lower pupillary border. In nearly all cases, however, it is safer to err on the low side. Patient dissatisfaction occurs more readily with a segment that is placed too high.
SEGMENT INSET: THE NEAR INTERPUPILLARY DISTANCE
The major reference point (MRP) of the lens is the location that contains the prescribed refractive and prism correction; the MRP is the same as the optical center if there is no prescribed prism. The MRPs of the distance portions of the lenses should be separated in the frame by the same amount as the patient's inter-pupillary distance (PD). However, the bifocal segments should be separated by the same amount as the patient's near PD. The monocular difference between the distance and near PDs is the segment inset, or the horizontal amount by which the center of each segment is nasally displaced with respect to the MRP.
Major influences on the segment inset include the distance PD (Table 1) and the near-viewing distance that is largely driven by the amount of the add (Table 2). However, other influences include the fitting vertex distance (back lens surface to corneal apex) and the power of the distance lens because of the base-in and base-out effects of converging through minus and plus lenses, respectively. In order to determine the segment inset it is best to measure the near PD with the patient viewing the examiner's eye at the intended near-viewing distance and with the intended total lens power and vertex distance.
Assumptions: No distance power, vertex distance 14 mm, 40 cm viewing distance.
PD, prism diopter.
Assumptions: No distance power, vertex distance 14 mm.
As presbyopia advances, a bifocal-wearing patient is no longer able to see clearly at intermediate viewing distances. In early presbyopia, the patient can clearly see at intermediate distances either by using their remaining accommodation and viewing through the distance portion of the lens, or by viewing through the near portion of the lens because the add power is low. With advanced presbyopia the nearest point of clarity through the distance portion of the lens recedes because of the reduced amplitude of accommodation, and the farthest point of clarity through the bifocal comes closer because of the increasing add power. This results in loss of intermediate clarity.
The loss of intermediate clarity with a bifocal typically occurs with a near add of +1.50 or greater. Intermediate viewing distances are important for shopping, general work around the home, computer work, viewing automobile instrument panels, card playing, playing musical instruments, etc. Trifocals are useful for many occupations. Many bifocal-wearing patients transparently adapt to this problem by adjusting their viewing distance to the object of regard or simply tolerating the problems. However, even if the patient is not aware of the clarity problem at intermediate distances, that does not mean they would not want a solution. Regardless patient awareness, consultation is indicated. One solution is to prescribe a PAL. Bifocal wearers can successfully change to a PAL. In the absence of desire to try a PAL, however, a trifocal lens is the appropriate solution.
Trifocal designs are variants of bifocal designs, three of which are shown in Figure 3A to 3C. If a patient has successfully adapted to a particular type of bifocal then the trifocal variant of that same design will likely be most successful. FT trifocals are the most commonly prescribed (Fig. 3A). The width of the segment can be chosen to match the current bifocal, or adjusted in size if necessary based on patient needs. The vertical dimension, or depth of the intermediate segment, is most commonly 7 mm. Larger depths are available and can be used for patients with extensive intermediate viewing needs such as at a computer. Similarly, there are executive trifocals (Fig. 3B), combination executive/FT trifocal (Fig. 3C), and round, curve-top, and Ultex trifocals (not shown). The features and benefits of the trifocal types mirror those of the bifocals as discussed above.
The fitting height of the trifocal is specified from the lowest edge of the lens to the top of the trifocal segment. The top of the trifocal segment is typically located at the lower pupillary margin but can vary slightly dependent upon patient needs as discussed above for bifocals. Compared to a bifocal, the depth of the intermediate segment, typically 7 mm, is at the expense of the distance and near-visual spaces. Larger intermediate depths (8 to 14) can be used for special purposes such as computer work, but often need to be fit somewhat higher. The larger trifocal depths (10 to 14) are usually reserved for special occupational needs and often do not serve well as general purpose lenses.
For most trifocals the intermediate add is 50% of the bifocal add, however intermediate add powers of 40% to 60% are also available in some designs. These other values can be used if there is reason to bias the intermediate viewing zone toward distance or near. For computer work a 60% intermediate portion can work better because the computer is typically located at the nearer end of intermediate vision.
Some presbyopic patients need to view overhead tasks at near or intermediate viewing distances. This can be true for car mechanics; painters and wallpaper hangers; some assembly line workers; pharmacists, librarians, and others with a need to view products on overhead shelves, etc. Typical bifocals or trifocals require the user to tilt the head severely to see these overhead objects. Double segments, samples shown in Figures 3D to 3F, are designed to meet such overhead viewing needs. The upper and lower segments usually have the same power. In some designs the upper segment is available in an add power that is intermediate to the bottom segment, of course designed for patients with overhead needs at intermediate distances. The typical separation between the top and bottom segments is 14 to 15 mm. The lens should be fitted by specifying the height of the lower bifocal segment as with standard bifocals; location of the upper segment is not specified.
|PROGRESSIVE ADDITION LENSES|
Since their introduction to the U.S. market in the 1960s,3 PALs have steadily increased their share of the multifocal market. Several studies have shown a large percentage of patients prefer PALs compared to bifocal lenses.4–6 PALs provide a continuous change of power from distance through intermediate to near that provides the wearer with a seamless visual space and eliminates the unusable area of visual space caused by the top line of a bifocal segment. A detracting feature of PALs is that the design necessarily results in unwanted astigmatism in the periphery of the lens, usually located in the lower diagonals relative to lens center.
PALs have both cosmetic and vision advantages compared to segmented multifocals. The cosmetic benefit results from the seamless design and is apparent. The vision advantage results from elimination of the bisecting region of unusable vision associated with the top of the bifocal segment, resulting in contiguous visual space from distance through intermediate to near. The vision advantage with PALs compared to bifocals is supported by a study that showed patient preference for PALs compared to a blended bifocal,7 both of which are seamless. The strong preference for PALs compared to bifocals is further supported by another study8 in which 265 habitual bifocal wearers were fitted with PALs: 92% of these patients preferred the PALs. Because progressive lenses have no image jump and no areas of intermediate blur, many wearers describe their vision with progressive lenses as more natural than with bifocals.
Progressive addition lenses are designed to provide distance viewing in straight-ahead gaze, a gradual progression of power in an intermediate corridor, and the full addition power lower in the lens. The power change is accomplished by increasing the curvature of the front surface (i.e., decreasing the radius of curvature) along the corridor toward the bottom of the lens. Because the add must be spherical in nature, the curvature must increase equally in all meridians (i.e., it must increase in the horizontal as well as the vertical meridian). As result, the horizontal curvature is flatter in the upper portion of the lens and steeper in the lower portion of the lens. To reconcile the curvature disparity and to make a seamless lens, surface curvature must be altered in the lower quadrants of the lens. This results in the unwanted astigmatism. Therefore, a necessary and undesirable side effect of the gradual progression of power is unwanted astigmatism in the periphery of the lens. The pattern of unwanted astigmatism is a defining characteristic of individual PAL designs.
The unwanted astigmatism limits the error-free viewing zones of the PAL, resulting in considerably narrower error-free distance, intermediate, and near-fixation fields than typical bifocal lenses. The magnitude and location of unwanted astigmatism are worse with higher adds (greater change in curvature required) and when the distance and near centers are closer to one another (curvature must change over a shorter distance). Primarily as a result of the latter, the near zone of a PAL is lower in the lens than for a FT bifocal. Whereas a very wide near zone is attained with a FT bifocal with approximately 16 to 20 degrees of ocular depression, most PALs require gaze depression of 30 to 35 degrees to obtain the near zone, one that is also considerably narrower than provided by a FT bifocal.
A common method to represent the optics of PALs is with contour plots of the spherical equivalent and unwanted astigmatism powers as shown in Figures 4 and 5. Because of the complex nature of the optics there are literally an infinite number of possible PAL designs and dozens of different designs are currently available. Although most current PALs defy clear categorization, it is instructive to consider the differences between a “hard” and “soft” design. The lens in Figure 4 shows characteristics of a hard design. Relatively speaking, a lens of hard design has wider error-free distance and near-viewing zones, the area of the lens with unwanted astigmatism is relatively small, and the magnitude of the unwanted astigmatism is large. The separation between the distance and near centers is shorter and the rate of power change along the corridor is greater. The lens in Figure 5 shows soft design characteristics: narrower error-free distance and near-viewing areas, larger area of unwanted astigmatism but of lower magnitude, distance and near centers are farther apart, and a lower rate of power change in the corridor.
To accommodate the convergence that occurs with viewing at closer distances, the central power corridor is nasally angled toward the bottom of the lens to accommodate ocular convergence at near-viewing distances. Older PAL designs used the same lens for right and left lenses but angled in opposite directions. This approach resulted in the pattern of zone widths and unwanted astigmatism being different to the two eyes. Most current PALs use different lenses for right and left (essentially mirror images of one another) in order to present the same optical characteristics to each eye.
The width and area of the error-free distance, intermediate, and near viewing as well as the magnitude and distribution of unwanted astigmatism can vary significantly across lens designs.9,10 There is considerable interdependence of the sizes and locations of the viewing zones and the magnitude of unwanted astigmatism that make it currently impossible to design a lens that is optimized for all optical attributes. Hence, a particular PAL design may optimize one viewing zone or characteristic, but it is at the expense of the other zones or characteristics.
The widths and areas of the three viewing zones (distance, intermediate, and near) and the magnitude of unwanted astigmatism have been reported.11 The mean measurements are shown graphically in Figure 6. Analysis showed that even the largest intermediate and near error-free zones are smaller than those required to view a typical computer screen or standard paper respectively,11 and also smaller than the normal amount of ocular rotation used to view noncentral targets.12 This means that the PAL wearer must learn to move their head more and their eyes less in viewing noncentral objects and/or tolerate some blurof typical noncentral foveally fixated objects. It is most likely that the head and eye movement patterns are altered by PAL wearers,13 but the high acceptance and preference rates for PALs also indicate that patients fairly readily are able to do so.
PRESCRIBING PROGRESSIVE ADDITION LENSES
For most patients a PAL satisfies general vision needs better than a segmented bifocal. PALs should be considered a first option for general visual use, unless cost is a consideration or the patient has specific occupational or other visual needs that require the wider error-free fields provided by segmented bifocals.
Current successful bifocal or trifocal wearers will remain successful with same lens design, although they would likely prefer the advantages of a PAL. A large percentage of successful bifocal wearers prefer a PAL if given the choice,8 although there is some risk in making the change. Determine if the bifocal-wearing patient has specific occupational or other vision needs that preclude recommending change to a PAL. Otherwise, the patient can be advised of the vision and cosmetic differences between the lens types in order to decide upon a possible change.
For a first-time PAL wearer, selection of the particular PAL design should consider the patient's specific visual needs. If a patient is successfully wearing a particular design of PAL, they will likely continue to be successful with the same design. However, consideration should be given to a different PAL design if it better meets their specific visual needs.
As a direct result of the trade-offs in PAL design and the fact that the various designs utilize different trade-offs, some PALs can be expected to provide better vision at distance, intermediate, near or various combinations of those distances. Ratings of 28 PAL designs based upon the widths and areas of the distance, intermediate and near-viewing zones and also the magnitude of unwanted astigmatism are presented in Tables 3 to 7.11 These tables identify those lenses that can best meet the specific visual needs of particular patients. Just as there is a range of optical characteristics among PALs, there is also a range of visual needs among patients. The clinical task is to match the two.
Table 3 shows lens ratings along the single attributes of distance zone, intermediate zone, and astigmatism. These ratings are useful for those patients for whom there is a single over-riding need for one of those attributes. For example, the distance rankings are used for patients who drive a lot or are involved in outdoor work and have only occasional need for intermediate and near vision. The intermediate rankings are used for patients who want a general purpose PAL but whose primary need is viewing a computer or performing a manufacturing task at intermediate viewing distances. However, such patients may benefit even more from an occupational progressive lens (see section on occupational progressive lenses [OPLs]). The rankings based on astigmatism are used for those patients who are very sensitive to unwanted astigmatism and for whom this is the most important lens attribute.
Higher ratings indicate larger and wider areas of vision and lower astigmatism magnitude. These ratings are useful for patients with a single over riding need for distance vision, intermediate vision, or reduction of unwanted astigmatism. (Reprinted with permission of Optometry.)
Table 4 shows ratings based only on the near-viewing zone. These ratings are useful for those patients for whom near vision is the single overriding need. This includes patients who perform extensive near tasks yet also desire a general purpose PAL. It also includes emmetropic patients who intend to use the lenses primarily as reading glasses but also desire a general purpose PAL. An OPL should be considered for these patients. The near ratings in Table 4 are directly derived from width and area magnitudes. As result, the ratings generally increase with greater fitting height because a greater fitting height necessarily results in a larger near-viewing zone for all lenses. The larger ratings for greater fitting heights represent the fact that the patient will have a larger and wider near-viewing zone with a greater fitting height. Only a few lenses provide any near addition for a fitting height of 16 mm. Note also that the lens rankings change somewhat as a function of the fitting height. This is because the manner in which the near zone changes toward the bottom of the lens is different for the various lenses
These ratings are useful for those patients for whom near vision is the single overriding need. Ratings increase with greater fitting heights commensurate with the greater near viewing zone sizes thereby attained. (Reprinted with permission of Optometry.)
The ratings in Table 5 combine viewing zones and are based on equal weightings of the distance and intermediate ratings and the intermediate and near (fitting height of 22 mm) ratings. The two columns on the left do not consider the amount of unwanted astigmatism, whereas those on the right include a 25% weighting of astigmatism. The distance/intermediate ratings apply to patients with primary visual needs at those two distances and for whom near viewing is not as important. This includes drivers or those with outdoor occupations and recreations who have distance and intermediate viewing needs but for whom near viewing is not as important. The intermediate and near category applies to patients who largely work indoors and primarily perform visual tasks at those viewing distances and for whom distance vision is not as important. The ratings that include astigmatism weighting apply to patients with sensitivity to unwanted astigmatism.
The distance/intermediate ratings apply to patients with primary visual needs at those 2 distances, near viewing is not factored. The intermediate and near category applies to patients that primarily perform visual tasks at those viewing distances and for whom distance vision is not as important. The ratings that include astigmatism weighting apply to patients with sensitivity to unwanted astigmatism. (Reprinted with permission of Optometry.)
The ratings in Table 6 apply for general usage needs. They combine ratings for distance, intermediate, and near-viewing zones or only distance and near zones; ratings are provided based on fitting height of both 18 and 22 mm. The ratings based upon all three zones apply for patients with general viewing needs at all three distances. If the intermediate viewing zone is not particularly important to the patient, then the ratings based on distance and near apply. Unwanted astigmatism is not factored into the ratings in Table 6. The ratings in Table 7 are the same as in Table 6 except that unwanted astigmatism is factored into the rating. These are for patients with general viewing needs and for whom reduced astigmatism is also important.
Distance/intermediate/near ratings useful for patients with general visual needs, distance/near ratings useful for patients without intermediate needs. Ratings calculated for fitting height (FH) of 18 and 22, representative of low and high fitting heights, respectively. (Reprinted with permission of Optometry.)
Same as Table 4 but with 25% weighting for unwanted astigmatism. Same use as Table 4 but for patients with sensitivity to unwanted astigmatism. (Reprinted with permission of Optometry.)
FITTING PROGRESSIVE ADDITION LENSES
Both the vertical and horizontal positioning of the PAL within the eye of the frame are critical. Because the near addition in a PAL is relatively low in the lens, a frame with a relatively large B (vertical) dimension should be selected, preferably larger than 30 mm. Frames with adjustable nose pads are also advisable, to enable postdispensing adjustment of the location of the lenses before the eyes. The frame must be properly adjusted to include pantoscopic angle, vertex distance, and vertical wearing position prior to measuring the lens location.
PALs are designed to have the fitting cross placed directly in front of the pupil with normal straight-ahead gaze. For each design this provides the optimal location of the viewing zones and the optimal binocular use of the lenses. The vertical height of the fitting cross should be measured from the pupil down to the lowest edge of the lens in the frame. The ratings in Table 4 show that considerably greater near areas and widths are attained with greater fitting heights. Only a few PALs provide a near-viewing zone with a fitting height of 16, and a fitting height of 22 provides considerably greater near zone than a fitting height of 18 for all of the lenses. Small frames, which result in shorter fitting heights, require careful selection of PAL design (far left columns of Table 2).
Even with a properly selected short-corridor PAL, fitting heights shorter than 18 necessarily result in some compromise of the near-viewing zone and the patient should be advised of such. The compromised near zone will be noticeable in conditions of low illumination (menu in a restaurant) because of a larger pupil, high reading demands (office work or reading a book in the evening), or viewing small print (phone book). Many patients consider these as reasonable trade-offs for the fashionable small frame. Separate reading glasses or OPLs can be prescribed for specific near-viewing tasks.
Accurate measurement of the PD is particularly critical for successful wearing of a PAL. This is because of the narrowness of the intermediate channel that is directly below the fitting cross. The narrowest portion of the average PAL (Fig. 6) is approximately 3 mm, or about the same size as the pupil of the eye. If the pupils of both eyes are to be able to simultaneously view through the channel, there is essentially no tolerance for error in the separation of the two lenses. Any error in the separation of the lenses relative to the PD of the patient results in one of the eyes viewing through an edge of the channel when the other is centered in it. Improper measurement of PD may be the largest reason for patient nonacceptance of PALs.
PD measurement with a pupillometer is probably the most accurate method. Accurate measurement of the total PD is the most important, however, it is also advisable to specify the split, or monocular components of the PD as enabled by a pupillometer.
ADAPTATION TO PALS
Some patients have difficulty adapting to PALs. However, this problem has lessened considerably with newer designs and the high acceptance rates of PALs indicate this problem is becoming uncommon. Adaptation problems can be related to spatial distortion resultant from the pattern of unwanted astigmatism in the lower quadrants of the lens. Such patients will report swimming sensations, spatial disorientation, warped appearance of lines, or vague complaints. Other patients may have difficulty with the width of the distance, intermediate, or near-viewing zones. Distortion is less and zones are wider for lower adds, therefore, adaptation is easiest for the beginning presbyope. Adaptation to a PAL will be more difficult for a first-time wearer with a higher add. As with most spectacle lens adaptation, it may take up to 2 weeks of continuous wear for adaptation.
Adaptation difficulties can often be managed by adjusting the pantoscopic angle, the vertex distance, or the fitting height (if the frame has adjustable nose pads). Verify that the distance between lens centers matches the patient's PD; mismatch can certainly cause adaptation problems. If the patient difficulties concern the width or area of one of the viewing zones, consult Tables 3 through 7 to determine if the complaints match the lens characteristics and whether another PAL design would resolve the problem. If the problems cannot be resolved, common industry practice is to replace the PAL lenses with bifocals with no additional charge to doctor or patient.
|OCCUPATIONAL PROGRESSIVE LENSES|
DESIGN AND OPTICAL PROPERTIES
OPLs utilize progressive power optics and are designed primarily to meet typical indoor viewing needs of presbyopic patients. OPLs provide near vision in the lower portion of the lens, a wide field of intermediate vision in the straight-ahead position, and far intermediate vision in the top of the lens. OPLs are not designed to meet typical distance viewing needs and they are not intended to meet general vision needs. The need for this type of lens has been driven by the large numbers of people working in offices and at computers. However, OPLs also meet the viewing needs of many people who work in other indoor environments.
As with PALs there is wide variation in OPL designs. Stylized designs of both a PAL and an OPL are shown in Figure 7 to demonstrate the principal differences between the two. Most OPL designs do not include any distance correction whereas a wide clear field of distance vision is important to PAL design. OPLs are designed to provide a wide field of intermediate vision, contrasted with the narrow intermediate vision provided by PALs. The unwanted peripheral astigmatism that necessarily accompanies any lens with progressive optics is typically located higher in the lens in an OPL compared to a PAL because indoor office work comprises more downward viewing than typical outdoor environments. The magnitude of the unwanted astigmatism will typically be less with an OPL than with a PAL. This is because the power change in an OPL is less (the OPL typically does not include the full distance power) and the poles of the power extremes are farther apart than for a PAL. Both of these factors serve to reduce the amount of unwanted astigmatism.
OPLs are commonly prescribed by writing the typical distance prescription with near addition and specifying an OPL design. The laboratory fabricates the lens to have the prescribed near power in the bottom of the lens. This is atypical because all other lenses are fabricated to have the prescribed distance power at a designed location near the center or upper portion of the lens. The powers provided in the middle and upper portions of the OPL are determined by the lens design relative to the near prescription.
Because the near power is used as the fabrication reference for OPLs, the power change that occurs in an OPL is specified from the near reference. This results in specification of a negative power progression that occurs with increasing height in an OPL, exactly the opposite manner of specifying power change in a PAL. This power change in an OPL is referred to as a power “degression.”
The amount of degression in an OPL, along with the add power, determines the power and hence the amount of distance blur in the top portion of the lens. Most of the OPL designs have a power degression less than the near add, which results in an add of 0.25 to 0.75 D in the top of the lens. This amount of plus in the top of the lens is usually acceptable for an indoor environment. Although a full distance power in the top of the lens may be needed for some patients, it may not be the best correction to meet the needs of many other patients and, in some ways, degression to the full distance power defeats some of the advantages of the OPL design. The lower degression (compared to the full degression to accomplish full distance power) enables a wider corridor of clear intermediate vision and lower magnitude of unwanted astigmatism. Selection of the amount of degression involves a trade-off between clarity of distance vision and the usable field of intermediate and near vision.
PRESCRIBING OCCUPATIONAL PROGRESSIVE LENSES
An important factor in the success of most OPLs is the far-intermediate power in the top of the lens. With far-intermediate correction in the top of the lens the patient can navigate hallways, recognize co-workers, attend meetings, etc. This is important because the patient does not need to remove the glasses each time they leave their desk as they would need to do with reading glasses. Most patients will not use computer glasses that need to be removed each time the leave their desk. The intermediate and near-viewing areas of an OPL enable the patient to function at their desk and computer, whereas the far intermediate zone in the top of the lens enables the patient to navigate the office while wearing the lenses.
Work at a computer is the most common task for which OPLs are designed and prescribed. Bifocals and PALs do not work well for computer users who require a lens addition for viewing intermediate distances. Early presbyopic patients with near adds of approximately 1.50 D or less have enough remaining accommodation to focus on the computer display through the distance portion of the lens. However, once the patient requires an add for intermediate viewing, they can no longer comfortably use a bifocal or PAL. A bifocal lens is usually prescribed for a 40-cm viewing distance and the bifocal segment is located for a downward viewing angle of approximately 25 degrees. The computer is typically at a distance of 50 to 70 cm and a downward viewing angle of 10 to 20 degrees. The bifocal wearer must awkwardly tilt the head back and lean toward the computer display to see the screen clearly through the near portion of the lens. A progressive addition lens is likewise limiting at the computer because the area of the lens containing the intermediate power is small and narrow, thereby requiring head instead of eye movements to scan the computer screen. An OPL has a wide corridor of usable intermediate power that is located well for computer viewing.
OPLs meet visual needs in many other situations in addition to computer use. They work well for many indoor needs such as housework, manufacturing facilities, medical offices, indoor sales, etc. They also work well instead of bifocals for pediatric patients. For pediatric patients, the OPL should be designed to provide some distance vision in the top of the lens for viewing the front of a classroom. OPLs are also a good substitute for single vision reading glasses, because they provide a good reading area but also far-intermediate vision in the top of the lens for viewing objects such as a television across the room.
Several OPLs are available in a series of discrete degression amounts. These include Access (SOLA International, San Diego, CA), Cosmolit Office (Rodenstock, Munchen, Germany), Office (Shamir Insight, Inc., aka Desktop by Hoya, San Diego, CA). The degression amounts available for each are shown in Table 8. The specific degression power is typically selected by the optical laboratory based on the near add power in the prescription as indicated in Table 8. The amount of add remaining in the top of the lens is shown in the right column of Table 8. The clinician can specify that the laboratory provide a nonstandard degression if patient viewing distances and intended usage vary significantly from typical indoor usage. For example, it is often possible to select a degression amount that matches the patient's near add, thereby providing the distance power in the top of the lens. This may be desirable for a child in a classroom or for a stockbroker to see the ticker display across the room. Although this approach provides a small area of distance correction in the top of the lens, it is too small in area to meet nonindoor distance viewing requirements.
Each degression value serves a small range of near addition powers, the resulting range of add power in the top of the lens for the add range is shown in the right column.
Gradal (Carl Zeiss Optical, Inc., Chester, VA) is an OPL that has a full range of degression powers in 0.25 D steps; the degression amount is selected by the laboratory so that +0.50 D add remains in the top of the lens, unlike the lenses in Table 8 in which the amount of add in the top of the lens varies dependent on near add.
Two OPLs, Tact (Hoya) and Technica (American Optical, New York, NY) are designed to have a small area of distance power in the top of the lens (i.e., the degression amount equals the add amount). The distance viewing area is small and located high in the lens. These lenses can be used for patients with limited but critical distance viewing needs while in an indoor environment.
|ANISOMETROPIA—VERTICAL IMBALANCE AT NEAR|
|Any spectacle-wearing patient with anisometropia will obtain different
amounts of prism to each eye when viewing away from the optical
center. For example, what differential prism is generated when the following
patient depresses their gaze 20 degrees to read?|
When the patient depresses their eyes 20 degrees, this represents viewing through a lens location that is 10 mm from the optical center (2 degrees = approximately 1 mm). By Prentice's rule, the amount of prism (in prism diopters or Δ) is equal to the distance from the optical center (in centimeters) multiplied by the power. Hence, the prism at that lens location in the right eye is 2Δ base-down (1 cm × 2 D) and in the left eye it is 5Δ base-down. This is a net of 3Δ base-down in the left eye.
This amount of induced differential prism might be expected to cause significant difficulty for the patient, but this is not always the case. It has been shown that most anisometropic patients will measure orthophoria in both straight-ahead and in down-gaze through spectacles.14 This seems to indicate that binocular eye movements can adjust to the differential prism demands. If the patient is wearing single vision lenses, the patient can also adjust to this problem by using more head movement than eye movements when viewing peripheral objects thus maintaining fixation close to the optical centers of the lenses and avoiding the differential prism effects. However, when a patient is prescribed multifocals, they are now required to view through a peripheral portion of the lenses in order to use the near portion of the lens. The bifocal-wearing patient cannot avoid the induced prism problem by using head instead of eye movements. Some bifocal wearing patients have difficulty with this and resultant discomfort while reading.
It can be determined if this is a problem for a particular patient by having them view downward through their spectacle lenses at a typical reading angle, interpose corrective vertical prism before their eyes (in the example above, this would be 3Δ base-up in the left eye), and note if the patient is more comfortable. Vertical phoria or fixation disparity can also be tested in down gaze through the glasses. Observe if the patient is a head mover with their glasses, this is an indication that differential vertical prism may be a problem.
One method of solving this problem is to prescribe a separate pair of reading glasses with the optical centers placed lower in the frame allowing the patient to read without encountering much differential prism.
The most common method of solving this problem is with bicentric grind, or what is referred to as “slab-off.” This is a special grinding technique applied to the back surface of one of the two lenses by the optical laboratory. Essentially the laboratory grinds the back surface with two different grinding centers, one higher than the other. This results in a horizontal line of demarcation that extends across the entire lens. There is a sudden prism jump at that line, and the amount of the prism jump is calculated to eliminate the induced differential prism at a given reading level. The amount of prism in the slab-off can be specified in the laboratory order, or the practitioner can specify the reading level at which prism equalization is desired and the laboratory will calculate the amount. The line is normally placed to be colocated with the top of the bifocal or trifocal segment. This procedure can also be performed on a PAL.
|FUTURE OF MULTIFOCAL LENSES|
|Multifocal lenses are, by far, the most common clinical treatment of presbyopia. Although newer surgical corrections of presbyopia will certainly gain usage in the future, it is likely that multifocal lenses will be a mainstay for quite some time. Progressive addition lenses will continue to gain market share and segmented multifocal lenses will become less common. Newer designs of progressive addition lenses will result in better visual function and special purpose PALs will also become more common.|