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Slide 1 |
With age, people become presbyopic (i.e., experience loss in power of accomodation). Reading small print, performing operations on small objects, or working with detailed technical drawings may become difficult. Reading glasses, which are spectacles with a magnification (plus power added to the distance correction), satisfy users whose demands are not complex and do not need to rapidly alternate between close and distant views. However, people who must read or manipulate small objects may find putting on and removing glasses cumbersome. For those who also require correction for distant vision, a switch between distance spectacles and reading glasses may be required.
Bifocals provide patients with the correction needed for most activities - distant and near. Bifocals are not new - they were invented in the 18th century. Later came trifocals with the addition of a third section for mid-range correction between close and distant vision.
Further addition of discrete corrective powers is not practical because the vertical range of view becomes smaller with each segment of correction and would be associated with an attendant "jump" at each change of lens power. An analytically minded patient or practitioner may ask, "Why not distribute the corrective change gradually from distant to near over the lower part of the lens?" The evolution of spectacles represents a logical extension from the monofocal to bifocal to trifocal, and now to a gradually varying lens power.
This next generation lens has a progressive addition of power and is called a progressive addition lens (PAL). Though not as old as the bifocal, neither is this a new idea - a U.S. patent was granted in 1938 to C.E. Evans for a lens of varying power.
The graduated power approach:
The sphere power of a lens is based upon the difference in the curvatures (inverse of the radii) of the front and back surfaces of the lens and on the index of refraction of the lens material. A lens has positive power and moves the resulting image closer to the object when the radius of the rear surface exceeds that of the front surface (i.e., the back surface is flatter than the front, a net convex lens).
Conversely, the lens has negative power and moves the resulting image farther from the object when the radius of the rear surface is less than that of the front surface (i.e., a net concave lens).
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Progressive addition results when the net power is made increasingly more positive and is produced on the lower portion of the lens where a more "plus" power is needed for close viewing. Generally, progressive addition is accomplished by beginning with the front surface radius of the upper (distance) portion of the lens and gradually reducing the radius (increasing the curvature) toward the bottom of the lens (Slide 1).
A PAL consists of at least two separate geometries that serve the different sight distances - both of which must be applied to a single surface. The radii operate in two dimensions (X and Y or height and width) to affect the resulting dimension (Z or depth).
The fundamental challenge in design is providing a transition between different shapes on a single surface without inducing optically significant discontinuities. Because the geometries may not be compatible, distortions may be evident in the area of transition.
Different manufacturers resort to different ways to blend the two prescriptions, and the blend becomes the initial objective of the design effort. Therefore, other requirements of the lens design must be subordinated to the goal of surmounting this hurdle.
Properties Associated with PALs
Most of the parameters used to describe conventional single-vision lenses apply to PALs. This tutorial, however, deals with properties exclusive to PALs.
Hard and soft lens design
Hard and soft does not refer to the physical properties of lens material, but to the concentration and rate of change of irregular, non-corrective astigmatism. Design is largely influenced by the rate of change of (or inversely, to the distance between) distant and near correction along a vertical path (or umbilical, central, line) as well as the rate of change laterally away from the umbilical within the reading area. A hard design is relatively abrupt, with the transition from distant to near vision taking place along a short vertical corridor. In a soft design, the transition takes place along a greater vertical distance.
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The hard design has an area of the lens free from irregular astigmatism (with its associated blur and distortion) but, where the astigmatism occurs, it is concentrated and abrupt.
Conversely, the soft design provides a smaller area of ideal vision by allowing distribution of astigmatism error over a larger area, but the astigmatism-induced distortion is less abrupt or noticeable to the wearer.
The graph in Slide 2 is often used to show lens power along the umbilical line in a simple one-dimensional diagram. For a more complete understanding of the lens characteristics, two-dimensional diagrams are required.
Mapping the distribution of astigmatism and spherical power
Unlike a single-vision lens, optical power of a PAL varies across the surface of the lens. Astigmatism also changes as a secondary result related to the rate and location in changes of power.
Several methods are used to describe the distribution of power and astigmatism as well as to show its rate of change. The most common are contour maps of these characteristics, in which lines of constant power or astigmatism are drawn across an outline of the lens.
Like contours of elevation on a topographic map, the lines are labeled to show located values of constant power (Slide 3), or astigmatism (Slide 4). The rate of change of either characteristic is observable in these contour diagrams because high rates of change are seen as close spacing of the contours.
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Index of refraction for lens materials
Raising the index of refraction of the material from which a lens is made enables the lens to be designed physically thinner than with a lower index material. The most often cited advantages of high index lenses are aesthetics and lighter weight. However, practical advantages exist as well, for the thinner lens reduces certain types of distortion.
With less thickness between front and back curves, reduced lateral (radial) offset of a ray results from traveling a shorter distance between the two surfaces, more nearly approaching the idealized textbook thin lens. This reduction can minimize the distortion in high-powered lenses, particularly the "pincushion" in plus lenses and the "barrel" in minus lenses.
For the same reasons, lenses with a greater base curve power can reduce off-center astigmatism. While not strictly limited to PALs, the reduction of any type of aberration will aid in the wearer's acceptance of the lens.
Asymmetric design
In PALs, the corridor of transition to near vision follows a near vertical line down the center. As the wearer adjusts his or her gaze from distant to near objects, the eyes converge, requiring that the corridor of best vision be positioned more nasal.
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In the past, PAL lenses were simply rotated to bring the near vision portions closer together. However, that approach resulted in uneven images between the eyes when the intermediate or distant gaze was shifted laterally. When looking to the right at an intermediate distance, for example, the left eye would be looking through a portion of the lens transitioning to near vision, while the right eye would be looking through a portion higher in the more distant correction area (Slide 5). Therefore, most manufacturers now provide separate right and left designs.
Progressive addition lenses can be a source of pleasure or a major aggravation for patients and practitioners. Progressive addition lenses are among the most frequent causes of patient complaints about glasses, and resolving such complaints can easily upset a busy office schedule. Patients mention problems adjusting to the glasses, trouble reading, peripheral distortion and blur, limited reading zone, and too much head movement required for clear vision. Proper choice of frames and lens design is worth the extra effort in the time it saves by reducing repeat evaluations.
Frames
Frames should be neither too large nor too small. Because the progressive corridor height is fixed by the manufacturer, when a small frame is chosen the optician must cut down the area for distance vision or reading or both, and a greater percentage of the lens area is occupied by the intermediate corridor than would occur with a larger frame. Even when the spectacles are fit perfectly, some patients find the reading zone too limited. In other instances, the distance area may have been minimized to the point that the patient is aware of the peripheral irregular astigmatic zones and is unable to scan side to side when driving. Although opticians sell PALs in frames as small as 26 mm (vertical), my colleagues and I advise no less than 30 mm and prefer 34 mm or greater.
Oversized glasses create a different set of problems for patients wearing PALs. The optician often makes these with a large distance zone which pushes the reading area toward the lower portion. This often results in the reading zone positioned too low in front of the eyes, forcing patients to turn their eyes excessively inferiorly or to raise their chin to look through the add in the lens. Added weight of the large lenses tends to cause the glasses to slip down the nose, further displacing the reading area below the normal line of sight.
Frames that are fit close to the face often position the reading zone at an extreme angle below the primary gaze so that the patient must infraduct the eyes too far for comfortable reading, similar to the situation when the frame slides down the nose. The optimal frame for PALs is at a moderate vertex distance.
Patient selection
The majority of patients experience a smooth adjustment to the use of PALs and will find PALs versatile in the work environment and home activities. However, some patients are less likely to achieve success than others.
Because effective use of PALs requires the patient to turn the head and face a target to avoid scanning into the peripheral zone of irregular astigmatism, patients who have problems with head movement are not the best candidates.
Fused cervical discs should be considered a contraindication for PALs, as should a neck brace or soft collar. Patients with strabismus that prevents eye movements into the necessary fields of gaze should also be told not to try PALs.
For example, a patient with thyroid orbitopathy who is unable to infraduct his or her eyes would have difficulty looking far enough down to use the small reading area at the bottom of the frame, but use of a flat-top bifocal segment positioned below the distance line of sight may be successful.
Patients who have a history of difficulty adjusting to new spectacles or changes of any type in the pre-presbyopic years may be poor candidates for PALs. The patient who struggles with every "better one or two" choice during refraction may also be distressed or disoriented by the peripheral distortions of PALs.
Myopes and progressives
Patients with myopia may be more sensitive to fluctuations in acuity than patients with hyperopia. As the hyperopic patient becomes presbyopic, he or she loses near acuity, but maintains good distance vision, albeit with occasional moments of blur from fluctuating accommodative effort.
A patient with myopia who is pre-presbyopic, wearing accurate corrective lenses, can see sharply at any distance, from infinity to near. As presbyopia approaches, the patient who is highly myopic must continue to wear spectacles for most activities, even as reading becomes more difficult. All but close targets are unresolvable if he or she removes the glasses.
Therefore, for a patient with myopia with emerging presbyopia, a PAL can match the pre-presbyopic state better than a bifocal, serving the entire visual range without major discontinuities. This may be of greater importance than the cosmetic advantage of a no-line bifocal.
The PAL experience may prove disappointing, however, unless the lens has been selected with attention to criteria specific to the needs of the patient with myopia. Each manufacturer designs PALs by unique, proprietary criteria, and all PALs do not provide the same visual experience.
Many lenses provide a small area for the prescribed distance correction, and another zone of good close vision, with most everything else in the transition corridor. For the patient whose corrective needs are primarily for reading, this may be satisfactory.
However, for a patient who is seriously nearsighted, a larger distance area is needed to provide adequate peripheral visual field, certainly most of the lens above the midline. Therefore (unless the patient is willing to purchase PALs only as reading glasses), it is imperative to ensure that the selected lens is a design that does not compromise distant vision.
Although PALs are especially beneficial for patients with high myopia, those with moderate ametropia (i.e., -1 D to -3 D) often find the small reading area difficult to use and frequently resort to removing the PALs to read without correction. Such patients are especially unlikely to adapt if a small frame is chosen.
Introducing patients to progressives
The ideal time to start patients with PALs is when they start their first pair of multifocal glasses. At the beginning of presbyopia, the add power will be lowest, resulting in a softer change from the distance zone to the reading area, and usually a wider reading area. The distortions in the lenses will be less than would be present with a strong add. Because a patient may have a significant amount of accommodative ability remaining, if he or she does not rotate his eyes fully into the maximum add area, then reading may still be clear.
Patients who start presbyopia in flat-top or executive styles of bifocals may have a difficult time switching to progressives later, especially if the conversion is not attempted until the "senior years" These authors try to discourage such patients from making the switch unless they are highly motivated to do so. (It is also quite a problem for progressive wearers to trade into flat-top bifocals. They almost uniformly feel the loss of the variable focus properties of the progressive and miss the ability to see well at the intermediate range.)
When prescribing progressive addition lenses as the patient's first multifocal glasses, it is suggested that one take the time to educate the patient regarding the plusses and minuses of these lenses. These authors promote progressives as the most versatile lenses available, capable of providing excellent vision at any distance from far away to the closest the bifocal power will allow.
We tell patients to expect the reading area to be smaller (both vertically and horizontally) than what they have been using in the past and to be lower before their eyes, requiring print to be held in a best position. Scanning an opened newspaper would not be as easy as a folded newspaper. We inform them that they will need to turn their head to point their nose where they want to see, and they may be conscious of the need to raise the chin to see through the zone best suited for a near target. We let them know their first sensation with the glasses may be a sense of peripheral distortion or "swimming at the edge of vision," but that this effect will disappear with continued use of the glasses as the brain adapts its expectations.
Listening to the patient
When dealing with patients new to PALs, the professional must understand the difference between complaints regarding a lack of clear vision (often a lack of constant distance correction in the case of patients with myopia) versus complaints dealing with "swimming" or distortion. The practitioner must not automatically assume that every complaint indicates that the patient has trouble getting used to the PALs. Repeat evaluation of the accuracy of the refraction, examination of the lens fit , checking of the optical center positions, and review of the environment in which patients use the glasses may be necessary to determine the cause of their complaints.
Adherence to fitting guides by manufacturers is more critical for PALs than for conventional lenses. Length of the transition corridor and selection of a lens compatible with the patient's frame selection (or advice on a frame's suitability) are essential to performance.
Because certain lenses emphasize performance in particular areas, knowledge of lens characteristics can be considered part of the fitting process. This last requirement is not always made easy by manufacturers.
Information from manufacturers and suppliers
A key factor in lens selection for patients with myopia is two-dimensionality of sphere power (Slide 3). However, such a map is frequently difficult to locate. Contour maps of astigmatism in two dimensions (Slide 4) are more readily obtainable, but sphere power is frequently diagrammed in manufacturer literature only as a single value along the umbilical line (Slide 2). Such oversight leaves out the key information needed for the patient with myopia; specifically, how much of the area needed and expected for distant vision will be provided with the needed spherical power.
Premium progressive lenses may represent either a combination of designs, or a new design, but often no actual parameters are released by the manufacturer. Therefore, one may find statements such as, the lens is "neither soft nor hard," but elsewhere stating that it has a "short progressive length."
Premium progressive lenses are of high quality in design and manufacture, but unless more information is delivered, neither a practitioner nor an educated patient has the ability to make a selection. An experienced dispenser becomes the critical determinant in the success of progressive addition spectacles.
