Progressive Addition Lenses
Both bifocal and trifocal lenses have an abrupt change in power as the line of sight passes across the boundary between one portion of the lens and the next; image jump and diplopia can occur at the segment lines. Progressive addition lenses (PALs) avoid these difficulties by supplying power gradually as the line of sight is depressed toward the reading level. Unlike bifocal and trifocal lenses, PALs offer clear vision at all focal distances. Other advantages of PALs include lack of intermediate blur and absence of any visible segment lines.
The PAL form has 4 types of optical zones on the convex surface: a spherical distance zone, a reading zone, a transition zone (or “corridor”), and zones of peripheral distortion. The progressive change in lens power is generated on the convex surface of the lens by progressive aspheric changes in curvature from the top to the bottom of the lens. The concave surface is reserved for the cylinder of the patient’s distance lens prescription, as in traditional minus cylinder lens designs.
However, there are certain drawbacks to PALs. Most notably, some degree of peripheral distortion is inherent in the design of all PALs. This peripheral aberration is caused by astigmatism resulting from the changing aspheric curves; these curves are most pronounced in the lower inner and outer quadrants of the lens. These distortions produce a “swimming” sensation with head movement.
The vertical meridian joining the distance and reading optical centers is free of surface astigmatism and provides the optimal visual acuity. To either side of this distortion-free vertical meridian, induced astigmatism and a concomitant degradation of visual acuity occur. If the lens is designed such that the peripheral distortions are spread out over a relatively wide portion of the lens, there is a concomitant decrease in the distortion-free principal zones. This effect is the basis of soft-design PALs (Fig 4-26). Conversely, a wider distortion-free zone for distance and reading means a more intense lateral deformity. This effect is the basis of hard-design PALs. If the transition corridor is lengthened, the distortions are less pronounced, but problems arise because of the greater vertical separation between the distance optical center and the reading zone. Therefore, each PAL design represents a series of compromises. Some manufacturers prefer less distortion at the expense of less useful aberration-free distance and near visual acuity; others opt for maximum acuity over a wider usable area, with smaller but more pronounced lateral distortion zones.
Figure 4-26 Comparison of hard-design and soft-design progressive addition lenses (PALs). These illustrations compare the power progression and peripheral aberration of these 2 PAL designs.
(From Wisnicki HJ. Bifocals, trifocals, and progressive-addition lenses. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 1999, module 6.)
Good candidates for progressive lenses may be patients with new presbyopia who have not experienced traditional lined bifocals or trifocals, those who only occasionally need intermediate vision, and those with strong aversions to lined lenses. Less successful candidates are those who are currently happy with other alternatives (lined bifocals/trifocals or single vision correction), anisometropia, and those requiring prism or high adds, a large field of view, or multiple near-fixation lines of sight, or those who have very mobile/dynamic eye movements (sports). Personality may play a role in acceptance of these lenses, and those factors should be taken into consideration.
Fitting of these lenses is a critical process. Measurement of pupillary distance should be performed monocularly, as any asymmetry must be accounted for. Frame selection is important. The frame must have adequate vertical depth to allow the full progressive add to be present. The distance and near reference areas should be marked with the frame in place. Frames with adjustable nose pads allow future adjustment of the PAL position. A shorter back vertex distance will give greater binocular field of vision. Greater pantoscopic tilt gives increased near visual field but must be accounted for in the effective lens power.
PALs are readily available from −8.00 to +7.50 D spheres and up to 4.00 D cylinders; the available add powers are from +1.50 to +3.50 D. Some vendors also make custom lenses with parameters outside these limits. Prism can be incorporated into PALs.
The best candidates for PALs are patients with early presbyopia who have not previously worn bifocal lenses, patients who do not require wide near-vision fields, and highly motivated patients. Patients who change from conventional multifocal lenses to PALs should be advised that distortion will be present and that adaptation will be necessary. Small-frame PALs can reduce the usable reading zone to a small area at the bottom edge of the lens. Also, the differential magnification through the progressive zone can make computer screens appear trapezoidal. Progressive designs are also available for indoor use, with large zones devoted to computer monitor and reading distances (eg, 23 inches and 16 inches from the eye).
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.