Figure 3-25 Bifocal lens styles. n = index of refraction.

With fused bifocal lenses, the increased refracting power of the bifocal segment is produced by fusing a button of glass that has a higher refractive index than the basic crown glass lens into a countersink in the crown glass lens blank. With all such bifocal lenses, the add segment is fused into the convex surface of the lens; astigmatic corrections, when necessary, are ground on the concave surface.

Trifocal Lenses

A bifocal lens may not fully satisfy all the visual needs of an older patient with limited accommodation. Even when near and distant ranges are corrected appropriately, vision is not clear in the intermediate range, approximately at arm’s length. This problem can be solved with trifocal spectacles, which incorporate a third segment of intermediate strength (typically one-half the power of the reading add) between the distance correction and the reading segment. The intermediate segment allows the patient to focus on objects beyond the reading distance but closer than 1 m. (See Clinical Example 3-1.)

Clinical Example 3-1

Consider a patient with 1.00 D of available accommodation. He wears a bifocal lens with a +2.00 add. His accommodative range for each part of the spectacle lens is

Distance segment: Infinity to 100 cm

Bifocal segment: 50–33 cm

He now has a blurred zone between 50 and 100 cm. An intermediate segment, in this case +1.00 D (half the power of the reading segment), would provide sharp vision from 50 cm (using all of his available accommodation plus the +1.00 D add) to 100 cm (using the add only). This trifocal lens combination therefore provides the following ranges:

Distance segment: Infinity to 100 cm

Intermediate segment: 100–50 cm

Near segment: 50–33 cm

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 sphere and cylinder of the patient’s distance lens prescription.

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 softdesign PALs (Fig 3-26). 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 3-26 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.)

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. The key to successful prescribing is careful patient selection.

The Prentice Rule and Bifocal Lens Design

There are special considerations when prescribing lenses for patients with significant anisometropias.

Prismatic effects of lenses

All lenses act as prisms when one looks through the lens at any point other than the optical center. The amount of the induced prismatic effect depends on the power of the lens and the distance from the optical center. Specifically, the amount of prismatic effect (measured in prism diopters) is equal to the distance (in centimeters) from the optical center multiplied by the lens power (in diopters). This equation is known as the Prentice rule:

= hD

where

= prismatic effect (in prism diopters)

h = distance from the optical center (in centimeters) D = lens power (in diopters)

Image displacement

When reading at near through a point below the optical center, a patient wearing spectacle lenses of unequal power may notice vertical double vision. With a bifocal segment, the gaze is usually directed 8–10 mm below and 1.5–3.0 mm nasal to the distance optical center of the distance lens (in the following examples, we assume the usual 8 mm down and 2 mm nasal). As long as the bifocal segments are of the same power and type, the prismatic displacement is determined by the power of the distance lens alone.

If the lens powers are the same for the 2 eyes, the displacement of each is the same (Figs 3-27, 3- 28). However, if the patient’s vision is anisometropic, a phoria is induced by the unequal prismatic displacement of the 2 lenses (Figs 3-29, 3-30). The amount of vertical phoria is determined by subtracting the smaller prismatic displacement from the larger if both lenses are myopic or hyperopic (see Fig 3-29) or by adding the 2 lenses if the patient is hyperopic in 1 eye and myopic in the other (see Fig 3-30).

hyperopic or if both eyes are myopic. If 1 eye is hyperopic and the other is myopic, the smaller amount of prismatic displacement is subtracted from the larger (see Fig 3-30). Image displacement is minimized when round-top segment bifocal lenses are used with plus lenses and flat-top segment bifocal lenses are used with minus lenses (Fig 3-31).

Figure 3-31 Image displacement through bifocal segments. (From Wisnicki HJ. Bifocals, trifocals, and progressive-addition lenses. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 1999, module 6. Reprinted with permission from Guyton DL. Ophthalmic Optics and Clinical Refraction. Baltimore: Prism Press; 1998. Redrawn b y C. H. Wooley.)

Image jump

The usual position of the top of a bifocal segment is 5 mm below the optical center of the distance lens. As the eyes are directed downward through a lens, the prismatic displacement of the image increases (downward in plus lenses, upward in minus lenses). When the eyes encounter the top of a bifocal segment, they meet a new plus lens with a different optical center, and the object appears to jump upward unless the optical center of the add is at the very top of the segment (Fig 3-32). Executive-style segments have their optical centers at the top of the segment. The optical center of a typical flat-top segment is located 3 mm below the top of the segment. The closer the optical center of the segment approaches the top edge of the segment, the less the image jump is. Thus, flat-top segments produce less image jump than do round-top segments because the latter have much lower optical centers.

Figure 3-32 Image jump through bifocal segments. If the optical center of a segment is at its top, no image jump occurs.

(From Wisnicki HJ. Bifocals, trifocals, and progressive-addition lenses. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 1999, module 6. Reprinted with permission from Guyton DL. Ophthalmic Optics and Clinical Refraction. Baltimore: Prism Press; 1998. Redrawn b y C. H. Wooley.)

Patients with myopia who wear round-top bifocal lenses would be more bothered by image jump than would patients with hyperopia because the jump occurs in the direction of image displacement. Thus, it is good practice to avoid prescribing round-top bifocal segments to patients with myopia.

Compensating for induced anisophoria

When anisometropia is corrected with spectacle lenses, unequal prism is introduced in all secondary positions of gaze. This prism may be the source of symptoms, even diplopia. Symptomatic anisophoria occurs especially when a patient with early presbyopia uses his or her first pair of bifocal lenses or when the anisometropia is of recent and/or sudden origin, as occurs after retinal detachment surgery, with gradual asymmetric progression of cataracts, or after unilateral intraocular lens implantation. The patient usually adapts to horizontal imbalance by increasing head rotation but may have symptoms when looking down, in the reading position. Recall that horizontal vergence amplitudes are large compared with vertical fusional amplitudes, which are typically less than 2Δ. We can calculate the amount of induced phoria by using the Prentice rule (Fig 3-33).

the management of heterophorias, approximately two-thirds to three-fourths of the vertical phoria should be corrected—in this case, 1.75Δ. This correction may be accomplished in several ways.

Press-on prisms With press-on prisms, 2.00Δ of base down (BD) may be added to the right segment in the preceding example or 2.00Δ of base up (BU) to the left segment.

Slab-off The most satisfactory method of compensating for the induced vertical phoria in anisometropia is the technique of bicentric grinding, known as slab-off (Fig 3-34). In this method, 2 optical centers are created in the lens that has the greater minus (or less plus) power, thereby counteracting the base-down effect of the greater minus lens in the reading position. It is convenient to think of the slab-off process as creating base-up prism over the reading area of the lens.

Figure 3-34 Bicentric grinding (slab-off). A, Lens form with a dummy lens cemented to the front surface. B, Both surfaces of the lens are reground with the same curvatures but removing base-up prism from the top segment of the front surface and removing base-down prism from the entire rear surface. C, The effect is a lens from which base-down prism has been removed from the lower segment only.

Bicentric grinding is used for single-vision lenses as well as for multifocal lenses. By increasing the distance between the 2 optical centers, this method achieves as much as 4.00Δ of prism compensation at the reading position.

Reverse slab-off Prism correction in the reading position is achieved not only by removing base-down

prism from the lower part of the more minus lens (slabbing off) but also by adding base-down prism to the lower half of the more plus lens. This technique is known as reverse slab-off.

Historically, it was easy to remove material from a standard lens. Currently, because plastic lenses are fabricated by molding, it is more convenient to add material to create a base-down prism in the lower half of what will be the more plus lens. Because plastic lenses account for most lenses dispensed, reverse slab-off is the most common method of correcting anisometropically induced anisophoria.

When the clinician is ordering a lens that requires prism correction for an anisophoria in downgaze, it is often appropriate to leave the choice of slab-off versus reverse slab-off to the optician by including a statement in the prescription, such as, “Slab-off right lens 3.00Δ (or reverse slab-off left lens).” In either case, the prescribed prism should be measured in the reading position, not calculated, because the patient may have partially adapted to the anisophoria.

Dissimilar segments In anisometropic bifocal lens prescriptions, vertical prism compensation can also be achieved by the use of dissimilar bifocal segments with their optical centers at 2 different heights. The segment with the lower optical center should be placed in front of the more hyperopic (or less myopic) eye to provide base-down prism. (This method contrasts with the bicentric grinding method, which produces base-up prism and is therefore employed on the lesser plus or greater minus lens.)

In the example in Figure 3-35 a 22-mm round segment is used for the right eye, and the top of its segment is at the usual 5 mm below the distance optical center. For the left eye, a 22-mm flat-top segment is used, again with the top of the segment 5 mm below the optical center.

Figure 3-35 Dissimilar segments used to compensate for anisophoria in anisometropic bifocal prescriptions.

Because the optical center of the flat-top segment is 3 mm below the top of the segment, it is at the patient’s reading position and that segment will introduce no prismatic effect. However, for the right eye, the optical center of the round segment is 8 mm below the patient’s reading position; according to the Prentice rule, this 2.50 D segment will produce 2.50 × 0.8 = 2.00Δ base-down prism.

Single-vision reading glasses with lowered optical centers Partial compensation for the induced vertical

phoria at the reading position can be obtained with single-vision reading glasses when the optical