Multifocal Intraocular Lenses
ROBERT J. SCHECHTER
Table Of Contents
OPTICS OF MULTIFOCAL INTRAOCULAR LENSES|
LENSES IN PRODUCTION
ALTERNATIVES TO MULTIFOCAL INTRAOCULAR LENSES
|Widespread use of intraocular lenses (IOLs) has changed the face of cataract surgery in the past decades. Optic rehabilitation of the postsurgical patient has eliminated the handicaps and inconveniences of spectacle and contact lens correction. However, the conventional IOL is a fixed-focus device currently not capable of accommodation-like changes in shape or power. It is also a single-focus device, restricting the practitioner to select only a single power for a particular patient. Interest has grown regarding the possibility of a multifocal IOL that would provide both near and distance vision without the need for spectacle correction.|
|OPTICS OF MULTIFOCAL INTRAOCULAR LENSES|
|Multifocal IOLs must incorporate some mechanism to focus light from distant
objects and light from near objects at the same time. Unlike spectacle
bifocal/trifocal lenses, the multifocal IOL refracts (or diffracts) light
from any object of regard through elements for both near and
distance vision at the same time (Fig. 1). Thus there must always be some light that is not in focus commingling with the light that is in focus. For distant objects, for
example, the “add lens” steals some of the light
that would have been focused and instead distributes relatively defocused
light onto the retina, decreasing image contrast.|
At the present time, these IOLs can produce multiple foci using either refractive or diffractive principles. Multifocal IOLs can be constructed, using these principles, in one of several different ways:
Multifocal designs can be classified based on whether they use refractive or diffractive optics to achieve multifocality (Table 1). Diffractive lenses combine an anterior spherical refractive surface with multiple posterior diffractive surfaces.
The mechanism of diffractive lenses is based on the Huygens-Fresnel principle.1 This states that every point of a wavefront can be thought of as being its own source of secondary so-called wavelets, subsequently spreading in a spherical distribution. The amplitude of the optic field beyond this point is simply the sum of all these wavelets. When a portion of a wavefront encounters an obstacle, a region of the wavefront is altered in amplitude or phase, and the various segments of the wavefront that propagate beyond the obstacle interfere and cause a diffractive pattern. A regular array of obstacles that has the effect of producing periodic alterations in the emerging wavefront is termed a diffraction grating. As the spacing between the diffractive elements decreases, the spread in the diffractive pattern increases. By placing the diffractive microstructures in concentric zones and decreasing the distance between the zones as they get further from the center, a so-called Fresnel zone plate is produced that can produce optic foci. Thus the distance power is the combined optic power of the anterior and posterior lens surfaces and the zero order of diffraction, whereas the near power is the combined power of the anterior and posterior surfaces and the first order of diffraction.
Diffraction can be interpreted as a spreading of wavefronts. If the scattered waves are in phase, they interfere constructively; if they are out of phase, they interfere destructively. By assigning appropriate dimensions for the 20 to 30 concentric zones on the posterior lens surface, approximately 41% of the light is in phase and focused for near vision, about another 41% is in phase and focused for distance vision, and the remaining 18% is lost.2 This diffractive optic effect is present at all points (or more precisely, at all sufficiently large areas) of the lens. This means that the multifocal performance of this lens should be unaffected by decentration, pupil eccentricity or deformation, or pupil size; distance and near vision are possible if any part of the lens optic is present behind the pupil.
Conversely, refractive lenses use only differing areas of refractive power to achieve their multifocality. They function by providing annular zones of different refractive power to provide appropriate focus for objects near and far. These lenses may have discrete zones of a near or distant power; thus they may be said to be bifocal. Others may have aspheric surfaces with transition zones resulting in powers that may vary continuously between a minimum for distance and a maximum for the strongest near add. These may be said to be multifocal. Refractive bifocal/multifocal IOLs may be affected by pupil size and decentration, to a greater or lesser degree depending on the size, location, and number of refractive zones. Postoperative pupil size might be a factor of importance in selecting which lens to use. One team investigated whether they could predict postoperative pupil size from preoperative factors.3 They concluded that they could not predict the pupil size with sufficient consistency to ensure a good match between pupil diameter and the zone sizes of various multifocal IOLs.
|LENSES IN PRODUCTION|
|Lenses such as the True Vista (Bausch and Lomb, Claremont, CA) and the
MF4 (IOLTECH, La Rochelle, France) are bifocal, with a central zone surrounded
by alternating annular spherical refractive surfaces on the anterior
surface of the lens to provide focus for both near and distance
vision. The True Vista lens has three zones, a central zone for distance
vision, surrounded by an annulus with a 4-D add for near vision, and
a peripheral zone with the distance power again (see Fig. 1).|
The MF4 is an example of a near dominant refractive bifocal lens, that is, a lens that may perform more satisfactorily for near vision tasks than for distance. It has a 4-D add in the center zone for near vision, surrounded by alternating rings for distance and near vision (Fig. 2).
The advantage of the refractive bifocal IOL is that it focuses most of the available light for a usable image, either distance or near. The disadvantage is that lens performance is sensitive to pupillary diameter and, to some degree, lens decentration. (Obviously, if the pupil is smaller than the central zone, the optics of the more peripheral zones have little chance to contribute to an image.)
The Array (Allergan Surgical, Irvine, CA) and Domilens Progress (Bausch and Lomb, Claremont, CA) lenses are multifocal lenses, designed to focus on intermediate, as well as near and far, objects. The Array is a distance-dominant, simultaneous-vision, zonal-progressive lens. It combines a posterior refractive surface with multiple anterior aspheric refractive rings, or zones, of continuously varying power (Fig. 3). Beginning with the 2.1-mm central zone, distance power is placed centrally in each odd ring (1, 3, and 5). The power gradually increases toward the periphery of the odd numbered rings to form a smooth transition with the even zones that emphasize near vision with a 3.5-D add. This provides continuous refractive power across the lens to minimize dependence on pupil size.
The smooth transition between the zones also minimizes any loss of available light that may occur as a result of diffraction where there is an abrupt change in diopter power, such as occurs in refractive bifocal lenses. Thus the Array lens focuses all available light and, unlike refractive bifocal lenses, performance is less sensitive to changes in pupillary diameter.4
The Domilens Progress 1 is a near-dominant refractive IOL with a central aspheric zone that provides near vision. Its progressive aspheric surface continuously increases to an add power of ±4.75 D, with distance vision through the periphery; it provides intermediate vision in between. Diffractive lenses such as the CeeOn (811E, 808X; Pharmacia Upjohn, Kalamazoo, MI) and the 3M (825X, 815LE; Alcon, Fort Worth, TX) are bifocal. The anterior surface of the lens is a spherical refractive surface. A diffractive surface is created on the posterior surface by removing wedge-shaped annuli of lens material (Fig. 4). This alters the phase of the incident light to form a diffraction grating. The zero and first orders of diffraction are captured by the retina. Distance focus is the combined power of the spherical anterior surface with the zero order of diffraction from the posterior surface. Near focus is achieved by the combination of the spherical anterior surface and the first order of diffraction from the posterior surface. About 41% of the light is focused for near vision and 41% of the light is focused for distance vision. The remaining light is lost to higher orders of diffraction that are unfocused on the retina. Thus the diffractive bifocal lenses have the disadvantage of being unable to use approximately 18% of the incoming light. However, they have the advantage of being less affected than refractive bifocal IOLs by lens decentration or pupillary diameter.
|Several laboratory studies have compared multifocal and monofocal lenses. Overall, there
is decreased contrast sensitivity for the multifocal
lenses when compared with findings for monofocal lenses.5–7 Conversely, the monofocal lenses tend to exhibit less depth of field than
the multifocal lenses.|
Multifocal lenses are designed to produce multiple focal points to provide vision at far, near, and intermediate distances. One team analyzed the light energy distribution for seven multifocal/bifocal IOLs: four bifocal IOLs (True Vista, Bausch and Lomb; P751E, Bausch and Lomb Iolab; CeeOn 811E, Pharmacia Upjohn; 3M 825X, Alcon) and three multifocal IOLs (Array, Allergan; Varifocal 200, Wright Medical; Domilens Progress 1, Domilens).8 The bifocal lenses had two focal points that showed approximately equal light intensity in two of the four bifocal IOLs (Iolab, 56.3% and 43.7%; Pharmacia Upjohn, 44.9% and 36.9%) and higher light intensity for the distance focus compared with the near focus for the other two (True Vista, 71.0% and 29.0%: 3M 825X, 71.7% and 10.3%). About 18% of the light was unfocusable (higher order optics) for both diffractive bifocal lenses (CeeOn and 3M 825X). The Array (20D) and Varifocal (20D) multifocal IOLs both had multiple foci between 20 D and 24 D, with the light intensity highest for the distance foci. The Array lens had a second illumination peak at 24 D for near vision. Because of its optic design (progressive aspheric surface), the Domilens Progress 1 did not permit the creation of independent focal spots but instead produced an infinite number of foci within the specified dioptric range. The highest illumination with this lens was between 20.00 and 20.26 D, decreasing at higher dioptric powers. However, all these multifocal lenses produced blur circles around the foci which, those authors state, could interfere with the quality of vision.
Resolution efficiency was not significantly different in a group of one monofocal, two multifocal, and three bifocal lenses measured in water using an optic bench apparatus.7 The two multifocal lenses were the Array and the Aspheric (Wright). The Annular (Pharmacia Upjohn), 3M 825X, and Morcher Diffractive were bifocal lenses. The resolution efficiency was defined as a measure of the resolving power of a lens expressed as a percentage of the resolving power of a perfect lens of the same power that is limited by diffraction only. Resolution efficiency ranged from 66% to 83%. Visual acuity and contrast sensitivity were also measured photographically. A water chamber containing the IOL was inserted into a specially built camera. Black and white negatives were taken of the high contrast (96%) Regan chart and the Pelli-Robson contrast-sensitivity chart. Negatives were examined under 40× magnification and the maximum number of letters that could be read was determined. This number was converted to the Snellen equivalent. Snellen equivalent acuity measurements reached 20/10 for the monofocal lens and ranged from 20/18 to 20/14 for the multifocal lenses. Near vision ranged from 20/23 to 20/56 for the multifocal lenses but reached only 20/80 for the monofocal lens. Using photographic testing, a 1.0 to 1.5 line improvement (Snellen equivalent) in the best-corrected distance acuity was demonstrated with the monofocal lens compared with the multifocal lenses. This difference appeared to result from decreased contrast sensitivity with the multifocal lenses. Contrast sensitivity is the reciprocal of contrast threshold, the lowest contrast (expressed as a percentage) at which a given size target is correctly recognized. Contrast sensitivity at a Snellen equivalent of 20/106 (corresponding to the angular size of the Pelli-Robson letters at 20 feet) was 8.3 for the monofocal lens, compared with a range of 2.9 to 4.8 for the multifocal lenses. Depth of field was assessed using minus defocusing lenses. The depth of field for multifocal lenses was two to three times greater than that of the monofocal lens. In a separate study comparing the True Vista (Bausch and Lomb) multifocal lens with a monofocal lens, the depth of field for the monofocal lens was -1 D to ±1 D compared with -1.5 D to ±4.5 D for the multifocal lens.5 Finally, in a test using a color chart, the monofocal lens separated the colors crisply, whereas the multifocal lenses tended to create color mixing between adjacent color bands. The eight colors seen with the monofocal lens became 15 colors with the multifocal lenses because of color mixing at the borders of adjacent bands.
The optic performance of one monofocal and five different style multifocal IOLs was evaluated in the laboratory.7 The resolution efficiency (and resolving power) were approximately equal in all six lenses. Several variables related to the modulation transfer function were superior in the monofocal lens. Defocusing with minus lenses revealed that the multifocal lenses had two to three times the depth of focus of the monofocal lens. The monofocal lens had a maximal resolution corresponding to 20/10 vision, whereas the multifocal IOLs were limited to 20/14 to 20/18. Testing in near vision, of course, revealed 20/23 to 20/56 with the multifocal IOLs (tested at 33 cm, which may not have been the exact near focus of the lenses), but no better than 20/80 with an (uncorrected) monofocal lens. Contrast sensitivities were 1.5 to 2.9 times lower in the multifocal IOLs compared with findings for the monofocal lenses. Photographs were taken through each of the lenses. The best-corrected visual acuity was 1 to 1.5 lines better with the monofocal lenses. Photography of a chart containing eight colors showed color mixing between adjacent colors with the multifocal IOLs, producing a display of fifteen colors. (This blend cannot be filtered out by the brain and/or retina; what is seen is a band of color mixture between the original colors.)
Pupil size may be more critical in the case of many types of multifocal IOLs. An optic mode of the human eye analyzed this effect for a concentric multifocal IOL with a base distance power and a 2-mm central bifocal add.9 This type of lens effectively consists of two lenses. The pupil size determines the ratio of distance lens to add lens in the optic aperture. Larger pupils allow relatively more of the distance portion of the lens to be exposed, which assists the imaging of distance objects and impedes the imaging of near objects. For a lens with a 2-mm round bifocal center, a pupil of 2.8 mm provides distance and near zones of equal areas. Pupils smaller than the add lens size actually restrict the bifocal IOL to function as a unifocal lens for close vision only. However, in the case of distance vision for pupils smaller than 2 mm, the pinhole effect compensated and still allowed image contrast of 90% to 95% of control. The image contrast decreased as the pupil was increased, reaching a minimum of 70% at 2.5 mm. As the pupil became larger than that, more of the distance portion of the lens became exposed and the image contrast increased again. For near vision, with pupil size smaller than 2.5 mm, the lens was essentially monofocal and image contrast was 100%. However, as the pupil enlarged, the contrast dropped rapidly to 25% for 6-mm pupils. Thus this bifocal IOL produced good image contrast of at least 70% in all situations except viewing near objects with a pupil larger than 3.5 mm (as in reading in a dimly lit room). Additionally, bear in mind that the pupil size as measured clinically is greater than the actual pupil size, because of the magnifying effect of the cornea/anterior segment curvature.10
|One study compared bifocal IOLs with a monofocal group. The bifocal IOL
was the True Vista, a three-zone refractive IOL with distance-central-distance
corrections in its three zones.11 Increasing pupil size resulted in a worsening of best-corrected vision
in both groups, although only for very large pupils in the monofocal
patients. In the bifocal group, a small pupil was found to eliminate much
of the contribution of the middle near ring and result in better distance
vision. With increasing pupil size, more near correction is available, which
reduces contrast at distance viewing. Corneal astigmatism
had almost no deleterious effect on the monofocal patients, although
it did have a significant effect on the bifocal group. This suggests
that bifocal lenses of this type might be less suitable for patients
with preexisting irregular corneal astigmatism and that implantation of
such lenses should be performed with attention to minimizing the postoperative
Another study compared the monofocal lens to the diffractive (Pharmacia Upjohn 808X) lens.12 Uncorrected near acuity was superior with the bifocal lens. Halos around lights were more common in the bifocal group. Contrast sensitivity differences were greatest in medium light intensities. Contrast sensitivity was tested in two refractive multifocal IOLs and one diffractive model.13 The diffractive IOL had approximately equal contrast sensitivities and near and distance, whereas the two refractive models (distance dominant?) had superior contrast sensitivity at distance. Contrast sensitivity and glare were tested for halogen light, relevant to night driving situations.14 The refractive IOL Array SA 40N was used. Reduced contrast sensitivity was noted only at the lowest spatial frequencies, and the results were interpreted as showing no significant additional risk for halogen glare disability in patients with this multifocal IOL. Another study compared contrast sensitivity in phakic, monofocal, refractive IOL, and diffractive IOL patients.15 The latter two groups had significantly worse contrast sensitivity scoring, although those authors found no significant superiority of one multifocal type over the other. Contrast testing of the Pharmacia Upjohn 811E/808X diffractive IOL showed that in eyes with good visual acuity there was only a slight reduction in contrast sensitivity at all spatial frequencies.16
Another investigator randomized 80 patients to receive either a refractive (Allergan array lens PA154N) or a diffractive (Pharmacia Upjohn 811E) IOL.17 The diffractive lens had a near add of 4 diopters. The refractive lens had five annular refractive zones. The distance power in each annular zone is placed centrally, gradually increasing in add power up to 3.5 diopters, then changing back to the distance power at the end of the zone. With this design, 60% of the light passes through the distance part of the optic, with the rest distributed for intermediate and near vision. Postoperatively, there was no significant difference between the groups in uncorrected or best-corrected visual acuity. However, near visual acuity was significantly better in the diffractive group. Defocusing curves showed no significant difference up to -1 diopter. At -1.5 diopter, which corresponds to intermediate distance, the refractive group showed better functional results. From -2.5 to -5, the diffractive group was superior. Overall patient satisfaction was higher in the diffractive group. There was no difference found for halos or glare. Patient satisfaction seemed to be more correlated with unaided reading ability rather than enhanced intermediate vision. A similar comparison was made between the AMO Array SSM 26NB and the 3M 825 diffractive IOL.18 The diffractive IOL provided significantly better near vision. It was pointed out that the diffractive IOL distributes approximately 41% of incident light to the near focus, compared with 25% to 38% for the AMO refractive lens. The same investigators then studied glare and contrast sensitivity with these lenses.19 The diffractive group showed decreased contrast sensitivity compared with findings in the refractive group, especially under glare conditions. Those authors, however, point out that this was at distance and speculate whether the diffractive group might well be superior if tested in this way at near.
Additionally, 100 patients with bilateral Array lenses were surveyed and contrasted with a matched group with bilateral monofocal IOLs.20 The multifocal group had a higher percentage who did not wear glasses for reading or did not wear glasses at all. Interestingly, patients with the multifocal IOL also were more likely than the monofocal group to never wear glasses for distance vision. The multifocal group had a trend toward more glare trouble during night driving.
An area where decreased contrast acuity creates concern is driving under low contrast conditions, such as low illumination conditions (driving at night, in fog, or in snowy conditions). In a unique study, (visual) driving performance was contrasted between 33 patients with bilateral refractive multifocal IOLs (Array) and 33 patients with bilateral monofocal IOLs at the Iowa Driving Simulator (Center for Computer Aided Design, University of Iowa, Iowa City, Iowa).21 In 26 of 30 performance measures, there was no significant difference between the refractive multifocal and the monofocal groups. Monofocal patients performed better than refractive multifocal patients in correctly recognizing warning signs at night in clear weather, in sign recognition distances for guiding and warning signs in fog, and in the detection distance for one of four hazards. Although those authors reported that there were no consistent differences between the two groups in driving performance and safety, patients with refractive multifocal IOLs should exercise caution when driving at night or in poor visibility conditions.
A new halogen glare test was used to simulate the glare of oncoming vehicle headlights to assess the glare disability in patients implanted with either a monofocal or refractive multifocal (Array) IOL.10 They found no difference in glare disability induced by halogen glare for patients with monofocal or refractive multifocal IOLs. Reduced contrast sensitivity was found in the refractive multifocal group compared with the monofocal group only at the lowest spatial frequency (three cycles per degree [cpd]) without halogen glare. Patients with refractive multifocal implants did report a greater mean limitation for driving at night compared with the monofocal group, a limitation that was slight and decreased with spectacle usage.22
Patients bilaterally implanted with a refractive multifocal (Array, n = 100) or a monofocal (n = 103) IOL were interviewed 7 to 8 months after surgery on the second eye.22 Patients rated vision overall and frequency of spectacle wear. Vision-related functional status was assessed using the Cataract TyPE Specification. A significantly larger percentage of the refractive multifocal group (41%) never wore spectacles compared with the monofocal group (11.7%). A significantly greater percentage of refractive multifocal patients also never wore glasses for distance vision (84.9%) compared with monofocal patients (52.4%). Even with best correction, refractive multifocal patients rated their vision as significantly better overall, and in the intermediate range, ratings for refractive multifocal patients compared with findings in monofocal patients. Refractive multifocal patients consistently reported less limitation in specific visual tasks without spectacles for distance and near vision activities, as well as for social activities. They also reported significantly more bother from halos/glare than patients in the monofocal group.
Similar results of decreased spectacle use in refractive multifocal patients compared with monofocal patients have been reported.4 In this study, 52% of 32 multifocal patients never wore spectacles, compared with only 25% of 30 monofocal patients. In a separate study, 81% of bilateral multifocal patients could function without glasses at near, compared with 56% of patients with a multifocal lens in one eye and a monofocal lens in the other eye and with 58% of unilateral multifocal patients.23
In a large multinational study of patients implanted bilaterally with a refractive multifocal IOL (Array) or a monofocal IOL, 32% of 127 refractive multifocal patients never wore spectacles, compared with 8% of 118 monofocal patients.24 Refractive multifocal patients rated their vision without glasses as significantly better overall, at near, and at intermediate distances compared with monofocal patients. They also reported better visual function for near tasks and social activities.
A decrease in spectacle dependency has also been reported for diffractive bifocal implants.12,25,26 A much higher percentage of patients with a an 808X implant (46% of 79) than a monofocal implant (9% of 70) did not use spectacles for near tasks. Thirty-seven percent of bifocal (N = 79) and 6% of monofocal (N = 90) patients did not use spectacles for near or distance vision.12 In an 8-year retrospective study of patients implanted with the 3M 825X lens, 54% of patients (39 of 72) did not use spectacles, a number that increased to 68% in patients with the multifocal IOL in both eyes (N = 25).26
In a study using the 3M 815LE and 3M 825X, 50% of the patients preferred wearing spectacles, even though 90% of patients had an uncorrected visual acuity of 20/40 or better.25 Spectacle wear had no correlation with the amount of refractive error or astigmatism.
Given that multifocal/bifocal IOLs focus a targeted object through both distance and near focal powers, there is always an in-focus image and a blurred image of the object at the retina. Clinical studies have demonstrated that this superpositioning of in-focus and out-of-focus images does not affect visual acuity assessed using standard measures. However, it does create a decrease in a patient's visual discrimination under low contrast conditions.15,16,23,26–30
Two measures are used to assess visual discrimination under low contrast conditions. In tests of contrast acuity, patients are asked to identify letters of increasingly lower contrast. For example, Regan charts are each printed with letters ranging from 100% contrast to 11% contrast. The ability to identify the low contrast letter is a function of recognition as well as detection. In measures of the contrast sensitivity function, recognition is less of a factor. Sine wave gratings of varying frequencies are presented to the patient. The patient identifies either whether they can see the gratings or the direction the gratings are pointing, depending on the test. The contrast sensitivity function equals one over the contrast threshold for discrimination of the sine wave grating. Because both measures indicate the ability of the patient to perform visual discriminations under low contrast conditions, studies using either contrast acuity or contrast sensitivity measures are reported in this review.
With the 3M 815LE lens, contrast sensitivity at distance focus was significantly lower in patients with bifocal IOLs compared with patients with monofocal IOLs. However, at near focus, the diffractive bifocal IOL had better contrast sensitivity.27 Contrast sensitivity was reduced at 6 to 18 cpd and only half of these patients were within the VisTech normal reference range at 6 cpd.26 In another study, contrast acuity was similar in diffractive bifocal (3M) and monofocal patients at 96% and 50% contrast.28 However, diffractive bifocal patients lost an average of 3.45 Snellen lines at 11% contrast compared with 2.65 lines for monofocal cases. In an electrophysiologic study of contrast sensitivity in patients implanted with the 3M 815LE lens,15 the investigators found reduced contrast sensitivity in the diffractive bifocal 3M eyes compared with monofocal eyes, although visual acuity remained high. Another group also found lower contrast sensitivity for the diffractive bifocal 3M lens compared with the monofocal lens29; however, they found that this had no effect on reading speed, except when text was very low contrast or letters were near the limits of the patient's visual acuity.
At 5 months after cataract surgery, patients with a diffractive bifocal (n = 115; CeeOn 811) or monofocal (n = 106) IOL were tested for contrast sensitivity using the Vision Contrast Test System.16 Although mean values for contrast sensitivity were within the normal range, the bifocal IOL group had slightly lower contrast sensitivity than the monofocal group. Interestingly, contrast sensitivity seemed to improve over time (mean 13 months) for the diffractive bifocal group. Contrast acuity was also lower in patients with the refractive multifocal Array IOL compared with monofocal lenses, but only at the lowest contrast level (11%).4 Low contrast visual acuity was reduced by about 1 line in patients with the Array lens.23 Another team investigated the contrast sensitivity and the glare disability sensitivity for the refractive multifocal Array and a monofocal IOL compared with a reference, normal, phakic group.30 There were no significant differences between the refractive multifocal and the monofocal groups in either measure. The results for both IOLs were toward the lower limit of the normal phakic range and did not result in clinical impairment. However, optimal vision for close work was provided by distance vision in conjunction with reading spectacles. Another investigation concluded that binocular implantation of the refractive multifocal IOL (Array) can alleviate the reduced contrast acuity at low contrast levels.31
In comparing a diffractive bifocal lens (3M 815 LE) with a refractive multifocal lens (Array), it was found that contrast sensitivity and glare sensitivity were at the lower limits of the normal range for both lenses.19 Overall, patients with the diffractive bifocal lens demonstrated decreased contrast sensitivity and greater glare disability than patients with the refractive multifocal lens.
COMPARISON WITH MONOFOCAL INTRAOCULAR LENSES
Refractive Bifocal Intraocular Lenses
The True Vista lens has a central distance zone, a near annulus with a 4-D add, and a peripheral distance annulus. With this lens, distance visual acuity is similar to a monofocal lens, and uncorrected and distance-corrected near acuity is superior than with a monofocal lens.32–35
In a study of patients implanted with the True Vista IOL, 98% of 145 patients had best-corrected distance acuity of 20/40 or better at 7 to 11 months after surgery. Near acuity was 20/30 or better for 92% of the patients with near spectacle add and for 78% of patients with distance-only spectacle correction. This compares with 98% of 91 monofocal patients who reached best spectacle-corrected distance acuity of 20/40 or better, 99% with best-corrected near acuity of 20/30 or better, and 6% with distance-only corrected near acuity of 20/30 or better.
Depth of focus was better with the multifocal lens. Using defocusing lenses, visual acuity with the bifocal lens dropped below 20/40 with a defocus of -4.0 D, compared with -2.0 D for the monofocal lens.34 When compared with monofocal IOLs, the bifocal implant had reduced contrast acuity at low (11%) contrast.33 Contrast acuity in the bifocal eye was lower at near focus than distance focus. This resulted from a decrease in image contrast at near focus,13,34,36 which those authors attribute to a smaller blur circle of higher light intensity around the near image on the retina than is found around the distance image.
Refractive Multifocal Intraocular Lenses
The Array and Domilens Progress lenses are refractive multifocal IOLs. The Array has been approved for use in the United States by the Food and Drug Administration and has been extensively documented in the literature. Several studies comparing the Array lens to monofocal IOLs have shown similar distance acuity, increased near acuity, and increased depth of focus for the Array IOL.4,23,24,37,38 Mean uncorrected near acuity was J3± (between 20/40 and 20/30) in patients with the Array refractive multifocal lens, compared with J7 (20/70) in patients with a monofocal implant (p <.0001).4 These results were supported by a later study of 456 participants in which distance acuity was similar for multifocal and monofocal eyes, but multifocal eyes had a mean two-line increase compared with monofocal eyes for uncorrected and distance-corrected near visual acuity.23 Another investigator assessed 15 patients and found that 70% of multifocal and 43% of monofocal patients achieved J3 (20/40) or better.38 A large multinational clinical trial reported that at 3 months postoperatively, bilaterally implanted refractive multifocal patients had significantly better mean uncorrected near visual acuity (20/26) than bilaterally implanted monofocal patients (20/40).24 Overall, 96% of the multifocal group (n = 127) and only 65% of the monofocal group (n = 118) achieved both 20/40 and J3 (20/40) uncorrected visual acuity. In another study, after a minimum follow-up of 18 months, no significant differences were found between 50 monofocal and 50 refractive multifocal eyes in mean distance visual acuity or brightness acuity test (BAT) at Regan 96% contrast.37 The refractive multifocal lens implants provided significantly better uncorrected near visual acuity (J3 versus J7) and wider depth of focus compared with the monofocal implants. Contrast acuity was significantly lower for the refractive multifocal lens patients at low contrast levels (Regan 25% and 11% charts).
The first clinical trial of the Domilens Progress 1 refractive multifocal IOL39 demonstrated that 65% of patients had uncorrected distance acuity of 0.8 (20/25) and near visual acuity of J2 (20/30). Depth of focus ranged from infinity to 19 cm from the corneal plane. There was a decrease in visual acuity of 0.01 Snellen lines in medium light and 0.05 lines in bright light. Contrast sensitivity was slightly less in multifocal than monofocal eyes and significantly less than phakic eyes (p <.05).
Distance, near, and intermediate vision were assessed for the Domilens Progress.1,40 In a prospective study of 59 eyes in 59 patients, the mean uncorrected distance acuity was 0.77 Snellen lines (20/26). Mean uncorrected near acuity was 4.75 Jaeger lines (between 20/50 and 20/40); distance-corrected near acuity was 4.00 (between 20/50 and 20/40). At 1 year, mean uncorrected intermediate acuity ranged from 0.58 Snellen lines (20/34) at 40 cm to 0.66 Snellen lines (20/30) at 120 cm. Distance acuity was not influenced by pupillary diameter. Patient satisfaction was high with distance and intermediate vision, but decreased dramatically for near activities such as reading or sewing in reduced light.
Diffractive Bifocal Intraocular Lenses
The CeeOn 811E (also 808X, Pharmacia Upjohn, Kalamazoo, MI) was compared with monofocal IOLs.8,12,16,41 In a study,42 all patients who received either the 808X diffractive bifocal implant (n = 79) or a similar monofocal (n = 70) implant had a best-corrected distance visual acuity of 0.5 (20/40) or better at 5 to 6 months after surgery. Best-corrected distance acuity of 1.0 (20/20) or better was achieved by 71% of patients with the bifocal implant and 80% of patients with the monofocal implant. Uncorrected near vision was J3 (20/40) or better for 9% of monofocal patients and 93% of bifocal patients. With distance correction, 4% of monofocal and 99% of bifocal patients read J3 or better.42 Contrast sensitivity was slightly decreased with the bifocal compared with the monofocal lens for both near and distance vision16,42; however, the lens provided good distance and near visual acuity.
Studies of the 3M diffractive bifocal lens indicate that it also provides distance vision comparable to monofocal lenses. In a study of 149 bilateral bifocal patients and 131 bilateral monofocal patients, 78% of bifocal and 74.8% of monofocal patients had uncorrected distance vision of at least 20/40.28 Another group reported 79% of 28 bifocal patients achieved uncorrected distance vision of at least 20/40 compared with 76% of 33 monofocal patients.43 In this same study, 86% of 23 bifocal patients achieved uncorrected near vision of J3 (20/40) or better, compared with 70% of 24 monofocal patients. Further, 30% of the bifocal patients achieved uncorrected near vision of J1 (20/20), compared with 4% of the monofocal patients.
Interestingly, in one study,44 the diffractive bifocal lens showed slightly lower distance visual acuity than the monofocal lens during the first 7 weeks after surgery. After 7 weeks, the visual acuity was comparable in both groups. The diffractive bifocal lens had a high frequency of posterior synechiae compared with the monofocal lens. Those authors hypothesized that this resulted from the configuration of the lens, which results in compression of the anterior capsule between the iris and the lens, and that this would tend to increase the formation of synechiae.
The 3M lens also has good long-term results. A study25 with a 2 1/2 year follow-up reported that 86% of patients with the 3M IOL achieved uncorrected distance vision of 20/40 or better and 94% had uncorrected near vision of 20/40 or better. Compared with patients with monofocal implants, the diffractive bifocal patients experienced significantly greater depth of focus. Comparable results were obtained26 in an 8-year retrospective study. Uncorrected distance acuity was 0.5 (20/40) or better in 89.6% of 76 eyes and uncorrected near vision was J3 (20/40) or better in 73.8% of 76 eyes. However, in a report (from 10 years earlier),45 three patients requested lens exchange of the diffractive bifocal lens for a monofocal lens because of disappointment with the quality of the acuity for distance and near vision. Lens exchange improved acuity two lines and eliminated complaints of monocular diplopia.
DIFFRACTIVE VERSUS REFRACTIVE COMPARISONS
In comparing a diffractive bifocal (3M) with a refractive bifocal (True Vista) lens,32 one group found that the diffractive bifocal lens had a slightly greater percentage of patients with best distance-corrected near vision of 20/40 or better compared with the refractive bifocal IOL (94.5% versus 86.0%). However, another group34 reported that distance acuity was better with the refractive bifocal lens compared with the diffractive bifocal lens, based on spectacle defocus results. They also reported that near acuity was better with the diffractive bifocal lens.
Comparison of the refractive multifocal Array lens with the diffractive bifocal 3M or CeeOn lens demonstrated that distance vision was similar for the two types of IOLs. In direct comparisons, both of these diffractive bifocal IOLs appeared to provide better near visual acuity than a refractive multifocal IOL.18,46–48 This difference may be attributable to the fact that the CeeOn 811E/808X and the 3M 825X IOLs have a ±4.0-D add, compared with the ±3.5-D add of the Array IOL.
In a comparative study of the 3M diffractive bifocal lens, the Iolab refractive bifocal lens, and the Array refractive multifocal lens, the Array lens was found to be the safest lens of the three in terms of overall visual acuity and optic aberrations.49 Seventeen percent of 47 patients with 3M 815LE/834LE eyes and 1 patients (2.5% of 40) with an Iolab eye could not read N5 (20/40), even with a reading add. Thirty percent of 47 patients with 3M 815LE/834LE eyes and 15% of 40 patients with Iolab eyes complained of shadow or ghosting when reading small print. All patients with Array eyes could read N5 uncorrected or with correction and only one (4.2%) Array patient complained of blur.
In all these clinical studies of multifocal IOLs, it should be remembered that these groups were not randomized. Patients are preselected in that they are prepared in advance to accept trade-offs of potentially reduced clarity in exchange for enhanced uncorrected near vision. Similarly, patients in the monofocal lens groups have specifically declined the risk/benefit ratio of multifocal IOLs and are much more likely to have been unhappy with multifocal IOLs had they received them.
|ALTERNATIVES TO MULTIFOCAL INTRAOCULAR LENSES|
|The advantage of the multifocal IOL is that it provides near and distance
vision without additional refractive correction. This return must be
weighed against the (possibly superior) optic performance of bifocal
spectacles or contact lenses. Another possibility is correcting one eye
for distance and the other for near; this eliminates the need for additional
correction at the cost of some binocularity.50 However, 86% of patients given a near focus IOL in one eye and a distance
focus in the other had some adjustment problem.|
It should be remembered that even a monofocal IOL may in some ways improve the visual performance of an eye at both near and distance. Even though the eye may not be focused for near vision, the net uncorrected near acuity may be superior to the best-corrected preoperative near acuity owing to the blurring effects of the cataract. The unfocused image may still be clear enough for desired near tasks.
An apparent accommodation has been noted in pseudophakic eyes.51 This covers an average power of about 1 D, without any significant difference between anterior and posterior chamber lenses. An object can be seen even if it is not focused exactly on the retina as long as its image does not subtend an angle greater than the separable minimum, about 60 degrees of arc. Pharmacologic mydriasis increases the area of these blur circles for a given object, thus causing a decrease in the depth of focus as the pupil is dilated.
There is another way to increase the effective depth of focus in a patient receiving a monofocal IOL. A zone of improved clarity exists between the two foci of the conoid of Sturm. Thus a surprisingly good depth of focus may be found in patients with postoperative myopic astigmatism. Monofocal IOL patients were studied who had an uncorrected visual acuity of 20/40 or better with a near acuity of J3 or better.52 The mean spherical correction of these patients was -2 D, with a mean cylinder of ±1.75 D. In an eye with an uncorrected astigmatism, an object of regard produces two foci corresponding to the two principal meridians. As the object is moved closer to the eye, both foci move posteriorly. In this situation, the posterior meridian will be close to the retina when imaging distant objects, whereas the anterior image will move closer to the retina (as both foci move posteriorly) when imaging near objects. Thus these patients will have both a far and near point with reduced blur (corresponding to the two foci of the conoid of Sturm), increasing depth of focus at the cost of a slight reduction in uncorrected visual acuity. As long as the retina is positioned within the interval of Sturm, the vision may be blurry but satisfactory.
A theoretic model was developed to analyze the optimal astigmatism needed to maximize depth of focus after IOL implantation.53 To see objects clearly in the range between 0.5 and 6 m distance, the best overall acuity is predicted to be a refraction of -1.00 sphere ±0.75 cylinder. For this refraction, a focal line will be present at the retina for object distances of 1 and 4 m. For objects between 0.5 m and infinity, one focal line will always fall within 0.5 D of the retina.
MULTIFOCAL INTRAOCULAR LENSES WITH CONCURRENT EYE DISEASE
It has been suggested that multifocal IOLs are contraindicated in patients with macular degeneration, who may need all the image clarity and contrast possible.45,54 (And, of course, all patients are at risk for developing age-related macular degeneration in the future!)
In general, patients with ocular diseases such as age-related macular degeneration, diabetic retinopathy, glaucoma, amblyopia, and corneal disorders are usually considered poor candidates for multifocal/bifocal IOL implants. Some reasons for exclusion include reduced contrast acuity of multifocal/bifocal lenses at low contrast levels, which may be functionally significant in compromised eyes.28,55 Additionally, the potential for visual recovery of these patients to 20/20 is not 100%. Another study suggested that patients with concurrent ocular disease benefit in distance-corrected near vision in eyes with a refractive multifocal IOL (Array) compared with a monofocal lens.56 In 133 eyes of 111 patients with cataract and concurrent ocular pathologies such as macular degeneration, glaucoma, and diabetic retinopathy, 40 eyes (49.5%) of the Array IOL group and four (7.7%) eyes of the monofocal IOL group achieved a best distance-corrected visual acuity of 6/12 (20/40) and N8 (20/57). This difference was statistically significant (p < .0001).
Multifocal IOLs make extra demands on both the physician and the patient. With monofocal IOLs, we have been accustomed to accepting any postoperative refraction resulting in emmetropia or mild myopia, the theory being that there will then be some distance at which the patient has good uncorrected visual acuity. With multifocal IOLs, the accuracy of the lens power calculation becomes much more critical. With monofocal IOLs, moderate degrees of postoperative astigmatism are acceptable (or perhaps even desirable for its increased depth of focus!); with multifocal IOLs, the amount of postoperative astigmatism should be minimized, perhaps even less than 0.5 D.57 Visual acuity decreases significantly in proportion to the diopters of astigmatism, especially at two diopters or more.58 At least for some types of multifocal IOLs, lens centering may be more critical than for the monofocal IOL. Similarly, a small tonic pupil may be a contraindication to the use of a refractive bull's-eye type of multifocal IOLs.59
A more difficult problem is in identifying which patients are appropriate candidates for the multifocal IOL. The same investigator who had to replace a multifocal IOL with a monofocal lens in three patients had three other patients in the same series who were so satisfied that they insisted on having a multifocal IOL implanted in their second eye.45 Although multifocal IOLs do double or triple the depth of focus, they do so at some cost in image clarity and contrast. It will be essential to identify those clinical situations for which this trade-off is desirable.
Another unanswered question is whether relatively minor degrees of capsule opacification adversely affect the already-reduced contrast sensitivity inherent in these lenses.60 Increased preoperative or postoperative astigmatism (or both) has been associated with decreased satisfaction with multifocal IOLs.54 For one thing, patients with postoperative astigmatism who must wear glasses anyway may not appreciate the benefit of a multifocal IOL. Some investigators point out that minimal-to-moderate preexisting astigmatism may also enhance the patient's depth of focus.54 These investigators thus consider a preoperative astigmatism of greater than 1 D to be a contraindication to multifocal IOL implantation. They also believe (on the basis of 1068 multifocal implants) that patients with contralateral clear lenses or with contralateral monofocal IOLs will not be completely content with a unilateral multifocal IOL; this observation certainly lends itself to different interpretations.
|Multifocal IOLs attempt to address a problem associated with aphakia and
pseudophakia (and indeed of normal presbyopia)—that of reduced
or absent accommodation. This problem may also be addressed by correcting
one eye for distance and the other for near; by leaving the patient
with a myopic astigmatic correction and resultant increase in depth
of focus; or, of course, with spectacle multifocal correction. Obviously, there
is no point in recommending multifocal IOLs for a patient who
wishes to continue to wear spectacles postoperatively anyway.|
It is incontestable that multifocal IOLs provide a greater depth of focus than monofocal lenses. Patients with multifocal IOLs are more likely to be able to see satisfactorily at both near and distance without spectacles. Conversely, there appears to be some unavoidable decrease in image quality and contrast sensitivity with multifocal lenses. The clinical significance of this increases with lower levels of illumination, lower levels of contrast, and smaller objects of regard.
The deleterious effects of macular degeneration may well be magnified by these lenses, although controlled clinical studies on this point may be impossible. Capsular pacification with a multifocal IOL may well cause a disproportionate decrease in acuity; of course, the neodymium:yttrium-aluminum-garnet (Nd:YAG) laser should be able to remedy this—but will a higher frequency of Nd:YAG capsulotomies be required in multifocal eyes?
Accurate calculation and selection of IOL power are more critical in multifocal IOL patients, and the preoperative and postoperative astigmatism must be low. Decentering may be more of a problem, at least in certain multifocal lens designs.
There are some patients for whom the trade-offs inherent in multifocal IOLs will be well worth it. These patients are satisfied to exchange some clarity of image quality for the increased depth of focus and reduced dependence on spectacles associated with multifocal IOL use. Equally important, there are patients for whom that trade-off is objectionable, perhaps patients with exacting demands for near vision or who drive a lot at night. As different types of lenses become available, differences between them may be of significance as far as which to choose for a given patient. Lenses with higher near-adds may provide better near vision and more patient satisfaction for fine visual tasks at near. (Will they be less satisfactory for “intermediate vision?”) They will also have a closer “near working distance” and may also have a larger “out of focus” blur at distance and thus possibly more problems with activities such as night driving. One of the future challenges of multifocal IOLs will be to develop optimal methods for identifying that subset of patients for whom the multifocal lens represents a viable clinical alternative, and perhaps which lens best suits the multifocal-plane needs of that individual.
3. Kock DD, Samuelson SW, Villarreal R et al: Changes in pupil size induced by phacoemulsification and posterior chamber lens implantation: Consequences for multifocal lenses. J Cataract Refract Surg 22:579–584, 1996
4. Steinert RF, Post CT Jr, Brint SF et al: A prospective, randomized, double-masked comparison of a zonal-progressive multifocal intraocular lens and a monofocal intraocular lens. Ophthalmology 99:853–861, 1992
23. Steinert RF, Aker BL, Trentacost DJ et al: A prospective comparative study of the AMO ARRAY zonal-progressive multifocal silicone intraocular lens and a monofocal intraocular lens. Ophthalmology 106:1243–1255, 1999
24. Javitt JC, Steinert RF. Cataract extraction with multifocal IOL implantation: A multinational clinical trial evaluating clinical, functional and quality of life outcomes. Ophthalmology 107:2040–2048, 2000