Alejandro Cerviño Optometry Research Group Department of Optics University of Valencia Dr Moliner, 50–46100–Burjassot Valencia SPAIN E-mail: firstname.lastname@example.org
During the past years, the wish to become independent of spectacles has been growing among cataract and presbyopic patients due to many factors, such as the increase in near visual demands, the aesthetic need for a spectacle-free image and ageing of refractive surgery patients, among others. This review assesses recently published studies that analyse visual and optical performance through different metrics of eyes implanted with multifocal intraocular lenses (IOLs), particularly hybrid IOL designs. The published evidence suggests that hybrid multifocal IOLs provide very good outcomes in a number of visual and optical performance parameters. Patients implanted with this type of IOL obtain a satisfactory full range of visual functions, including patients of particular characteristics such as highly ametropic or post-LASIK.
Ageing of the world's population has two major consequences in vision: presbyopia and cataract. Presbyopia results from the natural age-related decline in the ability of the eyes to change focus. There is a huge and still increasing proportion of the population that has passed the age of presbyopic establishment (people between 45 and 65 years represent 25 per cent of the total population). Cataracts occur in the elderly as a natural result of the ageing process causing a loss of vision. Cataract surgery has become the most frequent of all surgical operations (about five million in 2006). It is expected that the population older than 60 years of age will reach one-third of the whole world population. At this age, everyone is presbyopic and there is a considerable percentage of cataracts.
The correction of presbyopia and cataract through new designs of intraocular lenses (IOLs) offers a new approach with multiple ergonomic, aesthetic and practical advantages over spectacle lenses. The traditional approach consists of the replacement of the crystalline lens with a monofocal IOL, however, patients who undergo lens surgery with monofocal IOL implantation are not capable of focusing near objects sharply. For decades, this limitation has been solved with the use of reading glasses for near visual tasks.
Over recent years, the wish to become independent of spectacles is growing among cataract and presbyopic patients, along with the need for better visual performance for near visual tasks and for longer periods of time. One option to increase the depth of the focus allowing distance and near vision is monovision, that is, treating the dominant eye to provide distance vision and the non-dominant eye to provide near vision.1 Predicting success with this approach has proved to be difficult, as not all patients adapt to monovision due to the inherent loss of depth perception.2
Another option is the implantation of an accommodating IOL. These IOLs provide the advantage over monovision of maintaining binocularity for all distances. Clinical studies have shown that currently available accommodating IOLs provide an amplitude of accommodation that typically remains insufficient, besides being highly variable from patient to patient; all exacerbated by the high rate of posterior capsular opacification.3
Multifocality or pseudoaccommodation would be the other approach to seek for functional vision for all distances. Multifocal IOLs are designed to generate two separate focal points along the optical axis, thereby producing the functional equivalent of accommodation.4,5 The aim here is to provide good unaided distance and near vision as well as functional intermediate vision. Several clinical studies conducted using different multifocal designs have shown that patients primarily perceive only the focused image,6–8 however, some patients may experience unwanted photic images like glare, flare, streaks and halos.9,10 This occurs because a clear image is always accompanied by one or more blurred images.
To achieve multifocality, these lenses use either the principle of diffraction and/or refraction and they may be divided into three groups:11
1diffractive multifocal IOLs
2refractive multifocal IOLs
3hybrid multifocal IOLs.
Traditional diffractive IOLs feature concentric rings, covering either the anterior or posterior surface, which are separated from each other by a step height of about two microns. These rings serve as a phase grid leading to diffraction of the incoming light and therefore, allow the creation of the two foci independently of the pupil diameter.5,12 Height and size of the diffractive steps on the lens are used to separate the distance and near foci (addition) and the percentage of light distribution between foci. Figure 1 shows different full-optic diffractive IOLs.
Refractive IOLs are governed by a different optical concept. The anterior surface typically features two or more spherical zones of different radii of curvature. One zone of constant refraction will provide a focus for distance vision and another one will do so for near. The performance of refractive bifocal lenses is dependent on pupil size, as the relative percentages of the distance and near powers within the pupil depend on the size of the pupil.13Figure 2 shows two refractive IOL designs.
Clinical studies have shown differences in visual outcomes depending on the optical principle used. In refractive optics, the different zones of equal refractive power have a mutual focus. Phases of incoming light are incoherent, creating some destructive interference. This interference affects the intensity of the focused light and thus leads to a reduction in brightness and visual acuity.13 The retinal image with the multifocal refractive IOLs depends on pupil diameter because of the IOL's power profile.13 Diffractive IOLs are less pupil-dependent and have advantages over refractive IOLs in near vision,14 however, they can induce slight disturbances of night-vision and a reduction in contrast sensitivity.15–17
New optical designs applied to pseudoaccommodating IOLs combine refractive and diffractive optics to reduce the disadvantages of conventional IOLs with either refractive or diffractive designs.5,18 The present review will focus on the visual and optical performance of hybrid IOLs.
HYBRID MULTIFOCAL IOLS
The Acri.LISA 366D IOL, introduced into the market by Acri.Tec AG and the ReSTOR apodised lens, marketed by Alcon, use the hybrid concept.
The hybrid Acri.LISA 366D IOL is a single-piece IOL with a 6.0 mm foldable acrylate aspherical optic, an overall diameter of 11.0 mm and zero-degree haptic angulation. The surface is divided into main zones and phase zones; the phase zones assume that function carried out by the steps in the diffractive IOLs (Figure 3). The phase zones have a mean refractive power corresponding to the zero diffractive power of the main zones. The IOL power responsible for distance vision comes from both refractive and diffractive origins, simultaneously. The first diffractive power used for near vision is formed by in-phase interference of waves from the main zones. The two focal points are created by phase zones on the anterior surface of the IOL. The diffractive structure has a soft transition of the phase zones between the main zones. The adjusted phase zones were designed to reduce the disturbing light phenomena (for example, scattered light, halos) to improve retinal image quality, visual performance and comfort.
The design of the aberration-correcting lenses Acri.Smart 36A and Acri.Lyc 35A is used as a base for the Acri.LISA. Therefore, the IOL has an aspheric profile to correct the positive spherical aberration of the cornea. The optic is made of acrylate (refractive index 1.46) with a 25 per cent water content and ultraviolet wavelength-absorbing properties (Acri.Lyc material). The hydrophobic surface of the Acri.LISA IOL has sharp edges to reduce posterior capsular opacification. The IOL power varies from plano to +40.00 D and incorporates a +3.75 D near addition, corresponding to approximately +3.00 D in the spectacle plane. The Acri.LISA IOL is implantable through a very small incision (between 2.8 and 3.2 mm) and therefore, is suitable for microincision cataract surgery.
For the development of the AcrySof ReSTOR IOL, the main guideline was that near vision is less important under dim illumination conditions when pupils are large. A second one was that minimising the perception of halos and glare under dim illumination conditions is essential. At this point it was critically important that these two characteristics were actually complementary and this allowed the use of a unique optical feature, apodisation. This feature allows the energy balance of the optical system to vary with the pupil diameter in a way that is consistent with natural pupillary responses to distance and near visual requirements under various lighting conditions and compatible with the pupillary accommodative reflex.19
The term ‘apodisation’ derives from Greek, literally meaning ‘cutting feet off’.5 The apodisation property of the AcrySof ReSTOR IOL is defined by the gradual reduction in diffractive step heights and step widths from the centre towards the periphery, resulting in an energy proportion continuum for light directed to the two primary foci. For the AcrySof ReSTOR apodised hybrid IOL (Figure 4), the physical diffractive step heights are distributed in an almost continuously varying fashion from 1.3 microns tall centrally to 0.2 microns tall peripherally. There are 12 steps, with the first one having a diameter of 0.75 mm and the last one 3.6 mm. The central step heights create phase delays of approximately 0.5 of a wavelength for light in the aqueous. This divides the light energy fairly equally between the two lens powers, distance and near, for smaller pupils, with about 41 per cent of light going to each of the main foci. As the pupil becomes larger, the increasingly exposed steps are decreased in their step height attributing more light to the distance focus, while less light goes to the near focal point. This can be accomplished due to the progressive decrease in step height over the 12 steps, with the most peripheral step being just 0.2 microns tall, resulting in a gradual change in energy balance. In contrast, the outer region of the lens has no diffractive structure so all light goes to the distance power. This design results in a distance dominant lens for large pupils. Therefore, the AcrySof ReSTOR IOL combines the relative advantages of the refractive lens design with those of a diffractive lens design. Both design components together provide improved energy distribution control. The lens has two primary focal points, one at distance and the other at near, which are +4.00 D apart. This translates to the near point being equivalent to approximately a 3.20 D addition in the spectacle plane. The base lens curvature provides the distance power using its refractive shape.
The fine diffractive surface structure of the AcrySof ReSTOR IOL is fabricated on the AcrySof foldable hydrophobic acrylic material using manufacturing precision that can control surface features down to 0.2 µm. Like all foldable IOLs, it is inserted through a small incision, which minimises induced astigmatism to provide high-quality uncorrected vision. The diffractive surface is not damaged during folding or compression of the anterior surface or by transient compression by the folded over trailing haptic in the injector cartridge.5 An improvement of this concept was recently introduced by the same company through an aspheric platform by the name of AcrySof ReSTOR Aspheric IOL (SN6AD3 model) (Figure 5). The addition of asphericity aims to reduce unwanted visual phenomena, associated with multifocal IOL performance and to increase the range of focus improving image quality. The IOL has a symmetric biconvex design with an anterior aspheric optic to reduce whole-eye spherical aberration, that is, the IOL has a negative spherical aberration of -0.10 µm for a 6.0 mm pupil. The aspheric optics flattens the edge and reduces the central thickness (about 4.5 per cent thinner for a 20 D IOL compared to the spherical model). The nominal diffractive near addition is the same for both models (+4.00 D) and is equivalent to approximately 3.20 D at the spectacle plane (approximately 31.0 cm working distance). A new IOL with the same optical design was developed with a lower addition (+3.00 D), equivalent to approximately 2.40 D at the spectacle plane (approximately 41.0 cm working distance). The new aspheric bifocal +3.00 D model (AcrySof ReSTOR SN6AD1) was developed considering the near focus. For everyday living, the +3.00 D IOL may be more acceptable physiologically than the +4.00 D model. In addition, the lower addition may improve functional intermediate vision. The +3.0 D IOL has nine diffractive steps that are more widely spaced than the 12 steps of the +4.00 D IOL.
The findings on the visual and optical performance after hybrid multifocal IOL implantation will be discussed through careful review of the peer-reviewed literature. To ease the reading flow, the different aspects covered will be grouped into two main epigraphs, namely visual performance (distance, intermediate and near visual acuity, contrast sensitivity function and stereopsis) and optical performance (wavefront aberrations and retinal stray light). Additionally, the performance of these lenses in special patient groups will also be discussed for post-LASIK and highly ametropic patients.
Distance, intermediate and near visual acuity
Many studies20–35 have evaluated distance and near visual acuity after hybrid multifocal IOL implantation (Tables 1 and 2). In most of these studies, average monocular distance visual acuity (VA) and corrected for distance near vision (NV) after multifocal IOL implantation were better than 0.1 logMAR (about 6/7.5), with most eyes maintaining or improving both VA and distance corrected near vision. Several studies have assessed the performance of different types of hybrid multifocal IOLs. Alfonso and co-workers31 compared visual acuity after bilateral implantation of Acri.LISA 366D IOL and AcrySof ReSTOR SN6AD3 aspheric IOL, finding that both multifocal aspheric IOL models gave similarly good high-contrast visual acuity for distance and near. Maxwell and colleagues36 compared visual outcomes six months after bilateral implantation of AcrySof ReSTOR aspheric IOL with both +3.00 D and +4.00 D additions. In this study, the authors did not find differences for distance or near visual acuity between the two IOLs, however, the +3.00 D IOL increased the mean reading distance to approximately 40 cm over the 33 cm distance with the +4.00 D model.
Table 1. Peer-reviewed studies evaluating monocular and binocular uncorrected distance vision (UCV) and distance visual acuity (VA) after hybrid multifocal IOL implantation. The values of visual acuity are shown as mean and standard deviation in logMAR notation.Values marked with an * were originally reported in decimal notation but were converted to logMAR for comparison purposes.
STUDY (first author)
IOL = intraocular lens
UCV = uncorrected distance vision; VA = visual acuity
Table 2. Peer-reviewed studies evaluating monocular and distance-corrected near vision (NV) and uncorrected near vision (UCNV) after hybrid multifocal IOL implantation. The values of visual acuity are shown as mean and standard deviation in logMAR notation.Values marked with an * were originally reported in decimal notation but were converted to logMAR for comparison purposes.
STUDY (first author)
IOL = intraocular lens
UCNV = uncorrected near vision; NV = distance-corrected near vision
Fernández-Vega and associates23 and Petermeier and co-workers25 evaluated the visual acuity after AcrySof ReSTOR spherical IOL implantation in three groups divided as a function of pre-operative refractive error (hyperopia, emmetropia and myopia). In both studies, statistically significant differences were not found for VA and distance corrected near vision between groups, however, in the study by the Fernández-Vega and associates,23 myopic patients gained more lines of VA than hyperopic patients. The authors suggested that magnification and minification of the retinal image compared to spectacle acuity in myopic and hyperopic patients, respectively, may play a significant role in this finding.
Several studies27,33,37–39 have compared visual acuity between eyes with hybrid multifocal IOL and eyes with a monofocal IOL. All these studies show that patients who received a hybrid multifocal IOL obtained better uncorrected near vision and distance-corrected near vision than those who received a monofocal IOL with no decrease in distance visual acuity. Obviously, as the authors pointed out, these findings show that patients with hybrid multifocal IOLs obtained a satisfactory full range of vision and achieved higher spectacle independence than those patients with monofocal IOLs, which is the reason the patients wanted to be implanted with multifocal lenses.
An important requirement for achieving stability in the visual outcomes after multifocal IOL implantation is an appropriate centration and stabilisation of the IOL. One study22 analysed the effect of a capsular tension ring (CTR) on the visual acuity after Acri.LISA 366D implantation. In this study, differences were not found in post-operative visual acuity for distance and near; however, there was a statistically significant difference between groups in the post-operative sphere being better for the group of patients with capsular tension ring, confirming the hypothesis that a capsular tension ring plays an important role in stabilising the IOL.
All studies20,24–28,30,31,35,36,38 assessing intermediate visual acuity after hybrid multifocal IOL implantation show a statistically significant worsening of visual acuity as a function of distance from the test, although better performance than with monofocal IOL27,38 is expected. When analysing the results of intermediate visual acuity (from far to near) in these studies, two peaks of visual acuity may be observed corresponding to the two main foci of the lens with somewhat reduced acuity for intermediate distances. Recently, it has been reported20,30,31,35,36,38,40 that hybrid multifocal IOLs with aspheric design provide improved intermediate visual performance compared to those IOLs with spherical design. On the other hand, some of those studies30,36,38,40 show that the new AcrySof ReSTOR aspheric IOL with a +3.00 D addition provides better intermediate visual performance than AcrySof ReSTOR aspheric IOL with +4.00 D addition, with no decrease for either distance or near visual acuities. It seems that the lower addition of the IOL plays an important role in the improvement of visual acuity for intermediate distances.
All these points should be taken into account by the surgeon when choosing the hybrid multifocal IOL design for those patients, for whom intermediate vision is important. Blaylock and collaborators41 suggested that in patients who had bilateral implantation of AcrySof ReSTOR spherical IOL, 1.00 D overcorrection of the non-dominant eye to induce mild myopia improved intermediate visual acuity.
The visual performance parameter that better identifies the boundaries of human spatial vision is the contrast sensitivity function (CSF), which plots the reciprocal of the contrast threshold as a function of the spatial frequency. Thus, it gives information on visual performance for a range of object scales.
Many studies20,24,26,27,29,30,32,35–38,40,42–50 have measured the CSF after hybrid multifocal IOL implantation. It has been suggested that CSF under photopic and mesopic conditions, after multifocal IOL implantation, must be lower than that obtained after monofocal IOL implantation, but still within the normal range. The results found in those studies assessing and comparing CSF after hybrid multifocal and monofocal IOLs implantation agree with this statement.27,37,38,39,45
Decreased CSF in patients with multifocal IOLs compared with that obtained in patients with monofocal IOLs can be explained through the multifocal principle; the incoming light distribution is divided between two or more foci and therefore the image of a distinct focus is always overlapped by out-of-focus images generated by the multifocal principle.13 Despite this, other types of ocular aberration and the blending zones of the IOL tend to mask the differences between the monofocal and multifocal IOL in terms of CSF, such as the effects of the ocular longitudinal chromatic aberrations suggested by some authors.13
Montés-Micó and coauthors13 reported that patients with multifocal IOLs had worse CSF under dim conditions. Studies evaluating the CSF after hybrid multifocal IOL implants20,26,29,30,32,35,38,39,43,46–50 also found a reduction in contrast sensitivity when the luminance level was reduced, particularly for higher spatial frequencies. This trend agrees with classic data on the effect of the luminance level on contrast sensitivity.51
As previously explained in the introduction, in the ReSTOR IOL the light energy balance for both main foci (distance and near) depends on the pupil diameter. Taking this into account, Alfonso and colleagues46 assessed whether the pupil size was correlated with the CSF for all distances in eyes implanted with the ReSTOR IOL. They found that pupil size influences distance CSF under both photopic and mesopic conditions. Larger pupil sizes had the best distance CSF for all spatial frequencies under both bright and dim lighting conditions.
It has been suggested that visual function after multifocal IOL implantation takes several months to reach its maximum potential.52 To accelerate this learning process, Kaymak and associates53 investigated the efficacy of a special visual training program on the post-operative visual performance. The training method was based on orientation discrimination of tilted bars, which were presented on a monitor. Training consisted of six sessions over two weeks; time per session averaged 30 ± 5 minutes. Only one session per observer took place each day and sessions were repeated in intervals no longer than three consecutive days. The authors reported that CSF and near vision at different contrast levels showed a significant improvement through training and concluded that visual performance after multifocal IOL implantation can be significantly accelerated by a specific two-week training program.
Stereopsis contributes to the perception of depth and distance and participates in the process of recognition of solid objects. Stereoscopic acuity or stereoacuity is the ability to discriminate very fine differences in depth from geometric disparity. Good stereopsis at near is required for accurate hand-eye co-ordination when using tools, threading a needle, performing surgery or using a computer.54 Reduced stereopsis may cause symptoms of discomfort such as eyestrain, headaches and diplopia.55 It is quantified by the minimum geometric disparity that elicits a sensation of depth.
The visual system requires binocularity to achieve stereopsis and yet retains a single representation of a world viewed through two eyes. Fusion is the sensory process that unites the images from both eyes into a single image and fusion and vergence eye movements evolve to support stereopsis. Therefore, stereoacuity is a useful indicator of oculomotor system integrity.
Several factors affect stereopsis. Degrading factors include reduced contrast, refractive error, heterophoria56 (defined as failure of the visual axes from both eyes to remain parallel after elimination of visual fusional stimuli) in its various manifestations (horizontal, vertical, cyclophoria and anisophoric), aniseikonia57,58 (defined as the difference in perceived size between the images from both eyes) and age. As contrast is reduced, stereopsis degrades. Even a small amount of equal blur (0.50 D) in both eyes has been shown to reduce stereopsis.59 Unequal blur between the two eyes is a challenge for the stereoscopic system.60 It has been reported61 that stereoacuity is dependent on contrast and that the loss in stereopsis with age is strongly related to the loss in contrast sensitivity.62,63 Loss of light transmission through the ocular media is a likely factor for the decrease in contrast sensitivity and therefore, in stereopsis.
Multifocal IOLs enable projection onto the retinal plane of images set at distance and near. In both situations, the unwanted effect of the light in the out-of-focus image is to reduce the contrast of the in-focus image.13,52 Clear bifoveal images are needed for high-resolution stereopsis, and retinal image contrast reduction provided by multifocal IOLs may affect stereopsis.
Ferrer-Blasco and co-workers64 evaluated stereoacuity using the Titmus stereotest at 40 cm under photopic conditions before and after refractive lens exchange with AcrySof spherical ReSTOR IOL implantation in 15 non-cataractous hypermetropic patients. The authors showed that stereoacuity did not change after AcrySof ReSTOR IOL implantation. They reported stereoacuity mean values of 46.42 ± 1.36 and 48.67 ± 1.13 seconds of arc (p = 0.223) before and after surgery, respectively, with 95 per cent of patients maintaining the same level of stereoacuity after the surgery.
Souza and collaborators27 evaluated stereopsis using the Titmus stereotest after implantation of the AcrySof ReSTOR IOL in 25 patients and compared it with the monofocal SA60AT IOL in 15 patients. Ninety-two per cent of the patients in the ReSTOR group and 87 per cent in the monofocal group achieved a stereopsis of 50 seconds of arc or better. The authors did not find statistical differences between groups.
Cionni and colleagues65 studied stereopsis after unilateral implantation when the fellow eye was either phakic or implanted with a monofocal IOL or after stepwise bilateral implantation of AcrySof ReSTOR IOL showing the importance of bilateral implantation over monocular.
Elkington and Frank66 reported that 40 to 50 seconds of arc is considered normal stereoacuity measured at 40 cm. The results of these studies with AcrySof ReSTOR are close to this value. Therefore, it seems that the reduction in contrast of the in-focus image at near caused by the defocused image corresponding to the distance focal point does not seem to significantly affect stereoacuity.
An important issue to keep in mind is the type of test used to measure stereopsis in all these studies. Clinical stereotests, such as the Titmus, analyse local stereoacuity with large ranges between steps (from 800 to 40 seconds of arc in nine steps) without establishing a stereoscopic threshold. In addition, disparity is constructed for a specific examination and interpupillary distance, which obviously varies among patients. Cases of amblyopic patients with good outcomes using this test have been reported 67 (the standard deviation was equivalent to the stereoacuity measured and monocular cues were not eliminated). Then, the use of the Titmus test and others where disparity is constructed vectographically (such as the Random dot) may not be sensitive enough to detect subtle changes in stereoacuity produced by defocused images in eyes implanted with multifocal IOLs. The most sensitive and accurate test to determine stereoacuity is the Howard-Dolman method.68 Values for a normal phakic population using this method vary with age and a result of 14 seconds of arc or better for a phakic adult with a normal binocular system would be expected.56,69
Higher-order wavefront aberrations may degrade the retinal image even in the absence of sphero-cylindrical refractive error. For this reason, there has been an increasing interest in the measurement of the higher-order aberrations (HOA) of eyes that have been implanted with different types of IOLs.70–75 It would be expected that, at any pupil diameter, lower levels of HOAs are associated with better distance visual performance, although it is known that similar levels of root-mean-square (RMS) wavefront aberrations distributed between different Zernike coefficients do not necessarily imply equal performance.76,77 Several studies evaluated the optical performance after hybrid multifocal IOL implantation. Souza and colleagues,27 Rocha and co-workers45 and Ortiz and associates78 studied the optical performance of the AcrySof ReSTOR IOL and compared it with a monofocal IOL. Souza and colleagues27 chose the monofocal IOL AcrySof SA60AT, Ortiz and associates78 used the AcrySof MA60 monofocal IOL and Rocha and co-workers45 three monofocal IOLs (AcrySof MA30AC IOL, AcrySof SA60AT IOL and Mediphacos Acqua IOL). In all studies, the AcrySof ReSTOR IOL induced less spherical aberration than monofocal IOLs. Ortiz and associates78 found that the AcrySof ReSTOR had lower mean higher-order aberrations than the monofocal IOL for a 5.0 mm pupil diameter. In the other two studies, differences were not found between AcrySof ReSTOR IOL and monofocal IOLs in terms of higher-order aberrations.
Zelichowska and colleagues42 and Ortiz and associates78 compared optical quality of AcrySof ReSTOR with the ReZoom multizone refractive multifocal IOL. Both studies concluded that optical quality was significantly worse in patients with the refractive IOLs.
Three studies21,22,79 assessed the optical performance of Acri.LISA 366D IOL. All of them conclude that the optical quality provided by this multifocal IOL, using different metrics, was comparable to values reported for standard spherical monofocal IOLs. Alió and co-workers22 studied the effect of a capsular tension ring on the intraocular optical quality of the Acri.LISA IOL. They reported that patients with capsular tension ring implantation experienced a reduction in intraocular aberrations and a statistically significant increase in the Strehl ratio and modulation transfer function (MTF) values compared with patients without capsular tension ring. As pointed out by the authors, these results suggest better IOL stability and hence better optical performance when IOL implantation is combined with capsular tension ring implantation. Therefore the capsular tension ring could improve the intraocular optical quality.
Maxwell, Lane and Zhou80 compared the optical qualities of six IOLs for the correction of presbyopia with different designs (accommodating or multifocal and spherical or aspheric) using modulation transfer function (MTF) and the United States Air Force 1951 Resolution Target (AFT) optical bench testing with a modified International Organization for Standardization (ISO) model eye that matches the magnitude of the spherical aberration of the internal optics in young human eyes. The IOLs included three spherical designs (Crystalens AT-50SE, AcrySof ReSTOR SA60D3, and ReZoom NXG1) and three aspheric designs (AcrySof ReSTOR SN6AD3, Acri.LISA 366D and Tecnis ZM900). The authors indicated that AcrySof ReSTOR SN6AD3 aspheric IOL had superior optical properties for distance, as shown through modulation transfer function and AFT testing, among the six IOLs for presbyopia, including the monofocal accommodating IOL. The Acri.LISA IOL also performed very well, being second only to the ReSTOR aspheric IOL on the modulation transfer function test and displayed a high resolution on the AFT test. The ReZoom IOL had the poorest overall optical quality out of the IOLs examined.
Montés-Micó and colleagues81 measured the optical quality of healthy presbyopic eyes before and after implantation of the AcrySof ReSTOR Natural IOL. They concluded that higher-order aberrations after implantation of this multifocal IOL in healthy eyes seem to be similar to the pre-operative values.
Care must be taken when interpreting these studies that assess wavefront aberration in eyes with multifocal IOL implants, particularly those with diffractive designs. Charman, Montés-Micó and Radharkrishnan82 suggested that there are some likely problems when reporting wavefront aberration data for eyes with diffractive IOLs. The simultaneous distribution of light on the far and near foci means that two wavefronts of different curvature emerge from the IOL. This could make it difficult for the aberrometer to locate the sample centroids, from which it derives the aberration values. According to Charman, Montés-Micó and Radharkrishnan,82 the results of aberrometry depend on factors such as the power of the diffractive addition and the relative amplitudes of the distance and near wavefronts. Because diffractive efficiency falls and addition power increases as the measurement wavelength increases, it may be that Shack-Hartmann aberrometers that use longer wavelengths of infrared light are more likely to produce wavefront results that correspond to the wavefront produced by the distance power of a diffractive IOL.
Retinal stray light
Good post-operative visual acuity does not always guarantee patient satisfaction. Quality of vision includes, among other parameters, halos and glare. It is important to realise that the effect of stray light on vision is different from the effect of refractive blur. For proper understanding of patient complaints, it must be noted that a simple (monofocal) error of focus results in a blur circle on the retina when a distant point source, such as the headlamp of an approaching car, is viewed. This blur circle has a radius of p*D, where p is the radius of the pupil and D is the dioptric error. For a bifocal design, with approximately 50 per cent of the light in both foci, the approximate retinal image is the superposition of a sharp image and an image degraded with this blur circle. It is well known that patients see a halo-like circle of about that size around bright lamps. Therefore, the primary negative effect of bifocality is that (almost) 50 per cent of the light is spread out up to 0.4°. Because this area is relatively small and the amount of light spread over it relatively large, this blur circle is very bright. For that reason, it is sometimes termed ‘glare.’ This may be a confusing use of words because discomfort glare and disability glare are concepts defined by the International Commission on Illumination (CIE) to describe the effects suffered by the natural eye due to light spreading over angles greater than one degree in the natural eye. As disability glare was found to precisely correspond to retinal stray light, the CIE also set stray light to be the accepted definition of disability glare.
The amount of retinal stray light is different for each individual and in the young, normal eye is roughly caused one-third by the cornea (increased by corneal dystrophies),83 one-third by the lens (increased by cataracts),84,85 and one-third by the iris, sclera and fundus (sensitive to changes in pigmentation and vitreous turbidity).86 The stray light naturally increases with age due to increasing media opacification.87,88 Retinal stray light could be one possible source of unwanted visual phenomena associated with hybrid multifocal IOLs. Multifocality by definition worsens optical quality because of the secondary focus89 but an important question is whether, in addition to this small angle spread of the secondary focus, a large angle problem exists with such lenses. Three studies evaluated the stray light after hybrid multifocal IOL implantation. Hofmann and associates90 measured the retinal stray light using the C-Quant (Oculus Optikgeräte GmbH) system in 40 eyes implanted with the AcrySof ReSTOR IOL (SA60D3) and in 40 eyes implanted with the AcrySof SA60AT monofocal IOL. They also assessed patients’ complaint scores. The results of this study showed that patients implanted with the AcrySof ReSTOR IOL showed similar retinal stray light values compared to those implanted with monofocal IOL. Subjectively, patients with the diffractive IOLs claimed to suffer significantly more glare under every light condition, particularly under dim conditions.
De Vries and collaborators91 carried out a prospective open observational case series to measure stray light with the C-Quant system six months post-operatively in 66 eyes with the AcrySof ReSTOR SA60D3 IOL (multifocal group) and 40 eyes with a monofocal AcrySof SA60AT IOL (monofocal group). A comparison of stray light values in an age-matched population without cataract (control group) was also made. They found that retinal stray light values (log) were significantly lower for both types of IOL than for age-matched subjects from the control group. Mean retinal stray light values six months post-operatively were higher in patients with an AcrySof ReSTOR SA60D3 IOL than in patients with a monofocal AcrySof SA60AT IOL.
Cerviño and colleagues44 in a prospective study evaluated retinal stray light and contrast sensitivity six months after surgery in 67 eyes with either a monofocal or a multifocal IOL. Within the monofocal group a ThinOptx IOL was implanted in 12 eyes and an Acri.Smart 48 S IOL in 20 eyes. Within the multifocal IOL group, a ReZoom IOL was implanted in 13 eyes and an AcrySof ReSTOR SA60D3 IOL in 22 eyes. They did not find significant differences in stray light values between multifocal IOLs and monofocal IOLs but they did find that the retinal stray light value was significantly correlated to contrast sensitivity.
In all of these studies the C-Quant system was used to determine retinal stray light. Several studies show this instrument as repeatable and reliable for the assessment of retinal stray light in human eyes.92–94 The method measures stray light under photopic conditions; thus, differences due to IOL design could be minimised because of pupil constriction.
PERFORMANCE OF HYBRID MULTIFOCAL IOLS IN SPECIAL PATIENTS
From the early 1990s to 2004 over 17 million laser in situ keratomileusis (LASIK) procedures were performed worldwide, with eight million being carried out in the United States.95 It is known that the two most common human ocular afflictions are presbyopia and cataract96 and will occur in those patients who have had LASIK surgery. Most patients who have myopic or hyperopic LASIK are interested in remaining independent of spectacle for distance and near vision. Implantation of multifocal IOLs could be a good option for these patients when they develop cataract or presbyopia. Alfonso and co-workers97 assessed and compared the visual function of hybrid multifocal IOL in eyes that had myopic LASIK. They measured VA at 12.5, 25 and 100 per cent contrast levels under photopic and mesopic conditions, distance-corrected near vision and defocus curve, in 22 eyes with an AcrySof ReSTOR SNA60D3 IOL and 26 with an Acri.LISA 366D IOL after myopic LASIK, as well as in 32 phakic eyes after myopic LASIK (control group). The results of their study show that under photopic conditions and 100 per cent contrast all eyes had a VA of 0.1 logMAR or better and they did not find statistically significant differences between the three groups under these conditions. At 25 and 12.5 per cent, there were no statistically significant differences between both types of IOL or between the Acri.LISA and control groups. The control group had better VA than the spherical AcrySof ReSTOR group. Under mesopic conditions, the Acri.LISA group had better VA than the spherical AcrySof ReSTOR group for all contrast levels. The distance-corrected near vision was comparable for the two IOL groups and intermediate visual acuity was better for the Acri.LISA IOL group than for the spherical AcrySof ReSTOR IOL group. All groups were matched for age, spherical equivalent before LASIK, axial length and pupil diameter, while the corneal qualities were equal. The authors concluded that both IOL designs provide good visual acuity for distance and near. The Acri.LISA IOL provides better visual quality under mesopic conditions and better intermediate performance than the spherical AcrySof ReSTOR IOL, probably due to the asphericity of Acri.LISA IOL used, as pointed out by the authors.
Fernández-Vega and colleagues98 evaluated and compared the optical performance in eyes implanted with the spherical AcriSof ReSTOR and Acri.LISA IOLs after myopic LASIK, where they also analysed the efficacy and safety indexes of the procedure. A group of age-matched phakic patients who had myopic LASIK and a clear lens were also examined for comparison. Optical quality was assessed through the modulation transfer function for monochromatic light and measured with the Optical Quality Analysis System (OQAS, Visiometrics S.L). The modulation transfer function measurements were taken with dilated pupil (tropicamide 1%) and calculated for 3.0 and 6.0 mm pupil diameters. They analysed and compared the spatial frequency at 0.5 modulation transfer function and 0.1 modulation transfer function. For a 3.0 mm pupil, they failed to find differences between the control group and the Acri.LISA group or between the Acri.LISA group and the spherical AcrySof ReSTOR group. The results for the control group were better than for the spherical AcrySof ReSTOR for both spatial frequencies. For the 6.0 mm pupil, the Acri.LISA provided better optical quality than the spherical AcrySof ReSTOR IOL and in this study, patients implanted with the Acri.LISA IOL had similar optical quality values to age-matched phakic patients. The authors reported good safety and efficacy indexes for both groups and most of the eyes maintained or improved VA. The authors concluded that the implantation of a hybrid multifocal IOL in eyes that had myopic LASIK is a safe, effective procedure, although the Acri.LISA due to its aspheric design provided a better optical quality.
With regards to hyperopic LASIK patients, Alfonso and associates99 carried out a prospective study to assess safety, efficacy and predictability of spherical AcrySof ReSTOR IOL implantation in 41 eyes with prior LASIK for hyperopia. Their results showed good efficacy and safety indexes for both distance and near, and a mean spherical equivalent after surgery near emmetropia. They state that the implantation of spherical AcrySof ReSTOR IOLs in eyes with previous LASIK for hyperopia is safe, effective and predictable.
It has been reported that visual acuity decreases as refractive error increases.100,101 Taking this into account it is logical to wonder whether the power of the implanted IOL may affect visual quality. Alfonso and co-workers50 studied the differences in refractive and visual results obtained in high and low myopic eyes after AcrySof ReSTOR IOL implantation. The authors measured monocular visual acuity for distance and near, with and without distance correction, as well as photopic and mesopic contrast sensitivity six months after surgery in 76 eyes with low myopia and 76 eyes with high myopia. The authors found that uncorrected vision, uncorrected near vision and distance-corrected near vision were better in patients with low myopia than in patients with high myopia. Patients with low myopia also obtained better results for contrast sensitivity under both photopic and mesopic conditions, particularly for higher spatial frequencies.
A similar study was carried out by Fernández-Vega and co-workers47 assessing visual quality after bilateral implantation of Acri.LISA 366 IOLs in patients with high and low-moderate myopia. The parameters studied included distance, near and intermediate visual acuity, and distance contrast sensitivity under photopic and mesopic conditions after bilateral implantation of Acri.LISA 366D in 106 eyes with low-moderate myopia and in 198 eyes with high myopia. They did not find differences between groups for any of the parameters measured, concluding that bilateral implantation of the Acri.LISA 366D IOL provides a satisfactory and comparable full range of vision in patients with high and low–moderate myopia.
With regards to hyperopia, Alfonso and colleagues49 evaluated the differences in refractive and visual outcomes in high and low hyperopic eyes after implantation of the AcrySof ReSTOR IOL. They included 86 eyes with low and 86 eyes with high hyperopia, and analysed monocular visual acuity for distance and near, as well as photopic and mesopic contrast sensitivity six months after surgery. Their results show that this IOL provides good refractive and visual results for distance and near in both low and high hyperopic eyes. There was a correlation between IOL power and visual and refractive performance of the multifocal IOL. The authors discuss that worse visual performance was obtained in high hyperopic eyes, possibly because of minification of the retinal image.
Again, a similar study was carried with the Acri.LISA 366D by Fernández-Vega and associates,48 involving patients with low-moderate and high hyperopia. Monocular and binocular visual acuity, distance-corrected near vision, intermediate visual acuity and distance contrast sensitivity under photopic and mesopic conditions were measured in 100 eyes with low-moderate hyperopia and in 70 eyes with high hyperopia six months after surgery. Most of the eyes examined maintained or improved their monocular performance. Furthermore, good safety and efficacy indexes were obtained for both groups. Within the high hyperopia group, 21.43 per cent of the eyes lost one or two lines of monocular visual acuity and within the low-moderate hyperopia group, only nine per cent of the eyes lost one or two lines of monocular visual acuity. Differences were not found between groups in terms of binocular contrast sensitivity under photopic or mesopic conditions, however, the low to moderate hyperopia group obtained better monocular contrast sensitivity than the high hyperopia group at high spatial frequencies (12 and 18 cycles/degree) under mesopic conditions. It is concluded that bilateral implantation of the Acri.LISA 366D in patients with high hyperopia provides a satisfactory full-range of vision in terms of visual acuity and contrast sensitivity, comparable to those obtained in patients with low to moderate hyperopia. Again minification of the retinal image was attributed a role in explaining the slight differences observed monocularly.
To conclude, after reviewing all of these clinical studies it can be stated that hybrid multifocal IOLs provide a good visual and optical performance. Clinical results show that patients implanted with this type of IOL obtain a satisfactory full range of vision and achieve higher spectacle independence than those patients using other types of prescription. The new designs recently launched and for which the first clinical studies are now available, introduce modifications in asphericity and lower addition powers, which seem to improve visual performance in the intermediate distance range.
GRANTS AND FINANCIAL ASSISTANCE
This research was supported in part by Ministerio de Ciencia e Innovación Research Grants (#SAF2008-01114-E/# and #SAF2009-13342-E#)
The authors have no proprietary interest in any of the materials mentioned in this article.