Roger F. Steinert, MD,
Richard L. Lindstrom, MD,
David F. Chang, MD
Contents
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Monovision |
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Multifocal Intraocular Lenses |
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Multifocal Optics |
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Accommodating Intraocular Lenses |
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Optimizing Results |
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Summary |
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CHAPTER HIGHLIGHTS |
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The normal eye is constructed to allow the various refracting surfaces and ocular media to focus parallel rays of light coming from a distant object onto the retina. The eye can also adjust its refractive power through the accommodative process to bring near objects into focus. The cortex of the young lens is a soft, easily molded material contained in an elastic capsule. The traction of the zonular fibers opposes the natural tendency of the lens to assume a spherical shape. Contraction of the ciliary muscles relaxes the zonular attachment sites inward toward the lens equator, reducing the tension on the zonules and allowing the lens to move anteriorly and the highly elastic capsule to increase the convexity of the lens.[1] The resultant steepening of the anterior and posterior poles and anterior movement of the lens change the focal plane, resulting in accommodation.
For the cataract patient, intraocular lens (IOL) implantation surgery has overcome the loss of visual function associated with the removal of cataracts. However, because most IOLs are monofocal, the loss of accommodation becomes significant with surgery. Although the loss of accommodation is not absolute because increased depth of field secondary to small pupillary diameter[2,][3] or mild astigmatism[4–7] provides a degree of apparent accommodation (pseudoaccommodation), the need to correct the resultant loss of accommodation (presbyopia) is clinically apparent.
The most common treatment methods for presbyopia are reading glasses or, in cases of ametropia, bifocal spectacles. The difficulties in adapting to bifocal lenses can be considerable. As the add power is increased in strength, trifocal or multifocal reading segments may be required to enable a greater range of focus.
Three strategies expand the range of functional vision without spectacle aid:
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Monovision |
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Multifocal IOLs |
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Accommodating IOLs. |
Monovision
In typical monovision, monofocal IOLs are implanted such that one eye is focused for good uncorrected distance vision, while the other eye of the patient is focused at either intermediate or near distance, depending on the visual requirements and preferences of the patient. The mechanism of monovision is known as interocular blur suppression. The image processing system of the visual cortex suppresses the blurred image from one eye so that it does not interfere with the image from the in-focus eye. Successful monovision contact lens wearers have two orders of magnitude greater interocular blur suppression capacity than unsuccessful monovision contact lens wearers.[8] Suppression is less effective in dim illumination.
Monovision results in three inherent visual compromises:
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Reduced binocular visual acuity |
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Reduced stereopsis |
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Reduced contrast sensitivity. |
The impact on binocular visual acuity is modest, averaging 0.04 to 0.08 logMAR units reduction. That impact is increased when the dominant eye has residual oblique astigmatism.[8]The mean decrease in stereopsis is 37 arc sec with monovision contact lenses, but unsuccessful contact lens wearers averaged 50–62 arc sec greater reduction in stereoacuity than successful monovision patients.[9] Monovision reduces contrast sensitivity, especially at higher spatial frequencies (>4 cycles/°).[10] The improvement in a patient's contrast sensitivity under binocular viewing conditions (both eyes corrected for distance) is known as binocular summation. This diminishes to monocular levels at about 1.5diopters (D) of monovision, then further declines (known as binocular inhibition) until a low point at about 2.5D of monovision. With further defocus, the contrast sensitivity returns to the monocular levels.[11]
In refractive surgery, monovision can be simulated preoperatively with a trial frame or a contact lens trial. In a patient with significant visual impairment due to cataract, however, the patient cannot possibly appreciate the visual tolerance of monovision in a preoperative simulation. In refractive surgery, approximately two-thirds of patients have the ability to tolerate at least a modest amount of monovision, while the remainder dislikes experiencing any difference between their two eyes.[9,][12,][13] Some surgeons will only utilize a monovision strategy in patients who had monovision in contact lenses or with refractive surgery prior to the onset of cataract. Other surgeons have a strategy of creating monovision in most or all of their patients.[14] As is the case with refractive surgery, tolerance of monovision drops considerably when the near-focus eye exceeds −1.5D. Therefore, these patients must understand that they will gain intermediate vision but not necessarily be able to read fine print without spectacle assistance. Also, as in refractive surgery, a high degree of accuracy is required in the correction of the distance eye, because a patient's ability to function without spectacles will be highly dependent on a single eye.[13]
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Multifocal intraocular lenses
Bifocal and multifocal spectacles and some contact lenses are designed for alternating vision. The individual's gaze is directed through the portion of the lens containing the appropriate dioptric power for the target plane of focus. Usually the majority of the lens provides distance vision, and an inset dioptric add at the bottom of the lens is used for near vision. Such a design is not feasible for a multifocal IOL. An individual cannot “look through” different areas of an IOL.
Multifocal IOLs use the principle of simultaneous vision. Different areas of the IOL are designed with different focal planes, usually for near and distance vision. At any given time, one image is in focus at the retina, and the second image is highly defocused with very little structure. Distant objects are focused by the distance power of the lens and defocused by the near power. For near objects, the reverse is true; near objects are focused by the near power of the lens and defocused by the distance power.
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Multifocal optics
Multifocal IOLs produce simultaneous images using either diffractive or refractive optics. Although the term multifocal is widely used for these IOLs, most designs are actually bifocal.
All diffractive IOLs are inherently bifocal. These IOLs typically consist of an anterior refractive surface with multiple, concentric, microslope rings on the posterior surface (Figure 38-1A). The microslope rings diffract the incoming light, creating a diffraction pattern (Figure 38-1B). Distance and near foci are formed by the combination of the anterior refractive surface with the zero and first orders of diffraction, respectively, created by the posterior surface. In the Alcon ReSTOR design (Figure 38-2), the ratio of distance to near increases as the pupil size increases, due to a modification of the diffractive rings known as apodization.[15] Apodization is intended to reduce the problem of halo and glare at night. However, the ability to read in dim light, such as a menu in a restaurant, is also reduced. An alternative diffractive design is the Abbott Medical Optics (AMO) Tecnis multifocal, which has uniform distribution of distance and near focus at all pupil sizes (Figure 38-3). Both the Tecnis and the ReSTOR offer an aspheric refractive optic in an effort to reduce net spherical aberration and thereby reduce that contributing factor to halo and glare. Numerous multifocal IOLs are available internationally but not sold in the US due to FDA restrictions.[16]
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Figure 38-1 A, The optical principle of a diffractive intraocular lens, with refractive optics on the anterior surface curvature and diffraction optics on the posterior side of the optic. B, Two focal lengths are created by the zero order and first order diffractions. |
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Figure 38-2 Alcon ReSTOR Model SA60D3 |
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Figure 38-3 AMO Tecnis ZMAOO |
Refractive multifocal IOLs, in contrast, achieve more than one plane of focus by alternating zonular rings of different refracting power.
True multifocality has been attempted with the designs of the AMO Array (Figure 38-4) and ReZoom (Figure 38-5) and the now discontinued Domilens Progress IOLs. The Array and ReZoom are distance-dominant, simultaneous-vision, zonal-progressive lenses. They combine a posterior refractive surface with multiple anterior aspheric refractive zones of continuously varying power. Beginning with the central zone, distance power is placed centrally in each odd ring (1, 3, and 5). The power increases toward the periphery of the odd-numbered rings to form a smooth transition with the even zones (2 and 4) that emphasize near vision with a 3.5D add in the Array and a weaker, intermediate distance dominated near component in the ReZoom (3.2D at the IOL plane and 2.6D at the spectacle plane).
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Figure 38-4 AMO Array |
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Figure 38-5 AMO ReZoom |
The amount of light transmitted to the retina for image formation also differs among IOL designs. With diffractive IOLs, approximately 41% of the transmitted light is allocated to the distance focus and 41% to the near focus. (These ratios vary with aperture size in the apodization modification employed by the Alcon ReSTOR.) The remaining 18% of the transmitted light is lost to higher orders of diffraction that are not focused at the retina.[15,][17] In contrast, refractive IOLs transmit all of the available light to the retina. For bifocal refractive IOLs, the transmitted light is divided between near and distance foci. Multifocal IOLs focus the transmitted light for intermediate vision in addition to near and distance vision. For example, the Array MIOL allocates 50% of transmitted light to distance focus, 37% to near focus, and 13% to intermediate focus (based on a 4mm pupil). The decrease in the percentage of light transmitted through diffractive bifocal IOLs compared with the Array MIOL may result in decreased contrast acuity for patients’ distance and intermediate vision.[18]
Optical issues: contrast sensitivity, halos, and glare
Multifocal IOLs distribute the light energy into different focal planes. It is inevitable, therefore, that there will be loss of contrast sensitivity on clinical testing. Atebara and Miller demonstrated the physical basis for the patient's ability to decode the multifocal image presented to the retina as well as the unavoidable loss of contrast sensitivity due to the superimposition of the in focus and out of focus images on the retina (Figures 38-6A and B).[19]
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Figure 38-6 A, The optical physics of defocusing. The edges become blurred and the black center becomes gray. The illuminance curves under each image quantify these changes. B, A bifocal retinal image combines an in-focus and out-of-focus image. The illuminance curve is the sum of the two curves of the two images. Because the edge still has some sharp change from light to dark, the patient can accurately perceive the image and decode the letter being seen. The blunting of the edge of the image and the reduction in the total blackness (elevation of the bottom of the curve) both contribute to loss of contrast sensitivity. |
Because vision consists of continuous decoding and image processing by the visual cortex, rather than a pixel-by-pixel translation of the retinal image, patients with normal macular function typically tolerate the multifocal image better than would be expected by the basic physical optics of this lens design. In addition, many observations have concluded that patients experience improvement in the quality of vision over time. This is termed neuroadaptation.[20–22] A well-known piece of evidence for this phenomenon is shown in Figure 38-7. A patient of Michael Woodcock, MD, who was an artist received the Array multifocal IOL and painted the same scene immediately after surgery (see Figure 38-7A) and then again at 3 months (see Figure 38-7B). A marked improvement in the amount of glare and halo is evident, even though her eye did not physically change during that interval. Training regimens may be of benefit in encouraging neuroadaptation.[23]
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Figure 38-7 A, A patient's perception of night glare immediately after receiving an Array multifocal intraocular lens. B, The patient's perception 3 months later. Note marked reduction in glare around lights and the haze around objects such as trees |
Patients with bifocal/multifocal IOLs gain the ability to see at near but lose some contrast sensitivity. In most studies, the amount of contrast sensitivity loss was within tolerable limits for visual function. For example, using the contrast acuity charts of 96, 50, 25, and 11% thresholds, early studies (n = 291 patients total) consistently reported no significant difference in contrast sensitivity at high thresholds (96 and 50%) between the Array MIOL and monofocal IOLs.[26–29] These same studies found that at 11% contrast, contrast sensitivity was significantly lower (approximately 1 line) with the Array MIOL than with a monofocal IOL. However, when comparing binocular vision, contrast sensitivity at 11% did not differ significantly between the Array MIOL and the monofocal IOL.[26–29] The findings at 25% were mixed; some studies found a significant decrease in contrast sensitivity with the Array MIOL (n = 193 total patients),[27,][28] whereas other studies did not (n = 98 total patients).[26,][29] The reduction in low-contrast acuity had little effect on everyday visual tasks.[27]Bilateral implantation of the Array MIOL alleviated some of the reduced contrast sensitivity at low contrast levels.[26]
Modification of a monofocal IOL with the creation of an aspheric contour (negative spherical aberration) that compensates for the positive spherical aberration of the cornea has been reported to improve contrast sensitivity compared to a non-aspheric but otherwise equivalent monofocal IOL design for both the AMO Tecnis and Alcon AcrySof IOLs.[30–33] Whether an aspheric multifocal optic will result in the same improvement in contrast sensitivity has not yet been proven but similar improvement with asphericity seems likely.
Driving simulation studies can help define any potential for functional visual performance loss associated with a reduction in low-contrast acuity. At night, car headlights can decrease contrast sensitivity and increase glare. A prospective, masked, parallel-group comparison of 33 bilateral Array MIOL patients and 33 bilateral monofocal IOL patients assessed driving performance during night, night with glare, and fog conditions.[34] There was no significant difference between the groups for 26 of the 30 driving performance measurements. The four measures that were different were in favor of the monofocal subjects. These were the percentage of correctly recognized warning signs at night in clear weather, sign recognition distances for guide and warning signs in fog, and detection distance for one of four hazards (suitcase). The multifocal patients performed, on average, within safety guidelines (American Association of State Highway and Transportation Officials).[34] Schmitz et al.[35] compared the effect of glare from halogen lights (which are similar to oncoming automobile headlights) on contrast sensitivity in patients with an Array MIOL or a monofocal IOL. There were no significant differences in contrast sensitivity between the two groups in the presence of moderate or strong glare. A significant difference between the two groups was found only at the lowest spatial frequency (3 cpd) without halogen glare. Together, these studies suggest that driving vision with an Array MIOL is similar to that with a monofocal IOL. Nevertheless, patients with multifocal IOLs should exercise caution when driving at night or under poor visibility conditions. While US FDA approval of multifocal IOLs requires some type of driving simulation testing in order to gain approval, only the Array has published results in a peer-review journal.
Photic phenomena such as glare and halos occasionally occur in patients with refractive multifocal,[24,][25,][36–39] diffractive multifocal,[40,][41] and monofocal IOL implants.[36,][37,][42] As is the case with the inevitable loss of contrast sensitivity with multifocal optics, glare and halo are inevitable with multifocal IOLs due to the funadmental optical physics of this design. As shown in Figure 38-8, when a patient looks at a distant object, the portion of light energy that passes through the reading element in the IOL will come into focus anterior to the retina, and then diverge as a blur circle at the retinal plane. Under diffusely even illumination conditions, the lower light energy of the diffused out of focus image will not be perceived by most patients. The lower energy out of focus blur is masked by the higher light energy of the surrounding peripheral image. At night, those conditions are often different. When a patient looks at a headlight, for example, the headlight has high energy compared to the surrounding periphery, which is dark. The out-of-focus blur circle is, therefore, visible, and the patient perceives this as a halo. Efforts to reduce halos and glare include aspheric optics, aspheric blending and choice of refractive zone size, location, and power in refractive multifocal IOLs, and the apodization and ring placements in diffractive multifocal IOLs.
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Figure 38-8 Halo occurs due to the distant source light rays passing through the near-focus portion of the optic, coming to focus in front of the retina and then diverging outward as a ring as the light reaches the retina. |
With the original Array SA40N multifocal IOL, for example, one study reported moderate halos and glare in up to 46.2% and severe halos and glare in 14% of eyes.[43] In a multicenter study of the Alcon AcrySof ReSTOR SA30D3 diffractive IOL, severe halos were reported by 4.2% and glare by 8.5% of patients.[40] Measurement of straylight before and after cataract surgery found that straylight declined after cataract surgery more so with a monofocal IOL (Acrysof SA 60AT) than with a diffractive multifocal IOL (AcrySof ReSTOR SA60D3).[44]
Even though some patients experienced halos or glare with the Array multifocal IOL, they would choose to have the Array MIOL implanted again.[24,][25,][27] Further, overall quality of life and satisfaction were higher in multifocal patients than monofocal patients.[24,][25] Similarly, the second generation refractive and diffractive multifocal IOLs result in high levels of patient satisfaction, with excellent uncorrected distance vision. Because of the design of the add power, the Alcon ReSTOR typically gives higher levels of vision at near (effective add is 3.2D at the spectacle plane), whereas the AMO ReZoom is equal or stronger at intermediate distances (effective add is 2.6D at the spectacle plane).[45]
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Accommodating intraocular lenses
Accommodating IOLs have the potential to provide superior image quality compared to multifocal lenses, because competing retinal images are avoided. A lens that moves axially can also be expected to provide a full and continuous range of accommodative shifts. Distance, intermediate, and near focus should all theoretically be attainable with no loss of contrast sensitivity or unwanted nighttime images. The design objective of an accommodating IOL is to use the patient's own natural accommodation mechanism to modify the power of the IOL, in a manner that increases the effective power of the lens due to the action of the ciliary muscle.
The pioneering initial design to meet to this challenge was the CrystaLens AT-45 (eyeonics, Aliso Viejo, CA, now a division of Bausch & Lomb, Rochester NY). The original commercial design of the AT-45 (Figure 38-9A) has been replaced with the AT-50 (Figure 38-9B), featuring a larger optic (5mm instead of 4.5mm) and a square configuration to the haptics. These IOLs have two hinges positioned at the edges of the plate portion of the haptic.[46] The hinges are designed to allow the lens to flex anteriorly and posteriorly with contraction of the ciliary muscles. The design goal was to provide up to 2.5D of accommodation because the anterior movement of the optic would increase its effective power. Clinical experience with these lenses was markedly less powerful, however, and the US FDA labelling for the Crystalens states that 1.0D of accommodation is provided. A third generation of the Crystalens with an optic modification to provide more depth of focus – the “HD” – was introduced in 2008.
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Figure 38-9 A, The CrystaLens AT-45. B, The Crystalens AT-50 |
The concept of gaining accommodative benefit by anterior motion of the IOL optic is also the basis of the design of other lenses, including the HumanOptics and the Lenstec TetraFlex models.
Some monofocal IOLs have also been demonstrated to have anterior motion due to accommodative effort.[47]
The Synchrony IOL (Visiogen, Inc., Irvine, California) was the first dual-optic accommodating IOL to be implanted into human eyes. This is a single-piece foldable lens made of advanced generation silicone material. The lens is implanted through a 3.8mm incision with a pre-loaded disposable injector. The IOL has a 5.5mm diameter high-powered anterior optic (+32D) coupled with a 6mm diameter minus-power posterior optic (Figure 38-10). The amount of minus power is varied in order to correct each individual eye to emmetropia. The two optics are connected and separated by silicone struts that exert a specific amount of spring-like tension against the capsular bag, keeping it open at all times. The uncompressed lens complex is 3.8mm long axially and 9.8mm wide. When compressed, the total axial lens thickness decreases to 2.2mm. The IOL is sized so as to fill the capsular bag, where it should be confined by a centered capsulorrhexis that is smaller in diameter than the anterior optic (Figure 38-11).
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Figure 38-10 The Visiogen Synchrony dual optic accommodating intraocular lens |
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Figure 38-11 The Synchrony IOL implanted inside the capsular bag |
The Synchrony IOL is designed to utilize the natural mechanism of phakic accommodation according to the Helmholtz theory – namely zonular restraint and relaxation of the capsular bag shape. Specifically, the resting ciliary body maintains zonular tension, which is transmitted to the bag. This produces outward circumferential movement of the capsular equator, axial shortening of the capsular bag, and compression of the two Synchrony optics. This also generates strain energy that is stored in the connecting springs. With accommodative effort, the zonules relax, releasing their tension on the capsular bag. With relaxation of the bag, the spring-like struts move the anterior optic forward, thereby increasing the overall optical power of the IOL.
For an axially moving optic, the refractive shift achieved for each millimeter of movement is proportional to the dioptric power of the lens. According to ray tracing analysis, 1.5mm of anterior movement of a +32D IOL should theoretically produce a refractive shift of approximately 3.3D.[48] In contrast, the same movement of a +19D lens would only induce a change of 1.2D. The dual optic strategy seeks to provide every individual eye with a moving +32D lens, by varying the minus power of the coupled posterior optic. This design maximally leverages the accommodative effect of any axial movement of the front lens.
In late 2007, enrollment for the Synchrony IOL FDA clinical trial was completed and more than 450 eyes were implanted. The lens is now awaiting FDA submission and approval. Because of its mechanism of action, this IOL design dictates certain mandatory surgical objectives. The surgeon must create an astigmatically neutral temporal incision. A preloaded, disposable injector system is used to implant the IOL through a 3.8mm clear corneal or scleral pocket incision. The capsulorrhexis must be round, symmetric, intact, and approximately 4.5–5mm in diameter. With too large an opening, the springs can push the anterior optic partially out of the bag. Thorough cortical clean up is critical in order to maintain capsular flexibility. Polishing or vacuuming the underside of the anterior lens capsule is recommended in order to reduce the tendency for anterior capsule opacification (ACO). Because of the size and design of the lens, removal of viscoelastic is best performed with bimanual irrigation–aspiration, which can access the space in between the two optics.
Some objective evidence of actual lens movement is necessary to prove true ciliary muscle mediated accommodation. The front and back surfaces of each of the paired Synchrony optics are well visualized with high-frequency ultrasound biomicroscopy (UBM). By using a near card to stimulate accommodation in the contralateral phakic eye, separation of the optics can be consistently imaged by UBM (Figure 38-12A and B).
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Figure 38-12 A, The position of the Synchrony optics while the fellow eye is focused at distance. B, The position of the Synchrony optics with the fellow eye focused at near. Note the greater distance between the posterior optic and the back side of the anterior optic. |
As with all presbyopic lens technologies, proper patient selection, the ability to consistently attain emmetropia, and flawless surgery are important prerequisites for success. In addition, a lens system that requires a pliable and elastic capsular bag must further withstand the test of avoiding capsular fibrosis over time. Rabbit studies were performed by Werner and colleagues to study the rate of capsular fibrosis and capsule contraction with the dual optic bag-filling design of the Synchrony.[49] Bilateral eye implantations were performed with the Synchrony in one eye, and a silicone plate haptic IOL in the second eye as the control. There was no capsular fibrosis or opacification in the Synchrony eye, compared with extensive fibrosis, capsular contraction, and anterior and posterior capsule opacification in the control eye. A similar bilateral rabbit eye study was performed using two piggybacked hydrophobic acrylic IOLs as the control. Interpseudophakic opacification readily appeared in the piggyback IOL eyes, but not in the Synchrony eyes.
Many new concepts are under development in pursuit of a highly functioning and reliable accommodating IOL. The NuLens consists of two rigid PMMA plates and a flexible polymer. In principle, vitreous pressure on the back plate during accommodation forces increased anterior optic curvature of a soft optical material. A different approach to harnessing hydraulic fluid pressure is under development by PowerVision. Their “Fluid Vision IOL” has fluid in peripheral chambers that is designed to be pumped into the middle of a flexible fluid-filled optic during accommodation, thereby increasing its radius of curvature.
A different fluid concept is the basis of the “LiquiLens” from Vision Solution Technologies. Inside the optic are two fluids of different specific gravity and refractive index. When the eye rolls down to read something, the higher index of refraction fluid moves into the line of sight, increasing the optical power of the IOL.
The ultimate accommodating IOL would be a flexible polymer that could be injected into the capsular bag after removal of the crystalline lens. Many technical obstacles remain to be solved in order to implement such a lens, including removal of the natural lens leaving much of the capsule intact, prevention of capsular fibrosis and opacification from residual lens epithelial cells, and development of a non-toxic polymer that could be injected and then cured inside the eye while also assuming the correct shape and optical power.
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Optimizing results
Lens replacement with presbyopia-correcting IOLs is refractive surgery more than cataract surgery. This has implications for the surgeon's approach and management, and implications for a patient's expectations.
Selection of the “right” patient for a presbyopia IOL is more art than science, but following 10 basic rules will improve the likelihood of a satisfied patient.
The 10 principles of presbyopia intraocular lens practice
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The patient must be motivated to increase the range of daily functions that can be performed without spectacle correction. A patient who does not mind using glasses, or perhaps even prefers his or her appearance with glasses, is not likely to appreciate the optical compromise of a multifocal IOL or the extra expense of a premium IOL. The other aspect of this analysis is an assessment of personality. While there is no specific or absolute personality guideline, a patient should have a positive and optimistic personality in order to avoid postoperative disappointment with perceived lack of a perfect outcome. |
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A patient receiving a multifocal IOL must understand and be willing to be patient with the process, recognizing that it may take several months to adapt to the new visual perception system. |
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The surgeon and staff must assess the vision function needs and goals of each patient, differentiating distance, intermediate, and near visual tasks, and the lighting conditions under which those tasks are performed. These answers should then be correlated with objective measures of the pupils under photopic, mesopic, and scotopic lighting levels. These measurements will guide the selection of a specific multifocal IOL design, and also reveal whether the optic size of an accommodating IOL is adequate to avoid edge glare. |
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Because the goal of presbyopia IOL implantation is a wide range of functional vision without glasses, accurate IOL power measurement and calculation is critical. |
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Postoperative astigmatism greater than 0.75D begins to degrade the quality of uncorrected vision. The surgeon must plan the incision location and know the average astigmatic shift of that incision based on the preoperative corneal cylinder, and be comfortable with performing astigmatic keratotomy (see Chapter 24). |
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Because multifocal IOLs inherently involve optical compromise, they should be avoided in eyes with pre-existing optical impairments, such as maculopathy, amblyopia, glaucoma with central visual field loss, or in patients whose history suggests that those problems have a significant probability of developing. |
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The ocular surface is the dominant optical element in the eye. Careful preoperative assessment of aqueous tear deficiency and meibomian gland disease is important to pre-treat those conditions. If the quality of the surface cannot be improved, a presbyopia IOL may not be the best choice. |
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The surgeon must have a high level of skill so that complications are rare. A well-centered and correctly sized capsulorrhexis, thorough cortical removal, and intact capsular bag are essentials in obtaining a consistently good result with these IOLs. Because complications do occur for all surgeons, however, the patient should be aware that circumstances during surgery might lead to a change in the choice of implants. |
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Staged implantation means allowing enough time between the surgeries on the two eyes so that the visual function of the first eye can be accurately assessed. For many surgeons, that interval averages 2 weeks, but this may need to be longer if there is any uncertainty about the outcome of the first eye. By using the outcome of the first eye to guide the second, the surgeon can optimize the IOL power selection. Moreover, there is an opportunity to select a different IOL for the second eye if the patient feels the first IOL is deficient. This strategy is often called “custom matching” or “mix-and-match.” [50] |
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10. |
Do not promise, nor allow the patient to think, that glasses will never be needed. Under the best of circumstances this may occur. However, many patients with presbyopia IOLs gain a much wider range of function with unaided vision, but still require spectacles for specific needs. In the business world, this is referred to as “Under-Promise and Over-Deliver.” As a corollary, remember to celebrate success. The patient has no way of knowing what vision would have been like had a monofocal IOL been used. In an understated manner, make sure that a postoperative patient is shown the near vision that has been achieved compared to the typical outcome for a monofocal IOL. |
Approach to unexpected postoperative optical complaints
When a postoperative patient presents with complaints about vision with a presbyopia IOL, the surgeon must work methodically to determine the cause if the case was uncomplicated and the anatomical result looks normal. The process begins with taking a good history. The surgeon, not a technician, needs to listen carefully to the patient and ask non-leading questions in order to obtain an accurate picture of what images the patient perceives. For example, “glare” has vastly different meanings to different people.
The pupil is often overlooked as the source of difficulty. Because the staff may have dilated a patient before the surgeon first examines him, the patient may need to return for a second visit. The patient will appreciate that the surgeon and staff sincerely are concerned and are working hard to address the problem.
The optical work-up for complaints about clarity or about glare begins with a careful refraction, guided by corneal topography. Care must be taken to use a defogging technique in the refraction in order to determine the point where the distance vision ceases to improve, as it is easy to be deceived by “over-minusing” the patient and determining a manifest refraction that is not at distance, but rather somewhere within the near portion of the IOL optic. Corneal topographic mapping is important to detect astigmatism, both regular and irregular.
If distance vision is correctable to an acceptable level, but is inadequate in the uncorrected state, a determination can be made about improvement with strategies such as astigmatic kereatotomy or corneal laser refractive surgery. On the other hand, if uncorrected distance vision is adequate but intermediate or near vision is inadequate, the strategy of staged implantation allows the surgeon to select a different IOL power or IOL model to address the deficiencies of the first implanted eye (Figure 38-13).
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Figure 38-13 Algorithm for managing inadequate intermediate and near vision. |
A different challenge occurs when a patient perceives that vision is unacceptable at all distances and refractive correction does not improve the outcome. Figure 38-14 shows a decision algorithm for this situation. The ocular surface is the first area that should be carefully examined, as the impact of a poor-quality tear film has been underappreciated, and the findings are subtle. In addition to careful inspection of the lid margins and tear film break-up, the use of vital stains such as lissamine green or rose bengal are important. Ocular surface treatment begins with artificial tears and lid hygeine, but may need to include topical cyclosporine A and non-penetrating corticosteroid drops, punctal plugs, and systemic agents such as doxycycline, flaxseed oil, and omega-3 fatty acids. If the tear film is optimized, the underlying corneal contour is the next suspect. Corneal topography will reveal irregularities or unsuspected astigmatism. Next, attention turns to the deeper structures, with optical coherence tomography (OCT) being the key to detecting maculopathies, and visual field testing and color vision tests helping to detect optic pathway disorders (along with neuro imaging studies, if needed). Only after ruling out all of these issues the posterior capsule can be considered as the source, unless the opacity is unusual and unequivocal. More subtle posterior capsule opacities can most definitely impact the quality of vision in multifocal IOLs compared to monofocal IOLs because the multifocal optics have already reduced contrast sensitivity. However, once the posterior capsule has been opened by Nd-YAG laser capsulotomy, an IOL exchange becomes higher risk. Therefore, opening the posterior capsule should be an intervention reserved for cases when other sources of vision impairment have been excluded and posterior capsule opacity is the most likely source of the visual difficulty.
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Figure 38-14 Algorithm for diagnosing and managing complaints of poor quality vision with presbyopia intraocular lens. |
A different decision and management algorithm is needed to approach a patient with acceptable acuity but photic phenomenon such as halo and glare (Figure 38-15). The patient must first be asked to determine which eye is the source of the difficulty. Many patients have not thought to check. Have the patient draw the phenomenon. One person's halo is often quite different from another. Try to simulate the glare in the office, typically with light sources such as a muscle light. If the complaint can be duplicated in office, determining whether certain interventions are helpful can be efficiently pursued.
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Figure 38-15 Algorithm for diagnosing and managing complaints of halos, glare, and photic phenomena with presbyopia intraocular lens |
The workup continues with the measurements on the right side of the Figure 38-15 algorithm. Refractive errors and corneal distortions may be treatable sources of halo and glare. Astigmatism may require astigmatic incisions; many optical issues are best addressed with corneal laser refractive procedures.
One maneuver of particular benefit is the use of nighttime driving spectacles to reduce the perception of halos from oncoming headlights in patients with multifocal IOLs. After determining the optimum distance vision correction, added minus power is placed, typically −0.5 to −0.75D. Figure 38-16A shows that the cause of the halo is the light focused in front of the retina. Figure 38-16B illustrates that with added minus power correction, while the distance focus will be degraded slightly, the benefit is that the halo circle is now closer to the retina and, therefore, smaller. For some patients, this simple step will make the halos at night become tolerable.
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Figure 38-16 A, Halos caused by the defocused light from the near portion of the intraocular lens optic (see also Figure 38-8). B, A low-minus lens used for night driving will reduce the size of the halo by bringing the focal point closer to the retina. |
The relationship of the IOL to the pupillary aperture is the other main source of remediable glare and halo. This pathway is shown on the left side of Figure 38-15. The IOL may be well-centered in the capsular bag, but the pupil is eccentric to it, or the pupil may be too large or too small. Pharmacologic interventions that may reduce night glare symptoms are brimonidine 0.1% or 0.15% (Alphagan, Allergan), which does not actively constrict the pupil but rather acts to reduce natural mydriasis, and pilocarpine hydrochloride. For most patients, the weakest commercial concentration of 0.5% is adequate, but if the reaction is too intense, a compounding pharmacy can prepare lower concentrations under sterile conditions.
Lasers can be of assistance with pupil issues in several ways. A small pupil can be enlarged with photocoagulation spots applied as a ring outside the sphincter muscle. Alternatively, the Nd-YAG laser can create multiple small sphincterotomies (for details on Nd-YAG sphincterotomy, see Chapter 52). Donnenfeld reported a successful technique to shift the pupil to gain better centration (verbal presentation, American Academy of Ophthalmology, 2007). A green photocoagulator is set at 500μ spot size, 500milliwats, and 500ms. Three to five spots are applied in the mid-periphery of the iris in a circumferential arc in the direction toward which the pupil needs to move. Experience indicates that the performance of multifocal IOLs degrades with pupil mis-match greater than 0.3–0.5mm.
Opacification of the posterior capsule is another source of glare that can be treated. As is the case of suboptimal vision, however, the clinician must be wary of being seduced into attributing glare to the posterior capsule until other sources of glare have been ruled out and exchange of the IOL is not under consideration. Once the posterior capsule is open, an IOL exchange carries a much higher risk of complications such as cystoid macuar edema, retinal detachment, and endophthalmitis.
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Summary
Multifocal IOLs have the principal advantage that they function without moving parts, and, therefore, have more predictable visual performance. However, variables remain, especially related to pupil size and variable ability of patients to adapt to processing the multifocal image. Accommodating IOLs offer the potential for higher quality of vision, with absence of glare, halo, and loss of contrast, with optical quality identical to a monofocal IOL with the same optic. The principal challenge is harnessing the action of the ciliary muscle. Reduction in flexibility of the capsular bag as the eye heals can reduce or eliminate any IOL optic movement. As a result, many surgeons are utilizing a modified monovision strategy in conjunction with current accommodating IOLs.
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KEY POINTS |
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RULES FOR THE PRESBYOPIA INTRAOCULAR LENS SURGEON |
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