Intraocular lenses, part 2--the evolution.
Optometrists [COMMUNICATION][OCULAR DISEASE]
Dispensing opticians [COMMUNICATION][OCULAR ABNORMALITIES]
1 CET POINT
The rise in usage of mobile communication and advances in such technology has led to increasing visual requirements in the last decade and influenced demands for spectacle independence following cataract surgery. Consequently, intraocular lens (IOL) designs have progressed to provide better correction for refractive error but now also offer options for presbyopia. The introduction of premium IOLs offers specialist optics for enhanced near vision in the form of multifocal and accommodating designs. Presently, these IOLs are only available through private surgery but are gaining popularity and show promising results.
With a growing health conscious society, attention to the transmission properties of IOLs has also encouraged the introduction of ultraviolet and blue-light filtering IOLs. Industry is thus embracing a new generation of IOLs and this article will describe the detail of these evolving designs.
Multifocal IOLs aim to provide distance and near vision by distributing incoming light to different foci and are essentially 'bifocal' in nature (see Figure 1). Implementing the concept of simultaneous vision, superimposed images are formed on the retina. The brain selects the clearer image and suppresses the other. The effect is achieved by diffractive or refractive optics. Diffractive multifocals use the Huygen-Fresnel principle whereby eschelets on the IOL surface act as a diffraction grating creating different foci through constructive and deconstructive interference of incident light. Refractive IOLs achieve this through concentric circles of differing refractive power. Some light is of course lost in the process, which impairs contrast sensitivity, most notably in diffractive designs. Improvements in design have led to the development of apodized diffractive IOLs which incorporate both refractive and diffractive technology.
Generally, good functional vision and spectacle independence have been reported with implantation of multifocal IOLs. (1-3) Outcomes are most superior with bilateral implantation, (4-5) with surgeons occasionally implanting differing designs in fellow eyes to maximise performance. (6-8) The Array IOL (Abbott Medical Optics, Irvine, CA, USA) was one of the first multifocal IOLs to be approved by the FDA in 2000, followed by the ReZoom (AMO, Irvine, CA, USA) in 2005. Other popular multifocal IOLs include the Tecnis multifocal (AMO, Santa Ana, CA, USA) and AcrySof ReSTOR (Alcon, Fort Worth, Tx).
The most well known consequence of multifocal implantation is the complaint of photic phenomena such as glare and haloes; (3,9-16) it is estimated to occur 3.5 times more in comparison to conventional monofocal IOLs. (17) Encompassing aspheric design into multifocal IOLs, such as the Acrysof IQ SN60WF (Alcon, Fort Worth, Tx), may improve such symptoms by reducing ocular aberrations. (18) Multifocal IOL subjects are also more sensitive to the effects of astigmatism. (19,20) Dissatisfaction from multifocals may not always be due to photic phenomena; IOL decentration, posterior subcapsular opacification (PCO), pupil size and residual refractive error may also contribute to poorer outcomes. Severe cases of dissatisfaction may lead to explantation but is fortunately a rare occurrence. Prior to surgery, a comprehensive history is vital, paying attention to lifestyle and vocation. Multifocal IOLs should be avoided in night drivers and those regularly performing intricate near work must be advised that spectacle correction may still be necessary. The likelihood of imperfections must be highlighted to individuals with very high visual expectations to avoid disappointment.
Accommodating IOLs were developed in the 1980s following observations of exceptional near vision due to shifts in the position of plate-haptic IOLs. (21) Accommodating IOLs operate with contraction of the ciliary muscle, which remains functional until late in life. (22) These devices are distinguished by their various novel mechanisms, the categories of which will be explored next. Accommodating IOLs may be sought as an alternative to multifocal IOLs as they are devoid of photic phenomena and provide acceptable intermediate acuity. (23)
Single-optic IOLs are based on the Helmholtzian theory of accommodation. They consist of flexible hinged haptics, which push a single biconvex optic anteriorly on ciliary contraction. The shift in position is resultant of increased vitreal pressure, (23-24) and generates an increase in positive power. Axial shift may be expected in the order of 0.1 to 0.15mm in accordance with movement of the ciliary body apex but is variable and often limited by capsular fibrosis. (25) The physical space in the anterior chamber also serves as a restriction, (25) leaving a gain of only 0.75D of objective accommodation. (26) An element of pseudoaccommodation must thus contribute to the improved near range of focus. (27) Currently, based on this 'optic-shift' concept, the Crystalens (Bausch & Lomb, Rochester) remains the only FDA-approved accommodating IOL. It consists of a 4.5mm silicone optic with flexible hinges grooved into its plate-haptics. Non-FDA approved IOLs include the 1CU (Human Optics, Erlangen, Germany), and Tetraflex (KH3500, Lenstec, St Petersburg, FL). More recently the Trulign IOL (Bausch & Lomb, Rochester) has been introduced as a toric accommodating IOL. Singleoptic IOLs do show comparable distance vision to monofocal IOLs, (24-28) but have exceptionally high PCO rates in as little as 6-12 months post-implantation. (24-29)
A dual system, developed by Hara et al in 1990,30 is still relatively unexplored with little published research on its use. The design aims to provide greater accommodative power than single-optic designs. Dual-optic IOLs consist of a positive anterior optic adjoined by spring haptics to a negative posterior optic. In the relaxed state these optics sit close together (see Figure 2). On accommodation the haptics draw the optics apart allowing a potential increase of up to 4D in effective power.31 Literature describes two IOLs: the Synchrony (Visiogen, Irvine, CA, USA); and Safarazi IOL (developed by Shenasa Medical LLC with licensing rights acquired by Bausch & Lomb). Despite their potential, no lenses to date have received FDA approval.
Small physical changes in the crystalline lens generate significantly large changes in optical power. Curvature changes to the IOL optic may provide an alternative approach to inducing accommodation effects in the Pseudophakie eye. Such a system potentiates much higher levels of additional power through an estimated 8D increase in surface curvature. (32) Various conceptual designs have undergone animal testing and early human trials but at present none are commercially available.
The FluidVision IOL (PowerVision Inc, Belmont, CA) consists of fluid-filled haptics connected to a hollow optic; on ciliary contraction fluid is pushed into the optic increasing its curvature and dioptric power. Another design, the NuLens (Herzliya Pituah, Israel), contains a chamber of silicone gel with a posterior piston. On accommodation the capsular bag acts as a diaphragm activating the piston, which pushes the gel through an aperture, altering the surface curvature according the force of contraction by accommodation. Difficulty in sealing of the capsular bag following implantation has limited progression with curvature-change IOLs, which also increases the risk of PCO formation. Moreover, laser capsulotomy would pose significant risk of damage to the lens.
Good distance and near vision are well demonstrated in multifocal IOLs but intermediate vision is often variable, particularly in dim light. (6,34-38) In an attempt to overcome this problem, surgeons may implant an IOL with a low-power addition allowing a greater near vision range. However, for some this may still not provide the outcome they desire. Patients may be directed to the optometrist to be issued spectacles for intermediate vision. It must be stressed these patients do not require extra additional power but minus powered spectacles to extend their visual range. Commonly -1D spectacles are sufficient to aid intermediate tasks.
Figure 6 Evolution of the intraocular lens 1949 Ridley IOL Invention of the IOL by Sir Harold Ridley first Implanted in 45yr old woman at St Thomas hospital, London 1984-1993 Foldable IOLs First foldable IOL composed of silicone followed by hydrophilic and hydrophobic acrylic IOLs 1981 Blue-Filtering IOL First aprovai of blue light-filtering IOL 1994 Toric IOLs Conceptualised by Shimzu et al 2000-2004 Premium IOLs First MIOL and AIOL to receive FDA approval, Aspheric IOLs approved in 2004. 2010-Present day New Generation Introductions of the trifocal and Light adjustable Lens designs
To address the issue with intermediate vision, trifocal designs have been proposed by Swanson. (39) Such an example includes the Fine Vision IOL (PhysIOL, Liege, Belgium) (see Figure 3), which distributes light to three points of focus. Recent studies show significant improvements in intermediate vision and contrast sensitivity with high rates of patient satisfaction and reduced complaints of photic phenomena. (37,40,41) The IOL utilises apodized diffraction and incorporates a +1.75D addition for intermediate vision and +3.50D for near vision. Light loss is reduced to approximately 14% hence contrast sensitivity is reported as better than the existing bifocal designs. A smoothing function also reduces glare and haloes. Other IOLs of similar design include the AcrySof IQ PanOptix (Alcon), which received European CE mark in 2015 (see Figure 4) and the AT LISA tri 839MP (Zeiss, Germany).
Refractive surprises are not uncommon following refractive or cataract surgery but conventional posterior chamber IOLs cannot be adjusted once implanted. Despite the major advances in cataract surgery and biometric measures, only an estimated 55% reach the refractive target post-surgery, (42) with approximately 40% of patients dissatisfied with their vision due to poor refractive outcome. (43-44)
The Light Adjustable Lens (LAL, Calhoun Vision, Pasadena, CA), a three-piece silicone IOL, allows refractive adjustment in vivo. It consists of a light-sensitive silicone matrix polymer, which on exposure to UV light (365nm) induces polymerisation of macromers and local thickening in the area of irradiation. Unpolymerised macromers move to non-radiated regions causing a change in shape of the IOL. Adjustments are made using a digital light delivery device. Myopic refractive errors are corrected by irradiating the periphery of the IOL and hyperopic adjustments are made through irradiation of the centre. Both spherical and cylindrical adjustments of up to 2D are possible at any one time. Monovision can also be trialled allowing the patient to experience vision in daily life and return for any adjustments if necessary. With more skill, a multifocal profile may even be created. Alterations to refractive correction are performed two weeks postoperatively followed by two 'lock-in' procedures to set the refraction. Patients are instructed to wear UV protection for two to three weeks following the procedure; this is essential in obtaining the desired outcome. Concerns over UV exposure are addressed by the presence of UV absorber on the posterior surface of the IOL, avoiding harmful levels of transmission to the retina. Risk of corneal, (45) and retinal, (46) phototoxicity has also been established showing no associative damage. The LAL is an investigational device with current ongoing phase III trials, and as such, has not yet received FDA approval but is commercially available in the UK and other parts of Europe. Studies have shown good and stable refractive outcomes with high success rates and low levels of complications. (47-51)
Blue light-filtering IOLs
Brunescence or 'yellowing' of the crystalline lens with age allows some absorption of short-wavelength light (300-400nm). However, following cataract extraction this protective effect is lost. Since the 1980s, the majority of new-generation IOLs absorb UV light but still transit a proportion of blue light. It is reported that UV and blue light have potential to be harmful to the retina by inducing photochemical damage and increasing the risk of age-related macular degeneration (AMD) and even choroidal melanomas. (52) Photoxidative damage leads to the production of reactive oxygen species inducing damage to the retinal pigment epithelium eventually causing damage to photoreceptors. Blue light-filtering IOLs or 'yellow' IOLs incorporate chromophores and hence provide protection to the retina from such damage (see Figure 5). However, there is much speculation to the benefits of blue light-filtering IOLs, as the literature argues detrimental effects to colour vision, scotopic vision and the circadian cycle.
Blue light exposure plays a role in sleep regulation. Melanopsin is released in accordance to light exposure which in turn releases melatonin. Inhibition of melatonin increases alertness and vice versa. Blue-filtering IOLs are estimated to reduce melatonin suppression by approximately 27-38% and hence could affect sleeping patterns. (53) Impairment of scotopic sensitivity may affect night driving, (54) and possibly increase the risk of falls. (55) Limiting wavelengths of light may potentially alter colour perception although this along with contrast sensitivity has shown to be comparable to that of monofocal IOLs. (56) Conversely, colour vision under mesopic conditions may diminish. (57)
A causative link between AMD and shortwavelength light exposure is yet to be established in epidemiological studies. In addition, pseudophakes show no increase in risk of AMD, suggesting light exposure may not be a factor. (58-59) The use of blue-filtering IOLs, therefore, remains controversial.
Since the introduction of IOLs, designs have evolved considerably (see Figure 6). Manufacturers have adapted material and physical design for better stability on implantation and fewer post-operative complications. Monofocal designs are now well established in providing good distance vision with enhanced quality through asphericity. Premium IOLs are also rapidly progressing with the aim of restoring accommodation, though it is vital to manage patients' expectations with these presbyopia-correcting IOLs. With such complex optics, counselling candidates for corrective surgery is paramount as supported by NICE guidelines. (60-61) Both surgeons and optometrists alike must establish the visual needs of a patient and evaluate suitability. Modification of transmission properties also serves to protect the retina in the absence of the crystalline lens. Such advancements show encouraging results in the next step to surgical correction of presbyopia and refractive error following cataract extraction. Continuing research in this field has already shown great progress in the development of conceptual designs.
Exam questions and references
Under the enhanced CET rules of the GOC, MCQs for this exam appear online at www.optometry.co.uk. Please complete online by midnight on 10 March 2017. You will be unable to submit exams after this date. Please note that when taking an exam, the MCQs may require practitioners to apply additional knowledge that has not been covered in the related CET article.
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Course code: C-54753 Deadline: 10 March 2017
* Be able to explain to patients about advances in intraocular lens options (Group 1.2.4)
* Understand advances in intraocular lens design (Group 6.1.6)
* Be able to explain to patients about advances in intraocular lens options (Group 1.2.4)
* Be aware of advances in intraocular lens design (Group 8.1.5)
Dr Gurpreet K Bhogal-Bhamra PhD, MCOptom
* Dr Gurpreet K Bhogal-Bhamra is currently working as a research fellow at the Queen Elizabeth Hospital, Birmingham on work related to ocular surface disease and glaucoma treatments. She completed her PhD in 2012 with her thesis on the subject of surgical treatment of presbyopia with an emphasis on premium intraocular lenses.
Caption: Figure 1 Multifocal intraocular lens design with concentric circles to create differing foci. Rayner M-flex Multifocal IOL [c] 2016 Rayner Optical Limited
Caption: Figure 2 Dual-optic accommodating IOL
Caption: Figure 3 FineVision trifocal IOL
Caption: Figure 4 AcrySof IQ PanOptix IOL
Caption: Figure 5 Blue light filtering IOL
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|Author:||Bhogal-Bhamra, Gurpreet K.|
|Date:||Feb 1, 2017|
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