Elective surgery for refractive correction to gain complete or partial spectacle independence is now commonplace and even available on the high street. Advances in both surgical procedure and implant technology have made a wide range of surgical options available which can treat almost any refractive error if a patient finds the right surgeon, and is prepared to pay the fee. LASIK can deliver clear vision the next day which has been a major factor in driving refractive surgery popularity, although complex cases may require more than one surgical treatment and the final outcome may take several months to achieve.
There are several options available to presbyopic patients in the UK. It can be confusing to both practitioner and patient as the same procedure can be known by different names according to how it is marketed by refractive surgery providers. This can be found across both the laser and lens refractive marketplace. For example, refractive lens exchange is a generic term relating to phacoemulsification and IOL implant, but it is also called lens replacement and PRELEX. Some clinics also have their own branded treatment packages such as Clarivu or NuVu. This article will concentrate on the mainstream options available.
Refractive lens exchange (RLE)
This procedure is essentially the same as that for cataract surgery; phacoemulsification of the crystalline lens and the insertion of an intraocular lens (IOL) implant. The main difference is that the reason for surgery is elective rather than medical need, and the aim of surgery is to gain spectacle independence rather than to restore vision. It is often referred to as clear lens exchange with many patients opting for this type of surgery before the advent of cataract. Advances in surgical technique and implant technology have made cataract surgery one of the most commonly performed surgical procedures, with approximately 200,000 cataract procedures carried out in the UK per year.1 Advances in biometry equipment and corneal mapping techniques have made the calculation of IOL power and IOL alignment accurate enough for this procedure to be used successfully in refractive surgery.
RLE is considered to be a good option for patients who are presbyopic or have incipient presbyopia, and who will not get a visual correction for all their visual needs with laser refractive surgery. Patients undergoing refractive surgery generally want to be free from spectacles as much as possible and this desire to see clearly across a range of distances has driven the developments in IOL design and technology.
Multifocal IOLs technology
Multifocal IOLs utilise refractive or diffractive optics, or a combination of both. The side effects of significant glare and haloes are now much less common due to improved lens design. In addition to this, the use of different IOLs between the dominant and less dominant eye can sometimes increase patient satisfaction by providing clear vision over a wider range of distances. Micro-monovision, where the refractive target is shifted from plano to slight myopia can also enhance near vision without compromising the binocular distance acuity. Knowledge of different IOL designs along with their advantages and disadvantages, allows a surgeon to choose the most appropriate IOL for their patient. Patients are in turn prepared to put up with mild symptoms to gain spectacle independence resulting in a high degree of patient satisfaction.
Refractive IOL designs have concentric annular zones to provide distance, intermediate and near vision and typically provide a reading add at the spectacle plane of about 2.25D (Figure 1).
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Looking at the ReZoom design as an example, it consists of five alternating distance-near zones with the intermediate correction being provided by the aspheric transitions between adjacent zones (Figure 2).
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The zones used and the ratio of light distribution between distance, intermediate and near vision at any one time are pupil dependant. For example the large central zone is dedicated to distance vision in bright conditions, and the thinnest outer zone is dedicated to distance vision in low light when the pupil will be maximally dilated. However, glare and haloes can still be an issue in dim light conditions with bright light sources.
Diffractive IOLs consist of several concentric or annular zones with each step bending light differently to create a near focus that is separate to that of the distance (Figure 3).
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The power of the reading addition is governed by the width of the diffractive grating slits, the narrower the spacing, the higher the addition (Figure 4).
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This often makes up the central part of the IOL so as to provide pupil independent vision across a range of distances. The peripheral zone usually has no diffractive gradient and is therefore able to deliver clear distance vision when the pupil is dilated, ie in dim light.
The Tecnis Symfony uses an echelette style grating which optimises the energy for a specific wavelength by having an altered step angle of grating. This grating does not create two distinct focal points as seen with a normal diffractive IOL, but elongates the focal point to provide an extended range of vision (Figure 5).
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Diffractive IOLs are not limited to bifocals anymore. Trifocality can be achieved with several implants now well established and available in the UK. The Finevision IOL has two overlapping bifocal diffractive profiles; distance-intermediate, and a distance-near. It uses apodisation, a tapering of the height of the diffractive grating from the centre to the peripheral zones of the IOL to filter a greater proportion of light energy to the distance range in mesopic conditions. The Zeiss AtLisa IOL is a diffractive trifocal design that is not apodised, and so is less pupil dependent than the Finevision, but achieves it high patient satisfaction through a combination of having a ‘smoothed’ diffraction grating profile and an optic than changes from trifocal to bifocal in the periphery.
Hybrid IOL designs also exist such as the Tecnis Multifocal lens by AMO and the Acrysof ReSTOR lens by Alcon utilising refractive and diffractive technologies. The refractive element provides the distance vision whilst the diffractive the intermediate and near. While photopic phenomena are not eradicated by these lenses, it is much improved compared to older IOL designs using one or other of these technologies.
Sectorial IOLs
The Lentis Mplus represents a completely different approach to conventional multifocal IOL design. Unlike other IOL types that are rotationally symmetrical and based on the principles of diffraction, refraction, or a combination of both, this IOL has an aspheric, asymmetric distance-vision zone and a sector-shaped, near-vision zone (Figure 6).
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The lens has a main surface and a second embedded surface creating two defined focal points. The design of the surface-embedded segment allows the IOL to work independently of pupil size and ensures optimum adjustment of near and distance vision acuity. The lens design is such that light hitting the transition area of the embedded sector is reflected away from the optical axis to prevent optical aberrations interfering with the vision. Clinical trials reported excellent distance and near acuity with good contrast sensitivity. Halo and glare effects were rarely reported which led to CE approval being granted in March 2009. Good results have also appeared in peer reviewed literature.2,3,4
Accommodating IOLs are not very widely used. These IOLs (for example Crystalens, Tetraflex) are placed in the capsular bag just like other IOLs but their haptics are designed to allow the lens to shift anteriorly as the ciliary muscle contracts, thus simulating natural accommodation. This forward movement effectively increases the plus power of the lens and so allows the patient to read. The Tetraflex also makes use of the vitreous movement during ciliary muscle contraction to help flex the optic of the implant.
The advantage of an accommodating IOL is that unlike diffractive IOLs all available light is directed through the optic thus improving contrast sensitivity. The near vision is provided by the mechanical shift of the implant rather than by having simultaneous zones of vision and so glare and haloes are not an issue. However, the reading addition achieved is very much dependent on the ciliary muscle and it can take several months for a patient to achieve up to a +2.25D effective add for reading. Accommodation training exercises can also help, but require good patient motivation.
Monovision
Laser in situ keratomileusis (LASIK) and laser assisted sub epithelial keratectomy (LASEK) are refractive procedures that practitioners are generally the most aware of, and they are used in the main to correct myopia, hypermetropia and astigmatism. They are classed as ‘surface’ treatments and as such carry relatively low risk with respect to complications and tend to be more affordable than intraocular surgery options. So it is no surprise that some patients choose to have their reading vision corrected by monovision LASIK or LASEK, with existing ametropia (if any) being corrected, and the less dominant eye being left myopic to allow for reading. This is a particularly popular option with early presbyopes, where the reading add is still quite low (ie less than +1.75). This is a more suitable option for those patients that have minimal accommodation or crystalline lens changes. Monovision can also be achieved with IOL implantation.
The success of monovision in refractive surgery requires three factors; good patient motivation, clear ocular dominance in one eye for distance and a patient’s ability to accept compromise. Some patients can accept a surprising amount of disparity between their eyes, whereas others can tolerate none. Patients that are considering monovision must be carefully counselled regarding the decrease in binocularity and if they have not experienced monovision with contact lens wear before, a short trial with contact lenses can be useful in determining a patient’s likelihood of tolerating this type of correction.
Conductive keratoplasty (CK)
Conductive keratoplasty is a non-permanent refractive treatment most commonly used to correct presbyopia by monovision. It uses the principles of thermokeratoplasty to reshape the cornea and therefore adjust its refractive power. Under topical anaesthesia, radiofrequency energy is directed to the corneal stroma in a ring of spots around the periphery of the cornea. The stromal collagen tightens and scarring occurs in this area (Figure 7) creating a purse-string effect that steepens the central cornea. The depth of the shrinkage caused determines the degree of corneal correction achieved. The effects of CK typically lasts around five years, after which retreatment may be required. Conductive keratoplasty has also been used to reduce irregular astigmatism for keratoconic eyes, but the improvements have been temporary, even when combined with collagen crosslinking.5,6
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Laser blended vision
Although monovision can work very well for some patients (especially those with early presbyopia), many find that the blur created by an optimal near vision correction in their less dominant eye intolerable. Laser blended vision aims to overcome this by correcting one eye to see mainly for near vision and the other eye to see mainly for distance vision but neither eye is perfect for their respective distances. The eye corrected for near still has some distance vision and the eye corrected for distance still has some reading vision. Therefore both eyes need to be open and working together to achieve comfortable vision.
This correction is achieved by ablating the cornea to result in an exaggerated prolate profile. This increases depth of field and can compensate for up 1.50 dioptres of reading add. The dominant eye can then be treated for distance and intermediate vision, and the less dominant eye treated for intermediate to near vision. This decreases the amount of anisometropia between the two eyes which increases the likelihood of the patient being able to tolerate the correction. The depth of field is not enough to provide good vision over all distances in each eye individually, but between the two eyes a good compromise can be found.
Multifocal LASIK
Multifocal LASIK is not widely offered. It uses the principle of ocular aberrometry and aspheric laser ablation profiles to create a correction that is similar to that seen with multifocal contact lenses. True multifocality on the cornea is created using a multi-step laser treatment in which several independently calculated ablations are performed at various optical zones. Depending on the patient’s refractive error, there may be between three and eight different concentric zones. This option has not received FDA approval in the US. This may be due to the fact that the procedure may carry more risk in terms of a higher incidence of regression and an increased risk of glare, haloes and reduced visual quality when compared to traditional laser treatments.
Corneal inlays
Corneal inlays are tiny implants that provide a permanent correction for presbyopia (although they can be removed at a later date if needed). They are typically implanted into the non-dominant eye of patients that have good distance vision either naturally or by other surgical means. The use of femtosecond laser platforms and better biocompatibility of implant materials have significantly reduced, but not eradicated the issues with epithelial healing and long term biocompatibility that were found with earlier implants. There are several inlays available on the market and the ones described below are CE approved examples that each use a different principle to obtain their objective.
KAMRA Inlay – is an opaque disc approximately 3.8mm in diameter and 5µm thick with a tiny opening of only 1.6mm in the centre. Made of polyvinylidine fluoride (PVDF) and nano-carbon particles, it has 8400 micro-openings along the surface of the inlay to allow efficient nutrient flow. Figure 8 shows the KAMRA inlay and next to a standard soft contact lens for size comparison purposes. The KAMRA inlay provides a reading add of up to +2.00D by means of the pinhole effect. The inlay is placed at a minimum insertion depth of 170µm within the corneal stroma so that the refractive state of the anterior cornea is not altered. Long-term studies of up to four years have shown the inlay to be a safe and effective method of correcting presbyopia.7,8
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Raindrop Inlay – This is 2mm in diameter and less than half the thickness of a human hair. It is made of a micro-porous hydrogel that has the same refractive index and water content as human cornea (Figure 9).
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It does not introduce additional refractive power when implanted but when placed at a depth of approximately 120µm to 130µm in the cornea, it steepens the corneal profile in the non-dominant eye. The resultant hyperprolate shift within the central 2mm of the cornea creates the multifocality needed to correct presbyopia.
Flexivue Microlens – The hydrophilic polymer used in the Flexivue is similar to that used to make intraocular lenses. It has a higher refractive index than the cornea and generates a refractive change that corrects presbyopia in a similar way to that of a centre-distance contact lens. It is 3mm in diameter and approximately 20µm in thickness. The central 1.6mm disc area is a power neutral zone and reading additions from +1.50D to +3.00D in 0.25D steps can be chosen for the peripheral area depending upon the patient’s near add as measured at 33cm. This inlay also has a central perforation of 0.5mm that permits the transfer of glucose and other nutrients to corneal tissues.
Despite a decreased risk of haze with femtosecond laser created corneal flaps/inlay pockets and improved corneal nutrient supply by the use of thin permeable materials, inlays are still only suitable for a very selective group of patients. Patients must also bear in mind that long term follow up every six months is recommended to check for continued corneal clarity. If significant haze develops, the implant must be removed and the patient must seek alternative presbyopic correction. Some patients may find this risk acceptable, as dependence upon reading spectacles will have been delayed for a few years or more.
Conclusion
Today’s patients are surfing the internet for information and they will come across many clinics that offer a portfolio of surgical options. The optometrist has a key role in educating the patient, and in helping them to understand the potential refractive surgery options that are available. It is important to remember that presbyopic refractive surgery cannot (as yet) replace a patient’s lost accommodation. It can only provide some compensation. Well informed patients acknowledge this and are prepared to accept that some compromise is inevitable. After all, a few usually transient haloes may be preferable to the perceived nuisance of multiple pairs of spectacles.
The next article on this subject will look at the pre-operative surgical work up and the postoperative management of presbyopic refractive surgery patients.
References
1 Desai P, Minassian DC, Reidy A. National cataract surgery survey 1997-8: a report of the results of the clinical outcomes. Br J Ophthalmol, 1999;83:1336-1340.
2 4. van der Linden JW, van Velthoven M, van der Meulen I, et al. Comparison of a new-generation sectorial addition multifocal intraocular lens and a diffractive apodized multifocal intraocular lens. J Cataract Refract Surg, 2011 Nov 9.
3 Muñoz G, Albarrán-Diego C, Ferrer-Blasco T, et al. Visual function after bilateral implantation of a new zonal refractive aspheric multifocal intraocular lens. J Cataract Refract Surg. 2011 Nov;37(11):2043-52.
4 Alió JL, Plaza-Puche AB, Piñero DP, et al. Comparative analysis of the clinical outcomes with 2 multifocal intraocular lens models with rotational asymmetry. J Cataract Refract Surg, 2011 Sep;37(9):1605-14.
5 Kato N1, Toda I, Kawakita T, et al. Topography-guided conductive keratoplasty: treatment for advanced keratoconus. Am J Ophthalmol, 2010 Oct;150(4):481-489.e1.
6 Kymionis GD1, Kontadakis GA, Naoumidi TL, et al. Conductive keratoplasty followed by collagen cross-linking with riboflavin-UV-A in patients with keratoconus. Cornea, 2010 Feb;29(2):239-43.
7 Yilmaz OF, Alagöz N, Pekel G, et al. Intracorneal inlay to correct presbyopia: Long-term results. J Cataract Refract Surg. 2011 Jul;37(7):1275-81. Epub 2011 May 12.
8 Dexl AK, Seyeddain O, Riha W, et al. Reading performance after implantation of a small-aperture corneal inlay for the surgical correction of presbyopia: Two-year follow-up. J Cataract Refract Surg. 2011 Mar;37(3):525-31. Epub 2011 Jan 22.
Michelle Hanratty is the senior optometrist at Optegra Birmingham and an examiner for the College of Optometrists