Refractive error is one of the leading causes of visual impairment. Fortunately, today there are multiple approaches we can take to correct refractive error that have safe, effective profiles. In this article, Clinical Associate Professor Smita Agarwal discusses the evolution of refractive correction procedures and explores current and future techniques to correct for refractive errors.
With increasing life expectancy, most of us can expect to live with presbyopia for nearly half of our lives.1 The prevalence of myopia is also rising rapidly and with current trends, over half the world’s population will be myopic by 2050, and one-tenth will suffer from high myopia.2 Myopia comprises multifactorial aetiology including genetic, environmental, accommodative/vergence and peripheral retinal hyperopic defocus. To defuse the myopia tsunami, we need to manage younger myopes and treat older myopes. Myopia can be controlled at a younger age by prompt diagnosis, good visual hygiene, clinical intervention and increasing outdoor time. Older, stable myopes can benefit from refractive surgery if they do not like to wear glasses and/or contact lenses.
Refractive laser surgery reshapes the cornea to reduce refractive error, but in doing so, may produce variable effects on night vision and higher order aberrations
ADVANCES IN LASER REFRACTIVE SURGERY
Over the past 30 years, various refractive surgery interventions have come into vogue with their own relative nuances in techniques, risks and benefits.3 Laser refractive surgery procedures, including laser in-situ keratomileusis (LASIK), photorefractive keratectomy (PRK), small incision lenticule extraction (SMILE), and presbyopia correction, have now been established as reasonably safe, with excellent visual results for patients with ametropia.
Refractive laser surgery reshapes the cornea to reduce refractive error, but in doing so, may produce variable effects on night vision and higher order aberrations (HOAs).4
Although considered a cosmetic surgery, refractive surgery can provide spectacle independence in addition to improving quality of life and working performance.5 Over the decades there has been ample evidence supporting good visual outcomes and safety profiles with laser vision correction.6 Advanced corneal imaging, including corneal biomechanics, has led to improved patient selection and a reduced risk of complications, thereby improving safety and efficacy for patients.
The cornea represents two-thirds of the eye’s refractive power, hence corneal refractive treatment remains the most common modality to treat refractive errors. A review of nearly 100 studies, published since 2008, showed that 99.5% of patients undergoing corneal refractive correction achieved uncorrected distance visual acuity of 20/40 (meeting driving requirements) and 98.6% of patients had a refractive outcome within }1.0 dioptre (D) of the target, with a satisfaction rate of 98.8%.6 The risk of corneal infections is lower than the risk associated with long-term contact lens wear.7
Excimer lasers are used to ablate corneal tissue and reshape the cornea to treat lower order aberrations including myopia, hyperopia and astigmatism, as well as presbyopia. LASIK involves creating a flap of desired thickness with either a microkeratome or femtosecond laser, followed by the ablation of the underlying corneal stromal tissue. Currently, most corneal flaps are created by femtosecond (FS) laser rather than a blade/microkeratome. This achieves better precision, faster visual recovery and a lower risk of flap-related complications.8
Surface ablation procedures, such as PRK and transepithelial PRK (trans-PRK), use excimer lasers to ablate the Bowmans layer and the anterior corneal stromal tissue, once the epithelium has been removed, via traditional methods using alcohol (PRK) or by the excimer laser itself (trans-PRK). When appropriate, epithelial removal using the excimer laser is preferred, as it results in reduced pain and facilitates faster recovery postoperatively.9 Newer generations of laser platforms have improved speed, laser spot parameters and tracking ability, which enhances the visual outcomes.10 Customised treatments can improve vision beyond the sphere, and cylindrical corrections, by reshaping the cornea which can reduce HOAs, thereby improving glare and night vision.11 More recently, the FS laser has been developed to create a lenticule of desired thickness within the corneal stroma, which is then dissected and extracted to achieve optimal refractive correction via a procedure known as SMILE.
Although considered a cosmetic surgery, refractive surgery can provide spectacle independence in addition to improving quality of life and working performance
LASIK was first described in 1990 by Pallikaris et al, who used a microkeratome for flap creation and an excimer laser for tissue ablation.12 It lost its predominance in the late 1990s due to the emergence of cases of post-LASIK ectasia, where high refractive corrections were attempted and signs of subclinical keratoconus were missed.13,14 However, LASIK is still the most popular laser vision correction procedure in the world.15 It has been used for over three decades, and innovations in corneal imaging techniques and laser platforms have not only improved visual outcomes and patient satisfaction significantly, but also reduced the postoperative risks.
Improved technology for corneal flap creation helps with early visual recovery by reducing stromal inflammation and risks of corneal haze.16 LASIK can notoriously induce weakening of biomechanical strength and worsen dry eye. However, with proper patient selection and use of advanced corneal imaging techniques, the risk of ectasia can be minimised. Femtosecond laser flaps are more reliable regarding flap thickness, diameter, and side cut angles and hence, have a reduced risk of epithelial in-growth.17 Long-term visual results with LASIK are usually excellent, with similar outcomes to surface ablation.18 However, the risk of regression following hyperopic treatments is lower than with surface ablation as the presence of a flap prevents stromal healing and epithelial hyperplasia. Protocols have been optimised to reduce the risk of regression, including the use of nomograms, large optical zones, blend zones, altering centration of treatment and wavefront technology, as well as the application of mitomycin C (MMC) to the stromal bed.19 Newer generations of laser platforms feature wide optical zones, fast ablation times which reduce dehydration, high repetition rates which improve the quality of ablation, and faster, more accurate eye scanners, which help improve predictability and prevent regression. Regression of between 0.1 and 0.18D has been reported in 9.26 to 11.7% of eyes using lasers with this technology.20 Eye tracking technology has also been credited with smoother ablation surfaces and reduced incidence of epithelial hyperplasia.20 LASIK may be used to treat between -8.0D and +4.0D with up to +/-4.0D of astigmatism, but the range varies according to patient ocular profile, laser platform and surgeon preference.
Surface Ablation Laser Surgery/ Photo-Refractive Keratectomy
Photo-refractive keratectomy (PRK) was first described in 1988,11 and was the first procedure performed using the excimer laser for the correction of astigmatism and myopia. The most common indication is for patients with thin corneas. Surface ablation uses an excimer laser to remodel the corneal stromal tissue after the removal of the epithelium. For PRK, the corneal epithelium is removed mechanically by loosening the corneal epithelium using 20-30% alcohol. Preservation of the epithelium can reduce inflammation and pain, however the viability of corneal epithelium following treatment with a high concentration of alcohol is questionable.21 The corneal epithelium can also be removed mechanically by a motorised brush (Amolis scrubber, Figure 4) or removed directly by excimer laser ablation, in a procedure known as trans-PRK.22 Following treatment, the epithelium then regenerates on top of the ablated corneal stroma via the wound healing process.11 Unlike LASIK, no corneal flap is created, which potentially results in a biomechanically stronger cornea, minimising the risk of corneal ectasia.23 However, it also ablates the Bowman’s layer, and the keratocyte-rich anterior corneal stroma. This can induce scarring and corneal haze, potentially affecting refractive correction.24 Regeneration of the epithelium takes time, which can cause pain and visual fluctuations in the immediate post-operative period.25
Several excimer laser platforms now provide trans-PRK options, including SmartSurf (Schwind Amaris), Trans-Epi PRK (Bausch + Lomb Technolas), c-Ten (iVis Technologies) and Streamlight (Alcon Wavelight). Trans-PRK involves three key steps: an epithelial photoablation, a stromal photoablation, and a complementary photoablation to smooth out the corneal surface. This complete no-touch procedure requires no instruments, except for the laser itself, and does not involve alcohol solution. Healing with trans-PRK is much faster, with less pain and discomfort compared to PRK, and the outcomes are comparable to standard ablation treatments, LASIK and SMILE for mild-to-moderate myopia with or without astigmatism.26 Another role of trans-PRK is enhancement following LASIK and SMILE. This provides similar results to re-lifting the flap in LASIK and obviates the risk of epithelial in-growth, which is still shown to be as high as 13% after the flap re-lift. The application of low dose topical MMC, ranging from 0.02% to 0.04%, has been shown to reduce the risk of sub-epithelial fibrosis following surface ablation.27
Although the long-term predictability of surface ablation is comparable to LASIK, myopic regression due to epithelial compensation and corneal haze can be more common after surface ablation.28 It has been reported that following hyperopic treatments, the proportion of eyes that undergo PRK and achieve within }1.0D of the target (93.3% vs 92%) is similar to those undergoing LASIK, however the former are more likely to have peripheral postoperative corneal haze.29 It has also been reported that surface ablation may result in more regression compared to LASIK following high astigmatic corrections.30
Advances in wavefront technology have helped to lower pre-existing ocular aberrations and prevent the induction of new HOAs
Wavefront-Guided Excimer Laser Surgery
Despite a few possible side effects and rare complications, both LASIK and surface ablation produce excellent visual outcomes for the treatment of lower order aberrations (LOAs) such as low-to-moderate myopia, hyperopia and astigmatism.31,5 However, patients can complain of star bursts, glare and halos due to the induction of HOAs such as coma, trefoil and spherical aberrations.32 Advances in wavefront technology have helped to lower pre-existing ocular aberrations and prevent the induction of new HOAs. Wavefront-optimised (WFO) treatment helps to preserve the original spherical aberration of the cornea and customised wavefront-guided (WFG) treatments help to reduce surgically induced HOAs and/or compensate for the pre-existing aberrations in the treated eye.33 However, both treatments may have variable effects; HOAs may still increase with WFO treatments and WFG treatments may not predictably reduce HOAs.34 Visual improvement can be achieved using corneal topography guided treatments only, if the refractive error matches with the corneal topography, i.e. if the majority of the ocular aberrations are corneal as opposed to internal.35
Refractive Lenticule Extraction or Small Incision Lenticule Extraction
After the 2007 launch of the Visumax FS laser (Carl Zeiss Meditec, Jena, Germany) for corneal refractive surgery, refractive lenticule extraction (Re LeX) was introduced as a new form of flapless laser vision correction for myopia and astigmatism.36 Instead of using the excimer laser for corneal ablation, a lenticule of the desired correction is created within the cornea and then extracted following dissection through a much smaller corneal incision. You can watch a Re LeX video at mieducation. com/laser-refractive-surgery-options.
The potential advantages of small incision lenticule extraction (SMILE) over LASIK include reduced dry eyes,37 lower energy requirements, fewer induced HOAs,37 reduced corneal inflammation and keratocyte damage,38 and lower suction pressure during the procedure. The risk of post-operative ectasia is also lower than for myopic LASIK treatments, as the procedure removes less tissue per dioptre of correction,39 and the mechanical strength of the cornea is not compromised as a result. While most studies report slower visual recovery after SMILE in comparison to LASIK,40 SMILE is reported to be a relatively safe procedure, which yields predictable visual outcomes for patients with a moderate amount of myopia (<-5.0D) and low astigmatism (<-2.0D), achieving visual results comparable to LASIK.41 While efficacy and safety indexes have been reported to be similar for myopic SMILE and LASIK with or without astigmatism, eyes that undergo SMILE are reported to have less induced postoperative HOAs compared to eyes that undergo LASIK, and are more likely to achieve postoperative refraction within }0.5D of the target (80% vs 65% of eyes).42 While currently recommended for mildto- moderate astigmatism correction, not all currently available SMILE software has an inbuilt eye tracker to allow for compensation of cyclotorsion. Some surgeons opt for manual marking on the cornea, followed by manipulation of the cap to align with the horizontal plane. This manual approach has been shown to improve safety and visual outcomes, and reduce post-operative astigmatism and axis rotation.43 There is a steeper learning curve with SMILE surgery as it is more technically challenging and involves manual dissection of the lenticule within the cornea. Additionally, there is potential risk for the unsuccessful removal of the lenticule fragments as well as interface irregularities and iatrogenic stromal scarring, which can lead to compromised visual outcomes.44 As of 2018, SMILE has been approved by the United States Food and Drug Administration (FDA) for treatments of up to 10D of myopia and 3D of astigmatism. Higher corrections are prone to regression and under-correction due to inappropriate nomograms and epithelial changes. Epithelial thickening, seen after laser correction, is found to be positively correlated to the amount of myopia correction and may lead to potential regression.45 Further nomogram adjustments and software enhancements, including eye tracking and cyclotorsion compensation, may help improve outcomes of SMILE, along with energy optimisation to reduce aberrations.
Retreatments following under-correction can be performed by converting the original cap created by SMILE into a flap using Visumax Circle software or by PRK/trans- PRK. At three months after retreatment, one study reported that 100% of eyes were within } 0.25D of emmetropia, and all retreated eyes had uncorrected distance visual acuity (UDVA) of 20/20 or better.46
SMILE treatments will soon be available for hyperopia using a similar principal to myopic correction, with early results appearing promising. Reinstein published the first trial of hyperopic SMILE, reporting that 76% of eyes achieved a postoperative refraction within }1D of the target, with 18.2% of eyes losing lines of corrected distance visual acuity (CDVA) and UDVA improving in 89% of eyes.47 Overall, outcomes were similar to those reported for hyperopic LASIK treatments.48 However, further research is needed to improve the predictability and effectiveness of the procedure.
Technologies have advanced in recent years, with features such as eye tracking and cyclotorsion correction becoming standard in newer generations
While both SMILE and LASIK have been approved for astigmatic corrections, reports regarding the efficacy of each procedure vary depending on what laser platforms are used. Kanellopoulos reported that TG-LASIK gave superior refractive and subjective outcomes, compared to SMILE, in the treatment of myopic astigmatism, attributing this to eye tracking software, cyclo-rotation correction, and active centration control.49 These technologies have, however, become available in newer generations of FS lasers. By contrast, Zhao et al reported similar outcomes for both WFG-LASIK and optimised SMILE for moderate to high astigmatic treatments.50 Han et al also reported similar long-term outcomes for SMILE and AF-LASIK in myopic astigmatic eyes.42 WFO-PRK has been reported to give more accurate correction of astigmatism compared to WF-guided PRK,51 however astigmatic PRK treatments may be more prone to long-term regression compared to LASIK.30
PRESBYOPIA LASER CORRECTION
With an aging world population and an increasing life expectancy, presbyopia is the leading refractive disorder – the average person will spend almost half their life with presbyopia. Refractive surgery is still not very well established for presbyopia, despite a great demand, with over 2.5 billion presbyopes worldwide. A possible strategy is to induce monovision by either using LASIK, PRK or SMILE. However, monovision requires time for adaptation and causes reduction in stereopsis,52 making it unsuitable for a lot of professions. Multifocality can also be induced with a hyper-positive central cornea or midperipheral corneal zone corrected for near vision.53 A combination of inducing spherical aberration to increase the depth of field and micro-monovision has also been attempted with laser blended vision54 using special Presby-LASIK software available on most modern laser platforms. However, issues with long-term stability, loss of distance vision, and a limited compensation for the progressive decline in vision with increasing age and presbyopia, limit the adoption of these techniques.
Residual Refractive Error Following Implantation of Monofocal/Multifocal Intraocular Lenses
Multifocal intraocular lenses (IOLs) and/ or monovision are being increasingly used to improve near vision while providing a good level of distance vision. Patients with current presbyopia correcting lenses report less limitation in visual function and less spectacle dependency.55 Improved biometry, modern IOL formulae and surgical technique have dramatically enhanced refractive targets. However, refractive surprises can limit the outcomes of these lenses in this cohort of extra demanding patients. LASIK has been shown to be highly predictable and safe for correcting residual ametropia after lenticular based surgical options with or without multifocal lenses.56,57
Since the first approval of the Kremer excimer laser system in 1998, over 30 laser platforms have been FDA-approved for LASIK alone. Technologies have advanced in recent years, with features such as eye tracking and cyclotorsion correction becoming standard in newer generations. Most modern laser devices are equipped with sophisticated eyetracking systems which, by detecting and tracking the pupil and limbus, follow the eye movements and rectify, in real time, the position of the mirrors that direct the laser. To achieve this, cameras with acquisition frequencies of 1000Hz and latencies less than 3ms are used. Most of these devices perform the compensation for linear eye movements in the x-y axis, rolling eye movements and eye torsions around the visual axis or cyclotorsions. Some of the currently available laser platforms are described in Table 1.
Laser refractive surgery is rapidly evolving to improve efficacy and predictability of visual outcomes
COMPLICATIONS OF REFRACTIVE SURGERY
Laser refractive surgery is a commonly performed procedure with a low complication rate. Unsatisfactory outcomes may be associated with patient dissatisfaction, often caused by glare, starbursts, haloes, residual refractive error, irregular astigmatism and/or corneal haze. Dry eye is one of the most common side effects and is caused by corneal nerve damage and inflammation. Flap-related complications include flap displacement, stria, diffuse lamellar keratitis (DLK or Sands of Sahara) and epithelial ingrowth.58 These conditions can be easily treated by re-lifting the flap and flushing the interface. Occasionally, especially with higher refractive corrections and in thinner corneas, there is a potential risk of corneal ectasia due to a reduction in biomechanical corneal strength.59 SMILE has an edge owing to the absence of a corneal flap, meaning that the cornea can retain better corneal biomechanical strength. Careful preoperative evaluation of eyes to determine the risk of ectasia, by assessing individual topographical and biomechanical corneal characteristics, is key in preventing this complication. There are ample reports that thinner corneas, and those with suspicious topography, can be treated simultaneously or pre-treated with corneal cross-linking (CXL) to help prevent ectasia by strengthening the corneal stroma.
Laser refractive surgery is rapidly evolving to improve efficacy and predictability of visual outcomes. Advances in corneal imaging techniques and better engineering of laser platforms, that include eye tracking systems to compensate for cyclotorsion, help in better identifying at-risk patients and yield better visual outcomes. Combining refractive correction with CXL can improve the safety profile in eyes with thinner corneas. The advent of SMILE has allowed us to store the lenticule for the treatment of hyperopia,60 presbyopia61 and corneal ectatic disorders, including keratoconus, via lenticule implantation procedures known as stromal lenticule addition keratoplasty (SLAK).
Refractive surgery is now established as a safe and effective treatment for refractive error that produces predictable visual outcomes. It improves quality of life, and most patients are satisfied with their results. Rapid advances in laser technology and innovation have increased the variety of refractive surgical options available to patients. It is important for ophthalmologists and optometrists to be aware of the current advances, as well as the potential advantages and disadvantages of various techniques available, to provide a customised approach to each patient.
To earn your CPD hours from this article visit mieducation.com/laser-refractive-surgery-options.
Dr Smita Agarwal (MBBS; FRANZCO, Grad Dip in refractive surgery) is a comprehensive ophthalmologist with a special interest in refractive cataract surgery, complex cataract surgery including post-corneal refractive cataracts, and anterior segment eye diseases. She completed her ophthalmology training at Monash Medical Centre before undertaking her advanced training in cataract surgery at the same institution. She also completed her Grad diploma in refractive surgery through University of Sydney. Dr Agarwal has vast experience with a variety of intraocular lenses available and provides customised treatment suitable to each patient’s lifestyle and ocular health. She is the former Head of Ophthalmology at Wollongong and Shellharbour Hospitals and a visiting medical officer at various private hospitals in the Illawarra and Shoalhaven region. Dr Agarwal started her state-of-the-art laser center in Illawarra region, where she performs refractive laser treatment including PRK/LASIK/SMILE using the latest laser platforms. She works privately in the Illawarra and Shoalhaven regions and holds an academic post as a Clinical Associate Professor at the University of Wollongong and Senior clinical lecturer at University of Sydney. She is actively involved in the teaching of medical students, registrars and general practitioners in the area.
Dr Agarwal holds a deep sense of responsibility to the community, particularly those with limited access to adequate health care, and makes regular visits overseas, providing voluntary health care to underprivileged communities. She also has a keen interest in education and research, acting as a mentor for young researchers and keeping up to date in ophthalmology. She has been an invited speaker at various ophthalmology meetings, and has publications in peer reviewed journals.
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