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HomemiophthalmologyAdvances in Laser Vision Correction

Advances in Laser Vision Correction

Advances in technology are enabling surgeons to meet the needs of an increasingly broad patient group with truly personalised laser vision correction.

Laser vision correction is becoming increasingly customised to the eyes of individual patients. This may be surprising to many who don’t routinely perform laser eye surgery and expect it has always been this way. Indeed, it came as a major surprise to me when I undertook my fellowship subspecialising in laser and cataract refractive surgery six years ago. I had assumed that standard practice surely must be to measure the shape of the cornea, then use this information to plan and create a new, improved corneal contour with neutralised refractive error. I soon realised that what I had pictured as being normal, was a relatively rare practice of topographically guided laser ablation. Standard practice was, actually, to use corneal tomography to exclude abnormalities, such as keratoconus, but not incorporate this shape information into treatment planning.

Probably the most exciting use of this technology has been when it is used to treat severely abnormal corneas that simply would not be amenable to treatment by any other laser vision correction technique

Figure 1. An irregular cornea highlighted with
fluorescein staining after an intraoperative
accident with a microkeratome. This eye underwent topographically guided laser ablation and was returned to unaided visual acuity of 6/6.

I have been fortunate to have gained experience with the full range of increasingly customisable options of wavefront-assisted, wavefront-guided, and at my practice at Ashford Advanced Eye Care in Adelaide, truly topographically guided laser surgery. The ability to now safely and accurately treat normal or irregular corneas, damaged corneas and even those previously forbidden-to-treat keratoconic corneas, is an amazing advance for laser vision correction technology (Figure 1). This, and the other recent or near future advances in the field that I will discuss below, are broadening our ability to treat a wider variety of eyes and refractive errors. They are also making our pre-operative decision making easier and safer. While we still have to tell some patients that they are not suitable for laser vision correction, with these advancing technologies, that number is shrinking.

ADVANCES IN TECHNOLOGY

Corneal Epithelial Thickness Mapping 

You would think that the cornea – a thin, transparent layer of the eye – would not be able to hide many secrets from us. However, we continue to learn new things about its composition and behaviour. For example, a new anatomical corneal layer, known as ‘Dua’s layer’, was described as recently as 2013.1

We have many reasons to want a full understanding of corneal anatomy, biomechanics, healing responses and optics. The most important for laser refractive surgeons is to be able to assess for risk of ectasia. Whenever we perform laser vision correction, we remove tissue from the cornea, potentially weakening it’s structural integrity.2 Certain corneas will respond differently to this insult and our major concern is that rarely, a cornea will become ectatic post-laser, where it warps in shape. The resulting irregular astigmatism is difficult to manage and often the resulting best corrected visual acuity is reduced. Risk factors for postlaser ectasia have been identified, forming the basis for cut-off values of minimal safe residual corneal tissue post-ablation.3 Additionally, we are aware that keratoconic corneas and standard laser procedures can be a risky combination.4 Identifying definite keratoconus is relatively straight forward with use of corneal tomography and specialised algorithms.5,6 However, deciding whether borderline cases are safe to undergo laser surgery can be much more difficult. Advances in corneal epithelial thickness mapping have made decisionmaking much safer.7

Figure 2. Epithelial thickness maps of a patient with previous scarring from HSV infection. Overall pachymetry appears quite normal but epithelial thickness maps show there has been a lot of epithelial remodelling to compensate for underlying stromal thickness changes.

Corneal tomography gathers information about the shape of the anterior and posterior surfaces of the cornea as well as its thickness. Until recently, we have based our decisions about the presence of keratoconus or other irregular shape patterns on this information. However, this is massively oversimplifying such a complex living structure. Corneal epithelium rapidly grows to cover defects. This can be extremely helpful after injuries such as corneal abrasions, but it can also be very annoying, as in cases of epithelial ingrowth. That ability to cover abnormalities extends to three dimensions where epithelium varies in thickness to provide a regular, smooth curvature. We appreciate this as it usually gives us an optimal refractive surface for vision, but it can also hide quite significant irregularities below. You could compare it to the smooth ocean surface overlying a rocky seafloor of stromal irregularities, though it is not the perfect disguise so is more akin to laying a thick carpet over rough floor boards. Small irregularities in the underlying stroma can be smoothed over, but larger changes in contour will still be detectable, as they are with corneal tomography. Recently, optical coherence tomography (OCT) imaging has allowed us to separate and map corneal epithelial thickness (Figure 2). I use the Zeiss Cirrus HD-OCT to map the central 9mm of corneal epithelium. Knowledge of standard corneal epithelial thickness can be correlated with corneal tomography to identify the true shape of the underlying corneal stroma. It is the stroma which provides structural integrity and is most important in deciding whether a cornea is at risk for ectasia.8 Consider a scenario where a cornea has a normal appearance on corneal tomography, but epithelial thickness mapping indicates that there is atypical thickness inferiorly. This could indicate an inferior area of stromal thinning typical of a keratoconic cone and laser surgery should be avoided. Other interesting scenarios that we have come across, where we use this technology at Ashford Advanced Eye Care, have been epithelial thickness significantly above average, where a transepithelial photorefractive keratectomy (tPRK) planned without this technology would have undertreated the refractive error due to not reaching stroma until late in the ablation. We have also seen cases of early regression after myopic ablation that has been due to thickening of corneal epithelium post-treatment.9 It has been reassuring, for both surgeon and patient, to know that we expect the epithelium to remodel, become thinner and that the planned refractive outcome will likely be achieved. I expect that epithelial thickness mapping will become an essential imaging modality in corneal and refractive practices.

Topographically Guided Laser Treatments 

To fully appreciate the technological advancement of topographically guided laser surgery, it is helpful to understand some additional laser related terminology:

  • Standard laser ablations do not makeany customisable adjustments to atreatment other than for the refraction of the patient.
  • Wavefront optimised treatments allowthe surgeon to make pre-programmedadjustments for certain features, such as spherical aberration, and
  • Wavefront guided ablations can take intoaccount multiple measured higher orderaberrations and adjust the treatment to attempt to minimise these in combination with the refractive ablation.

There are many available combinations of laser systems, wavefront analysers and corneal tomographers, which will provide surgeons with reconstructions of corneal shape, wavefront analysis, and options to plan treatments in attempts to optimise higher order aberrations. These additional pieces of information, from various diagnostic devices and abilities of laser planning software, create some potential benefits but also require more surgeon understanding and input into each treatment. The vast majority of eyes will do very well with standard laser treatments.

Figure 3. Dr Ben LaHood performing SMILE.

Reducing the presence of higher order aberrations after laser surgeries can potentially provide better quality vision and fewer side effects, such as night time halo or glare.10 However, there is one major difficulty that all of these complex treatment options face: measuring corneal tomography, using this to construct a wavefront, and then using this information to not only plan how to treat a refractive error but also how to adjust the shape of the cornea to a more regular shape, is extremely complicated. It is nearly impossible to correctly predict the refractive impact of small alterations to the shape of the cornea and still expect to provide a precise refractive correction. Alcon recently integrated new clinical decision support software to help with this decision making. The Phorcides Analytical Engine quickly integrates information from multiple measurement devices and helps surgeons determine a precise and customised plan for their patients. This has the potential to improve vision quality outcomes for laser vision correction on the Alcon laser platform.11 

Truly topographically guided laser vision correction is exceedingly rare. In this scenario, corneal tomography is measured and based on an analysis of that precise shape, and a laser plan is constructed to alter that contour without any extra steps involving wavefront analysis. I use such a topographically guided laser system from Ivis. This advanced technology really comes into its own when dealing with irregular corneas. For example, it is possible to regularise the central visual axis of keratoconic corneas with minimal tissue ablation.12 This may allow for better fitting of contact lenses, lighter, more normal spectacle lenses, or potentially less dependence on refractive correction. Minimising tissue ablation to literally just a few central microns reduces the risk of further ectatic change occurring. It also allows treatment of relatively thin corneas while maintaining enough stromal thickness to perform corneal crosslinking if necessary.13 Probably the most exciting use of this technology has been when it is used to treat severely abnormal corneas that simply would not be amenable to treatment by any other laser vision correction technique. A perfect example of this was a patient who underwent laserassisted in situ keratomileusis (LASIK) overseas when the microkeratome cut out. The patient was left with a step across his visual axis, where the junction between normal cornea met the half of his cornea where a LASIK flap had been cut off. A single treatment with topographically guided trans-epithelial phototherapeutic keratectomy produced an unaided visual acuity of 6/6 and a very happy patient.14 This advancement of topographically guided laser vision correction has massively broadened the scope of which eyes can be safely treated. We now have the ability to manage keratoconic corneas, damaged, irregular corneas and even those corneas which have had post-laser ectasia.

Figure 4. The Zeiss Visumax laser for performing SMILE, which will soon be joined by other laser platforms able to remove corneal lenticules.

Hyperopic SMILE 

When patients come to see me to discuss laser vision correction, I usually start by explaining to them that no matter what their refractive error or pathology, it would be very unusual for me not to be able to come up with an appropriate solution for them. However, it may not always be laser vision correction. Myopic refractive errors have many potential treatment options including PRK, LASIK and small incision lenticule extraction (SMILE) (Figure 3). Treatments for hyperopes, however, have been more limited. The reasons why may not seem obvious at first. Basically, keratorefractive treatments for myopia involve making the central cornea flatter. This is very easy to do by ablating tissue away from the central corneal stroma, leaving a gentle curving change in contour. To treat hyperopia in the same way involves removing tissue to make the central cornea steeper – a much more challenging task. This means ablating a donut of tissue around the periphery of the optical zone and steepening the area within. Apart from the potential downsides in terms of visual side effects, the human body simply does not like sharp changes in curvature and will often fill in this donut of ablated tissue, leading to regression.15 Some refractive surgeons will not perform hyperopic laser treatments for this reason. Regression rates for hyperopic treatments with PRK is higher than for those with LASIK and up until recently, hyperopic SMILE treatments have not been an available option.16 Personally, I am willing to perform hyperopic LASIK treatments, but not PRK, unless the patient specifically requires a surface treatment and is fully understanding of the risk of regression. Improvements in presbyopia correcting IOL options mean I am performing a lot more lens based surgery for hyperopia than laser vision correction.

SMILE involves removing a lenticule of tissue from the mid stroma of the cornea. Myopic treatments involve removing a convex piece of tissue to flatten the central cornea. Hyperopic SMILE will mean removing a concave piece of corneal stroma. Being able to offer another laser vision correction option for hyperopes will be great as we know that SMILE offers benefits over LASIK in terms of dry eye and corneal sensitivity during recovery.17 Preliminary studies indicate that SMILE offers hyperopes a safe, effective alternative to LASIK.18 This is certain to be an advancement we will see on the Zeiss Visumax laser soon. While there is no certainty, it is likely that impending updates to the Zeiss Visumax laser system will address concerns about a lack of automated treatment centration and cyclotorsion control.

New Options in Lenticule Extraction Laser Surgery 

For over a decade, Zeiss and its VisuMax laser have had the monopoly on being able to perform SMILE surgery (Figure 4). This huge length of time speaks volumes about the research and development required to create a new, safe and effective laser vision correction technique. SMILE appears as though it is here to stay, with advantages of less post-operative dry eye and without concerns of flap complications. We are now seeing competitors enter the lenticule market. The Schwind ATOS laser, with its Smartsight application, has recently been approved for market as has the Ziemer Femto LDV Z8 laser, with it’s corneal lenticule extraction for advanced refractive (CLEAR) correction program.19 Both devices offer the ability to modify centration after docking the laser, and compensate for cyclotorsion. These features are not available on the current generation of VisuMax laser, but this is likely to change with the upgraded model later this year. An interesting addition to the Ziemer Z8 laser is intraoperative OCT, which allows precise visualisation of the stromal lenticule.20 This will be a welcome sight for any surgeon who has struggled to find the correct plane during dissection of a corneal lenticule during SMILE. It will also broaden the range of treatable corneas to more difficult cases, such as those post-transplant surgery.

Laser Adjustable IOLs 

Figure 5. Proposed laser adjustment technology from Perfect Lens LLC, currently under development.

While not keratorefractive laser vision correction surgery, laser adjustable IOLs are on the horizon and they are going to be a major game changer (Figure 5). I am obsessive about trying to minimise any pre-operative variables that could lead to post-operative refractive error. I take biometry on multiple devices and check for consistency; assess and treat any irregular tear film concerns; use the most advanced IOL calculation methods; and yet, while I am very happy with my refractive outcomes, they could always be better. So where is that error coming from? There are so many tiny sources of error, it is hard to know where to start and once you start thinking about them, it is amazing that our outcomes are as good as they are. Variability in tear film will give variable keratometry measurements; our understanding of corneal biomechanics is insufficient to predict surgically induced astigmatism accurately; we can only estimate the final effective lens position; the IOL sphere and cylinder have ranges of tolerance during production; and finally, we assess our outcome accuracy with subjective refraction. There is so much potential noise that we will never reach the asymptote of 100% of patients having zero residual refractive error after cataract surgery. I am sure that we will become incrementally better at predicting appropriate IOL powers with each passing year, but the cheat code for achieving perfect refractive outcomes will surely be to fine-tune the outcome post-operatively. It simply makes sense to have an IOL sitting in a stable position, a stable corneal shape and an accurate subjective refraction before trying to refine the refractive outcome. This is analogous to our aims with laser vision correction where we get around concerns about corneal or lenticular astigmatism and visual processing by using a subjective refraction to give the whole visual system what it requires.

Light adjustable IOLs have been available for some time and have proven popular.21 The potential downsides of this technology have been needing to stay away from UV light post-operatively and that a specific IOL needs to be implanted for any post-operative adjustment to be possible. Laser adjustment offers so much more, with the ability to adjust both the sphere and cylinder powers of a wide range of previously implanted IOLs. What excites me most, however, is the ability to impart multifocality on an IOL and then reverse it again. Imagine being able to tell a patient that they can test out whether they like multifocality, and if in the future they were to change their mind or develop a pathology such as macular degeneration which could degrade their image quality, multifocality could be reversed. With a single laser treatment, that eye could be returned to excellent quality, monofocality. This technology is currently under investigation and hopefully will not be far from availability.22, 23 

Custom Corneal Cross-linking 

Similar to laser adjustable IOLs, crosslinking is on the periphery of what might be classified as laser vision correction techniques. I have generally thought of corneal cross-linking as being a technique for salvaging irregular, ectatic corneas to prevent further warping.24 But if you consider how cross-linking works, there is also potential for refractive correction without removing tissue. Corneal crosslinking acts by stiffening corneal stroma. The process typically involves removing the corneal epithelium, applying riboflavin to soak into the corneal stroma, and applying UVA light. The stiffening effect produces changes in corneal contour, often reducing irregular and regular astigmatism quite significantly. It therefore makes sense that this process could be refined and the ability to change corneal shape harnessed to alter refractive properties. This nonablative keratorefractive procedure is known as photorefractive intrastromal cross-linking (PiXL) and has the ability to treat low magnitude myopic or hyperopic refractive error.25 PiXL can also be performed without removing corneal epithelium, meaning that this treatment is comfortable and safe with no stromal tissue being removed.26 Unlike LASIK or SMILE, corneal nerves are not severed but dry eye symptoms do seem to still be relatively common after PiXL. Twelve month data for low hyperopic treatments were published last year and indicate good stability, so it will be interesting to watch this technology as the range of treatments continues to grow.27 

CONCLUSION

Laser vision correction has always seemed exciting and futuristic. One of my vitreoretinal colleagues told me recently that he is jealous of me being a refractive surgeon as when he tells people he is an eye surgeon, they always ask excitedly if he does laser surgery!

Lasers capture people’s imagination and our ability to take someone with high refractive error and eliminate their need for glasses overnight does seem like magic. This area of ophthalmology continues to evolve and it is incredibly exciting to be on the cutting edge of that. Evolution of species is never perfectly smooth, but usually occurs with stepwise bursts of small variations over time. Laser vision correction is much the same. We are seeing incremental improvements in laser customisation with topographically guided treatments, safer pre-laser assessment with epithelial thickness mapping, and upgrades in lenticule extraction techniques. We are also seeing the emergence of new technologies with laser adjustable IOLs and refractive cross-linking. It will be extremely interesting to see what is coming next.

Dr Ben LaHood (MBChB, PGDipOph, FRANZCO) is an ophthalmologist specialising in refractive cataract and laser vision correction in Adelaide Australia. His research interests include astigmatism management, biometry and intraocular lens calculation. He practices privately at Ashford Advanced Eye Care and in public at The Queen Elizabeth Hospital. Dr LaHood also hosts two popular podcasts, Ophthalmology Against The Rule and The Second Look. Both are widely and freely available on Spotify, Google and Apple Podcasts. 

To earn your CPD hours from this article, visit mivision.com.au/advances-in-laser-vision-correction

References 

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  2. Dawson DG, Randleman JB, Grossniklaus HE, O’Brien TP, Dubovy SR, Schmack I, Stulting RD, Edelhauser HF. Corneal ectasia after excimer laser keratorefractive surgery: histopathology, ultrastructure, and pathophysiology. Ophthalmology. 2008 Dec 1;115(12):2181-91. 
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  12. Mulè G, Chen S, Zhang J, Zhou W, Selimis V, Stojanovic A, Aslanides IM. Central corneal regularization (CCR): an alternative approach in keratoconus treatment. Eye and Vision. 2019 Dec;6(1):1-7. 
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  14. LaHood BR, Goggin M, Ryan TG, Beheregaray S. Topography-guided transepithelial phototherapeutic keratectomy to treat a partial laser in situ keratomileusis flap amputation over the visual axis. JCRS Online Case Reports. 2019 Jun 1;7(3):33-5. 
  15. Yan MK, Chang JS, Chan TC. Refractive regression after laser in situ keratomileusis. Clinical & experimental ophthalmology. 2018 Nov;46(8):934-44. 
  16. Spadea L, Sabetti L, D’Alessandri L, Balestrazzi E. Photorefractive keratectomy and LASIK for the correction of hyperopia: 2-year follow-up. 
  17. Denoyer A, Landman E, Trinh L, Faure JF, Auclin F, Baudouin C. Dry eye disease after refractive surgery: comparative outcomes of small incision lenticule extraction versus LASIK. Ophthalmology. 2015 Apr 1;122(4):669-76. 
  18. Pradhan KR, Reinstein DZ, Carp GI, Archer TJ, Dhungana P. Small incision lenticule extraction (SMILE) for hyperopia: 12-month refractive and visual outcomes. Journal of Refractive Surgery. 2019 Jul 1;35(7):442-50. 
  19. Fuest M, Mehta JS. Advances in refractive corneal lenticule extraction. Taiwan J Ophthalmol [Epub ahead of print] [cited 2021 Jun 19]. 
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  21. Schojai M, Schultz T, Schulze K, Hengerer FH, Dick HB. Long-term follow-up and clinical evaluation of the light-adjustable intraocular lens implanted after cataract removal: 7-year results. Journal of Cataract & Refractive Surgery. 2020 Jan 1;46(1):8-13. 
  22. Werner L, Ludlow J, Nguyen J, Aliancy J, Ha L, Masino B, Enright S, Alley RK, Sahler R, Mamalis N. Biocompatibility of intraocular lens power adjustment using a femtosecond laser in a rabbit model. Journal of Cataract & Refractive Surgery. 2017 Aug 1;43(8):1100-6. 
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  24. Wan Q, Wang D, Ye H, Tang J, Han Y. A review and meta-analysis of corneal cross-linking for post-laser vision correction ectasia. Journal of current ophthalmology. 2017 Sep 1;29(3):145-53. 
  25. Nordström M, Schiller M, Fredriksson A, Behndig A. Refractive improvements and safety with topographyguided corneal crosslinking for keratoconus: 1-year results. British Journal of Ophthalmology. 2017 Jul 1;101(7):920-5. 
  26. Sachdev GS, Ramamurthy S, Dandapani R. Photorefractive intrastromal corneal crosslinking for treatment of low myopia: clinical outcomes using the transepithelial approach with supplemental oxygen. Journal of Cataract & Refractive Surgery. 2020 Mar 1;46(3):428-33. 
  27. Stodulka P, Halasova Z, Slovak M, Sramka M, Liska K, Polisensky J. Photorefractive intrastromal crosslinking for correction of hyperopia: 12-month results. Journal of Cataract & Refractive Surgery. 2020 Mar 1;46(3):434-40.

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