How and Why LASIK has Gotten Better and Better Over the Years
The technology behind LASIK has advanced at an extraordinary pace over the quarter of a century since its introduction. Our expert panel reviews what this has meant in terms of patient outcomes
Karl Stonecipher asserts that the biggest unsolved post-LASIK problems are night vision issues and dry eye. “My hope is that we can solve night vision problems with better wavefront sensors, but I’m not sure how we can address the dry eye issue.” One solution, he suggests, is to encourage LASIK at a younger age, when vision stabilizes in the patient’s mid-20s: “Younger LASIK recipients tend to have less dry eye, and also fewer night vision problems.”
John Marshall is clear on the benefits of the new technology: “Changes in the way in which we operate lasers have significantly reduced the problems commonly associated with early systems, and the number of patients that complain of glare or halos has gone down significantly.” Stefanie Schmickler agrees: “Night-time halos have been largely eliminated by new excimer laser systems – I hardly ever hear of any such problems from my patients. It’s really not that big an issue anymore.” Further, the introduction of higher-accuracy, modern corneal topographers, and faster-scanning excimer lasers have significantly reduced the need for secondary touch-up procedures. As Schmickler asserts, “One result of modern laser systems is that re-treatments are hardly ever necessary.” She does, however, have some caveats regarding outcomes: “Ensure correct technique during preoperative examination – no tilt – and check that there has been a one or two week break from wearing contact lenses prior to the surgery, as this may influence the results.”
Julian Stevens also observes that outcomes have improved as a consequence of better technology and the optimization of laser ablation (and other) techniques; he points to identification and understanding of the various factors that can affect manifest refraction. These include pupil size (illumination): “We know that most eyes have some spherical aberration, and we can see a difference between refraction under dim and bright light conditions.” Wavefront sensing techniques also contribute to higher-accuracy and therefore better outcomes, he says.
John Kanellopoulos believes that laser refractive surgery is being brought to a new level of safety and efficacy through use of topography-guided LASIK. “This technology will help us refine what the clinical refraction should be; it really is a new world for us in terms of accuracy.” But he also emphasizes the importance of getting the message out: “Our retina surgeons tend to be unaware of the efficacy of LASIK – we first have to educate our fellow ophthalmologists, and then move on to general medicine and the general public.”
However, Robert Maloney is skeptical that improvements in lasers or measurements will ever entirely eliminate photic phenomena. He points out that post-LASIK night glare is largely due to spherical aberration, which itself is an inevitable consequence of flattening the cornea without changing its periphery. “Larger optical zones and smoother surfaces will probably help, but we’ll never completely overcome the fact that the cornea is flatter in the middle and steeper at the periphery,” he concludes. Similarly, some unexpected phenomena relating to the wound healing process may be unavoidable. Nevertheless, Marshall remains optimistic: “We are developing treatments which will facilitate wound healing and make it more predictable.”
Dan Reinstein agrees with Maloney that spherical aberration changes when refractive error is corrected, but points out that there is a huge neural processing reserve in the brain for spherical aberration. "The reason why night vision disturbances after LASIK are extremely rare now compared to the 1990s is not because we are no longer inducing spherical aberration, it is because we are controlling the increase much more effectively. We are now able to maintain it below the threshold levels where the brain cannot filter them." He also notes that this very increase in perfectly filtered spherical aberration produces an enhanced depth of field in the eye and therefore not only delays the onset of the symptoms of presbyopia, it can in fact be used to treat presbyopia, something that he points out he has had on his own eyes.
What about corneal ectasia? Maloney’s view is that ectasia screening is a very important part of practice, and that improvements in screening have been behind the dramatic reduction in the incidence of post-LASIK ectasia. “We’re more cautious about topography; anterior segment OCT is a great help, and epithelial thickness mapping is promising and likely to become more widely used.” At the same time, says Maloney, ectasia risk factors – such as youth – now are better understood. “Big Data” has been enormously helpful in this regard: he cites evidence from Steve Schallhorn and Optical Express which shows that although younger people are at a higher risk of ectasia after keratoconus, that risk is still very low. “These people are still good candidates if other factors are normal,” Maloney asserts. Schmickler emphasizes the importance of being very familiar with the patient and their family: “Investigate pre-existing illnesses; for example, neuromyelitis may contribute to keratoconus or to ectasia after LASIK,” she says.
Reinstein states that one of the most sensitive screening tools we have now for keratoconus screening and ectasia prevention is the epithelial thickness profile – shown in published studies to be more sensitive and more specific than tomography and topography alone. He emphasizes that the combination of topography, tomography and epithelial thickness maps gives us a far superior method of avoiding ectasia in the hard-to-detect cases. There are multiple OCT scanners on the market now that enable epithelial thickness to be mapped preoperatively with a resolution sufficient for detecting most cases that would be found using the gold standard of VHF digital ultrasound scanning.
Can we go beyond screening, and actively prevent ectasia? Kanellopoulos points out that corneal crosslinking (CXL) is now accepted by the global ophthalmological community as the definitive means of halting ectasia. Its ability to modulate corneal biomechanics, he states, has resulted in its use for better biomechanical profiles and more stable outcomes: “We and others use it as an adjunct in a significant proportion of routine LASIK cases.” Indeed, in Greece he has found that one patient in 40 has signs of ectasia as shown by epithelial maps and sophisticated corneal imaging. “Obviously, that makes us very cautious when giving LASIK to patients under 30 with refractive errors of 6 D of myopia – and that’s why we favor reinforcing the cornea.” Indeed, Kanellopoulos relates that his clinical and laboratory data indicate that crosslinking reinforces the cornea by almost 100 percent. “Furthermore, our hyperopia data strongly suggest that LASIK-CXL pattern crosslinking provides a refractive effect similar to that of excimer ablation.” Hence, LASIK-CXL – or “LASIK Extra” – is now routine procedure for all hyperopes seen in his practice. “It gives us more stable keratometries in hyperopes – stability which persists for many years after the procedure,” he claims. He also emphasizes its utility in routine LASIK procedures involving higher refractive errors in young patients (below 30 years of age) – a population which historically has been associated with a high incidence of keratoconus. In summary, states Kanellopoulos, “LASIK-CXL is not just a means to treat suspicious corneas – it is a general mechanism for enhancing the safety profile of routine LASIK procedures.”
In this context, Marshall points out that the current interest in LASIK Extra is reflected in a number of ongoing clinical trials, and reminds us of a large study from the Singapore National Eye Centre, based on the work of Jerry Tan, showing excellent stability up to two years in very high order corrections – up to -18 D. That said, refractive crosslinking still needs a lot more work, cautions Marshall: “It’s at the point PRK was after about three years of work – the algorithms require significant development. And it’s likely to be somewhat limited in scope – 2.5 D will probably be the upper limit of refractive change achievable through crosslinking alone.” Kanellopoulos agrees that LASIK-CXL is not yet mainstream and may not be suitable as a routine procedure in all parts of the world; he adds that more outcomes data is required before it can be accepted to be as safe as LASIK. “I doubt it will replace excimer ablation in correcting refractive errors,” he says, “but I do think it will be a useful adjunct, especially in those cases where laser may not be the best choice.”
Maloney has a cautious view on LASIK and crosslinking, informed by its relative side-effect profile: “There are reports that maybe one in 1,000 cross-linked patients develops corneal opacification.” His feeling is that ectasia nowadays is a very rare complication, and it may not always be of benefit to prevent it with a procedure which itself is also associated with a rare complication, i.e., corneal opacification. He concludes: “I don’t see it becoming routine, but I could certainly see it used in special cases.”