Murat V. Kalayoglu, M.D., Ph.D.
Contributing Editor
Wavefront–guided LASIK systems continue to improve, and are used more and more frequently over conventional LASIK systems. Wavefront LASIK offers several advantages over standard systems. For one, in some clinical trials, the standard deviation of refractive correction is narrower with wavefront systems, suggesting higher treatment accuracy over conventional LASIK. In addition, wavefront–guided systems tend to induce fewer higher order aberrations compared with their conventional counterparts. Since qualitative changes in vision are governed in large part by such aberrations, patients may report increased contrast sensitivity, fewer glare and less halo effects following wavefront–guided LASIK compared with conventional LASIK. Indeed, in quality of vision questionnaires following wavefront–guided LASIK treatments, patients tend to report improvement in contrast, glare and haloes.
Wavefront technology has been used by astronomers for decades in order to reduce higher order aberrations induced by the earth’s atmosphere. The technology has recently been incorporated into several major LASIK systems in order to improve treatment outcomes and address higher order aberrations within the visual system. Overall, wavefront–guided LASIK systems perform well. However, relatively few head to head outcome trials have been performed between available platforms. Trials that have been conducted suggest that there are more similarities than differences in outcome metrics measured by the different wavefront LASIK systems.
Key metrics that have been compared include subjective assessments such as best corrected visual acuity, uncorrected visual acuity, contrast sensitivity and glare. In addition, several objective metrics such as percent induction of higher order aberrations, ablation depth, and topographic refractive error have been compared. Stability of these metrics over time is also measured.
Head to head comparisons of wavefront–guided systems is difficult for several reasons. For one, the National Institutes of Health assigns a low priority for funding comparison trials of FDA approved, commercially available devices used in cosmetic eye surgery. Most available data have been gathered by private funding, often through device manufacturers. In addition, each device has unique components that differentiate it from its competitors. For example, VISX’s Star S4 and WaveScan WaveFront System, which recently gained FDA approval to treat mixed astigmatism, has added a new Fournier wavefront algorithm upgrade to calculate ablation shapes with more precision and individualize treatment to each patient. Additionally the VISX Customvue system offers Iris Registration and Activetrak™ to compensate for intra-operative cyclotorsional movement, and to ensure treatment is delivered to each eye accurately. Such added features differentiate each wavefront–guided system and allow tailored algorithms to match the preferences of each refractive surgeon. Indeed, it is not unusual for a surgeon to experience improved outcomes after taking advantage of a system’s custom features.
Currently, all wavefront–guided LASIK systems are highly reliable. All machines reach the desired ablation zone target with very little variability (+/- 0.1 mm). There are some differences in ablation depths, but all systems tend to reach depths of approximately 50 – 80 um. In head to head comparisons, the systems do not demonstrate statistically significant differences in refractive outcomes. There may be subtle differences in how long patients take to reach refractive stability, but nevertheless all machines demonstrate stability within 6 months.
Standard metrics such as best corrected visual acuity and uncorrected visual acuity may soon be refined to emphasize other metrics such as improvement in contrast sensitivity, increased low contrast acuity, and individualized improvements in each patient’s postoperative uncorrected visual acuity in relation to their preoperative best corrected visual acuity. As device manufacturers incorporate additional technology to enhance their wavefront – guided LASIK platform, the gold standard outcome is likely to move beyond an uncorrected visual acuity of 20/20; eliminating higher order aberrations will help realize “super-human vision” in the near future.
Today’s systems are impressive but still years beyond this goal. Currently, less than one third of individuals treated with wavefront–guided LASIK achieve uncorrected visual acuity better than 20/20. Following treatment with available wavefront systems, there is little improvement in best corrected visual acuity. Furthermore, today’s platforms do not improve contrast sensitivity appreciably; in fact, some devices may worsen contrast sensitivity. Finally, rather than eliminating and correcting aberrations, all devices continue to induce higher order aberrations, albeit at lesser levels compared with conventional LASIK systems. A 10-20% induction of higher order aberrations is still expected using wavefront systems. Much of this induction is due to mechanical disruption of the cornea during flap creation, and will likely be reduced as non-mechanical techniques are developed using femtosecond lasers or similar upcoming technologies.