Eye | 2021
Cochrane Corner: wavefront-guided laser vision correction
Abstract
The author performs both wavefront-guided and wavefrontcompensated laser vision correction in private medical practice. In the last 3 years, he has received travel and/or accommodation expenses for expert user meetings sponsored by Schwind Eye Tech Solutions GmbH (Kleinostheim, Germany) and Staar Surgical Inc (Monrovia CA). Li et al. [1], are the latest group to evaluate results for wavefront-guided and wavefront-optimised laser vision correction (LVC) in a systematic review. Using Cochrane methodology, they found no significant differences in pooled visual results from 33 randomized trials including 1499 patients. But does this mean that clinically important differences do not exist? The jargon is confusing. Higher order aberrations are elements of defocus that remain after correction of sphere and cylinder. Zernicke decomposition is used for classification and ranking. In this system, 3rd and 4th order aberrations (coma, trefoil, and spherical aberration) and the total score, expressed as a root mean square function, are the most widely studied clinical indices. Aberration scores are commonly expressed in microns for the eye measured and depend on pupil size at the time of measurement. Using a simple conversion formula described by Thibos et al. [2], they can also be expressed as equivalent dioptric values, and this helps add clinical meaning. The first task in LVC is to correct for aberrations induced by treatment. This is what “wavefront-optimised” or, more correctly, “wavefront-compensated” (WFC) LVC treatments seek to do. WFC treatments build in corrections based on mean changes in higher order aberration terms observed in a patient sample treated with a given magnitude of sphere and cylindrical correction. All modern LVC systems incorporate some form of wavefront-compensation; but newer systems using larger optical zones, smoother ablation profiles, more accurate treatment registration, and faster eye-tracking are more effective. Wavefront-guided (WFG) treatments layer-on an additional element of individual customisation by targeting higher order aberrations measured preoperatively. Since these total around 0.25D [2] in normal eyes, any gains from wavefront-guidance in routine LVC treatment are likely to be small. But WFG treatment may also help to enhance accuracy of sphere and cylinder outcomes. In their first analysis, Li et al. [1] have categorized some older laser systems, with more basic wavefront-compensation, as “conventional treatments,” comparing results with newer WFC or WFG excimer laser platforms. Results are largely historic, with the last trial included reporting in 2012; and results are likely to have been skewed against the newer laser systems by relative immaturity of nomogram development. Other systematic reviewers have detected a trend towards better results for newer LVC technology [3], and this trend is also evident, although not picked up, in better visual results for the later trials analysed by Li et al. Systematic reviews of randomized trials are less easy to interpret for technologies in evolution than for drug treatments. Both WFC and WFG excimer laser platforms have evolved significantly in the time period (late 1990s–2019) Li et al. study [1]. Most of the trials they include in their comparison of WFG and WFC treatments compare an older WFG excimer laser platform (CustomVue Star S4 IR excimer laser; Johnson& Johnson Vision, Inc., Santa Ana, California) with a newer WFC platform (Wavelight Allegretto Eye-Q 400excimer laser; Alcon Laboratories, Inc., Fort Worth, Texas) incorporating significantly faster ablation and a larger optical zone size. If wavefront compensation is more effective for the newer laser, any gains from wavefront-guidance will have been masked. To determine whether WFG treatment improves visual results, both the mechanism of wavefront compensation and the maturity of nomogram development need to be standardised between trial arms. Otherwise, you are, in effect, comparing apples and pears. * Bruce D. Allan [email protected]