Optical and visual quality of different refractive surgery procedures

In recent years, more patients want to correct their refractive error by means of refractive surgery. Although the most popular option to correct refractive error is laser in situ keratomileusis (LASIK), actually, other options, such as phakic intraocular lens (IOL) implantation, are growing.

The Visian Implantable Collamer lens (ICL, STAAR surgical, Nidau, Switzerland) is a posterior chamber phakic intraocular lens approved by the US FDA for the treatment of moderate to severe myopia. Implantation of this lens is a good alternative for patients with high levels of myopia or hyperopia or with thin corneas, which may not be good candidates for LASIK.¹

The observer viewed the visual test generated on a microdisplay through the adaptiveoptics system and artificial pupil. So, it is possible to measure the visual acuity and contrast sensitivity achieved for different wavefront patterns. This technology allows us to determine the impact on the visual performance of different surgical techniques before the surgical procedure actually takes place in the same patient and under the same conditions.

The first study

The other aberrations that we evaluated (coma, trefoil, tetrafoil and secondary astigmatism) were minimal. Differences in spherical aberration and total higher order aberrations (HOAs) root mean square (RMS) values were found to be between lowmedium and high powers, but these differences were clinically negligible on visual quality after its implantation.

Besides, we have to take into account that these ICLs should be implanted, so the negative spherical aberration that has the ICLs could benefit many patients, because normally corneas have a positive spherical aberration.

In our research,³ to bear out the outcomes of the first study, we used the adaptive optics visual simulator to simulate different powers of ICLs (–3, –6 and –15 D) for small- and largeincision surgery from the ICLs' wavefront aberrations. Visual acuity and contrast sensitivity were measured in 11 observers for 3 and 5 mm pupils.

Regarding the effect of the incision surgery, we found significantly better outcomes for small-incision surgery, because the larger the incision size, the greater the HOAs that were induced. So, it should be preferable to implant a toric ICL through a small incision instead of a spherical ICL through a large incision.

As mentioned earlier, the ICL power affects the optical quality. In this study, we did not find differences between –3 and –6 D ICL, but they did become apparent for –15 D ICL. However, these losses are offset by the effect of spectacle magnification, which occurs when myopic patients undergo ICL implantation surgery. This can be attributed to the refractive correction of moving from the spectacle plane to the eye.