Review on Centration, Astigmatic Axis Alignment, Pupil Size and Optical Zone in SMILE

Abstract

The advent of “flapless” small-incision lenticule extraction (SMILE), employing all-in-one technology, has resulted in a revolutionary breakthrough in refractive surgeries. SMILE has been gaining popularity due to fewer potential complications, such as postoperative dry eyes and greater biomechanical stability, etc. However, attention must be given to 1) the centration on the corneal vertex, 2) the proper alignment of the astigmatic axis, and 3) the relationship between pupil size and treatment diameter, to achieve good SMILE results. There is no pupil-tracking system to ascertain the accuracy of centration during the SMILE surgery. To improve the centration accuracy, our center uses two corneal topographers (Pentacam and Sirius) to measure and determine corneal vertex. Proper predicted optical zone diameter is not clearly defined yet in SMILE. Some scholars insist that mesopic pupil size should be taken into consideration when setting the predicted optical zone. Meanwhile, the issue of “functional optical zone” still has many unresolved issues and warrants further studies.

Changes in effective optical zone after small-incision lenticule extraction in high myopia

“…EOZ was de ned as the diameter of the largest ring that the corneal refractive power difference between the ring and the pupil center was smaller than 1.50 D 22 . In our study, four measurement methods showed that EOZs were signi cantly reduced in eyes with high myopia as compared to the POZ at 3 months after SMILE surgery (Table 1), which is consistent with the ndings in other reported studies 2,3,11,19,24 . This reduction could be attributed to the postoperative corneal oblate changes 15 and the corneal healing responses, including the increased epithelial thickness 25 , the loss of laser energy around the cutting zone and the changes in biomechanical properties after surgery 8,26 .…”

“…Previous studies suggested that the appearance of these symptoms is directly related to the reductions in the EOZ. 2,18,19 .…”

Corneal Higher-Order Aberrations Measurements: Precision of SD-OCT/Placido Topography and Comparison with a Scheimpflug/Placido Topography in Eyes After Small-Incision Lenticule Extraction

“…The main advantage of this technique over laser-assisted in situ keratomileusis (LASIK) is the lack of flap-related complications. In addition, many studies have proven that SMILE is characterized by good efficacy, predictability, safety, stability, and patient comfort [ 7 , 8 ]. However, it has also been reported that SMILE induces different degrees of HOAs, such as coma, spherical aberration (SA), trefoil, and astigmatism II, which might be related to decentration [ 7 , 9 , 10 ].…”

Comparative study of functional optical zone: small incision lenticule extraction versus femtosecond laser assisted excimer laser keratomileusis

“…Studies have shown that this is directly associated with the size of optical zone [7][8] . The size of the optical zone of human eyes is equal to the overlapping area between the pupil and cornea, which corresponds to the cornea area that light goes through to form images on the retina [9] . To ensure a smooth optical surface and optimal visual quality after refractive surgery, the laser treated area consists both an optical zone and a transition zone.…”