Methods of Corneal Vertex Centration and Evaluation of Effective Optical Zone in Small Incision Lenticule Extraction

Abstract

Inappropriate small incision lenticule extraction (SMILE) centration methods can affect the decentration of the effective optical zone (EOZ) after operation, which can subsequently lead to the decline of postoperative visual quality. We aimed to provide an overview of corneal vertex (CV) centration methods and an evaluation of the size and decentration of the EOZ in SMILE. We described the CV centration methods for patients with myopia, myopic astigmatism, hyperopia, and large kappa angle. The measurement methods of the EOZ were evaluated from the aspects of corneal morphology and corneal refractive power. Additionally, we summarized the advantages and disadvantages of measuring decentration based on topographic mapping and intraoperative video-captured images. Finally, we discussed the relationship between the EOZ and visual quality. Based on our review, clinicians should consider the following when choosing CV centration methods and evaluating EOZ postoperatively. First, the tear film mark center or topographic map comparison method is preferred for the correction of myopia, low myopic astigmatism, hyperopia, and large kappa angle (>0.2 mm). Triple marking centration is recommended for high myopic astigmatism (-3.5 diopters). Second, the total corneal power better reflects the change in refractive power than the topographic method. The measurement of the area rather than the diameter of the total corneal refractive power is more suitable for the evaluation of non-circular EOZs after high myopia astigmatism (< -2.0 diopters). Third, for the evaluation of decentration, the tangential curvature difference map method is preferred as it is not influenced by offset pupils. Finally, a large EOZ after SMILE may improve patient tolerance to decentration.

Comparison of toric intraocular lens tilt and decentration measurement using dynamic Purkinje-meter and anterior segment optical coherence tomography

“…Keratometric or topographic axis is used for alignment in keratometers and corneal topographers; this axis contains the center of curvature of the anterior surface of the cornea and is also called the corneal vertex normal, as it intercepts the anterior surface of cornea at the point called corneal vertex 23 . The corneal vertex is defined as the intersection of the cornea with the line connecting the fixation point and the 1st Purkinje image 24 . Vertex normal is used as a reference axis in anterior segment OCT, which was the point of interest of this study.…”

“…Vertex normal is used as a reference axis in anterior segment OCT, which was the point of interest of this study. It has been proven that the corneal vertex is the closest point on the cornea to the intersection of the ideal visual axis with the cornea [24][25][26] . So, for that reason, the values of IOL tilt and decentration obtained from AS OCT (with vertex normal as reference) and from our Purkinje-meter (with visual axis as reference) are comparable.…”

“…The reference used by this AS OCT is a corneal vertex normal. This axis intercepts the cornea at the point called corneal vertex 23 , which has been proven to be the closest point on the cornea to the intersection of the ideal visual axis with the cornea [24][25][26] .…”

The Impact of Back Optic Zone Design in Orthokeratology on Visual Performance

“…This is represented by the corneal light reflex when the cornea is illuminated coaxially with fixation. 16 …”

Correlational Analysis of the Effective Optical Zone with Myopia, Myopic Astigmatism, and Spherical Equivalent in LASIK, PRK, and SMILE

“…In comparison to other methods that were trialed, this was the most reproducible across various levels of myopia and cylinder, along with having particular utility in determining decentration. 17 By employing this comprehensive approach, we were able to provide a more nuanced impression of EOZ shape without the need for analyzing multiple Pentacam map types, while preserving the integrity of each parameter of interest.…”