This study describes the 3-year changes in anterior and posterior corneal irregular astigmatism and curvature after small incision lenticule extraction (SMILE).
Study design
Researchers studied 50 eyes of 28 patients who underwent SMILE for myopic astigmatism with the VisuMax femtosecond laser. The 3-year postoperative measurements of anterior and posterior curvature with a Scheimpflug camera were compared with preoperative values. Anterior and posterior corneal Fourier parameters (spherical component, regular astigmatism, asymmetry and irregularity) and curvature data were evaluated. Patients were divided into 2 subgroups: low myopia (spherical equivalent, SEQ, ≥ -6 D) and high myopia (SEQ ≤ -6 D).
Outcomes
As expected, the investigators found an increase in radius of anterior curvature (i.e., corneal flattening) in all eyes, which was strongly correlated with preoperative SEQ and lenticule thickness and volume. For the posterior cornea, however, the high myopia group alone showed a significant decrease in flattest radius, corneal astigmatism, spherical component, regular astigmatism and irregularity.
Changes in posterior astigmatism were significantly correlated with changes in anterior radii (R=0.349, P=0.014), SEQ (R=0.396, P=0.0049) and lenticule thickness (R=20.414, P=0.0031). Changes in posterior corneal irregularity were correlated with changes in anterior and posterior radii (R=20.3, P=0.034, and R=0.449, P=0.0012, respectively), changes in preoperative SEQ (R=0.284, P=0.0477), and lenticule thickness (R=20.311, P=0.0298).
Limitations
The study did incorporate bilateral eyes of some patients, which may have skewed the statistical interpretation. Furthermore, corneal incision location and size varied between 2.5 and 4.0 mm, depending on access to the eye due to brow anatomy.
Clinical significance
In high myopes undergoing SMILE, there appears to be a steepening of the posterior cornea at 3 years and a decrease in posterior corneal astigmatism. Due to the inverse association between posterior corneal changes and refractive effect, this would likely cause an increase in total corneal astigmatic power, which may lead to undercorrection in high refractive treatments. Total irregularities increased despite the compensatory effect of the posterior corneal surface.