Many types of methods can be used to measure corneal thicknesses, such as ultrasound pachymetry, anterior-segment optical coherence tomography (AS-OCT), Pentacam measurements, Orbscan measurements, and non-contact specular microscopy techniques. However, the values are not always consistent between the different instruments. In a long-term follow-up study, it was recommended to uniformly use the same kind of measuring instrument [3–5]. Non-contact specular microscopy is a commonly used method for corneal thickness measurement, with advantages including noninvasiveness, ease of operator use, and good examiner-independent reproducibility . Non-contact specular microscopy can be an ideal technique for studying the corneal thickness changes after photorefractive keratectomy, because it is noninvasive, which is especially important during the early postsurgery stage. Changes of CCT values are very important for observation of corneal tissue healing responses after surgery.
In this study, we used non-contact specular microscopy to follow-up the CCT changes within 6 months post thin-flap LASIK treatment. We found that CCT values declined significantly at 1 day postsurgery, and continued to decline at 1 week. CCT values began to increase over time. Several processes occurred during the early postoperative period, including resorption of fluid introduced by intraoperative irrigation, biomechanical hydration shift, epithelial thickness modulation in response to laser ablation, and interface reflectivity changes. However, in many reported cases, the systematic changes are small after 1 week , and the posterior stroma is significantly thickened after 1 week postsurgery [8, 9]. Peng et al.  reported that just after LASIK surgery, keratocytes were activated by cytokines that induced collagen fiber synthesis. Keratocyte activation was strongest at 1 to 2 weeks, and persisted until 3 months after LASIK surgery [8, 9]. This could cause the increase in posterior stromal thickness, and may be why the CCT values continued to increase after 1 week postsurgery.
In this study, we show that from 1 week postoperatively, SE continually shifted to the myopic side over time. Corneal wound repair is believed to be a contributing factor in the gradual increase of corneal thickness and postoperatively in the development of refractive regression [11, 12]. Epithelial hyperplasia after photorefractive keratectomy (PRK) has been suggested to contribute to the loss of the postoperative refractive effect [13–16]. However, after LASIK surgery, epithelial changes were significantly reduced, and were not the main cause for refractive instability . Avunduk et al. reported no significant changes in epithelial thicknesses at any time point after LASIK treatment . MØller-Pedersen et al. demonstrated activated keratocyte-mediated rethickening of the photoablated stroma of myopic individuals . They further demonstrated that corneal rethickening caused myopic regression mediated almost solely by stromal rethickening; only a minor contribution appeared to originate from restoration of the postoperative epithelial thickness . In the present study, we found a significant increase in the CCT between 1 week and 6 months after surgery (422.6 μm versus 435.6 μm). The spheroequivalent refraction changed to the myopic side between these time points (0.54 D versus 0.37 D), but the difference did not reach statistical significance, and no significant correlation was detected between the SE value changes and the CCT value changes at different examination time points. Normally, it would be expected that a 10 μm to 15 μm rethickening of the posterior stroma would produce a 1 D myopic shift. However, in the present study, the much greater rethickening created only a small amount of refractive change. It is difficult to explain this refractive change with activated keratocyte-mediated rethickening of the photoablated posterior stroma. Avunduk et al. suggested that the most probable explanation is that the refractive change is induced by different refractive characteristics of activated keratocytes during LASIK surgery. Therefore, the anterior and posterior curvature and the refractive index may be shifting postsurgery, and these factors may also play a role . However, we do not have any corneal curvature measurements to support this hypothesis.
Li et al. found that CCT increased within 12 months after LASIK surgery, and was correlated with age, preoperative SE, and corneal bed thickness (r = −0.554, r = 0.382, r = −0.352, respectively) . Our present observations were similar, however, in our study the correlations were all weak (r < 0.2). The reason for this difference is unclear, and further study is needed to confirm this finding. In elderly patients, tissue regeneration ability is reduced therefore the possibility of refractive regression is lower. It is not recommended to overcorrect on LASIK in patients over 40 years old . To prevent the occurrence of refractive regression after surgery in patients with high myopia, it is recommended to expand the optical zone diameter as large as possible, leaving sufficient corneal stromal tissues. We also retained a certain degree diopter that was corrected after surgery with eye glasses .