CXL with riboflavin and UVA is used for slowing or halting the progression of KC and is shown to be effective [15, 16]. However, the long-term efficacy of epi-on CXL on progressive KC treatment is not fully understood, especially that there are only few studies demonstrating the stability of the PCE.
Changes in PCE can represent the shape of the posterior cornea and provide an evaluation of the shape and corneal stability in the post-CXL stage. Posterior elevation is a sensitive parameter to monitor corneal remodeling after CXL [17]. A positive change in the posterior corneal surface indicates an ectatic change in the cornea.
In the present study, PCE was analyzed, and this study aimed to determine whether or when corneal posterior morphology was stable following CXL. To the best of our knowledge, only few studies have reported the safety and efficacy of CXL by PCE in different corneal zones and local points in different quadrants.
We analyzed PCEs using elevation maps provided by Scheimpflug tomography. In this study, no significant forward displacement of PCE was found in different corneal annular areas and local points in different quadrants, which suggests that cross-linking surgery is safe and effective. Grewal et al. [18] and Henriquez et al. [19] have also analyzed the corneal changes after CXL with Scheimpflug imaging and found no significant changes in the PCE. Moreover, Steinberg has demonstrated a statistically significant increase in the posterior elevation at the apex 2 years after CXL [12]. At 36 months, the posterior elevation of the thinnest point did not change significantly [20]. Both of these studies mainly focus on the corneal curvature and corneal anterior surface, but studies assessing the description of PCE are rare.
In this study, we analyzed the changes and characteristics of PCEs in circular areas and compass points of different diameters after CXL to determine the effectiveness of the operation and evaluate the stability after the operation.
In the current study, some characteristic changes in PCE were found after the cross-linking surgery. In Method 1, apart from the mean ΔPCE values at 1 month after CXL and that of the annular diameters Φ0–2 mm at 6 months being positive after CXL, the values of all areas were negative at all times. This suggests that there is a backward displacement in the PCE 1 month after CXL. Moreover, Method 2 showed a forward displacement of PCE at the T side of 2 mm and 4 mm 1 month after surgery and then a backward displacement of PCE. This is possibly due to the rarefaction of keratocytes associated with stromal edema at 1 month after treatment [21]. After 2 to 3 months, the population of cell increased, and the edema decreased gradually [21].
Except for a 4 mm point on the T side, there was no significant change in PCE at the other 33 points involved in this study after CXL compared with before CXL. For the 4 mm point on the T side, the PCE at 1 month after CXL showed a forward displacement compared with that before CXL, and the PCE at 12 months after operation showed a backward displacement compared with that at 1 month after CXL. There was no significant difference in PCE at 12 months after CXL compared with before CXL. This is consistent with Magli’s findings that no significant difference was found in the posterior elevation at the thinnest location and at the apex from preoperative value at 1 year after treatment [22]. However, Magli’s study mainly evaluates the safety of cross-linking surgery. Except for the thinnest point and apex, it does not involve the study of the changes in the PCE at other different points. In addition, the PCE at the TI side of 6 mm and 8 mm at 6 and 12 months after surgery showed a significantly forward displacement than that at 1 month after surgery. The PCE at the T side of 2 mm shows a backward displacement at 6 months after CXL compared with 1 month after CXL. The overall change was not significant. Consistent with KC development, our study found that the PCE at the T and TI side changes evidently.
The abovementioned study results show that the PCE in the local point can be more sensitively detected before and after CXL than that in the circular area. The possible reason is that the annular area is the mean value of the PCE difference of the raw data points, which will reduce the sensitivity of detection.
The PCE of the thinnest point was correlated with other corneal points, except the points at the N and NI sides of 4 mm. Previous studies have found that the flattening effect of CXL is augmented in the eyes with thinner corneas. CXL may affect deeper stromal tissues of thinner corneas compared to cases with thicker corneas [17]. This may be attributed to the fact that changes in corneal biomechanics after cross-linking surgery cause more collagen fibers in the deep corneal stroma that can be cross-linked. Based on our study and previous studies, we found that the cone of KC tends to occur on the T side [23]. Therefore, the corneal thickness of the T side is thinner than that of the nasal side; thus, the cross-linking effect of the temporal cornea is stronger than that of the nasal cornea.
The main limitation of this study is that we did not analyze the PCE at the thinnest corneal point, the cone apex, and the maximum K point.
In summary, our data confirm that transepithelial corneal CXL is a potent treatment for preventing corneal ectasia progression in KC eyes. The PCEs of KC after CXL had certain characteristic changes at 1 year in the studied cohort. The local point assessment of the PCE of KC is more valuable than that of the circular area. Compared with the nasal points of the cornea, the change in temporal points was more significant after CXL. CXL surgery may cause different degrees of cross-linking effect in different parts of the cornea. The cross-linking effect of the temporal cornea is stronger than that of the nasal cornea. The changes of corneal collagen fiber structure in different areas after CXL need further observation.