Iatrogenic keratectasia is the foremost concern for both corneal laser surgeons and patients, as it is one of the most devastating complications after corneal laser surgery. To date, several cases of postoperative ectasia have been reported with the worldwide acceptance of choosing SMILE as one of the main kinds of refractive surgeries.(17–19) Thus, monitoring posterior corneal elevation to predict corneal stability and prevent iatrogenic keratectasia is of great significance.(20) In the current study, we reported posterior corneal elevation changes after SMILE with different optical zones.
For PCE and four points in the central 2 mm, the overall trend was the same for both groups: all ten points showed backward changes 3 years postoperatively as elevation data decreased. The results in our study proved that the central posterior corneal area presented no forward protrusion after SMILE regardless of optical zone size and were concurrent with previous studies. Researchers evaluated PCE changes after SMILE in eyes with myopia more than − 9.0 diopters and found that it decreased with a subtle value of -0.16.(21) The elevation value in the 2 mm optical zone also demonstrated a minor retreating change of -0.33.(21) In eyes with suspicious tomographic features that underwent SMILE, the 3-year change in PCE was − 1.22, indicating no forward displacemen.(22) Interestingly, in another study comparing posterior corneal elevation changes in eyes 10 years after laser in situ keratomileusis (LASIK) to unoperated normal eyes, the results revealed that points in the central 2 mm zone showed backward displacement after LASIK while increasing change in normal eyes.(23) Investigators have attributed this to hyperopic shift after corneal laser surgery: removal of central corneal tissue in corneal refractive surgery leads to relaxation of the peripheral corneal lamella, resulting in redistribution of corneal biomechanical balance.(7) The results in the current study further add evidence to this theory that hyperopic shift may also occur after SMILE.
According to published studies, the phenomenon of region-dependent variations from the central to the peripheral was also noted after corneal laser surgery: the central area retreated, but the peripheral area exhibited a forward change.(24) However, previous studies used a single mean value of several elevation data representing the whole corneal region, arousing some questions in the interpretation of these results. To fill this gap and illustrate the posterior corneal changes in detail, we studied all single points of different angles in various regions.
In the 6 mm corneal annulus, points at 45° and 135°elevated with increasing changes; points at 225°and 315°showed decreasing changes, with a more statistically significant change occurring at 315°. The results were in alignment with each other when comparing two groups together. All points in the inferior part were not statistically elevated. This seems counterintuitive, as theoretically, the inferior part of the cornea is more prone to forward protrusion because of biomechanical property loss after corneal laser surgery. Previously, we studied the mean value of 13 points in the corneal superior area (MPE-Superior) and the inferior area (MPE-Inferior) classified by preoperative corneal thickness. Changes in MPE-Superior in all eyes with different corneal thicknesses were all positive values; changes in MPE-Inferior in the thin cornea and thick cornea groups were negative values.(24) The current and the abovementioned studies both indicated that the posterior elevation changes in the peripheral region could be further divided into two parts: the inferior region showed a backward change, and the superior part exhibited a forward change.
In general, posterior corneal elevation changes after SMILE in the two groups showed similar patterns, suggesting that the optical zone did not play a decisive role in posterior corneal alteration.
To better understand the factors contributing to iatrogenic keratectasia and provide practical advice for clinical application, we reviewed published cases of ectasia following SMILE. Some researchers have proposed that ectasia postoperatively could be an inherent tissue-specific variation. Huang reported a case of a female who developed bilateral ectasia after SMILE starting in the first trimester of her pregnancy.(25) This case highlights the role of hormonal changes in ectasia occurrence and progression. Shetty found reduced expression of lysyl oxide (LOX) and collagen type I alpha (COLIA1) in the ectatic eye compared to the normal control eye after SMILE.(26)
In addition to these inherent reasons, some criteria could be adhered to avoid this serious complication: (1) A large percentage of reported ectatic cases after SMILE showed forme fruste keratoconus before surgery, reinforcing that contraindictions for LASIK are not allowed for SMILE. (2) Strict comprehensive examinations are indispensable before the application of SMILE, and the measurement of corneal biomechanical properties enables a more sensitive diagnosis of fruste and subclinical keratoconus. (3) Reinstein reported a case of ectasia implying no anatomical risk factors preoperatively, but the patient confirmed that she frequently and vigorously rubbed her eyes.(27) Patients should be educated that eye rubbing is related to ectasia and must be avoided postoperatively. (4) The classical Ectasia Risk Score System (ERSS) and metric of percent tissue altered (PTA) are valuable in evaluating corneal stability after SMILE and ruling out potential suspicious cases. (28–30)
There were some limitations in this study. First, we matched the two groups of eyes in terms of the preoperative thinnest corneal thickness and ablation depth, but there was a significant difference in residual bed thickness. This should be considered because it may affect the accuracy of the results. Second, this was a retrospective study and lacked data on corneal biomechanical properties. Combining corneal biomechanical changes and corneal morphology variations would provide deeper insight into corneal stability after SMILE. Third, the sample size was relatively small and eyes were only classified into two groups. A prospective multicentre study with a large sample size is warranted to further validate the results of the current study.
In conclusion, our results could make it conceivable to consider SMILE as a safe procedure causing no corneal forward protrusion with difference optical zones 3 years after the procedure. Comprehensive preoperative measurements are essential for ensuring corneal stability and avoiding ectasia postoperatively.