Previous reports have shown that the SMILE procedure showed predictability, efficiency, stability and safety.[1–4] The predictability and efficiency of excimer laser surgeries, that have been implemented about 30 years ago, is known to be affected by a number of factors, including ablation parallax, corneal hydration, and corneal curvature.[18] However, the SMILE procedure that was first performed 10 years ago induces less corneal epithelial damage than LASIK and PRK, by using a femtosecond laser spherical intrastromal lenticule extraction, which causes less thermal damage to corneal tissues, and varying levels of surgical method and wound recovery compared with LASIK and PRK.[16, 17] Corneal biomechanics and wound healing properties of the cornea undermine the predictability and stability of refractive surgery and contribute to discrepancies between attempted and achieved visual outcomes after LASIK, surface ablation and SMILE.[19] However, the factors affecting visual outcomes after SMILE procedure are still unknown.
In the present study, we demonstrated that visual outcomes such as refractive predictability, efficacy, stability and safety at 12 months postoperatively were not significantly different between preoperative flat and steep corneas. However, eyes with high myopia were less corrected than eyes with moderate and low myopia especially in steep corneas. These results differ from that of several previous studies.[8, 9]
Kim et al.[9] demonstrated the clinical SMILE outcome of high-myopia patients, including efficacy, predictability, and safety, which were comparable to that of patients with mild-to-moderate myopia. In this study, we treated all the eyes with the same nomogram regardless of the degree of keratometry and preoperative myopia, which contributed to an additional 10% diopter adjustment of the attempted treatment spheres. Consequently, there was no difference in the refractive results between flat and steep corneas for 12 months after SMILE, whereas eyes with high myopia were less corrected than eyes with moderate and low myopia especially in steep corneas. Therefore, when correcting eyes with high myopia especially in steep corneas, adjustment of the following nomogram is needed. The greater the amount of myopia corrected, the greater is the percentage of myopia that needs to be corrected.
Although the correlations between preoperative keratometry and postoperative spherical equivalent in all patients for the 12 months after SMILE were not significant, the present study shows that the flatter or steeper the cornea in the flat corneas or all the low myopia corneas, the more overcorrected or undercorrected is the postoperative refraction at 1 month postoperatively. Our results are similar to those reported previously suggesting that spherical equivalent refraction undercorrection was predicted by increasing patient age (0.10 D per decade; P < 0.01) and steeper corneal curvature (0.04 D per D; P < 0.01).[13] In addition, our study shows that the flatter the cornea, the greater the postoperative keratometric change in flat corneas in 3 to 12 months.
In our study, due to undercorrection of myopia after SMILE, the nomogram was adjusted for the additional myopia requiring correction. A possible reason for under-correction after SMILE might be associated with that the achieved lenticule diameter is larger than the programmed with the VisuMax femtosecond laser.[20] In addition, as the degree of corrected myopia increased, the degree of undercorrection increased after SMILE due to weaker mechanical and structural properties of the cornea in the higher myopic corneas after surgery, which was linked to more deformable corneal surfaces.[21]
Geometrically, in this study the eyelid pressure in flat corneas exposed to lenticule side cut is less than in steep corneas, so the flatter the cornea, the greater is the postoperative keratometric change in flat corneas between 3 to 12 months. These keratometric changes affect the postoperative refraction for a year after SMILE.
Our study had limitations. Therefore, the findings must be interpreted cautiously. The sample size was small because the patients were divided into three subgroups, limiting confidence in the conclusions.
To summarize, in early phase after SMILE for myopia, the corneas were flatter in the preoperative flat corneas or all the low myopic corneas, and they were more overcorrected. However, most importantly, preoperative corneal curvature does not influence visual outcomes at 1 year after SMILE.