The reasons for the failure to fully correct astigmatism when using SMILE are not fully understood. There have been several possible causes of astigmatic undercorrection after SMILE suggested by many authors. Because of the lack of an active eye tracker, the centration of treatment in SMILE is purely subjective and may vary greatly between surgeons. In addition, there is no standardized method to determine the center of the optic zone, which can be the pupil center or the corneal vertex. Yu et al11 found that the decentration was 0.27 ± 0.09 mm from the corneal vertex, and Lazaridis et al12 reported a mean decentration of 0.31 ± 0.21 mm after SMILE. However, laser in situ keratomileusis (LASIK) procedures that harbor an active eye tracker also cause decentration of ablation. Some authors reported no significant difference in decentration between LASIK and SMILE.,12,13 and Chan et al13 found no significant association between decentration and astigmatic correction after LASIK and SMILE. Cyclotorsion of the eye from standing to supine position is another possible cause of astigmatic undercorrecion after refractive surgery.14 Moreover, because Visumax does not harbor a cyclotorsion compensation system, cyclotorsion is one of the biggest concerns when treating eyes with high astigmatism through SMILE. Some authors reported axis misalignment after SMILE and the superiority of LASIK in correcting low-to-moderate astigmatism, which was explained by the lack of a cyclotorsion compensation system in SMILE.4,15,16 To compensate for cyclotorsion, manual compensation using corneal marking in the sitting position and rotation of the con after docking was suggested during SMILE, with some reports of better astigmatic correction.17,18 However, according to an observation reported by Ganesh et al,17 86% of eyes with a high cylinder demonstrated 5 degrees or less of cyclotorsion with none having 10 or more degrees, and the manual marking method could inherently introduce inconsistency ranging from 3.8 to 6.0 degrees. Moreover, a recent study yielded comparable results between SMILE and LASIK when treating over 3.0 D of astigmatism without the use of the manual corneal marking method during SMILE.19 Besides, there are other factors that may influence astigmatic correction by SMILE, including ocular residual astigmatism, angle kappa, anterior corneal curvature, preoperative axis of astigmatism, and the technique of lenticule extraction.13,20−23 Several adjustments of current treatment nomograms have been suggested, leading to a 10% increment in the magnitude of astigmatism correction.4,24 Currently, however, there are no standardized nomograms for astigmatism correction.
Although many causes have been suggested for astigmatic undercorrection after SMILE and methods have been proposed to remediate them, none of them has been proved to be true or effective. However, there is a clear tendency that the greater the preoperative astigmatism, the higher the degree of undercorrection.4–6 Ivarsen and Hjordtal6 thus reported that an undercorrection of 13% per diopter of attempted correction was observed after treatment of low astigmatism, compared to 16% after treatment of high astigmatism after SMILE. Furthermore, Pederson et al4 found that 94% and 63% of patients with 0.5 to 1.0 D and 3.0 to 4.0 D preoperative astigmatism had an ME of less than 0.5 D, respectively.
In the current study, we focused on refraction. Preoperative high astigmatism is one of the biggest risk factors for astigmatic undercorrection, and high astigmatic cornea can cause decentration during suction because of the discrepancy between the corneal curvature and the contact glass surface during SMILE.13 We thus performed FTAK prior to SMILE to reduce the amount of astigmatism, and hence expect better predictability of SMILE. The steep cornea was flattened by FTAK, which was expected to be helpful in centrating during suction. Furthermore, we also aimed to reduce the lenticule thickness by performing FTAK, thereby reducing astigmatism without altering the spherical equivalent.25 Moreover, because currently, SMILE is limited to a refractive correction of up to 5.0 D for astigmatism, we could make SMILE applicable to patients who had more than 5.0 D astigmatism preoperatively. We previously reported good efficacy and predictability for the combination of FTAK and SMILE in patients with high astigmatism.7 However, the follow-up period was of only 6 months, the number of eyes that were included were 13, and no vector analysis was conducted. Herein, we evaluated the effectiveness of astigmatic correction by FTAK and SMILE, both individually and combined, in patients with over 4.0 D astigmatism preoperatively using Alpin’s vector method during a 2-year follow-up period.
The current study demonstrated that when combined, FTAK and SMILE procedures yielded good efficacy, safety, and predictability in correcting eyes with high astigmatism. In our study, an average undercorrection of DV of 0.34 D x 178˚ was found 24 months after the combined FTAK and SMILE procedures. Pederson et al4 reported an average undercorrection of DV of 0.31 D x 91˚ for eyes with a mean astigmatism of 1.81 ± 1.0 (range: 0.75 to 4.0 D) at 1 year after SMILE. Considering that the preoperative mean astigmatism in our study was 5.48 ± 1.17 D (range: 4.0 to 9.0 D) and 54.7% of eyes had over 5.0 D of astigmatism preoperatively, and were thus inoperable by SMILE alone, the combined FTAK and SMILE procedures showed good effectiveness of astigmatic correction. According to prior studies, the amount of undercorrection after SMILE tends to increase with higher preoperative astigmatism.4–6 In our study, a tendency toward increased undercorrection with a high level of preoperative astigmatism was not statistically significant after FTAK and SMILE combined (p = 0.489), whereas it was statistically significant after FTAK and SMILE individually (both p < 0.001). We suggest that this was because the astigmatism was reduced after FTAK, so the TIA of SMILE was relatively low and the predictability of SMILE was increased. Furthermore, 79% of eyes achieved a UDVA of 20/20 or better, 96% of eyes had a CDVA that was equal to or better than preoperative CDVA, and the postoperative SE was − 0.36 ± 0.42 D at 24 months after the combined procedures. No complication was observed during the 2-year follow-up period.
Astigmatic keratotomy (AK) is an effective procedure for the correction of naturally-occurring astigmatism as well as any residual astigmatism in patients who underwent LASIK, lensectomy, or keratoplasty.26–28 However, the predictability of AK is poor, especially when correcting higher astigmatism.27 Recently, many authors reported better results using femtosecond laser technology with AK.29–31 Loriaut et al29 reported a mean CI of 0.9 of femtosecond-assisted AK in post-keratoplasty astigmatism, but 50% of eyes were overcorrected. Although the CI of FTAK was 0.63 in our study, only 3 eyes (4%) were overcorrected with FTAK, including 2 eyes that were overcorrected by less than 0.5 D (Fig. 4). That was because we did not have 1 CI as our target for FTAK. Rather, we performed a FTAK procedure to reduce astigmatism so that the accuracy of astigmatic correction with SMILE could be improved. In our study, the mean safety index was 1.02 ± 0.10 after FTAK, which was lower than the safety indices of 1.59, 1.29 reported by Loriaut et al29 and Fadlallah et al30, respectively. We suggest that this is because these two previous studies performed AK on post-keratoplasty eyes whereas virgin eyes were used in our study, and preoperative CDVAs were 0.5 ± 0.3 and 0.51 ± 0.26 logMAR in Loriaut et al29 and Fadlallah et al30’s studies, respectively, which was lower compared to -0.03 ± 0.07 logMAR in our preoperative CDVA. Even though we used manual incision, full-thickness incision with a uniform keratome was used to avoid inaccuracies in incision depth and length, and we achieved satisfactory results in astigmatism reduction with FTAK, with a SIA of 3.38 ± 1.18 D at 1 month postoperatively. Importantly, we did not note any complications such as wound leak, endophthalmitis, or retinal detachment after FTAK during the follow-up period.
There may be some concerns about refraction change after FTAK as a result from corneal healing.26,30 Some authors reported postoperative increase in astigmatism after partial thickness AK on post-keratoplasty eyes, but we performed full-thickness incision, which is preventive of astigmatic regression, on eyes with naturally-occurring astigmatism. Furthermore, there was no significant change in postoperative astigmatism 1 month after FTAK in our previous studies about FTAK.7,32 All eyes, in our study, had a stable refraction at 4 weeks after FTAK, so all SMILE procedures were performed within 4 ~ 6 weeks after FTAK. Finally, inaccuracy in astigmatic correction of astigmatic keratotomy may also be a big concern. However, in our study, because residual astigmatism was subsequently corrected with SMILE, any potential inaccuracy did not pose any problems.
In this study, at 1 month after FTAK, SE was not changed compared to preoperative SE (p = 0.357). And the mean DV of FTAK was 2.27 ± 0.97 D, which may also be considered as the TIA for SMILE. The amount of astigmatic correction by SMILE was comparable with the results of other studies.4–6 In the current study, an undercorrection of 10.3% was observed after SMILE, and there was a tendency towards increased undercorrection with a high level of TIA. We suggest that this is because the astigmatism range was 0.25 to 4.25 after FTAK, which was comparable to previous studies, and FTAK was done on the peripheral corneal zone, thereby saving the optic zone.
Two limitations of this study was the lack of evaluation of the causes underlying astigmatic undercorrection and its retrospective design. A prospective study examining the relationship between decentration and the amount of astigmatic undercorrection or a comparative study evaluating the efficacy of SMILE alone versus SMILE combined with FTAK in eyes with 4.0 ~ 5.0 D are thus warranted to corroborate our findings.