Residual refractive error is one of the complications in patients with a history of successful PK. Several surgical and non-surgical methods have been used to manage this complication, which either did not have an acceptable effectiveness or were associated with serious consequences. According to the literature, there is no standard method for treatment of post-keratoplasty refractive error (10). On the other hand, SMILE, as a safe, efficacious, predictable, and stable method for correction of myopic and astigmatic error (14–16), has raised hopes for finding a new method for correction of high residual refractive error after keratoplasty. To the best of our knowledge, this is the first case series with a mid-term follow-up investigating the efficacy and safety of SMILE for correction of post-DALK refractive error in patients with moderate myopia (mean: 4.7 D) and high astigmatism (mean: 5.6 D). Although the present study had a small sample size and a single-arm design without a control group, the results provide valuable information regarding the application of this procedure for correction of post-DALK residual refractive error. Massoud et al (10) reported that the short-term (6-month) results of this procedure were acceptable in patients with a history of PK that had high myopia (mean: 6.8 D) and moderate astigmatism (mean: 3.1 D).
According to the two-year results, SMILE was a safe procedure for correction of post-DALK residual refractive error. The mean CDVA improved from 0.23 LogMAR to 0.13 LogMAR indicating an acceptable safety index (1.53). In the study by Massoud et al (10), CDVA reduced from 0.73 LogMAR to 0.82 LogMAR 6 months after the procedure, safety index was 1.12, but none of the cases experienced a reduction in CDVA. However, in the present study, CDVA reduced in one of the patients in the first month, which improved gradually from the 3rd month to the 24th month. Considering the trend of improvement after the third month, it is expected that patient gains the preoperative or even a better visual acuity in the next follow-ups. A study that analyzed the results of post-PK refractive surgery procedures found a 0.1 LogMAR loss of CDVA after PRK (from 0.45 to 0.36) and a 0.04 LogMAR loss of CDVA after LASIK (from 0.39 to 0.35) (17). Since the prevalence of CDVA loss ≥ 1 line is 1.04% after SMILE and 0.51% after LASIK (18) in normal subjects with no history of PK, it seems that our results are acceptable compared to other corrective methods in subjects with a history of PK as well as normal individuals.
The results showed an efficacy of about 0.7. According to Fig. 1, postoperative UDVA was one line better than the preoperative CDVA in one of the cases (patient number 3), and postoperative UDVA reached preoperative CDVA despite stromal rejection in another case (patient number 5). In three eyes, postoperative UDVA was one line less than preoperative CDVA. Attention should be paid to the precision of the examiner and patient status due to the subjective nature of the test. In the remaining five cases, differences of 2 to 4 lines were observed. In these cases, although the postoperative UDVA did not reached to preoperative CDVA, it improved 1 to 4 lines. Massoud et al (10) reported a 6-month efficacy of 0.93. The difference might be due to the shorter follow-up duration. In this regard, Fadlallah et al (19) found that the efficacy of femto arcuate keratotomy for correction of post-PK residual refraction was 0.81 after one year and 0.67 after two years. An efficacy of 1.04 has been reported in normal subjects with moderate to high myopic astigmatism. It seems that the efficacy of this procedure is lower in post-PK cases compared to normal subjects.
In the present study, SE and spherical error decreased by almost 2.0 and 1.2 D after two years, respectively. Massoud et al (10) reported a mean reduction of 4.5 and 5.4 D in SE and spherical error after six months, respectively. There were no cases of over-correction in our study; however, considering the over-correction cases in the above study, they could have reached different results in a longer follow-up. Kovoor et al (17) found a spherical error reduction of 3.6 D after PRK. This decrease was 1.94D in a study by Shen (20).
One reason for the lower spherical correction in our study could be the lower preoperative sphere. The mean preoperative sphere was 1.9 D in our study, 5.3 D in the study by Massoud et al (10), 5.1 D in the study by Kovoor et al (17), and 2.6 D in the study by Shen et al (20).
Two years after SMILE, astigmatism had an arithmetic mean reduction of 0.75 to10.0 D in eight cases and remained unchanged in two cases (0.25 D). According to the results of vector analysis, in our study SIA, TIA, and DV was 2.7 D, 5.6 D, and 2.9 D, respectively. These values were reported 2.1 D, 2.6 D, and 1.1 D by Massoud et al (10). The IoS was 0.6 in the present study, indicating that SMILE corrected up to 60.0% of post-PK astigmatism. Regardless of the follow-up differences between the two studies, the mean error of astigmatic correction increases in the presence of high astigmatism, and this direct correlation is confirmed in normal subjects (21). Studies have shown that in normal corneas, the probability of residual astigmatism increases by up to 16% for each one-diopter increase in astigmatism (14). The difference in axis orientation of TIA, SIA, and DV indices indicated that axis rotation of astigmatism is one of the reasons for under-correction. In addition, according to the available nomogram, astigmatic correction in patients with no history of keratoplasty requires 10% adjustment (19, 21). This is while this nomogram is related to cases with no history of keratoplasty and it seems that it requires re-adjustment for refractive error correction in patients with history of keratoplasty. In addition, lack of automated cyclotorsion control and centration in Visumax could be other reason for under-correction in SMILE compared to other methods (22).
HOAs, except trefoil, showed a non-significant increase in the current study. HOA changes had a similar trend to normal corneas undergoing SMILE (9). Two points should be kept in mind. The first one is the contribution of these changes to the quality of vision and visual function, since visual performance is affected by adaptation, scatter, and neural adaptation (23). The satisfaction of the patients with the surgical outcome indicated that the increase in aberrations did not reduce the quality of vision or the reduction in the quality of vision was not significant compared to the increase in visual acuity. The second point is the repeatability of aberration measurements in grafted corneas. To the best of our knowledge, no study has evaluated the repeatability of these indices (measured by the Sirius) in subjects undergoing DALK. A study reported that a within-subject standard deviation of 0.01 to 0.05 µmin these indices in patients with keratoconus (24). However, post-PK corneal irregularities reduce the repeatability of the measurements. If these data were available for grafted corneas, comparison of the two-year changes of these indices with their measurement error could improve our conclusion.
Finally, according to the two-year results, SMILE is a safe, effective, predictable, and stable method for reducing post-DALK refractive error. Under-correction of astigmatism may be due to axis rotation during surgery. Refinement of SMILE treatment nomogram for post-DALK cases seems necessary. This procedure can facilitate the use of spectacle or even contact lenses through improving vision and reducing refraction. However, considering the possibility of graft rejection, longer durations of corticosteroid therapy and more intensive monitoring are required in the post-operative period.