The results of this study suggest that FLACS for patients with a previous history of CRS was more effective and safe than conventional PCS in terms of vision, refractive outcomes, and safety outcomes at 3 months’ follow-up. Postoperative ORA was significantly lower and the reduction of refractive astigmatism and ORA was significantly higher in FLACS than in conventional PCS.
Because the target IOL (D) had been set not only for far vision or emmetropia but also for near vision, uncorrected visual acuity (UCVA), BCVA, and prediction error were analyzed. UCVA, BCVA, and prediction error were not significantly different between FLACS and conventional PCS. The Barrett True-K formula is one of the most accurate IOL calculation formulas for patients with a previous history of CRS,17 and the prediction error of Barrett True-K formula was − 0.24, which was similar to those of our study (-0.30 in FLACS and − 0.25 in conventional PCS).
Postoperative ORA was lower, and reduction of refractive astigmatism and ORA were significantly higher in FLACS than in conventional PCS. A multicenter, randomized study showed that FLACS had no advantage in corneal astigmatism, as in our study,18 but it did not analyze the effect of FLACS on refractive astigmatism and ORA. ORA is composed of posterior corneal astigmatism, lens astigmatism, and retinal astigmatism.19 The preoperative and postoperative posterior corneal astigmatism was not significantly different in our study (0.80 ± 0.40 and 0.88 ± 0.45 in FLACS; 0.84 ± 0.49 and 0.88 ± 0.51 in conventional PCS; p = 0.653 and 1.000, respectively). In a previous report, ORA, which also included a part of retinal astigmatism, was found to be inversely correlated with axial length and positively correlated with SE and corneal astigmatism.20 In our study, axial length, SE, and both anterior and posterior corneal astigmatism, were not significantly different between the two groups. Therefore, the difference in ORA between the two groups may be explained by lens astigmatism due to lens tilt and decentration caused by the different capsulotomy methods between FLACS and conventional PCS.6, 7 Previous studies have reported that horizontal and vertical IOL tilt and decentration were significantly higher in manual capsulotomy and that the results showed a correlation with changes in refraction values between 1 month and 1 year after surgery. IOL tilt and decentration influence visual acuity, dysphotopsia, and coma-like aberrations.21–24 Moreover, CRS also induced aberrations in a previous study25: the root-mean-square wavefront error increased 1.9-fold in a 6.5 mm pupil and significantly in a 3.0 mm pupil, and positive spherical aberration was increased 4-fold after myopic LASIK. Oblate corneas that underwent myopic correction benefited from aspheric IOLs with negative spherical aberration which compensates the positive corneal spherical aberration,26 and aspheric IOLs are shown to produce more optical quality degradation if tilted or decentered.27 Considering the visual impacts of capsulotomy method and CRS, femtosecond laser may provide a higher quality of vision and have a greater impact on patients after CRS.
Trauma vulnerability is a concern in patients with a history of CRS.28, 29 The physical and thermal energy in the process of FLACS could damage the previous CRS-operated tissue.30–32 However, the complication rates between FLACS and conventional PCS for patients with a history of CRS were not different intraoperatively and postoperatively in our study. The overall intraoperative and postoperative complication rates were 2.8% and 12.5% in FLACS in a previous study,5 which was similar to that of our study. Specifically, posterior capsule tear rates were 0% for FLACS, as in the previous UK reports.5, 33 An intraoperative pupil contracture of 3.9% and incomplete laser capsulotomy of 5.9% were the challenges of FLACS.5, 34, 35 In the UK reports, 9.7% of the FLACS group and 8.2% of the PCS group experienced postoperative anterior uveitis.5 In this study, only 1 out of 204 participants developed postoperative anterior uveitis. The intense anti-inflammatory treatment for a month in our hospital may decrease the complication.
There are certain limitations in our study. First, the results of this study did not involve the patients' subjective symptoms. In a Chinese report, FLACS resulted in dry eye at postoperative day 1, week 1, and month 1.36 However, in the UK reports, the health-related quality of life and vision questionnaires did not show a significant difference between FLACS and conventional PCS.5, 37 After CRS, there was a possibility of visual disturbance and other subjective dissatisfaction,38 and the additional laser treatment could affect the subjective outcomes. Second, our study showed that FLACS had an advantage on ORA and that refractive astigmatism in FLACS was lower than that in conventional PCS. The influence of IOL-induced astigmatism on ORA and aberration was estimated and not directly measured, although posterior corneal astigmatism and the factors affecting retinal astigmatism were well-controlled and showed no differences. Further assessment of the patients’ subjective outcomes and analyses for ORA and aberration are required to investigate the effectiveness of FLACS after CRS.
In conclusion, at 3 months’ follow-up, FLACS was effective in terms of vision and refractive outcomes and was free from adverse effects. The competitive edge of FLACS in postoperative ORA, with reduction of refractive astigmatism and ORA, may provide better visual quality than conventional PCS. Considering the results and other previously known advantages, FLACS is effective and safe in patients with a previous history of CRS.