Basic information of patients
Table 1 summarizes the basic information of all patients included in this study. SMILE surgery was successful for correcting myopia and myopia astigmatism in all eyes, and all surgeries were completed without intraoperative or postoperative complications.
Table 1 Preoperative demographics of patients with astigmatism
Characteristic
|
Value
|
No. of patients
|
37
|
No. of eyes
|
55
|
Sex (M/F)
|
15/22
|
Age (y)
|
22
(range, 17–40)
|
Manifest spherical equivalent (D)
|
-6.48 ± 1.46
(range from -3.63 to -9.88)
|
Manifest refractive cylinder (D)
|
-2.41±0.54
(range from -2.00 to -4.50)
|
UCVA (logMAR)
|
1.10
|
BCVA (logMAR)
|
-0.18
|
Intraocular pressure (mmHg)
|
15
|
D Diopter, UCVA Uncorrected visual acuity, BCVA Best corrected visual acuity
Effectiveness
Following SMILE, uncorrected visual acuity (UCVA) improvement occurred postoperatively in all enrolled patients (Table 2). There were significant differences in logMAR (logarithm of the minimum angle of resolution) UCVA between the 1-month and 3-month follow-up visits (P < 0.001). There were no significant differences in UCVA (P = 0.884), best corrected visual acuity (BCVA, P = 0.516), and the efficacy index (P = 0.690). Figure 2 shows the cumulative percentage of eyes that achieved definite cumulative levels of UCVA at 2 days and 1 and 3 months post-surgery. The median efficacy index at 2 days and 1 and 3 months post-surgery was 0.67, 0.80, and 0.83, respectively.
Table 2 Changes in visual acuity and diopter before and after SMILE surgery
Parameter
|
Preoperative
|
Postoperative
|
|
|
2 days
|
1 month
|
3 months
|
UCVA
|
|
|
|
|
logMAR
|
1.10
|
0
|
-0.08
|
-0.08
|
≥20/25
|
0%
|
83.64%
|
98.18%
|
100%
|
≥20/20
|
0%
|
52.73%
|
83.64%
|
87.27%
|
|
|
|
|
|
BCVA
|
|
|
|
|
logMAR
|
-0.18
|
|
-0.18
|
-0.18
|
≥20/25
|
100%
|
|
100%
|
100%
|
≥20/20
|
100%
|
|
96.36%
|
98.18%
|
|
|
|
|
|
SE(D)
|
-6.48±1.46
|
|
-0.25
|
-0.25
|
|
|
|
|
|
Cylinder(D)
|
-2.41±0.54
|
|
0
|
0
|
D Diopter, UCVA Uncorrected distance visual acuity, BCVA Best corrected distance visual acuity
Safety outcomes
The Wilcoxon test showed that the median safety index was 1 at 1 and 3 months post-surgery, and there was no significant difference between the two follow-up visits (P = 0.401). Figure 2 shows the UCVA at the 1-month and 3-month follow-ups as compared with the preoperative BCVA in cumulative eyes and change in lines of BCVA.
Predictability
The mean spherical equivalent (SE) was -0.14 ± 0.35 D (range, -0.75 D to +0.50 D) and -0.15 ± 0.36 D (range, -0.75 D to +0.50 D) at 1 and 3 months post-surgery, respectively, while the percentage of eyes with postoperative SE within ±0.5 D and ±1.0 D were both 87.27% and 100% at 1 and 3 months (Fig. 3a).
The mean cylinder was -0.15 ± 0.33 D (range, -1.00 D to +0.50 D) and -0.14 ± 0.31 D (range, -1.00 D to +0.75 D) at 1 and 3 months postoperatively, respectively; the percentage of eyes with postoperative cylinder within ±0.5 D and ±1.0 D were both 89.09% and 100% at 1 and 3 months (Fig. 3b). There was a significant statistical association between |TIA| and |SIA| at 1 month and 3 months post-surgery (r = 0.947, Fig. 4a and 0.914, Fig. 4b, respectively).
The vector method
The double-angle plots demonstrate the TIA, SIA, and DV at the 3-month follow-up of 55 eyes (Fig. 5). The arithmetic mean TIA was 2.00–4.50 D in the small-incision lenticule. The TIA centroid coordinates were (x: 2.10 ± 0.99, y: -0.26 ± 0.81), which indicated that the average astigmatism was with-the-rule before surgery. Postoperatively, the centroid coordinates of DV were (x: 0.08 ± 0.28, y: 0.07 ± 0.19) at 1 month, and (x: 0.09 ± 0.23, y: 0.01 ± 0.24) at 3 months, and the mean astigmatism in vector form was -2.12 D × 7.06° preoperatively, -0.11 D × 41.19° at 1 month post-surgery, and -0.09 D × 6.34° at 3 months post-surgery. Table 3 shows the comparison of vector analysis between 1 and 3 months post-surgery.
Table 3 Comparison of vector analysis at 1 and 3 months after SMILE surgery
Parameter
|
Postoperative 1 month
|
Postoperative 3 months
|
P
|
SIA
|
2.35±0.51
|
2.36±0.57
|
0.819
|
DV
|
-0.15±0.33
|
0.14±0.31
|
0.919
|
CI
|
0.98±0.07
|
0.98±0.07
|
0.904
|
IOS
|
0.08±0.12
|
0.08±0.13
|
0.971
|
AofE
|
-1.16±2.89°
|
-0.08±3.56°
|
0.028*
|
|AofE|
|
1.29±2.83°
|
1.81±3.06°
|
0.278
|
MofE
|
-0.06±0.20
|
-0.05±0.19
|
0.819
|
FI
|
0.97±0.07
|
0.97±0.07
|
0.427
|
CI Correction index, FI Flattening index, IOS Index of success, MofE Magnitude of error, SIA Surgically induced astigmatism, TIA Target induced astigmatism
*P < 0.05 indicates significant difference
The Wilcoxon signed-rank test showed that there were no significant differences in the SIA (P = 0.819), DV (P = 0.919), CI (P = 0.904), IOS (P = 0.971), |AofE| (P = 0.278), MofE (P = 0.819), and FI (P = 0.427) between 1 month and 3 months post-surgery (Table 3).
The absolute AofE value deviated from the intended direction (Table 4). A positive value indicates a counterclockwise rotation from its intended axis, while a negative AofE value indicates a clockwise rotation. Table 4 shows that the AofE was significantly different between 1 and 3 months post-surgery: 45 eyes (81.82%) had |AofE| < 5°, while 10 eyes (18.18%) had |AofE| > 5° to ≤10°.
Table 4 Postoperative astigmatism at the 3-month follow-up of 55 eyes
Postoperative cylinder (D)
|
Absolute shift in axis (n)
|
|
≤5°
|
>5°to≤10°
|
>10°
|
total
|
0*
|
38
|
--
|
--
|
38
|
>0.00 to≤-0.50
|
6
|
5
|
--
|
11
|
>0.50 to≤-1.00
|
1
|
5
|
--
|
6
|
Total
|
45
|
10
|
--
|
55
|
Axis shift was determined from the postoperative to preoperative cylinder axis
*Shifts were determined as 0 for eyes with zero residual cylinder magnitude
At 3 months post-surgery, the residual cylinder was -0.14 ± 0.31 D (range, -1.00 to +0.75 D), the CI was 0.98 ± 0.07, the IOS was 0.08 ± 0.13, and the FI was 0.97 ± 0.07, which indicated slight undercorrection. Spearman correlation analysis at 3 months post-surgery showed a clear positive correlation between the |DV| and |AofE| (r = 0.737, P = 0.000, Fig. 6); a clear positive correlation was observed between the |MofE| and |AofE| (r = 0.694, P = 0.000, Fig. 7), which was the same as the relevance between the IOS and |AofE| (r = 0.699, P = 0.000, Fig. 8). Meanwhile, there was negative relevant relation between the FI and |AofE| (r = -0.725, P = 0.000, Fig. 9), so there was a tendency toward undercorrection as the AofE between the corrected astigmatism and the target corrected astigmatism increased. However, there were no significant correlations between the |DV|, |MofE|, IOS, FI, CI, and |TIA| at 1 and 3 months post-surgery (P > 0.05), which indicated that the outcome of astigmatic correction mainly depends on the AofE instead of preoperative astigmatism.
The influence of binocular differences and repeated measurements were corrected using generalized estimation equations, and the influencing factors of UCVA and the absolute DV value were analyzed at 3 months post-surgery. The inclusion factors were: sex, age, preoperative intraocular pressure, preoperative spherical diopter, preoperative cylindrical diopter, preoperative astigmatic axis, preoperative anterior corneal surface curvature Km, intraoperative corneal cap thickness and diameter, lens thickness and diameter, and residual stromal thickness. The main factor affecting the logMAR UCVA at 3 months post-surgery was the preoperative astigmatic axis (P < 0.05). The influencing factors of the absolute DV value at 3 months post-surgery were: preoperative spherical diopter, preoperative cylindrical diopter, intraoperative lens thickness, lens diameter, and preoperative anterior corneal surface Km (P < 0.05).
Corneal HOA before and after surgery
Table 5 shows the changes in the anterior corneal surface HOA before and after surgery. The t-HOA, spherical aberration, vertical coma aberration, and trefoil 30° increased significantly 3 months post-surgery compared with preoperative measurements (P < 0.05), while no significant difference was found for trefoil 0° and horizontal coma aberration before surgery or 1 and 3 months post-surgery (P > 0.05).
Table 5 HOA of the anterior corneal surfaces at 4 mm in diameter preoperatively and at 1 and 3 months postoperatively
Parameter
|
Preoperative
|
Postoperative
|
|
|
1 month
|
3 months
|
t-HOA RMS
|
0.11
|
0.23*
|
0.21*
|
Spherical aberration
|
0.03±0.03
|
-0.07±0.07*
|
-0.06±0.07*
|
Vertical coma
|
-0.01±0.07
|
-0.07±0.10*
|
-0.10±0.09 *
|
Horizontal coma
|
-0.01±0.05
|
-0.01±0.08
|
-0.00±0.09
|
Trefoil 0°
|
0.00±0.03
|
0.02±0.07
|
0.02±0.07
|
Trefoil 30°
|
-0.02±0.04
|
0.01±0.08**
|
0.03±0.09*
|
*P < 0.001, **P < 0.01
t-HOA RMS Root mean square of total higher-order aberration
P < 0.05 indicates significant difference
Generalized estimation equations were performed between the anterior corneal surface HOA at 3 months post-surgery and the refractive diopter before surgery and 3 months after the surgery. The inclusion factors were: absolute preoperative spherical diopter value, absolute preoperative cylindrical diopter value, absolute spherical diopter value 3 months post-surgery, and absolute cylindrical diopter value 3 months post-surgery. The t-HOA at 3 months post-surgery correlated positively with the absolute values of the preoperative spherical diopter and cylindrical power (P < 0.05). The absolute values of the spherical aberration and the vertical coma at 3 months post-surgery correlated positively only with the preoperative cylindrical diopter (P < 0.05). This showed the t-HOA, spherical aberrations, and vertical coma aberrations increased at 3 months after the surgery as the preoperative astigmatism increased.