General Results

All patients underwent successful surgery without intraoperative or postoperative complications. All patients achieved visual acuity of 0.8 and above at 1 week and more than 6 months after surgery. The diopters were all within ± 1 D, and 95.2% (532 eyes) were within ± 0.5 D. No patient had refractive regression > 0.5 D. No patient had corneal ectasia or secondary keratoconus during follow-up.

Height of the PCS

At the point of corneal vertex, the height of the PCS was 1.200 ± 2.629 µm before the operation, 0.224 ± 3.242 µm 1 day after the operation, 0.209 ± 2.773 µm 1 week after the operation, and 0.163 ± 2.681 µm 1 month after the operation. There were significant differences when comparing the height of the PCS at 1 day, 1 week, and 1 month after the operation with the height before the operation (F = 52.01, P < 0.0001). The apex of the posterior surface of the cornea gradually moved backward at 1 day, 1 week and 1 month after the operation. There was no significant difference when comparing the PCS height at 1 week with 1 day after surgery (P = 0.999), 1 month with 1 day after surgery (P = 0.948), and 1 month with 1 week after surgery (P = 0.928). (Fig. 1)

At the 2 mm circle, the height of the PCS was 1.623 ± 1.607 µm before the operation. 0.908 ± 2.033 µm 1 day after the operation, 1.086 ± 1.664 µm 1 week after the operation, and 1 month after the operation was 1.063 ± 1.625 µm. There were significant differences when comparing the height of the PCS at 1 day, 1 week, and 1 month after the operation with the height before the operation (F = 44.63, P < 0.0001). One day after the operation, the posterior surface of the 2 mm circle moved backward and then moved forward to approach the preoperative height. There was no significant difference when comparing the PCS height at 1 week with 1 day after surgery (P = 0.070), 1 month with 1 day after surgery (P = 0.168), and 1 month with 1 week after surgery (P = 0.967). (Fig. 1)

At the 4 mm circle, the height of the PCS was − 1.733 ± 1.389 µm before the operation. -1.177 ± 1.730 µm 1 day after operation, -1.278 ± 1.423 µm 1 week after operation, and 1 month after operation was − 1.321 ± 1.379 µm. There were significant differences when comparing the height of the PCS at 1 day, 1 week, and 1 month after the operation with the height before the operation (F = 32.15, P < 0.0001). One day after the operation, the posterior surface of the 4 mm circle moved forward compared with the preoperative value and then moved backward to approach the preoperative height. There was no significant difference when comparing the PCS height at 1 week with 1 day after surgery (P = 0.532), 1 month with 1 day after surgery (P = 0.151), and 1 month with 1 week after surgery (P = 0.830). (Fig. 1)

At the 6 mm circle, the height of the PCS was − 6.085 ± 2.714 µm before the operation, -4.885 ± 4.246 µm 1 day after the operation, -4.983 ± 2.751 µm 1 week after the operation, and − 5.093 ± 2.568 µm 1 month after the operation. There were significant differences when comparing the height of the PCS at 1 day, 1 week, and 1 month after the operation with the height before the operation (F = 30.27, P < 0.0001). One day after the operation, the posterior surface of the 4 mm circle moved forward compared with the preoperative value and then moved backward to approach the preoperative value. There was no significant difference when comparing the PCS height at 1 week with 1 day after surgery (P = 0.950), 1 month with 1 day after surgery (P = 0.607), and 1 month with 1 week after surgery (P = 0.681). (Fig. 1)

Analysis of the influencing factors of the height change of the PCS

The difference between the corneal posterior surface height of the corneal vertex at pre-operation and 1 day, 1 week, and 1 month after the operation was calculated. Analyze the correlation between the change and possible factors. (Table 1)

The change in the height of the posterior surface of the corneal apex was correlated with the preoperative K2 value (r = 0.124, P = 0.003) and the preoperative Km value (r = 0.090, P = 0.033) at 1 day after the operation.

The change in the height of the posterior surface of the corneal apex was correlated with the preoperative spherical equivalent power (r = 0.101, P = 0.017), lenticule thickness (r= -0.112, P = 0.008), corneal cap thickness/corneal thickness (r = 0.086, P = 0.043), lenticule thickness/corneal thickness (r= -0.106, P = 0.012), preoperative K1 value (r = 0.124, P = 0.003), preoperative K2 value (r = 0.136, P = 0.001) and preoperative Km value (r = 0.119, P = 0.005) at 1 week after the operation.

The change in the height of the posterior surface of the corneal apex was correlated with the preoperative K1 (r = 0.148, P = 0.000 4), K2 (r = 0.141, P = 0.001), and Km value (r = 0.142, P = 0.001) at 1 month after the operation.

The difference between the posterior surface height of the corneal 2 mm circle at pre-operation and 1 day, 1 week, and 1 month after the operation was calculated. Analyze the correlation between the change and possible factors. (Table 2)

The change in the posterior surface height in the 2 mm circle was correlated with the preoperative K1 (r = 0.087, P = 0.040), K2 (r = 0.137, P = 0.001), and Km values (r = 0.001) 0.103, P = 0.015) at 1 day after the operation.

The change in the posterior surface height of the corneal 2 mm circle was correlated with the preoperative K1 (r = 0.103, P = 0.015), K2 (r = 0.123, P = 0.004) and Km values (r = 0.123, P = 0.004) 0.104, P = 0.014) at 1 week after the operation.

The change in the posterior surface height of the corneal 2 mm circle was correlated with the preoperative K1 (r = 0.107, P = 0.011), K2 (r = 0.109, P = 0.010), and Km values (r = 0.109, P = 0.010) 0.109, P = 0.010) at 1 month after the operation.

The difference between the posterior surface height of the corneal 4 mm circle at pre-operation and 1 day, 1 week, and 1 month after the operation was calculated. Analyze the correlation between the change and possible factors. (Table 3)

The change in the posterior surface height of the corneal 4 mm circle was correlated with lenticule thickness (r = 0.088, P = 0.038) and corneal cap thickness/corneal thickness (r= -0.084, P = 0.047) at 1 day after the operation.

The change in the posterior surface height of the corneal 4 mm circle was correlated with the preoperative K2 value (r= -0.099, P = 0.020) at 1 week after the operation.

The difference between the posterior surface height of the corneal 6 mm circle at pre-operation and 1 day, 1 week, and 1 month after the operation was calculated. Analyze the correlation between the change and possible factors. (Table 4)

The change in the posterior surface height of the corneal 6 mm circle was correlated with the preoperative K2 value (r= -0.090, P = 0.033) at 1 day after the operation.

The change in the posterior surface height of the corneal 6 mm circle was correlated with the preoperative K1 (r= -0.124, P = 0.003), K2 (r= -0.187, P < 0.0001), and Km values (r = -0.153, P = 0.000 3) 1 week after the operation.

The change in the posterior surface height of the corneal 6 mm circle was correlated with the corneal thickness (r = 0.085, P = 0.044), preoperative K2 (r= -0.095, P = 0.025), and Km value (r= -0.090, P = 0.033) at 1 month after the operation.