Although angle kappa compensation combined with personalized femtosecond laser combined with excimer laser in situ keratomileusis has a good theoretical basis, there is still a significant gap between the actual and ideal visual quality.Factors influencing angle kappa include axial length, ACD and corneal curvature radius,but the proportion of each factor has not been confirmed[11, 12].Moreover, the size and central position of the pupil in corneal refractive surgery cannot be ignored.In order to ensure the accuracy of surgical positioning, it is necessary to ensure the matching with the preoperative pupil size and position, iris texture recognition, corneal limbus recognition[13, 14].
At present, the target of non-interference pupil center-corneal reflex line-of-sight tracking technology based on image processing is the pupil center of the operated eye, and the direction of the visual axis is estimated by calculating the vector between the pupil center and corneal reflex.The laser cutting area is mostly centered on the coaxially sighted corneal light reflex point. If the positions of the two are different, the locked pupil center needs to be adjusted accordingly. The centrifugation used is a fixed value, but the ideal centrifugation should also change with the dynamic changes of the pupil.Therefore, according to the pupil size, center position and dynamic change of angle kappa of the patients who are suitable for excimer laser surgery, obtained the individualized pupil center displacement curve. During the operation, the pupil dynamics were monitored to adjust the angle kappa and calibrate the ablation centration in real time to ensure that each scanning-spot excimer lase was in the correct position.
In this study, the average shift distribution between the center of the cornea and the center of the pupil was 0.322 ± 0.194 mm (range: 0.005 to 0.956 mm), and 64% of the eyes were ≤ 0.40 mm. We also measured P-Dist during the operation, and showed that the average P-Dist = 0.225 mm, the minimum 0.010 mm and the maximum 0.580 mm, 80% of the eyes is ≤ 0.30 mm, and the coaxially sighted corneal light reflex point is mainly biased which is Upper temporal side of the pupil center (34%).The angle kappa is not a fixed value,which will change under different conditions.Influenced by factors such as intraoperative illumination, surgical stimulation, eye type, emotional tension, and accommodation convergence caused by close gaze indicator lights, the dynamic changes of pupil size and central position can be caused[20, 21].Theoretically, when the size and the center of pupil continuously change during surgery, the angle kappa also changes significantly[7, 22].Therefore, we speculate that there is difference in diopter and visual quality after different angle kappa compensation percentage.
The results of this study indicate that the positions of the pupils’ center of the left and right eyes were significantly different in low light conditions. The X axis of the left eye is -0.046 ± 0.091 mm, and the X axis of the right eye is -0.152 ± 0.084 mm. The right eye deviated from the left eye to the temporal side by 0.106 ± 0.095 mm under dark light.The Measured Centroid Shift of the left eye is 0.127 ± 0.103 mm, and that of the right eye is 0.176 ± 0.139 mm, with statistical difference.We speculate that the majority of the population is right-handed, and the head deviates to the right side when working in close range. Due to the inconsistent gaze target distance, the left eye needs more accommodation force to achieve the same state as the right eye (the right eye is closer), causing smaller pupils, shallower anterior chambers and larger angle kappa relative to the right eye.
The changes in the pupil center positions of the left and right eyes guide us the left eye should adjust a larger proportion of angle kappa when positioning in excimer laser surgery. Because the angle kappa offset of the left eye is greater than that of the right eye.If positioned at the center of the cornea, the right eye can appropriately adjust the ablation centration at a position slightly within 0.152 mm from the corneal center to the temporal side to find the ablation centration point which is most closest to the visual axis.To avoid the reduction of postoperative retinal imaging quality caused by unnecessary eccentric ablation[24, 25].
Our results also show a positive correlation between lateral WTW and pupil diameter change. The related results have not been reported at home and abroad.This conclusion guides us to treat patients with large cornea during the operation to ruduce the dynamic changes in pupil size whit a darker environment as much as possible in order to improve the efficiency of pupil matching.There is no correlation between the SE and the change of pupil diameter, but there is a negative correlation between the SE and Measured Centroid Shift.It has been reported in the literature that the refractive state has a significant correlation with the angle kappa, which is consistent with the results of this study.Therefore, We speculate that the dynamic change of pupil center of Measured Centroid Shift is small for people with high myopia.Therefore, it can be understood that the dynamic change of angle kappa is correspondingly reduced, and the efficiency of pupil matching is higher. It is suggested that the proportion of compensating angle kappa vector is higher during operation.
This study firstly explored the offset distribution between the pupil center and the corneal co-reflective point, and its correlation with other biomechanical factors such as the eyeball.It further confirms the importance of accurately locating the position of the ablation centration and the necessity to reasonably adjust the angle kappa vector ratio under different myopia degrees and eye type.This study also has limitations,the insufficient samples and the individual variability may affect the results.The relationship between angle kappa compensation and visual quality needs to be further studied. The digital relationship between various individualized cutting modes still needs further exploration.