Abstract: Objective To observe and describe the distribution characteristics of angle Alpha and angle Kappa in adults with normal axial length, as well as their correlations with age and ocular biological parameters.Design Retrospective study. Participants A total of 9,272 cases (18,544 eyes), with an age range of 6~95 years (33.27±25.70 years), who underwent Lenstar 900 examination at Hefei Aier Eye Hospital from January 2018 to December 2022 were included. All participants had an axial length of 21~26 mm and no history of refractive surgery, contact lens wear, ocular trauma, or other ocular surgeries. Methods Participants were divided into 10 groups by age (≤10 group, >10~20 group, >20~30 group, >30~40 group, >40~50 group, >50~60 group, >60~70 group, >70~80 group, >80~90 group, >90 group). Each eye was measured consecutively 3 times using the Lenstar 900, and the average value was taken for analysis. The Lenstar 900 expresses the angle Kappa as the distance between the pupil center and the corneal vertex. The angle Alpha is represented by the distance between the corneal center and the corneal vertex. The Kolmogorov-Smirnov test was used to verify the data distribution; non-normally distributed data were expressed as median (P25, P75). The Wilcoxon signed-rank test was applied to compare differences in angle Alpha and angle Kappa between the left and right eyes, as well as between genders. Spearman correlation analysis was used to explore the relationships between angle Alpha/angle Kappa and ocular parameters. A linear mixed-effects model was used to control the correlation between the two eyes of the same subject, and nonlinear regression was used to fit the age-related change curves of angle Alpha and angle Kappa. Main Outcome Measures The distribution of angle Alpha and angle Kappa, and their correlations with age and ocular biological parameters. Results Both angle Alpha and angle Kappa showed a non-normal skewed distribution. For angle Alpha, 71.70% of eyes had a value <0.5 mm, and 93.97% had a value <0.8 mm. For angle Kappa, 94.86% of eyes had a value <0.5 mm, and 99.39% had a value <0.8 mm. Angle Alpha in both eyes was mostly distributed in the superotemporal quadrant, with the largest value in the superotemporal quadrant; angle Kappa in both eyes was mostly distributed in the inferotemporal quadrant with the largest value in the inferotemporal quadrant. The median angle Alpha was 0.42 (0.26, 0.53) mm for the right eye and 0.41 (0.26, 0.52) mm for the left eye (P=0.004); it was 0.41 (0.26, 0.53) mm in males and 0.40 (0.25, 0.52) mm in females (P=0.001). The median angle Kappa was 0.24 (0.14, 0.31) mm for the right eye and 0.23 (0.13, 0.30) mm for the left eye (P＜0.001); it was 0.23 (0.13, 0.30) mm in males and 0.24 (0.13, 0.31) mm in females (P=0.825). There was a moderate positive correlation between angle Alpha and angle Kappa (r=0.352, P<0.001). Neither angle Alpha nor angle Kappa showed a significant linear correlation with age; instead, they exhibited a non-linear relationship of first decreasing and then increasing. The regression equations were as follows: angle Alpha=0.5076-0.0101Age+0.0001Age2 and angle Kappa=0.2139-0.0014Age+0.00003Age2. The smallest values of both angles were observed in the 20~30 years age group. Additionally, angle Alpha and angle Kappa were positively correlated with lens thickness (r=0.166， P＜0.001； r=0.215， P＜0.001), negatively correlated with anterior chamber depth (r=-0.223， P＜0.001； r=-0.234， P＜0.001), axial length (r=-0.295， P＜0.001； r=-0.162， P＜0.001), and white-to-white distance (r=-0.079， P＜0.001； r=-0.119， P＜0.001), and showed no correlation with central corneal thickness and corneal curvature radius. Conclusion Alpha and Kappa angles are primarily distributed on the temporal side of the visual axis, both exhibiting a trend of initially decreasing and then increasing with age. Alpha and Kappa angles measured by Lenstar 900 both show non-normal distribution, Alpha angles are greater individual variation compared with Kappa angles. Alpha and Kappa angles exhibit significant correlations with the ocular structural parameters.

Key words: Alpha angle, Kappa angle, Anterior chamber depth, Axial length, Lenstar 900