A total of 56 subjects were enrolled in this study, including 26 (46.4%) men and 30 (53.6%) women. Their mean age was 24.7 ± 1.8 years old, all subjects were Chinese, with a right eye stereoscopic equivalent of -2.05 ± 0.46 D.
Sagittal heights at chord lengths of 10.0, 12.3, and 15.0 mm
The sagittal heights at 10.0 mm chord were 1734 ± 71 and 1779 ± 67 µm for the horizontal and vertical meridians, respectively, 2664 ± 122 and 2653 ± 96 µm at 12.3 mm chord, and 3680 ± 168 and 3672 ± 141 µm at 15.0 mm chord, respectively. The differences between two meridians were not statistically significant (P = 0.222, 0.382, and 0.632 respectively, Table 1).
Table 1
Comparison of sagittal heights at different segments and chord lengths (n = 56)
Chord length (mm) | Nasal-temporal (µm) | Superior-inferior (µm) | Mean value (µm) | Difference value (µm) | t valuea | P value | Absolute value of difference (µm) |
10.0 | 1734 ± 71 | 1779 ± 67 | 1756 ± 72 | -45 ± 56 | 1.05 | 0.222 | 54 ± 40 |
12.3 | 2664 ± 122 | 2653 ± 96 | 2658 ± 110 | -11 ± 100 | 0.88 | 0.382 | 70 ± 67 |
15.0 | 3680 ± 168 | 3672 ± 141 | 3676 ± 155 | 8 ± 100 | 0.42 | 0.632 | 117 ± 95 |
a Independent-samples t-test |
The absolute values of the difference between horizontal and vertical meridians were 54 ± 40, 70 ± 67, and 117 ± 95 µm at 10.0, 12.3, and 15.0 mm chord, respectively. Besides, 57.1% (32) of subjects had an absolute value of less than 50 µm at 10.0 mm chord, and 50.0% at 12.3 mm chord. This percentage decreased to 19.6% at 15.0 mm chord, as shown in Fig. 1.
CSJ angle
The CSJ angles from low to high were 176.53 ± 2.14° (nasal), 178.66 ± 1.84° (inferior), 179.13 ± 1.20° (temporal), and 179.31 ± 1.68° (superior), with statistically significant differences (F = 32.01, P < 0.001, Table 2).
Table 2
Comparison of measurements in different segments (n = 56)
Segment | Corneoscleral junction angle (°) | Corneal angle at 12.3 mm chord (°) | Scleral angle at 15.0 mm chord (°) | Difference of two angles (°) | t value | P value |
Nasal | 176.53 ± 2.14 | 40.31 ± 2.80 | 35.37 ± 3.10 | 4.94 ± 3.34 | -0.48 b | 0.020 |
Temporal | 179.13 ± 1.20 | 39.46 ± 2.41 | 38.35 ± 2.47 | 1.10 ± 3.43 | 1.45 b | 0.030 |
Superior | 179.31 ± 1.68 | 37.20 ± 2.26 | 38.26 ± 3.37 | -1.06 ± 3.42 | -3.47 b | 0.020 |
Inferior | 178.66 ± 1.84 | 37.50 ± 2.28 | 35.30 ± 4.71 | 2.20 ± 4.59 | -1.32 b | 0.020 |
F value | 32.01a | 21.31 a | 13.37 a | - | - | - |
P value | <0.001 | <0.001 | <0.001 | - | - | - |
a One-way analysis of variance test; b Independent-samples t-test |
The differences were − 2.60 ± 0.32°, -2.78 ± 0.32°, and − 2.13 ± 0.32° on the nasal side compared with the temporal, superior, and inferior sides, respectively, with statistically significant differences (all P < 0.001). There were no statistically significant differences in the temporal, inferior, and superior sides of pairwise comparisons among the three groups (all P > 0.05) (Table 3).
Table 3
Pairwise comparison of different parameters among four segments (n = 56)
Segment | Corneoscleral junction angle | Corneal angle at 12.3 mm chord | Scleral angle at 15.0 mm chord |
| Difference value (°) | P value | Difference value (°) | P value | Difference value (°) | P value |
Nasal-Temporal | -2.60 ± 0.32 | <0.001 | 0.85 ± 0.46 | 0.007 | -2.98 ± 0.66 | <0.001 |
Nasal - Superior | -2.78 ± 0.32 | <0.001 | 3.11 ± 0.46 | <0.001 | -2.89 ± 0.66 | <0.001 |
Nasal- Inferior | -2.13 ± 0.32 | <0.001 | 2.81 ± 0.46 | <0.001 | 0.07 ± 0.66 | 0.922 |
Temporal- Superior | -0.18 ± 0.32 | 0.572 | 2.26 ± 0.46 | <0.001 | 0.10 ± 0.66 | 0.892 |
Temporal - Inferior | 0.47 ± 0.32 | 0.142 | 1.96 ± 0.32 | <0.001 | 3.05 ± 0.66 | <0.001 |
Superior- Inferior | 0.65 ± 0.32 | 0.042 | -0.30 ± 0.46 | 0.532 | 2.95 ± 0.66 | <0.001 |
Least-significant difference test |
The peak value of nasal CSJ angle was < 179°, and the peak values of the temporal, inferior, and superior CSJ angles all occurred at the range of 179–181° (Fig. 2).
Moreover, 87.5% of CSJ angles on the nasal side were < 179°, and the temporal, inferior, and superior CSJ angles were predominantly 179–181° (57.1%, 55.4%, and 57.1%, respectively).
Corneal and scleral angles
The corneal angles of 12.3 mm chord from high to low were 40.31 ± 2.80° (nasal), 39.46 ± 2.41° (temporal), 37.20 ± 2.26° (inferior), and 37.50 ± 2.28° (superior), with statistically significant differences (F = 21.31, P < 0.001, Table 2). The differences were 0.85 ± 0.46°, 3.11 ± 0.46°, and 2.81 ± 0.46° on the nasal side compared with the temporal, superior, and inferior sides, respectively, with statistically statistically differences (P = 0.007, < 0.001, and < 0.001, respectively). The differences were 2.26 ± 0.46° and 1.96 ± 0.32 on the temporal side compared with the superior and inferior sides, respectively, which were statistically significant (both P < 0.001). No statistically significant difference was found between the superior and inferior sides (P = 0.532) (Table 3).
The scleral angles of 15.0 mm chord l from high to low were 38.35 ± 2.47° (temporal), 38.26 ± 3.37° (superior), 35.37 ± 3.10° (nasal), and 35.30 ± 4.71° (inferior), with statistically significant differences (F = 13.37, P < 0.001, Table 2). The differences were 2.89 ± 0.66° and 2.95 ± 0.66° on the superior side compared with the nasal and inferior sides, respectively, which were statistically significant (both P < 0.001). The differences were 2.98 ± 0.66° and 3.05 ± 0.66° on the temporal side compared with the nasal and inferior sides, respectively, which were statistically significant (both P < 0.001). No statistically significant differences were found between the superior and temporal sides and the nasal and inferior sides (P = 0.892 and 0.922, respectively), as shown in Table 3.
Comparison between corneal angle and scleral angle
With comparing the changes between the corneal angle at 12.3 mm chord and the scleral angle at 15.0 mm chord, the overall morphology of the ocular surface varying from the corneal limbus to the anterior sclera could be obtained. The corneal angle was lower than the scleral angle on the superior side (difference value was − 1.06 ± 3.42°), with a statistically significant difference (P = 0.020), and the corneal angles were higher on the nasal, inferior, and temporal sides (the difference values were 4.94 ± 3.34°, 2.20 ± 4.59°, and 1.10 ± 3.43°, respectively), with statistically significant differences (P = 0.020, 0.002, and 0.020, respectively), with the largest difference on the nasal side (Table 2, Fig. 3).