Objective To study asphericity in the horizontal direction of the anterior corneal surface in high myopia. Methods This was a retrospective case series study. The corneal topographies of 38 cases (right eyes) with high myopia were collected from the Orbscan Ⅱ topography system and the data of the anterior corneal surface in the tangential map were filtered out. The Q-values of the 360 semi-meridians were calculated by linear regression using the tangential radius in corneal topography and the coordinate rotation technique. The data in the range of ±45° around the horizontal principal meridian were then studied. The Q-values of the semi-meridians in intervals of 15° from the principal meridian were selected (the semi-meridians of 315°, 330°, 345°, 0°, 15°,30° and 45° on the nasal side; 135°, 150°, 165°, 180°, 195°, 210°, 225° on the temporal side).The average Q-values of the quadrants of all 15 semi-meridians were calculated (the quadrants of 315°-329°, 330°-344°, 345°-359°, 1°-15°, 16°-30°, 31°-45° on the nasal side; 135°-149°,150°-164°, 165°-179°, 181°-195°, 196°-210°, 211°-225° on the temporal side). All the subjects were divided into two groups according to equivalent refractive power: high myopia group (-6.00-9.00 D,26 cases); ultra-high myopia group (over -9.00 D, 12 cases). Then an independent samples t test was used to compare the differences between the two groups. Results The Q-values of the selected semi-meridians in the horizontal direction: the nasal side, 315° was -0.17±0.05, 330° was -0.22±0.06, 345° was -0.30±0.09, 0° was -0.36±0.08, 15° was -0.37±0.10, 30° was -0.32±0.09, 45°was -0.26±0.10; the temporal side, 135° was -0.26±0.11, 150° was -0.34±0.12, 165° was -0.36±0.12, 180° was -0.38±0.12, 195° was -0.41±0.12, 210° was -0.40±0.10, 225° was -0.36±0.11.The average Q-values of the quadrants: the nasal side, 315°-329° was -0.19±0.05, 330°-344° was -0.26±0.08, 345°-359° was -0.33±0.09, 0° was -0.36±0.08, 1°-15° was -0.37±0.08, 16°-30° was -0.34±0.09, and 31°-45° was -0.30±0.10;the temporal side, 135°-149° was -0.30±0.11, 150°-164°was -0.35±0.11, 165°-179° was -0.37±0.12, 180° was -0.38±0.12, 181°-195° was -0.40±0.11,196°-210° was -0.41±0.11, and 211°-225° was -0.38±0.11. The average Q-values of the quadrants (15° on eac side around the horizontal meridian) for the high myopia group: the nasal side was -0.34±0.09 and the temporal side was -0.35±0.12; for the ultra-high myopia group: the nasal side was -0.31±0.08 and the temporal side was -0.34±0.13. The differences between the two groups (the nasal side: t=-1.058, P=0.297; the temporal side: t=-0.162, P=0.873) were not statistically significant.Conclusion The horizontal direction of the anterior corneal surface is a prolate ellipsoid. This means it becomes gradually flatter from the central to perimeter zone, and the trend declines gradually from the horizontal to oblique meridian. The influence of refractive power on asphericity is quite weak.%目的 研究高度近视患者角膜前表面水平方向的非球面特性.方法 回顾性研究.使用OrbscanⅡ角膜地形图仪采集38例高度近视患者右眼角膜地形图并导出角膜前表面正切图中的数据(θ,Ft,R),利用线性回归拟合法及坐标旋转技术,以正切曲率半径计算得到角膜前表面360条半子午线截痕的Q值,对水平主子午线上下45°范围数据进行研究.取研究范围内间隔15°的半子午线Q值(鼻侧315°、330°、345°、0°、15°、30°、45°,颞侧135°、150°、165°、180°、195°、210°、225°,总共14条半子午线的Q值).将研究范围划分为间隔15°的区间(鼻侧315°～329°、330°～344°、345°～359°、1°～15°、16°～30°、31°～45°,颞侧135°～149°、150°～164°、165°～179°、181°～195°、196°～210°、211°～225°,总共12个区间),并分别计算出每个区间的Q均值.将研究对象按等效球镜度数分成高度近视组(-6.00～-9.00 D,26例)、超高度近视组(-9.00 D以上,12例),采用独立样本t检验比较两组间的差异.结果 水平方向所取半子午线的Q值分别是:鼻侧315°为-0.17±0.05,330°为-0.22±0.06,345°为-0.30±0.09,0°为-0.36±0.08,15°为-0.37±0.10,30°为-0.32±0.09,45°为-0.26±0.10;颞侧135°为-0.26±0.11,150°为-0.34±0.12,165°为-0.36±0.12,180°为-0.38±0.12,195°为-0.41±0.12,210°为-0.40±0.10,225°为-0.36±0.11.水平方向所分区间的Q均值分别是:鼻侧315°～329°为-0.19±0.05,330°～344°为-0.26±0.08,345°～359°为-0.33±0.09,0°为-0.36±0.08,1°～15°为-0.37±0.08,16°～30°为-0.34±0.09,31°～45°为-0.30±0.10;颞侧135°～149°为-0.30±0.11,150°～164°为-0.35±0.11,165°～179°为-0.37±0.12,180°为-0.38±0.12,181°～195°为-0.40±0.11,196°～210°为-0.41±0.11,211°～225°为-0.38±0.11.Q值的区间(水平主子午线上下15°)均值,高度近视组:鼻侧为-0.34±0.09,颞侧为-0.35±0.12;超高度近视组:鼻侧为-0.31±0.08,颞侧为-0.34±0.13,两组间鼻侧(t=--1.058,P=0.297)、颢侧(t=-0.162,P=0.873)差异均无统计学意义.结论 角膜前表面水平方向为长椭球面,即从中央到周边逐渐趋于平坦.且自水平主子午线向斜向子午线方向延伸时,这种趋于平坦的趋势逐渐减弱.屈光度对非球面特性的影响较小.
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