正切地形图推算Q值对近视人群角膜前表面形态的区间性分析

Interval analysis of anterior corneal surface by Q-value calculation on tangential topography

背景角膜Q值是描述角膜前表面非球面形态特性的重要参数，而角膜作为光学曲面，是由一系列通过光轴正切面的截痕集合而成。所以只有正切面对于曲面所截的截痕才能代表其真实的光学形态，也就是说用正切曲率半径推算的Q值能真正代表角膜前表面的非球面特性。目的运用角膜地形图正切曲率半径推算角膜前表面子午线截痕Q值，分析青年近视受试者角膜前表面水平方向非球面特性。方法16～30岁的近视受试者90例90眼行OrbscanⅡ角膜地形图检查，均取右眼，受试者平均等效屈光度为（-5．45±2．75）D，按近视度数不同分为低度近视组30眼、中度近视组30眼、高度近视组30眼。采集并导出角膜前表面间隔0.1mm的360条半子午线正切曲率Ft值。将所需计算半子午线截痕上的所有Ft值代入正切曲率半径公式，应用线性回归方法并通过计算机编程的模型处理系统计算得到各半子午线Q值，并进行区间整合，同时对每位研究对象进行检影验光、眼压测量、裂隙灯检查、角膜曲率计检查及眼底检查。结果所有受检眼角膜的半子午线确定系数（R^2）均〉0．5。90眼水平方向鼻侧区间和颞侧区间Q值均数分别为-0．32±0．11和-0．30±0．12，差异有统计学意义（t=2．009，P〈0．05）；角膜顶点曲率半径（r0）值均数分别为（7．78±0．27）mm、（7．72±0．25）mm，差异无统计学意义（t=-1．016，P〉0．05）；不同近视度组鼻侧、颞侧区间平均Q值差异均无统计学意义（鼻侧：F=0．192，P=0．825；颞侧：F=0．912，P=0．406）。水平鼻侧和颞侧半子午线平均Q值分别为-0．33±0．14和-0．30±0．13，r0平均值分别为7．76±0．30和7．74±0．24，r0与水平鼻侧半子午线平均Q值呈弱正相关（r=0．320，P〈0．05），与水平颞侧半子午线平均Q值无明显相关（r=0．104，P〉0．05）。水平鼻侧半子午线与鼻侧区间及水平颞侧半子午线与颞侧区间Q值比较，差异均无统计学意义（t=0．349、-0．373，P〉0．05）。结论用线性回归方法以角膜地形图正切曲率半径可以推算出角膜前表面非球面系数Q值和区间Q值整合的结果。本组近视受试者角膜前表面水平方向形态趋向于长椭圆。

Background Q-value is used to express the asphricity of the anterior corneal surface. As a optical surface,the optial morphology of cornea is composed of a series of tangential sections that cut-off through the optical axial. Therefore,tangential section could represent the true optical morphology of the corneal surface, and Q-value calculation by tangential radius can better represent the asphericity. Objective This study was to calculate Q-value of meridian section of the anterior corneal surface by tangential radius of the tangential topography and analyze the corneal asphericity in horizontal interval. Methods Ninety right eyes of 90 myopia subjects aged 16-30 years and with mean spherical equivalent of （- 5.45± 2.75 ） D received corneal topography examination using Orbscan Ⅱ system. The subjects were assigned to low myopia group,moderate myopia group and high myopia group based on their diopter and 30 eyes for each group. The tangential curvatures on meridian section at a 0. 1 mm interval from apex to periphery of the anterior surface were accepted and the Q value of the semimeridian section was calculated by linear regression mathematical formulas of the tangential radius. Mydriatic optometry, intraocular pressure,keratometer and fundns examinations were performed on the subjects. Written informed consent was obtained from each subject before any medical examination. Results The coefficients of determination in all the semimeridians were over 0. 5. The average calculated Q-values in the nasal and temporal horizontal interval were -0.32±0. 11 and -0.30±0. 12 ,with a significant difference between them（ t = 2. 009, P〈0.05 ）. The vertex radius of curvature was （7.78 ±0. 27 ）mm and （7.72±0.25）mm respectively in the nasal and temporal horizontal interval, showing a significant difference（ t =- 1. 016, P〉0. 05 ）. No significant difference was seen in Q values of both nasal and temporal areas among three myopic groups （ nasal ： F =0. 192, P =0. 825 ; temporal ： F =0. 912, P = 0. 406 ）. The average Q value of the nasal and temporal principal meridian was -0.33±0. 14 and -0.30±0. 13 respectively,and the r0 was 7.76±0. 30 and 7.74±0.24 respectively. A weak positive correlation was found between r0 and mean Q value of nasal meridian （ r = 0. 320, P 〈 0. 05 ） , but no significant correlation was found between r0 and mean Q-value of temporal meridian （r= 0. 104 ,P〉0. 05 ）. No significant differences were seen in the Q values between nasal meridian and nasal zone （t = 0. 349, P〉0.05 ） as well as between temporal meridian and temporal zone （t =-0. 373, P〉0.05 ）. Conclusions The study analyzes the calculated Q-value of the semimeridian section in horizontal area with myopia by linear regression mathematical formulas of tangential radius on tangential topography. The anterior surface of the cornea is proved to be prolate ellipse in shape in the subjects with myopia.