RESUMO
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 16-30years 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 fundus 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 differenees 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 borizontal 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.