| Since it was first discovered that the corneal is the major refracting surface of the ocular refractive system, more and more investigators have devoted themselves to the research of this field, while to the research of the corneal shape especially. With the deepening of the investigation, the complexity of the cornea is being realized little by little. In fact, the normal cornea is an optical and anatomic structure which is sophisticated and precise extraordinarily.The widespread applications of various computer-assisted corneal detecting instruments, accelerated the development of the clinical and scientific investigations in the corneal domain greatly. But some inevitable defects reduce their measuring accuracy, such as the confusion of paraxial and nonparaxial systems, the narrowness of datum information and the 1 imitated applicable coverage.Cornea is the curved surface which has its own specific mathematical model theoretically. And this model is the mathematical formularepresenting the surface shape of the object, namely geometric model. It' s the foundation to give an analysis, calculation or description of the object, and the key to detect the nature of the curved surface. It reflects the essential qualities of the curved surface to some degree, therefore, to make up the mathematical model of the body is the chief task of the accurate analysis.Computer Aided Geometric Design, curved planar reformation chiefly, is just the way to construct the mathematical model according to the existing body. And its accelerated development and extensive application bring a brand-new approach to the research of the corneal shape. Objective To detect the inner characters of the corneal shape from a mathematical point of view and construct its correct geometric model, in order to provide the reliable theoretical guide and basis to the scientific and clinical investigations, the design and improvement of the detecting instruments and so on. To devise the three-dimension reconstruction and display software of the model so that it will be more stereoscopic, visual and vivid and can be used to display the corneal models of various human groups. Methods1. Data collection Anterior corneal radius of curvature, posterior corneal radius of curvature, total corneal radius of curvature and corneal thickness of the points located 1. 5mm, 2. 5mm, 3. 5mm and 4. 5mm away from the corneal apex on certain meridians, including 0° , 30° , 60° , 90° , 120° , 150° , 180° , 210° , 240° , 270° , 300° and 330° meridians, were measured with OrbscanIIcorneal topography system,2. Coordinate Cartesian coordinate was established: the origin 0 was placed at the corneal apex, and the optical, horizontal and vertical axises were defined as the Z , X and Y axises respectively. Then the locationsof all the points on the cornea were determined referred to the coordinate.3. Datum processing SPSS 10. 0 for windows and Microsoft Excel of Office 2000 were used for statistical analysis. The statistical methods included Descriptive statistics, Independent-Sample T Test and Bivariate Correlations.4. Mathematically modeling(1) Mathematical model of anterior and posterior corneal surface Based on Bennett' s notation about conic sections and quadric theory, the corneal shape was analyzed and described with the quadric formula: y2=a1z +a2z2.(2) Mathematical model of corneal shape factor(SF) The equation ez=l -(2rnz -y2)/ z2 which derives from Bennett' s notation about conic sections (y2=2r0Z - (1-e2) z2), was used to define the shape factors of the steepest and flattest meridians on anterior and posterior corneal surfaces.(3) Distributions of corneal curvature With the formula F' =Fn+(Fh-F,n*. Sin2a which is about the curvatures of oblique meridians on toric surface, the theoretical values of anterior, posterior and total corneal radius of curvature of the oblique meridians were obtained. The concordance between the theoretical and the actual values were detected in order to determine the distributing rul...
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