A Computed Tomographic Study of the Relation Between Ocular Axial Biometry and Refraction

Abstract

Ocular biometry in 255 eyes (131 subjects) with computed tomographic (CT) scan revealed that the length of the anteroposterior axis was 23.63 ± 0.92 mm in hyperopia, 24.62 ± 0.38 mm in emmetropia, and 26.68 ± 0.75 mm in myopia, and the respective lengths of the horizontal transverse axis were 24.61 ± 0.53, 24.91 ± 0.37, and 25.12 ± 0.73mm. There were significant differences among groups of different refrac-tions, and the axial length increased with the severity of myopia. It was evident that refractive errors were chiefly caused by varying axial lengths. We propose the parameter of the ocular diametrical ratio of the anteroposterior axis to the horizontal transverse axis, and accordingly hyperopia has a ratio under 1, myopia a ratio over 1, and an emmetropic eye is apparently spherical with a ratio in the vicinity of 1. The refractive state is determined by the ocular diametrical ratio. As is commonly known, refractive errors are mainly related to ocular axes [1]. Our previous studies also confirm this concept [2]. But the commonly used figures are the average values of the ocular anteroposterior dimensions. The lengths of ocular axes with same refractive power may be different. The anteroposterior dimensions of myopic eyes may be lower than the average values in emmetropic eyes. On the other hand, few anteroposterior dimensions of hyperopic eyes are greater than the average values in emmetropic eyes. With modern diagnostic techniques, such is computed tomographic (CT) scan, it becomes possible to measure the parameters of two dimensions on living eyes. Materials and Methods The CT Max-640 System (GE Medical Systems, USA) and KeratometerK-l (Canon, Japan) were used respectively to investigate ocular CT image in vivo and corneal curvature. Ocular refraction and visual acuity were obtained by retinoscopy (under cycloplegia) and conventional tests, respectively. For the CT Max-640 System measurement , the subject stared straight ahead lying on his face, slightly shut his eyes, and