Anterior corneal asphericity calculated by the tangential radius of curvature

Abstract

Abstract. We propose a method of calculating the corneal asphericity (Q) and analyze the characteristics of the anterior corneal shape using the tangential radius. Fifty-eight right eyes of 58 subjects were evaluated using the Orbscan II corneal topographer. The Q-values of the flat principal semi-meridians calculated by the sagittal radius were compared to those by the tangential radius. Variation in the Q-value with semi-meridian in the nasal and temporal cornea calculated by the tangential radius was analyzed. There were significant differences in Q-values (P < 0.001) between the two methods. The mean Q-values of the flat principal semi-meridians calculated by tangential radius with −0.33 AE 0.10 in the nasal and −0.22 AE 0.12 in the temporal showed more negative than the corresponding Q-values calculated by the sagittal radius. The Q-values calculated by tangential radius became less negative gradually from horizontal semi-meridians to oblique semi-meridians in both nasal and temporal cornea. Variation in Q-value with semi-meridian was more obvious in the nasal cornea. The method of calculating corneal Q using the tangential radius could provide more reasonable and complete Q-value than that by the sagittal radius. The model of a whole anterior corneal surface could be reconstructed on the basis of the above method.

The analysis of corneal asphericity (Q value) and its related factors of 1,683 Chinese eyes older than 30 years

“…This study found that the corneal Q values of the anterior surface at 3.0, 5.0 and 7.0mm aperture diameters were -0.29 ± 0.18, -0.28 ± 0.18 and -0.29 ± 0.18, respectively, while some previous studies reported the corneal Q value to be -0.33 (Carney (Australian) [13], Ying (Chinese) [14]), -0.22 (Cheung (Chinese) [15], Scholz (German) [16]), -0.24 (Dubbelman (Dutch) [17]), -0.08 (Horner (Indian) [18]), -0.20 (Fuller (American Caucasian) [6]), -0.26 (Fuller (Africa-American) [6]), -0.346 (Davis (American) [19]), and -0.30 (Zhang (Chinese) [9]). In these studies, the maximum corneal Q value was -0.08, while the minimum value was -0.346.…”

“…Only Ying [14] used the same instrument as we used, and all other studies used different methods. For example: Scholz [16] utilized Tracey iTrace visual function analyzer with integrated EyeSys vista corneal topography, which uses a placido-based videokeratoscope that calculates the Q value by combining the anterior and posterior surfaces as a whole.…”

Scleral topography analysed by optical coherence tomography

“…This might be explained by the fact that Choi and colleagues calculated the axial (sagittal) radius of the sclera while in the studies of Hall et al [11,12] and in this study the tangential scleral radius was assessed. As the tangential radius requires no assumption concerning the location of the optic axis, the tangential radius gives a truer picture of off-axis irregularities than the sagittal radius [26][27][28]; hence the tangential radius seems to be more useful then the sagittal measurement in the detection of topographic abnormalities and surface shape changes, as in accurate GP base curve selection [28][29][30][31][32].…”

Group Analysis ofQValues Calculated with Tangential Radius of Curvature from Human Anterior Corneal Surface

“…The tangential radius of curvature ( r t ) is a true radius of curvature that can better represent the corneal shape and local curvature changes. In 2012, Ying et al [ 6 ] proposed a method for calculating the corneal asphericity ( Q ) and analyze the characteristics of the anterior corneal shape using the tangential radius. In 2013, Zheng et al [ 7 ] elucidated the 360-semimeridional variation rule of the Q value using the polynomial fitting and constructed a customized 3-D model of the anterior corneal surface.…”