Purpose: To evaluate changes in posterior corneal asphericity (ΔQ) using the tangential radius of curvature after femtosecond laser–assisted in situ keratomileusis (FS-LASIK) and small incision lenticule extraction (SMILE) procedures. Methods: One hundred twenty right eyes of myopic patients who underwent either FS-LASIK or SMILE procedures were analyzed using Sirius 3D corneal topography for assessment. The tangential radius was employed to calculate both preoperative and postoperative posterior corneal Q-values across each semimeridian. After both surgical interventions, the ΔQ value variations across the 360° semimeridional regions of the posterior corneal surface were compared. Results: A marked postoperative increase in the posterior corneal Q-value was documented. No significant differences were noted between the postoperative Q-values or ΔQ-values of the two surgical approaches. Among patients with moderate myopia, postoperative Q-value exhibited considerably lower increases and ΔQ-value significantly smaller than their counterparts with high myopia. Moreover, the fluctuation in ΔQ across semimeridional regions was less evident in patients with moderate myopia than in those with high myopia. Notably, the degree of ΔQ fluctuation across semimeridional regions was similar between both surgical categories. These data offer insights into variations in the posterior corneal surface after refractive surgeries depending on the degree of myopia, illuminating their clinical relevance. Conclusions: Both FS-LASIK and SMILE introduce notable changes to posterior corneal asphericity among patients with different myopia intensities. Furthermore, the influence on the asphericity across the entire posterior surface is similarly distributed between FS-LASIK and SMILE techniques. [ J Refract Surg . 2024;40(3):e133–e141.]

The anterior corneal asphericity (Q) with the tangential radius is calculated, and a three-dimensional (3-D) anterior corneal model is constructed. Tangential power maps from Orbscan II are acquired for 66 young adult subjects. The Q-value of each semimeridian in the near-horizontal region is calculated with the tangential radius. Polynomial fitting is used to model the 360-semimeridional variation of Q-values, and to fit the Q-values in the near-vertical region. Furthermore, a customized 3-D anterior corneal model is constructed. The 360-semimeridional variation of Q-values is well fitted with a seventh-degree polynomial function for all subjects. The goodness of fit of the polynomial function was >0.9, and the median value was 0.94. The Q-value distribution of the anterior corneal surface showed bimodal variation. Additionally, the Q-values gradually become less negative from the horizontal to the vertical semimeridians in the four quadrants. The 3-D surface plot of the anterior corneal surface approximated a prolate ellipsoid. Using a method to calculate the Q-value with the tangential radius combined with polynomial fitting, we are able to obtain the Q-value of any semimeridian. Compared with general models, this method generates a complete shape of the anterior corneal surface using asphericity.

In this paper,we derive a nonlinearequation of corneal asphericity (Q) usingthe tangential radius of curvature (rt) on every semi-meridian. We transform the nonlinear equation into the linear equation and then obtain theQ-value of cor-neal semi-meridianby the linear regression method.We find the 360 semi-meridional variation rule of theQ-value us-ing polynomial function. Furthermore, we construct a new 3D corneal model and present a more realistic model of shape of the anterior corneal surface.

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 ± 0.10 in the nasal and -0.22 ± 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.

角膜的屈光力主要取决于前表面的屈光力,建立角膜前表面的精确数字模型对于临床应用具有重要意义.本研究在应用角膜地形图的正切曲率半径建立角膜前表面子午线截痕模型的基础上,将角膜地形图仪采集到的数据定位在以角膜中心顶点为原点的Cartisian坐标系内,提出了采用坐标旋转来求解截痕曲线参数的具体算法,进而利用计算机编程语言绘制出了360°全方位前表面子午线截痕曲线及其Q值分布曲线,并分析了子午线角度与其截痕形态的变化规律以及Q值的分布规律,为可透气硬性接触镜(RGP)的后表面设计、用Q值引导LASIK角膜切削术等临床应用提供较为直观且有整体感的角膜数字模型.