Total Corneal Power Estimation: Ray Tracing Method versus Gaussian Optics Formula
Total Corneal Power Estimation: Ray Tracing Method versus Gaussian Optics Formula
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DOI：
10.1167/iovs.09-4982
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发表时间：
2011-03-01
影响因子：
4.4
通讯作者：
Roberts, Cynthia J.
中科院分区：
文献类型：
--
作者：
Wang, Li;Mahmoud, Ashraf M.;Roberts, Cynthia J.
目的。利用角膜地形图数据，评估在正常眼、先前接受过激光原位角膜磨圆术/光屈光性角膜切除术（LASIK/PRK）的眼和理论模型中，用射线追踪法（TCP）计算的角膜总度数与高斯公式（GEP）计算的角膜总度数之间的差异。使用平均瞬时曲率计算94只正常眼、61只近视- iasik /PRK眼和9只远视- iasik /PRK眼中心4-mm区域的TCP和GEP。建立了角膜模型，计算了折射光和平行光在角膜后锥面的入射角。我们还构建了不同参数的角膜模型，以研究平均TCP和GEP （4mm区域）之间的差异，并进行了光学设计软件验证。正常眼和近视- lasik /PRK眼的TCP值倾向于小于GEP，而某些前表面曲率最高的远视- lasik /PRK眼的TCP值则相反。TCP与GEP的差异与术后眼的前表面瞬时曲率半径和后/前比值有关，而与正常眼的差异无关。在模型角膜中，平行光线的后入射角大于折射光线的后入射角，导致对负有效后角膜度数的高估；TCP和GEP之间的幅度差异随着后/前曲率比的减小而增大，与临床结果一致。在屈光手术后的眼睛中，使用高斯公式及其近轴假设计算后角膜度数会导致计算总角膜度数的误差。这可能会在屈光手术后使用高斯公式计算人工晶状体度数时产生误差。（Invest Ophthalmol Vis Sci. 2011;52:1716-1722） DOI:10.11.67/iovs.09-4982
PURPOSE. To evaluate with the use of corneal topographic data the differences between total corneal power calculated using ray tracing (TCP) and the Gaussian formula (GEP) in normal eyes, eyes that previously underwent laser in situ keratomileusis/photorefractive keratectomy (LASIK/PRK), and theoretical models.METHODS. TCP and GEP using mean instantaneous curvature were calculated over the central 4-mm zone in 94 normal eyes, 61 myopic-IASIK/PRK eyes, and 9 hyperopic-IASIK/PRK eyes. A corneal model was constructed to assess the incident angles at the posterior conical surface for both refracted rays and parallel rays. Corneal models with varying parameters were also constructed to investigate the differences between mean TCP and GEP (4-mm zone), and an optical design software validation was performed.RESULTS. The TCP values tended to be less than GEP in normal and myopic-IASIK/PRK eyes, with the opposite relationship in some hyperopic-LASIK/PRK eyes having the highest anterior surface curvature. The difference between TCP and GEP was a function of anterior surface instantaneous radii of curvature and posterior/anterior ratio in postrefractive surgery eyes but not in normal eyes. In model corneas, posterior incident angles with parallel rays were greater than those with refracted rays, producing an overestimation of negative effective posterior corneal power; differences in magnitude between TCP and GEP increased with decreasing ratio of posterior/anterior radii of curvature, consistent with clinical results.CONCLUSIONS. In eyes after refractive surgery, calculating posterior corneal power using the Gaussian formula and its paraxial assumptions introduces errors in the calculation of total corneal power. This may generate errors in intraocular lens power calculation when using the Gaussian formula after refractive surgery. (Invest Ophthalmol Vis Sci. 2011;52:1716-1722) DOI:10.11.67/iovs.09-4982