Young Graeme
*MPhil, PhD, FCOptom, FAAO Visioncare Research Ltd, Farnham, United Kingdom.
Optom Vis Sci. 2014 Jul;91(7):e167-76. doi: 10.1097/OPX.0000000000000306.
To evaluate the effect of varying lens and ocular topography parameters on soft contact lens (SCL) fit, using a novel computer spreadsheet model. Although SCLs are worn by more than 100 million ametropes, the factors governing their fitting characteristics are poorly understood.
A spreadsheet-based computer model used a novel ellipto-conical corneal model coupled with population data on corneoscleral topography obtained in a previous clinical study. The model calculated lens edge strain (circumferential elongation) as a predictor of lens tightness. The following parameters were systematically varied: corneal curvature, corneal diameter, corneal shape factor, corneoscleral junction angle, lens base curve (BC), and diameter.
The ellipto-conical corneal model showed closer concordance with actual measurements of corneal sagittal height than a simple elliptical model (limits of agreement, ±0.20 vs. ±0.25 mm; p = 0.0015). For an eye with average ocular parameters wearing a typical SCL design (BC, 8.60; diameter, 14.2 mm), the model calculated an edge strain of 2.7%. For the same SCL, the tightest fit (8.5% strain) was found with the eye showing the combination of smallest, flattest, most aspheric cornea. Conversely, the loosest fitting (-2.6%) was found with the eye showing the combination of largest, steepest, least aspheric cornea. A change in BC of 0.4 mm typically resulted in changes in edge strain of less than 2.5%, whereas a change in diameter of 0.5 mm resulted in a change of less than 2%. Using the typical SCL design and average corneal model, wide variations in corneoscleral junction angle did not critically affect lens fit. More extreme combinations of SCL and ocular parameters resulted in edge strain likely to result in a tight (>6%) or loose fit (<0%).
A novel ellipto-conical corneal model in conjunction with spreadsheet mathematical modeling proved to be a useful tool for attempting to understand the factors governing SCL fit.
使用一种新型计算机电子表格模型，评估不同的镜片和眼表地形参数对软性接触镜（SCL）适配性的影响。尽管超过1亿屈光不正患者佩戴SCL，但对其适配特征的影响因素却知之甚少。
基于电子表格的计算机模型使用了一种新型椭圆圆锥角膜模型，并结合了先前一项临床研究中获得的角膜巩膜地形的人群数据。该模型计算镜片边缘应变（圆周伸长）作为镜片贴合度的预测指标。系统地改变以下参数：角膜曲率、角膜直径、角膜形状因子、角膜巩膜交界角、镜片基弧（BC）和直径。
与简单椭圆模型相比，椭圆圆锥角膜模型显示出与角膜矢高的实际测量值更接近的一致性（一致性界限，±0.20对±0.25毫米；p = 0.0015）。对于佩戴典型SCL设计（BC，8.60；直径，14.2毫米）的具有平均眼部参数的眼睛，该模型计算出边缘应变为2.7%。对于同一SCL，在显示最小、最平坦、最非球面角膜组合的眼睛中发现了最紧的适配（8.5%应变）。相反，在显示最大、最陡峭、最不非球面角膜组合的眼睛中发现了最松的适配（-2.6%）。BC改变0.4毫米通常导致边缘应变变化小于2.5%，而直径改变0.5毫米导致变化小于2%。使用典型的SCL设计和平均角膜模型，角膜巩膜交界角的广泛变化不会严重影响镜片适配。SCL和眼部参数的更极端组合导致边缘应变可能导致紧适配（>6%）或松适配（<0%）。
一种新型椭圆圆锥角膜模型与电子表格数学建模相结合，被证明是试图理解影响SCL适配因素的有用工具。
