Mathematical model for evaluating soft contact lens fit.

PURPOSE

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.

METHODS

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.

RESULTS

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%).

CONCLUSIONS

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.