Post-lens tear film thinning in rigid gas permeable lenses

An attempt is made to gain a greater understanding of lens binding during overnight wear. To do this, a mathematical model is used to investigate the motion of a rigid gas permeable (RGP) lens normal to the eye surface, when the eye is closed. Graphs of the displacement of the lens as a function of time are computed for different lens designs. The rate of apical tear film thinning is found to depend upon the central fitting relation, with lenses that are fitted flatter giving the highest rate of thinning. The model also suggests the relevance of other factors such as eyelid muscle tension and tear-layer composition during sleep to the clinical phenomenon of lens binding.