A new model of central lid margin apposition and tear film mixing in spontaneous blinking

To investigate tear film spreading and central lid position in spontaneous blinks and to propose a model of the central lid position of the closed eye in such blinks.

In vivo: lid margin position and geometry of 15 subjects (9 female; median age=45 years) were evaluated by high-speed video and slit-lamp microscope video in consecutive spontaneous blinks. Upper lid (UL) tear meniscus (TM) depth was observed in the open and almost closed eye. Eyelid geometry, position and UL TM depth were analysed by Image-J Software. Lid margin thicknesses were measured with a Scheimpflug camera. In vitro: tear film spreading and lipid layer formation were simulated on a lubricated glass plate and videoed by high-speed camera (JVC, GZ-GX1BE, Japan).

In vivo: the median central lid margin thickness was not significantly (p=0.258) different between UL (1.8mm) and LL (1.7mm) in the opened eye. During blinking, UL remained perpendicular to the corneal surface, while LL tilted in and thinned. A scaled model diagram was created and revealed an over-blink of the UL over the LL (>0.7mm) and a height offset of the posterior lid margin of >0.7mm. In vitro: the LL TM fused with the UL TM even before full lid touch due to capillary bridge building.

The central UL overlaps the central LL during spontaneous blinking. This provides the appearance of complete closure. The space that results from the lack of lid margin apposition influences the fusion of the upper and lower TM and ultimately tear film mixing.