The Role of Overriding Preseptal Orbicularis Oculi Muscle in Development of Involutional Lower Eyelid Entropion: Microscopic Viewpoints.

The aim of this study was to demonstrate the extent of preseptal orbicularis oculi muscle (OOM) override onto the pretarsal OOM in the lower eyelids. In this experimental microscopic study, 22 exenterated specimens from 22 Japanese cadavers were prepared as full-thickness sagittal sections. All exenterated specimens were devoid of lower eyelid entropion. The tarsal dimension and distance from the lower tarsal edge to the tip of overriding OOM were microscopically measured. The rotation axis of the tarsus was presumed to pass through the tarsal centroid and the distance from an estimated tarsal centroid to the lower tarsal edge was calculated. Consequently, 2 eyelids did not exhibit any overriding of the OOM. In the remaining 20 eyelids with OOM override, the average distance of the overriding OOM was found at a level covering 48.0% of the whole tarsal height. In 15 out of the 20 eyelids, the tip of the overriding OOM was located lower than the centroid. These microscopic findings suggest that overriding of the preseptal OOM does not always produce an inward rotational force on the tarsus.