Width of the orbicularis oculi fibers extending to the upper lip with the lateral and inferior lengths of the orbicularis oculi at the lateral canthus level: application to botulinum neurotoxin type A injection for crow’s feet

Purpose The aim of this study was to determine the width of the fibers that extend from the orbicularis oculi muscle (OOc) to the upper lip, and the lateral and inferior lengths of the OOc at the lateral canthus level. Methods The OOc was investigated in the 40 hemifaces of 20 Korean cadavers. The lateral fibers of the OOc (OOc lat) were traced to determine whether or not these fibers extended to the upper lip. Results The OOc lat extended to the upper lip at the lateral canthus level in 31 of the 40 specimens (77.5%), whereas some inferolateral fibers of the OOc that extended to the upper lip were observed near the level of the lower margin of the OOc in the other 9 specimens (22.5%). The mean ± SD and maximum widths of the OOc lat that extended to the upper lip at the lateral canthus level were 6.9 ± 3.3 mm and 14.3 mm, respectively. Conclusion The obtained data will be helpful to distinguish the muscles that underly the wrinkles around the lateral canthus for safer and more-efficient BoNT-A treatments for crow’s feet.