Topography and location of the depressor anguli oris muscle with a reference to the mental foramen

The labiomandibular fold (LMF) is the area of the face that extends from the mouth corner to the mandibular border, and its prominence tends to increase with age. The LMF can be formed by the medial or lateral border of the depressor anguli oris (DAO). The aim of this study was to demonstrate the topographical anatomy between the DAO and mental foramen, thereby providing critical information for the safest and most effective site at which to inject botulinum toxin type A (BTX-A). Thirty-four hemifaces from Korean adult cadavers were dissected. The maximum width between the medial borders of the bilateral DAO, parallel to the intercheilion horizontal line, was 59.9 ± 4.6 (mean ± SD) mm below the lower lip. The minimum width between the medial borders of the attachment of bilateral DAO was 29.7 ± 4.8 mm at the mandibular border. The mental foramen was located in the middle third from the cheilion to the mandibular border in 28 cases (90.3%), and it was mostly confined within the DAO muscle coverage in 21 cases (67.7%). The buccal branch of the facial nerve entered through the middle third of the lateral border of DAO and then distributed. Concomitantly, the marginal mandibular branch of the facial nerve entered through the lower third of the lateral border of DAO in 17 cases (60.7%). These results represent additional reference data for identifying the position of the mental foramen on the facial skin, and will be useful for providing criteria for the most effective site for injecting BTX-A when treating the LMF.