Anatomical features of the incisivus labii superioris muscle and its relationships with the upper mucolabial fold, labial glands, and modiolar area

The current study examined the incisivus labii superioris muscle (ILS) and its morphologic and spatial relationships with the surrounding structures, especially focusing on the upper mucolabial fold, labial glands, and modiolar area. ILSs were investigated in 52 specimens obtained from formalin-fixed Korean adult cadavers (26 left sides, 26 right sides of the face; 15 men, 11 women; mean age, 70.8 years). ILSs were observed in all specimens (100%). The ILS has an oblique and linear origin from the incisive fossa of the maxilla to the point just medial to the origin of the levator anguli oris muscle (LAO). The arising fibers of the ILS arched and covered the prominent labial glands at the superior margin of the orbicularis oris muscle (OOr). After the ILS coursed laterally along the anterior part of the upper mucolabial fold, it divided into superficial and deep inserting fibers in 48 specimens (92.3%) and it did not divide in 4 specimens (7.7%). The superficial inserting ILS fibers and the ILS fibers that did not divide blended with the medial fibers of the LAO to converge toward the modiolus. The deep inserting fibers of the ILS blended with the lateral deep fibers of the OOr in the 48 specimens (92.3%), and the deep inserting fibers continued to descend to converge toward the modiolus in 20 of those specimens (38.5%). These observations indicate that the ILS may assist to compress the labial glands and the upper oral vestibule, controlling modiolar movements and thereby integrating the movements of the mouth and lips.