Orbicularis oris and buccinator as a sphincter complex of the oral fissure – re-evaluation using isolated muscle specimens.

Abstract

Abstract Purpose This study aimed to clarify the structural arrangement of the orbicularis oris (OOr), the buccinator (BC), and the other perioral muscles around the modiolus. Methods The perioral muscles in seventeen faces were dissected in situ and/or in isolated muscle specimens, and their layers were reconstructed schematically upon pantomographic view of the skeleton to evaluate their actions. Results The BC was composed of three parts including upper and lower oblique parts (BC-UO, BC-LO) in its superficial layer and a middle transverse part (BC-MT) in its deep layer. The superior and inferior OOr (SOOr and IOOr, respectively) were composed of an inner marginal part (SOOr-IM, IOOr-IM) and an outer labial part (SOOr-OL, IOOr-OL) in each. The perioral muscles as a whole were arranged in three layers. The first layer, with the depressor anguli oris (DAO) connected at the modiolus with the SOOr-OL, could mainly depress and deform the upper lip, as well as move the modiolus medially, upward, or downward. The second layer, with the BC-UO and the BC-LO connected at the modiolus with the IOOr-OL, could mainly elevate and deform the lower lip, as well as move the modiolus medially or laterally. The third layer, with the BC-MT continuous with the SOOr-IM, IOOr-IM, and IOOr-OL, could bring the upper and lower lip margins nearer to close the oral fissure lightly. Conclusions The actions of the individual perioral muscles on the movement of lips were predicted to open and close the oral fissure, and widen and narrow the lip width.