30.3.4Muscles of the Cheek and Perioral Region

The muscles of the mouth constitute an intricate framework of interconnected synergistic and antagonistic muscle groups that enable individual to communicate both verbally and nonverbally with the world. These muscles may be further subdivided into muscles that function to elevate or depress the mouth. The lip elevators and depressors typically insert into the orbicularis oris, a large muscle surrounding the circumference of the mouth which does not possess attachments to bony or cartilaginous structures, allowing for a significant amount of mobility [4, 13].

The muscles of the upper mouth consist of six structures that serve to elevate the lips or retract and elevate the angle of the mouth. The buccal branch of the facial nerve provides motor innervation to these muscles. The four lip elevators in this group consist of the levator labii superioris alaeque nasi, the levator labii superioris, the zygomaticus minor, and the zygomaticus major muscles. Two muscles that function to retract and raise the oral commissures are the levator anguli oris and risorius muscles [4, 6].

The levator labii superioris alaeque nasi muscle originates on the medial maxillary bone and divides into two heads. The medial portion of the levator labii superioris alaeque nasi inserts into the alar cartilage and skin overlying the alar rim and aids in flaring the nostril. The lateral division of this muscle inserts at the medial aspect of the orbicularis oris muscle as well as the skin of the upper cutaneous lip and aids in elevation of the upper lip. The levator labii superioris arises at the infraorbital portion of the maxilla and inserts more laterally into the orbicularis oris muscle and the skin of the upper lip. It also functions to raise the upper lip. The zygomaticus major muscle originates laterally on the zygoma beneath the orbicularis oculi muscle. It travels inferiorly and medial over the masseter and buccinator muscles to insert at the lateral orbicularis oris muscle and skin at the oral commissure. It not only elevates the lip but also draws it laterally, enabling one to smile. The zygomaticus minor muscle originates at the zygomatic bone inferiorly and medially to the zygomaticus major. It travels superiorly and parallel to this muscle to insert medially to the angle of the mouth at the orbicularis oris muscle and overlying skin and functions identically to its counterpart. The zygomaticus muscles are also predominately responsible for creation of the nasolabial fold [6].

The levator anguli oris muscle lies more deeply than the remaining lip elevators, originating from the canine fossa of the maxilla and inserting at the orbicularis oris and overlying skin just inferior to the angle of the mouth. The risorius muscle is a thin, poorly developed muscle which is occasionally absent in individuals, especially African-Americans. It originates in the soft tissue overlying the parotid gland and inserts at the corner of the mouth. It may also be continuous with the platysma. These muscles draw the corner of the mouth laterally. The modiolus is formed from the convergence of the lip elevators, depressors, and the orbicularis oris muscles, and it is typically located approximately 1 cm lateral to the oral commissure [13].

The buccinator muscle spans across a large portion of the cheek, originating posterior and medially to the last molar at the alveolar process of the maxilla and the medial mandible at the convergence of the body and ramus. This muscle inserts into the orbicularis oris muscle as well as the skin of the lips and the labial mucosa and functions to press the cheek against the teeth while chewing, preventing food accumulation. It also acts to prevent overdistension of the cheeks in situations of increased intraoral pressure (i.e., playing a musical instrument). The buccinator and orbicularis oris muscle act synergistically, allowing one to whistle [4].

The orbicularis oris is a large, circumferential muscle surrounding the mouth. It serves as a sphincter, allowing for pursing and puckering of the lips and pulling the corners of the mouth inward. It is also important in speech and annunciation, aiding in the pronunciation of the letters M, V, F, P, B, and O. This muscle originates from and interdigitates with numerous muscles surrounding the mouth as well as the modiolus [4, 14].

The muscles of the lower lip include the depressor labii inferioris, the depressor anguli oris, and the mentalis muscles. The depressor labii inferioris originates from the mandible medial to the mental foramen and inserts into the orbicularis oris as well as the skin and labial mucosa. It functions to depress and retract the lower lip [2]. The depressor anguli oris originates from the mandible, lateral to the mental foramen and inferior to the canine tooth and first and second premolars, and inserts into the orbicularis oris and skin. This muscle acts by depressing the lower lip and pulling the corner of the mouth laterally. Over time, this muscle may contribute to the formation of marionette lines. The mentalis muscle originates from the mandible, traveling inferiorly to insert on the skin of the lower chin. It enables wrinkling