The second antagonistic pair of facial expression muscles comprises the levator muscle and the depre
The second antagonistic pair of facial expression muscles comprises the levator muscle and the depressor muscle of the angle of the mouth with their accessory muscles.
The levator muscle of the angle of the mouth (musculus levator anguli oris; also known as musculus caninus) starts wide and flat in the canine fossa below the infraorbital foramen and extends downward and forward to the angle of the mouth, where a few fibers attach and others pass into the orbicular muscle to the lower lip. Downward to the oral vestibule, a few fibers reach into the ligaments of the cheek and are responsible for their mobility. The muscular activity of this muscle will appear in the functional impression: It raises the angle of the mouth and releases the canine (baring the teeth).
The zygomatic muscle (musculus zygomaticus) lifts the angle of the mouth and draws it upward and outward. It originates at the external surface of the temporal process of the zygomatic bone, hence above the masseter attachment in front of the zygomatic arch. It covers the superoanterior edge of the masseter, where it extends downward and obliquely forward to the angle of the mouth and radiates into the upper lip with a few fibers. Its muscular activity draws the angle of the mouth upward as an expression of pleasure and laughter; therefore, it, not the risorius muscle, ought to be called the "laughter muscle." Its activity is also reflected in the functional impression because it tightens the upper and lower lips.
The levator muscle of the upper lip (musculus levator labii superioris; also musculus quadratus labii superioris) is another accessory muscle that lifts the angle of the mouth forward but has no
influence on the retention of a complete denture. This multiheaded muscle can be divided into three parts:
- The caput angulare originates as the medial part on the frontal process of the maxilla and a small part at the nasal wing; it runs vertically downward into the upper lip and into the nasolabial sulcus and, when activated, pulls up the upper lip and the nasal wing.
- The caput infraorbitale has its origin at the edge of the infraorbital foramen and converges downward toward the upper lip and raises the lip.
- The caput zygomaticum has its origin on the facial surface of the zygoma and radiates medially downward into the upper lip and nasolabial sulcus in order to raise this area.
The depressor muscle of the angle of the mouth (musculus depressor anguli oris; or musculus triangularis) is the antagonist for the levator of the angle of the mouth, the zygomatic muscle, and the upper lip levator. Its origin is at the lateral edge of the mandible between the canine and the second premolar, and with its converging fibers it runs obliquely upward and forward to the angle of the mouth, the fibers passing into the orbicular muscle. It draws the angle of the mouth downward as an expression of sadness, depression, and pessimism and thereby also has an effect on the nasolabial sulcus.
A denture must not be developed too much in this cheek area, and the dental arch must remain narrow enough because the muscle lies very close to the rows of teeth here.
The depressor muscle of the lower lip (musculus depressor labii inferioris) is the first accessory muscle, originating below the mental foramen but above the attachment of the depressor muscle of the angle of the mouth on the mandibular body. Its fibers lie below those of the depressor of the angle of the mouth, where they run upward and forward to the lower lip to draw it downward; it has no prosthetic significance.
The chin muscle (musculus mentalis) is the second accessory muscle, and it limits the scope for extension of the mandibular denture border because, when active, it narrows the mandibular vestibular fornix; if severe resorption of the mandibular bone has taken place, it can produce pressure points at the denture border. Its origin is at the alveolar eminences of the mandibular incisors, where the inferior incisive muscles also originate. It runs downward to the chin and raises it by muscular action, but when strongly contracted it can raise the lower lip or the orifice of the mouth to create a pout.