The levator labii superioris muscle is a three-part muscle useful for facial expression and dilation of the mouth. It runs down alongside the lateral aspect of the nose. Its primary function is in raising the upper lip. It is also involved in movements such as facial expressions, actions of disgust, expression of sadness, nasal flaring, retching (vomiting), disdain, and even to show oral content. Its general origin is on the lateral aspect of the nose and extends to the zygomatic bone. The levator labii superioris receives its blood supply from the facial artery and the infraorbital branch of the maxillary artery. The buccal branch of the facial nerve or cranial nerve VII innervates the levator labii superioris.
The levator labii superioris muscle divides into three parts: the medial, central, and lateral portions of the muscle. The medial portion can further subdivide into a (nasolabial) furrow portion, an alar portion, and a lip portion (the long portion). The primary function of the levator labii superioris is the elevation of the upper lip. It may also evert the upper lip.
The muscles of the face start developing between the third and eighth weeks of embryonic life. The muscles originate as a thickening of the mesoderm layer of the second branchial arch. The infraorbital lamina and occipital platysma are the first laminae to develop. Both infraorbital laminae give rise to the levator labii superioris, among many other facial muscles.
The levator labii superioris receives its vascular supply from two arteries: the facial artery and the infraorbital artery. The angular artery is the terminal branch of the facial artery, which, in turn, is a branch arising from the external carotid artery. The infraorbital artery is a branch of the maxillary artery. The infraorbital artery runs through the infraorbital foramen.
The buccal branch of the facial nerve or cranial nerve VII innervates the levator labii superioris.
As noted above, the levator labii superioris muscle divides into three parts: the medial, central, and lateral portions of the muscle.
To recap, the medial portion is also called the levator labii superioris alaeque nasi muscle (sometimes called the angular head of the labii superioris muscle). The levator labii superioris alaeque nasi can further subdivide into a furrow portion (nasolabial), an alar portion, and a lip portion (also known as the long portion). The three parts lie on top of the nasalis muscle. The origin of the levator labii superioris alaeque nasi is the superior anterior process of the maxilla (just above or below the medial palpebral ligament) and inserts into the region to the side of the nostril and upper lip. The main action of all three parts is to elevate the upper end of the nasolabial furrow, the posterior end of the nasal wing, and the middle portion of each side of the upper lip.
The central/intermediate portion (infraorbital head) arises from the lower margin of the orbit directly above the infraorbital foramen. The attachment is mostly to the maxilla, with a few of the other fibers attaching to the zygomatic bone. The fibers ultimately converge and insert into the muscular area of the upper lip between the angular head and the levator anguli oris.
The lateral portion, which forms the zygomatic head (and is also known as the zygomaticus minor muscle), arises from the malar surface of the zygomatic bone directly behind the zygomaticomaxillary suture and heads downward and medial in the direction of the upper lip.
One study performed in Taiwan found that 31 out of 32 adult cadavers had the zygomaticus minor muscle fibers inserting somewhere into the region of the superior lip and arises by the creation of the muscle fibers that arise from the zygomatic region and muscle fibers that extend from the muscle of the orbicularis oculi. In 14 of the cadaver specimens (43.8%), some fibers of the zygomaticus minor fibers blended with the lower border of the orbicularis oculi muscle. The fibers previously mentioned were then attached to the palpebral ligament medially, a part of the maxilla, the muscle of the levator labii superioris alaeque nasi, and the depressor supercilia muscle as well.
Excessive gingival display, or “gummy smile,” is often caused by a hyperactive upper lip and hyperactivity of the muscles of the upper lip, including but not limited to the levator labii superioris muscle. Correction of a gummy smile can occur in multiple ways, often depending on the severity of the excessive gingival display, the cause, and the amount of jaw protrusion. If the cause is due to hyperactivity of the upper lip muscles, an effective surgical intervention is a myotomy of the levator labii superioris muscle and lip repositioning followed by orthodontic treatment.
Trauma or cutaneous malignancy surgeries can cause alteration in facial appearance by causing defects of the nose. A novel technique using pedicled levator labii superioris alaeque nasi flap has been described and has utility in single staged reconstructions.
There are other alternative ways to treat a “gummy smile,” which causes an excessive gingival display. Injecting botulinum toxin-A has been demonstrated to be a reasonable alternative treatment for patients with uncontrolled gingival disease (gummy smile) caused by overactive upper lip muscles that cause lip elevation. This treatment is a newer method and is much cheaper compared to performing surgery and the physical and emotional stress experienced by patients. The shortcoming of botulinum toxin-A treatment is that it will need repeated injection as the drug effect lasts only a few months), and it is only useful if a hyperactive upper lip is the cause of the excessive gingival display.
A more recent, less invasive, and safer technique is by infiltrating hyaluronic acid. This treatment has shown to be feasible and long-lasting.
A weakness of the levator labii superioris muscle is more often unilateral but can also present bilaterally and ranges from being slightly affected to a full loss of function. Most commonly, it results from damage to the facial nerve or the buccal branch of the facial nerve. It is crucial to perform a full neurological examination and determine the cause of the weakness. Post-operative damage to the facial nerve (cranial nerve VII) can lead to weakness of the levator labii superioris muscle.
As an interesting side note, levator labii superioris muscle transposition has been shown to treat and is an alternative treatment of chronic oral maxillary sinus fistulas in horses.
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