Introduction

A muscle used in facial expression, primarily for smiling, the levator anguli oris elevates the angles of the mouth. The levator anguli oris originates roughly 1 cm inferior to the infraorbital foramen from the canine fossa of the maxilla and is located in the deepest layer of mimetic muscle. The levator anguli oris muscle falls in the hyoid muscle “facial expressions” category. These muscles are associated with skeletal elements that develop together with the neural crest cells of branchial arch two. Blood supply to the levator anguli oris comes from various small branches of the labial, infraorbital, and facial arteries. Levator anguli oris receives innervation by the terminal buccal branches of the facial nerve.

Structure and Function

The levator anguli oris is a facial muscle used to elevate the corners of the mouth. The zygomaticus major and the levator anguli oris control the elevation of the commissure. The upper lip gets raised when the levator labii superioris and the zygomaticus minor are present. The vector of the zygomaticus major orients in a superolateral and posterior direction and thus, additional contraction of the levator anguli oris results in the alteration of the overall vector into a more superior direction, which provides increased elevation of the commissure than the zygomaticus major alone. The levator anguli, located along the buccinator in the fourth and deepest layer of the mimetic muscle, originates roughly 1 cm inferior to the infraorbital foramen from the canine fossa of the maxilla. Its fibers travel in anteroinferior direction to insert into the modiolus, allowing the muscle fibers to combine with fibers of the zygomaticus major, orbicularis oris, risorius, buccinator, and depressor anguli oris. As the muscle fibers of levator anguli oris coalesce with zygomaticus major, they reach the modiolus and form an average angle to one another of 37 degrees.[1]

Embryology

The levator anguli oris muscles form alongside other structures of the face. The differentiation stages of most head muscles are neural crest-dependent. Branchiomeric muscles are composed of three subcategories: mandibular, caudal brachial, and hyoid.  Innervated by branches of the mandibular nerve and associated with skeletal elements that develop within branchial arch 1, the mandibular muscles are mainly masticatory muscles.[2] However, the completion of myogenesis and synaptogenesis is unnecessary until biting movements begin; this is due to the masseter muscle’s primary function in the biting movements of the jaw.[3] The caudal branchial muscles have evolved into actions relating to swallowing and head movements. Gill movements and common ancestry of both cyclostomes and gnathostomes are additionally associated with caudal branchial muscles. The hyoid muscles relate to skeletal elements that develop together with branchial arch two neural crest cells and are innervated by cranial nerve VII. Muscles of facial expression comprise the hyoid muscles category. The muscles of facial expression are unique to mammals, derived from second branchial arch myoblasts. The origin of the neural crest-myogenic mesoderm may have relation to the gnathostomes and cyclostomes since moveable gills and jaws were necessary to create respiratory and food-capturing opportunities. These actions led to the emergence of vertebrates and thus, neural crest cells remain as the dominant population contributing to their craniofacial connective tissues. Mesodermal populations that contribute to head muscle development are in place by the closure of the cephalic neural tube. The prechordal mesoderm becomes fully enclosed by the surface ectoderm during the elongation of the head process and body folding initiation.[2] 

Blood Supply and Lymphatics

The branches of the external carotid artery deliver blood supply to the majority of the face, while the internal carotid artery contributes a smaller amount.[4] The facial artery, the main branch of the external carotid artery, originates in the carotid triangle of the neck. Various small branches of the labial, infraorbital, and facial arteries supply blood to the levator anguli oris.[1] Aside from the levator anguli oris muscle, the labial artery gives off septal branches to the nose and runs between the mucosa and orbicularis oris. The infraorbital artery runs in the infraorbital canal and leaves through the infraorbital foramen to reach the face.[4] 

Following the facial vein, the lymph from the medial canthus deposits into the submandibular lymph nodes, but additionally into the buccinator and parotid lymph nodes. The submental lymph nodes primarily drain the perioral region. Venous drainage of the face mainly occurs through the facial vein. This vein remains as the major venous drainage of the superficial areas of the face, receiving the deep facial vein, which drains the pterygoid venous plexus of the infratemporal fossa, the masseteric and parotid veins, and veins from the inferior eyelid and lips.[4] 

Nerves

The facial nerve, composed of approximately 10000 fibers, is predominantly myelinated and innervates the facial expression muscles, in addition to the posterior belly of the digastric, stylohyoid, and stapedius muscles. Levator anguli oris innervation is by the terminal buccal branches of the facial nerve from its anterolateral aspect.[5]

Surgical Considerations

A branch of the facial artery vascularizes the levator anguli oris. Knowledge of the facial nerve is important for both surgical and non-surgical interventions involving the face. This need gets further emphasized by the facial artery and its tributaries having close topographical connections toe the facial expression muscles, such as the levator anguli oris.[6]

Another surgical consideration involving the levator anguli oris is when a surgical procedure resulting in the removal of a nasal skin or mucosa, such as for tumor removal, occurs. These procedures can utilize the levator anguli oris muscle to reconstruct the nasal defect. Denewer A. et al. proposed a method where they used a levator anguli oris myocutaneous flap or levator anguli oris myocutaneous mucosal flap to repair small unilateral defects and unilateral compound loss of skin and mucous membrane, respectively.[7]

The levator anguli oris muscle is also a component of interest when undergoing lip repositioning surgeries.[8]

Clinical Significance

The levator anguli oris muscle also serves as a good landmark for anatomical isolation of the deep medial cheek fat as it has been described as a partition of medial and lateral portions of the deep medial cheek fat. This characteristic makes it an ideal area for injections directed at the deep medial cheek fat; specifically, aiming for the space between the alar crease and levator anguli oris.[9]

Other Issues

Koltsidopoulos et al. detailed the findings of an intramuscular hemangioma within the levator anguli oris muscle in a 26-year-old man who presented for progressive swelling of the right cheek. The lesion was confirmed through histopathological examination after excision through an intraoral approach. Hemangiomas are rare benign malformations of blood vessels which occur in skeletal muscles.[10]