The lips function to provide the ability to eat, speak, and express emotion and, as a sensory organ, to symbolize sensuality and sexuality. To accomplish this multitude of functions, lips require a complex system of muscles and supporting structures. The surrounding complex of anatomical relationships of the muscles attached to the lips may be organized by classifications of aging or by anatomical region.
CLASSIFICATIONS OF AGING
Aging lips can be classified into three categories (Beer, 2007):
Group 1-Nice shape and definition (Figure 1): These clients have a nice shape (vermillion) and definition (vermillion border) but wish for enhancement. The vermillion is composed of numerous capillaries, which give its characteristic pink color. Around the mouth is the vermillion border, a fine white line that accentuates the color difference between the vermillion and normal skin. Lip enhancement typically involves injection of the wet-dry junction to gently inflate and cause lip eversion. Injection into the lateral upper lip border should be done to avoid the fade-away lip. The client may also require injections into the vermillion border to further highlight or define the lip. The injections may be performed by linear threading (needle or cannula) or serial puncture, depending on the preferred technique of the provider.
Group 2-Atrophic lips (Figure 2): These clients have atrophic lips, which may be due to aging or genetics, and are seeking augmentation to make them look more youthful. After an assessment and counseling as to the limitations that may be achieved, a treatment plan is established. The treatment would begin with injection into the wet-dry junction to achieve desired volume; additional injections may be performed into the cupid's bow and/or philtral columns to further contour the lips.
Group 3-Lip atrophy and vermillion disappearance (Figure 3): The perioral lines are observed at the edge of the white roll of the lip where the orbicularis oris is attached to the dermis with no interposed fatty layer. These lines typically start in the 30s and increase in length and depth with aging. They may be more apparent with increasing sun exposure, smoking (free radical theory of degradation), lifestyle (poor nutritional diet, exercise, and rest habits), and genetic predisposition.
Other contributing factors of the aging lip include soft-tissue volume loss, maxillomandibular resorption, and repetitive pursing of the lips. These contributing factors result in lengthening of the upper lip with loss of definition of the lip margin, flattening of the philtral columns, loss of projection of the cupid's bow, and creation of the marionette line.
These clients have loss of lip definition and/or perioral lines and require more attention to technique and other adjunctive treatments. Injection treatment may involve filling each individual perioral line and/or defining the lip border by injecting and strengthening the vermillion border to minimize the appearance of the perioral lines.
ANATOMICAL REGION
There are 12 facial muscles that affect the shape and function of the perioral area.
Muscles in the perioral region can be classified according to their origins and insertions, as well as by their locations, with respect to the major structures.
Group 1-Muscles that insert into the modiolus: The modiolus is a fibrous meeting point where seven muscles connect. It is located lateral and slightly superior to each angle of the mouth. It derives its motor nerve supply from the facial nerve and its blood supply from labial branches of the facial artery (Table 1).
Group 2-Muscles that insert into the upper lip: These muscles originate from the maxilla below the infraorbital foramen and insert into the orbicularis muscle of the upper lip. The nerve supply is from the facial nerve, and these muscles act to elevate the upper lip. Along the upper vermillion-skin border, there are two medial elevations known as the philtral columns. The philtral columns form a midline depression called the philtrum and are responsible for the formation of the cupid's bow. The philtrum extends from the vermilion superiorly to the columella (Table 2).
Group 3-Muscles that insert into the lower lip: These muscles originate from the lower border of the mandible and insert into the skin of the lower lip. The nerve supply is from the facial nerve, and they act to depress the lower lip. The labiomental crease passes horizontally in an inverted U-shape across the lower lip, which intraorally corresponds to the depth of the gingivolabial sulcus; the labiomental crease can become more prominent as we age (Table 3).
INNERVATION
Cranial nerve V (trigeminal nerve) is the largest of the cranial nerves and the most important sensory nerve of the face. It branches into three divisions (Table 4): ophthalmic (V1), maxillary (V2), and mandibular (V3).
The sensory innervation to the perioral region is from the maxillary and mandibular branches.
The infraorbital nerve, which is the terminal branch of the maxillary nerve, exits the infraorbital foramen approximately 4-7 mm below the orbital rim. It travels beneath the levator labii superioris and superficial to the levator anguli oris; it innervates the side of the nose, ala, columella, medial cheek, and upper lip.
Branches of the mandibular nerve innervate the lower lip and chin. One of the branches is the inferior alveolar nerve, which travels through the body of the mandible to exit at the mental foramen. This foramen is located below the second mandibular bicuspid and has 6-10 mm of lateral variability.
The infraorbital and mental nerves exit though a foramen along with its corresponding artery. To prevent complications when injecting dermal fillers, identify these nerves by putting pressure on the foramen with a fingertip, which causes a soreness or sensitive pressure point.
Cranial nerve VII (facial nerve) is the major motor nerve of the facial muscles. It divides into five prominent branches following a pattern much like the outstretched fingers placed on the side of the face. The branches from the top include temporal, zygomatic, buccal, mandibular, and cervical. The perioral muscles are innervated primarily from the buccal and mandibular branches of cranial nerve VI; they pass through the parotid gland and form multiple interconnections as the exit the parotid gland (Table 5).
THE VASCULAR PATHWAY
Injection into or occlusion of a blood vessel can be a serious complication resulting from the use of dermal fillers. Understanding the vascular structure of the face can help decrease the risk of complications. The facial artery should be considered for embolization following injections of the cheek, nasolabial folds, and lips. Being able to visualize the path of the facial artery will help determine where to place the needle and the depth of placement (Table 6).
The external carotid artery passes up the side of neck to the mandible where it divides into the left and right branches. The left branch travels under the parotid gland and passes in front of the ear as the superficial temporal artery. The right branch of the carotid passes in front of the masseter muscle and behind the depressor anguli oris muscle, through a palpable notch in the mandible. At this point, the vessel is known as the facial artery (Figure 4).
The facial artery passes forward and upward across the cheek to the angle of the mouth, where it gives rise to the inferior labial artery (lower lip) and the superior labial artery (upper lip). According to a study by Lee et al. (2015), the area where this bifurcation occurs can be seen at approximately 1.5 cm superolateral to the corner of the mouth, at a depth of approximately 3-5 mm. This can be roughly measured by placing a thumbnail beside the corner of the mouth. (Figures 5 and 6). The superior and inferior labial arteries form a circular vascular network around the mouth, with several small blood vessels branching out radially.
When performing lip augmentation, the provider not only must be aware of the location on the inferior and superior labial arteries but must also understand that these blood vessels are not always as they are in a textbook illustration. Most commonly, the inferior and superior labial arteries are located posterior to the wet-dry border and under the orbicularis muscle. Injecting a dermal filler above the orbicularis oris muscle would help avoid inadvertent injection into the labial artery. However, there are branches of the labial arteries located above the orbicularis oris muscles in the area of the cupid's bow. Caution must be used when augmenting the cupid's bow, as well as constant examination of the skin for any evidence of blanching (Figures 7 and 8).
The facial artery then continues along the side of the mouth and across the cheek. It is located deep to the SMAS, platysma, risorius, and zygomaticus major and minor muscles but superficial to buccinators and levator anguli oris (Figure 9).
As the facial artery approaches the alar base of the nose, it gives off the lateral nasal artery that supplies the ala and dorsum of the nose; at this point, the facial artery is known as the angular artery. The angular artery then ascends along the side of the nose, passes to the medial angle of the eye, and anastomoses with the nasal dorsal branch of the opthalmic artery (Figure 10).
CONCLUSION
It is imperative that health care professionals understand the importance of facial anatomy when providing treatments and educating clients for a surgical or nonsurgical procedures. Visualizing the anatomy will help prevent complications and improve patient outcomes.
Although anatomy is often considered to be a static discipline with few changes over time, new research is always coming to light that reveals new opinions regarding both gross and microanatomy. This article should be considered a snapshot in time and that updated information may come to light that may change the material contained within this article.
ACKNOWLEDGMENTS
The author wishes to acknowledge Dr. Claudio DeLorenzi and Dr. Jack Kolenda for their excellent preparation of the cadaveric specimens; the skilled technicians at the Surgical Skills Lab at Mount Sinai Hospital in Toronto, Ontario, for their invaluable assistance with the cadaver lab; and Deborah Elias, RN, for her assisatnce in the cadaver workshops. Most importantly, the author would like to express gratitude to the specimens who graciously donated their bodies for medical research.
REFERENCES