KeywordsFacial compartmentsFacial agingVolume lossAesthetic nursingFacial anatomyBone structureFacial muscles
2.1 Facial Anatomy
Facial structure is comprised of skin, subcutaneous fat , fascia , muscle, and bone. The face has 40 muscles and each has a specific function that results in expression (Vigliante 2005). The amount of facial volume contributes to the projection and fullness of the face. Natural appearance, proportioned features, and firm resting tone are attributes of an aesthetically pleasing face.
Generally, the aging process begins to change the appearance of the face at approximately 35 years old; however, aging takes place at a relatively constant pace throughout life (Pessa 2000). These changes are a result of bone resorption or thinning, muscle atrophy, and skin changes that happen simultaneously. As the bone begins to recede, flattening and shortening of the face occur, and the tissues soften and begin to fall since they are no longer supported adequately by the bone (Mendelson 2012). It is essential for practitioners to understand facial anatomy and the impact of the aging process, so the practitioner can reproduce the appearance of youth in the aging face.
Typically, the face is divided into three parts: the upper face, mid-face, and lower face. The upper face is comprised of the forehead, eyebrows, and eyes; the mid-face is comprised of the nasal area and cheeks; the lower face is comprised of the mouth, chin, and platysma (Vigliante 2005). These three regions should be assessed individually and collectively so that the entire face is balanced.
Some important structures lie within the framework of the face. Facial vessels, nerves, ducts, and glands are contained within the face and should be considered (Cohen 2008; DeLorenzi 2014; Scheuer et al. 2017). Understanding key facial structures is essential to avoid serious complications such as emboli, vascular compromise, or nerve impairment as a result of treatment.
The anatomy of the skin includes the epidermis , dermis , and subcutaneous tissue including fat (Kolarsick et al. 2011; Habif 2016; Nea 2016). The skin is a living, complex organ that protects the underlying tissues from environmental toxins, ultra-violet radiation, and injury (Nea 2016). In addition, the skin regulates temperature, fluid loss, homeostasis, and vitamin D production and monitors conditions for immune responses (Nea 2016). It is also the heaviest organ of the body with the dorsal and extensor surfaces being thicker than the ventral and flexor surfaces (Habif 2016).
Several specialized cell types control how the skin responds to the environment: (1) melanocytes synthesize pigment, (2) Langerhans cells are involved in the immune response, and (3) Merkel cell function plays a role in tactile sensation (Kolarsick et al. 2011; Habif 2016). Melanocytes are located deep in the epidermis and account for the varying degrees of pigmentation as a result of sun exposure. The location of the melanocyte in the skin makes it a challenge to cosmetically lighten unsightly dark lesions that result from sun exposure. Patients must be informed that in general, topical medications and treatments will lighten and blend these pigments but may not completely resolve the hyperpigmentation. Langerhans cells phagocytize antigens in the skin and stimulate cell migration in the event of invasion by foreign organisms (Kolarsick et al. 2011). Merkel cells are more numerous in areas of high tactile sensitivity such as the lips, fingertips, and hair follicles. When stimulated, Merkel cells secrete a chemical signal that generates an action potential in the adjacent afferent neuron that relays the signal to the brain (Kolarsick et al. 2011).
2.2.3 Subcutaneous Fat
The third and deepest layer of the skin is the subcutaneous fat layer. The subcutaneous fat layer contains the bulb and matrix of the hair follicle, larger vessels, dermal collagen, and adipose tissue (lipocytes) (Kolarsick et al. 2011). This layer is of varying thickness depending on the area of the body; for example, increased thickness is found on the waist and abdomen vs. the eyelid. The function of the subcutaneous layer is to act as a shock absorber, insulation of underlying tissues, provide buoyancy, and store energy (Kolarsick et al. 2011; Nea 2016). In addition, the subcutaneous tissue is considered an endocrine organ because it converts androstenedione into estrone and also produces leptin, a hormone that influences body weight (Kolarsick et al. 2011).
2.3 Skin Appendage: Hair
The hair follicles are referred to as appendages of the skin as are the sweat glands and nails (Kolarsick et al. 2011). Human beings have all of their hair follicles at birth however, the size changes under the influence of androgens; no hair follicles are formed after birth (Kolarsick et al. 2011; Habif 2016). There are three types of hair as follows. (1) Terminal hairs are thick, heavily pigmented hairs on the head, beard, axillae, and pubic areas. The terminal hair follicles become larger in puberty but shrink during the lifespan at the temporal region. (2) Lanugo hair is fine hair found on the newborn. (3) “Peach fuzz” or vellus hair covers much of the body and is not influenced by androgens as are the terminal hairs (Kolarsick et al. 2011; Habif 2016).
Hair growth varies depending on body area. The eyelashes have double or triple rows of few hairs with an average growth phase of 1–6 months. The scalp has approximately 100,000 hairs and has a range in growth phase of 2–6 years, shedding up to 100–150 hairs per day (Habif 2016). This explains why hair on the head can grow to great lengths. Hair on the arms and legs remain in the growth phase for approximately 30–45 days (Habif 2016).
Hair diameter is determined by the number of hair cells entering the root sheath of the follicle. The curvature of the root sheath determines the shape or shape of the hair itself. For example, oval follicles create curly hair as in people of African descent whereas round follicles create straight hairs in people of Asian descent (Kolarsick et al. 2011; Habif 2016; Nea 2016; James et al. 2006).
Hair color is determined by the amount, size, and distribution of melanosomes interspersed among the matrix cells contained in the hair shaft (Habif 2016). Darker hair has larger melanosomes from an increased number of melanocytes. Conversely, graying hair has fewer melanocytes and produces less or no melanosomes (James et al. 2006).
Hair growth cycles are important to understand especially when considering laser hair reduction treatment. Although there are three phases of hair growth, each follicle behaves independently (James et al. 2006). The three stages of hair growth are anagen, catagen , and telogen (Kolarsick et al. 2011).
Anagen is the active growth stage where the hair cells are tightly bound and forced out through the skin (Habif 2016). Anagen phase varies depending on the location on the body. For example, the scalp hair follicle has an anagen phase of approximately 2–6 years (James et al. 2006). The hair in the anagen growth phase contains the highest number of melanocytes which absorb high levels of heat (Lin et al. 1998). Laser hair reduction is most effective during the anagen phase because the heat from the laser is absorbed by the melanosomes and this heat is transferred to the hair bulb. Damage occurs to the hair follicle from the absorbed heat and results in impaired production of matrix cells, if matrix cells are produced at all (Lin et al. 1998; Sadick and Prieto 2003).
Catagen is the involution phase of the hair follicle when the growth and metabolic processes of hair production regress. Cell division in the matrix ceases, the lower portion of the follicle shrinks and ascends (Habif 2016). This phase lasts about 2–3 weeks in all body areas (Nea 2016).
Telogen phase describes the resting phase of the hair follicle and the hair ceases to grow any longer. The telogen phase lasts approximately 3–5 months on the scalp whereas other body hair has a longer telogen phase and accounts for the shorter hairs in place for longer periods of time (James et al. 2006). This phase also includes the shedding of the hair from the body called exogen (Nea 2016; James et al. 2006). These phases of hair growth are important to know as it relates to laser hair reduction.
Fascia is defined as loose connective tissue found beneath the subcutaneous layer of the skin that encloses and separates muscles. Fascia covers every structure of the body and provides form to all tissues and organs (Seely et al. 1989). The fascia is able to support and penetrate blood vessels, bone tissue, and meninges and holds the muscle cells in place and serves as a passageway for vessels and nerves to reach muscle cells (Seely et al. 1989; Bordoni and Varacallo 2018). Fascia tissues allow the muscles, nerves, vessels, and joints to glide over one another; so the body can move in real time and into different positions while also having the ability to repair its structure and adapt to mechanical stress (Bordoni and Varacallo 2018).
On occasion during BoNT/A injections in the forehead, as the needle passes through levels of skin, fascia can be pierced and create an audible crunch sound that both the patient and practitioner can hear. To avoid alarming the patient, this phenomenon is best explained before injection into a treatment naive patient to prepare them for this sound.
2.5 Facial Muscles
The muscles of the face are different than muscles located in the rest of the body and are associated with the dynamic lines of the face. The facial muscles are generally thin and superficial and insert into or affect the skin (Vigliante 2005). The muscles most relevant to facial aesthetics include the frontalis, procerus, orbicularis oculi, corrugator supercilii, levator labii superioris alaeque nasi, nasalis, levator labii superioris, zygomaticus major, zygomaticus minor, risorius, levator anguli oris, orbicularis oris, depressor anguli oris, mentalis, and platysma. All of these facial muscles can be affected by BoNT/A (Carruthers and Carruthers 2005).