If either skin undermining alone or subperiosteal undermining alone is performed, the surgeon can, to some extent, ignore the anatomy. These two planes of dissection are safe. Manipulation of the tissues between these two planes, however, necessitates an understanding of and constant attention to the anatomy to avoid complications.
There are five layers of critical anatomy: skin; subcutaneous fat; the superficial musculoaponeurotic system (SMAS)–muscle layer; a thin layer of transparent fascia; and the branches of the facial nerve (Fig. 49.2). These five layers are present in all areas of the face, forehead, and neck, but they vary in quality and thickness, depending on the anatomic area.
The first two layers, the skin and subcutaneous fat, are selfexplanatory. The third layer (SMAS) is the most heterogeneous (2). It is fibrous, muscular, or fatty, depending on the location in the face. The muscles of facial expression are part of the SMAS layer (e.g., frontalis, orbicularis oculi, zygomaticus major and minor, and platysma). In the temporal region, this layer is not muscular but is fascial in quality and is represented by the superficial temporal fascia (or temporoparietal) fascia.
FIGURE 49.2. The anatomic layers of the face. Although the quality of the layers differs in various areas of the face, the arrangement of layers is identical. The facial nerve (cranial nerve [CN] VII) branches innervate their respective muscles via their deep surfaces.
The fourth layer consists of a layer of areolar tissue that is not impressive in thickness or strength, but is always present and is a key guide to the surgeon as to the location of the facial nerve branches. In the temporal area, this layer is known as the innominate or subgaleal fascia; in the cheek, it is the parotid– masseteric fascia; and in the neck, it is the superficial cervical fascia. Once under the SMAS, the facial nerve branches can be seen through this fourth layer. If the layer is kept intact, it serves as reinforcement to the surgeon that he or she is in the correct plane of dissection. If a nerve branch is encountered without this fascial covering, the surgeon must be aware that the dissection is too deep and nerve branches may have been transected. Just as it is convenient, but not totally accurate, to think of the galea–frontalis–temporoparietal fascia–SMAS– orbicularis oculi–platysma as a single layer, it is useful to think of the subgaleal fascia–innominate fascia–parotid/masseteric fascia–superficial cervical fascia as a single layer.
The fifth layer is the facial nerve, which is discussed in detail below.
If the surgeon remembers that the facial nerve branches innervate the respective facial muscles via their deep surfaces, the safe planes of dissection become obvious. Dissection in the subcutaneous plane, superficial to the SMAS–muscle layer, is safely performed anywhere in the face, whether it is the temporal region, cheek, or neck. Dissection deep to the SMAS, superficial to the facial nerve branches, requires care.
There are three to five frontal (or temporal) branches of the facial nerve that cross the zygomatic arch and innervate the frontalis muscle, orbicularis oculi, and corrugator muscles via their deep surfaces (3). Because the layers of anatomy, although present, are compressed over the arch, these branches are vulnerable to injury in this region. Dissection in this region can either be performed superficial to the nerve branches in the subcutaneous plane, or deep to the branches on the surface of the temporalis muscle fascia (deep temporal fascia) (4).
The zygomatic branches innervate the orbicularis oculi and zygomaticus muscles. One must remember that although the facial nerve branches travel deep to the SMAS layer, at some point these branches turn superficially to innervate the overlying muscles. Any dissection in the sub-SMAS plane in the cheek, whether as part of a composite rhytidectomy or standard dissection of the SMAS as a separate layer, necessitates a change of surgical planes at the zygomaticus major muscle to avoid transection of the branch to this muscle. The dissection plane changes from sub-SMAS to subcutaneous by passing over the superficial surface of the zygomaticus major and thereby preserving its innervation.
The buccal branches lie on the masseter muscle and are easily visualized through the parotid–masseteric fascia. Some buccal branches merge with branches of zygomatic origin to innervate the procerus muscle and provide additional innervation of the corrugator muscle. Consequently, the corrugator muscle receives innervation from the frontal, zygomatic, and buccal branches.
Earlier publications indicated that the marginal mandibular branches were located above the inferior border of the mandible in many cases. More recent studies demonstrate that, in fact, these branches are always located caudal to the inferior border of the mandible. The cervical branches innervate the platysma muscle.
Anatomic studies indicate that there are fewer crossover communications between the frontal branches and marginal mandibular branches, which helps to explain why injuries to these nerves are less likely to recover function in their respective muscles than injuries to the zygomatic or buccal branches.
In at least two areas of the face the anatomic layers are condensed and less mobile with respect to each other. These “ligaments” are areas where the skin and underlying tissues are relatively fixed to the bone (5). The zygomatic ligament (previously known as the McGregor patch) is located in the cheek, anterior and superior to the parotid gland and posteroinferior to the malar eminence. The mandibular ligament is located along the jaw line, near the chin, and forms the anterior border of the jowl.
The retaining ligaments restrain the facial skin against gravitational changes at these points. The descent of tissues adjacent to these points form characteristic aging changes such as the jowl. In addition, some surgeons feel that the ligaments must be released in order to redrape tissues distal to these points.
Although the platysma muscle is a component of the previously discussed SMAS/–muscle layer, it deserves special attention because of its clinical importance. The medial borders of the two muscles decussate to a variable degree in the midline of the neck, helping to explain the variability of aging patterns in the neck (6). The medial borders of the muscles tend to become redundant with age and contribute to the appearance of bands in the submental region.
The malar fat pad is part of the subcutaneous layer of the face. It is superficial to the SMAS layer represented in this region by the zygomaticus muscles. The malar fat pad appears to descend with age, leaving a hollow infraorbital region behind it and creating larger nasolabial folds and deeper nasolabial creases. Each of the major facelift techniques include a method to mobilize the malar fat pad and restore volume to the upper part of the face and malar region. As is discussed below, the extended SMAS technique involves mobilizing the malar pad in continuity with the SMAS layer. Other techniques involve mobilizing and repositioning the malar pad independent from the SMAS dissection.
The buccal fat pad is deep to the buccal branches of the facial nerve, anterior to the masseter muscle, and superficial to the buccinator muscle. Access to the buccal fat pad is achieved by performing a sub-SMAS dissection in the cheek and spreading it between the buccal branches of the facial nerve or through the mouth, by a stab wound in the buccinator muscle. Despite occasional indications to remove the fat pad in patients with very full faces, removal of cheek fat tends to ultimately make the patient look older. As a general rule, rejuvenation of the face involves redistribution, not removal, of fat.
The facelift operation inevitably disrupts branches of sensory nerves to the skin. Normal sensibility always returns eventually but numbness may persist for months postoperatively. The only named sensory nerve that is important to preserve is the great auricular nerve. With the head turned toward the contralateral side, the great auricular nerve crosses the superficial surface of the sternocleidomastoid muscle 6 to 7 cm below the external auditory meatus (7). At this point it is 0.5 to 1 cm posterior to the external jugular vein. The vein and nerve are deep to the SMAS–platysma layer, except where the terminal branches of the nerve pass superficially to provide sensibility to the skin of the earlobe. Transection of the great auricular nerve will result in permanent numbness of the lower half of the ear and may result in a troublesome neuroma.
The tear trough or nasojugal groove is an oblique indentation running inferiorly and laterally from the medial canthus. This groove is a subject of much attention at the present time. Although it is probably better included in a discussion of eyelid surgery, it deepens with age and is a frequent complaint of patients interested in facial aesthetic surgery (see Chapter 48). In general, facelift procedures do not address the tear trough. Redraping of orbital fat or microfat grafting is usually required.