Procedural Dermatology Volume I: Reconstruction (eBook)

1 Facial, Scalp, Neck, Hands, Lower Extremities, and Genital Anatomy

Shauna Higgins, Marissa B. Lobl, and Ashley Wysong

Summary

This chapter discusses the anatomic areas of the face, scalp, neck, hands, lower extremities, and genitalia that are relevant to minimally invasive and surgical procedures performed for medical, oncologic, and cosmetic indications.

Keywords: dermatologic surgery cosmetic procedures anatomy danger zones

1.1 Introduction

Dermatologic surgery has expanded significantly with advances in minimally invasive and surgical procedures for medical, oncologic, and cosmetic indications. Procedurally relevant knowledge of anatomy is crucial to planning procedures, achieving optimal outcomes, and minimizing adverse events.

1.2 Head and Neck

1.2.1 Cosmetic Units and Facial Fat Pads

The face is broken down into a number of cosmetic subunits that share common characteristics such as skin color, texture, thickness, and presence or absence of hair.1 The cheek, temple, chin, and eyelids exist as their own well-defined units, with the nose and ear being subdivided into several smaller units (Fig. 1.1).1 At the borders of these units exist junction lines that include the melolabial fold that separates the cheek from the upper cutaneous lip, the mentolabial crease that divides the chin from the lower cutaneous lip, the nasolabial fold, the nasofacial sulcus, the hairline, and the jawline.2 These cosmetic units and their junction lines have a number of surgical implications. For example, junction lines generally serve as ideal locations to place incisions and closures.2 Surgical closures should also generally be confined to a single cosmetic subunit.1 When not possible, skin should be recruited from adjacent subunits and scar lines should be placed within junction lines or parallel to relaxed skin tension lines.1 When a defect involves several cosmetic subunits, consider repairing each subunit independently.1 Additionally, when the majority of a cosmetic subunit has been removed, for example, in tumor extirpation, consider removing the remaining portion and replacing the entire unit.1

Cosmetic subunits and their corresponding junctions also reflect variations in tissue composition and three-dimensional structure and contour.2 It is essential to thoroughly evaluate each patient for individual concavities, convexities, and transition zones. For example, variations in presence and density of fat pads such as the buccal and orbital pads can influence contour and may have implications for the depth and prominence of junction lines. Results of a 2018 study of 30 cadaver specimens revealed 7 bilateral distinct superficial (subcutaneous) facial fat compartments (when excluding the 3 subcutaneous compartments of the forehead): superficial nasolabial, superficial medial cheek, superficial middle cheek, superficial lateral cheek, jowl, and superficial superior temporal and superficial inferior temporal.3 Increased age was shown to have a significant influence on the inferior displacement of the superficial nasolabial and jowl compartments (p < 0.001).3 Several of these compartments are illustrated (Fig. 1.2). Wysong et al used magnetic resonance imaging scans of men and women to demonstrate measurable decreases in volume in the infraorbital area and the medial and lateral cheek areas, and concluded that facial soft tissue undergoes significant deterioration during the aging process and is different between men and women.3,​4,​5 Thus, this inevitably manifests in changes in volume and tissue laxity that can be addressed with injectable soft-tissue fillers. Of note, a 2018 cadaveric study utilized upright computed tomographic scanning to simulate the effects of gravity and reported that the superficial (subcutaneous) fat compartments behave differently upon injection of filling material.3 Whereas the inferior aspect of the nasolabial, middle cheek, and jowl compartments descended on filling, this effect was not observed for the medial or lateral cheek compartment or either of the superficial temporal compartments.3 Thus, in a clinical setting, care must be taken when injecting volumizing material into the subcutaneous plane.3 Targeting specific superficial fat compartments, such as the superficial nasolabial fat compartment, can result in an effect opposite to that desired: instead of reducing the nasolabial crease depth by the implantation of soft-tissue filler, a worsening of its appearance and a deepening of the crease can be noted.3 On the contrary, injections of soft-tissue filler into the superficial temporal compartments or the superficial medial cheek compartment (also called the malar fat pad) have been associated not with descent but with an increase in the local volume and an increase in the soft-tissue projection capable of inducing a lifting effect in the middle and/or lower face.3

The suborbicularis oculi fat (SOOF) has also been reported to be of particular clinical and procedural relevance, as nerves and vasculature such as the infraorbital and zygomaticofacial nerves course through the SOOF.6 The infraorbital nerve travels through the medial SOOF or deep medial cheek fat, whereas the zygomaticofacial nerve and artery travel through the lateral SOOF.6 Knowledge of this anatomy can help the surgeon accurately place nerve blocks and avoid bruising from bleeding complications.6 Further, because nerves travel within the medial and lateral SOOF, it is important to avoid performing multiple or crisscross passes with the needle in this plane, a technique that increases risk of nerve injury.6

1.2.2 Superficial Landmarks

The frontal, maxillary, zygomatic, and mandibular bones give rise to the prominent bony surface landmarks on the face that include the orbital rim, the zygomatic arch, the mastoid process, and the mentum (Fig. 1.3).2 On the orbital rim, several important foramina can be located. This includes the supraorbital and infraorbital foramina, with the former being located and palpable on the underside of the superior orbital rim 2.5 cm or approximately one thumb-breadth from midline.2 The supraorbital neurovascular bundle emerges from this foramen and includes the supraorbital artery, vein, and nerve.2 The infraorbital foramen can generally be located 1 cm below the infraorbital rim and is where the infraorbital artery, nerve, and vein emerge from the skull.2 The zygomatic arch serves as the prominent bone of the lateral cheek.2 Its posterior aspect helps define the superior pole of the parotid gland, the superficial temporal artery, and the temporal branches of the facial nerve.2 Of note, the temporal branch of the facial nerve is most superficial and vulnerable in the area around and just above the zygomatic arch. The mastoid process is the bony prominence palpated posterior and inferior to the postauricular sulcus.2 It serves as the landmark for the emergence of the facial nerve trunk from the stylomastoid foramen.2 After exiting the stylomastoid foramen, the trunk of the facial nerve travels through the nook of the neck for 1 to 1.5 cm, typically located midway between the cartilaginous tragal pointer of the external auditory canal and the posterior belly of the digastric muscle before entering the parotid gland. The mental protuberance of the mandible forms the prominence of the chin.2 The mental foramina, which are found on either side of the mandible along a vertical plumb line with the supraorbital and infraorbital foramina, are the route by which the mental nerve and artery exit the skull.2

1.2.3 Muscles

There are two types of muscles in the face: the muscles of facial expression, also known as the mimetic muscles, and the muscles of mastication (Fig. 1.4). The muscles of facial expression are unique in that they are the only muscles to insert directly into the skin and interdigitate with other muscles. They can be described by the cosmetic subunits on which they act.

The muscles acting around the eyelids include the frontalis, corrugators, and orbicularis occuli.2 The frontalis muscle of the upper face/forehead acts as one unit to wrinkle the forehead and raise the eyebrows.2 It secondarily works to raise the eyelids via its interdigitation with the orbicularis oculi muscle.2 Injury to the frontalis muscle results in ipsilateral flattening of the forehead and often brow depression.2 The corrugator supercilii muscle is located underneath the bilateral eyebrows and is a common target of botulinum toxin injections.2 It has a large transverse head and an oblique head, the latter of which inserts into and depresses the skin of the medial brow to form the scowl lines of the glabella.2 The depressor supercilii has also been described as having a similar...