Undermining Planes by Anatomical Region

Undermining is the process of separating tissue layers to create a mobile tissue plane that allows wound edges to advance toward each other for tension-free closure. The correct undermining plane varies by anatomical region and is determined by the location of critical structures (nerves, vessels, muscles) relative to the skin surface. The fundamental rule is to undermine in a plane that provides adequate tissue mobility while remaining superficial to vital structures. In general, undermining should extend at least as far from the wound edge as the diameter of the defect. The "1:1 rule." Blunt undermining with scissors (spread technique) is preferred over sharp undermining in areas near nerves because blunt dissection pushes nerves aside rather than cutting them.

The following table provides a rapid reference for the safe undermining plane in each facial and neck region. Each plane is defined by the layer immediately superficial to the critical structures that must be protected.

The eyelid has the thinnest skin on the body (0.5-1.0 mm) with virtually no subcutaneous fat between the skin and the orbicularis oculi muscle. Undermining is performed directly above the orbicularis oculi muscle. Essentially in the preseptal/pretarsal plane between skin and muscle. Because there is almost no subcutaneous fat, the plane is entered immediately upon penetrating the dermis. The orbital septum lies deep to the orbicularis oculi and must never be violated. Penetration of the septum exposes orbital fat and risks damage to the levator aponeurosis (upper lid) or inferior oblique muscle (lower lid). The corrugator supercilii and procerus muscles are deeper structures in the glabellar region that are not typically at risk during eyelid undermining.

The scalp has five distinct layers, memorized with the mnemonic S-C-A-L-P: Skin (thick, hair-bearing), subCutaneous connective tissue (dense, fibrous, contains the major blood vessels and nerves), Aponeurosis/Galea aponeurotica (the flat tendon connecting frontalis anteriorly and occipitalis posteriorly), Loose connective tissue (the subgaleal/subaponeurotic space. The key surgical plane), and Periosteum (adherent to the outer calvarium). The subgaleal space (loose connective tissue layer) is the preferred plane for scalp undermining because it is relatively avascular, extends over the entire calvarium, and allows wide, easy mobilization of the entire scalp as a unit. The emissary veins traverse the subgaleal space, connecting the extracranial veins to the dural sinuses. These can be a source of bleeding and, rarely, a route for infection spread intracranially.

Lip undermining requires special attention because the labial artery (superior and inferior branches of the facial artery) courses WITHIN the orbicularis oris muscle, not above or below it. The artery runs approximately 3-5 mm deep to the vermilion-cutaneous junction within the muscle belly. The safe undermining plane is above the orbicularis oris muscle. In the subcutaneous plane between skin and muscle. Undermining through or below the orbicularis oris risks transecting the labial artery, which can cause significant hemorrhage. The labial artery is the dominant blood supply to the lip and must be preserved when possible.

Nasal undermining varies by subunit. On the distal nose (tip, ala, dorsum), undermining is performed deep to the superficial musculoaponeurotic system (SMAS) and the nasalis muscle, in the submuscular plane directly on the perichondrium or periosteum of the underlying cartilaginous/bony framework. This deep plane allows mobilization of the entire skin-muscle envelope as a composite unit. On the proximal nose (bridge, radix, sidewall), undermining may be in the subcutaneous plane above the nasalis, particularly for small defects. The nasal SMAS is continuous with the facial SMAS and contains the nasalis, procerus, and depressor septi muscles.

The cheek is divided into upper and lower zones with different undermining considerations. The lower cheek (below the zygomatic arch) requires undermining in the mid-subcutaneous fat, above the SMAS. The SMAS in the cheek is a critical protective layer. The facial nerve branches, parotid duct (Stensen's duct), and facial artery all lie deep to the SMAS. Undermining deep to the SMAS risks injury to these vital structures. The parotid duct (Stensen's duct) runs along a line from the tragus to the mid-upper lip, crossing the masseter muscle approximately 1 cm below the zygomatic arch. It penetrates the buccinator muscle to open into the oral cavity opposite the second upper molar.

The temple is the single most dangerous region for undermining on the face because the temporal branch of the facial nerve runs in a superficial plane within the temporoparietal fascia (superficial temporal fascia). Unlike all other facial regions where deeper undermining is safer, the temple requires SUPERFICIAL subcutaneous undermining. Above the temporoparietal fascia. The superficial temporal artery and vein also run in the temporoparietal fascia and are at risk during temple surgery. The sentinel vein (zygomaticotemporal vein), which crosses the zygomatic arch, is a useful landmark. The temporal branch of CN VII typically runs 1 cm posterior to this vein.

Forehead undermining plane depends on defect size. For small forehead defects, undermine in the deep subcutaneous fat just above the frontalis muscle. This preserves frontalis function and is adequate for small advancement or rotation flaps. For large forehead defects requiring extensive mobilization, the subgaleal plane (identical to the scalp subgaleal plane) provides maximal tissue mobilization. The galea aponeurotica (scalp) transitions to the frontalis muscle in the forehead. The subgaleal plane is continuous across this transition. For eyebrow undermining, the plane is in the deep subcutaneous fat BELOW the hair follicle bulbs but above the frontalis muscle. This plane preserves eyebrow hair growth while allowing tissue mobilization. Undermining through or above the bulb level will damage hair follicles and cause permanent eyebrow alopecia.

Neck undermining is performed in the superficial subcutaneous fat, above the platysma muscle. The platysma is a thin sheet of muscle that covers the anterior and lateral neck. It is the equivalent of the facial SMAS in the neck. Critical structures lie deep to the platysma: the external jugular vein (superficial but beneath platysma laterally), the spinal accessory nerve in the posterior triangle, and the cervical plexus sensory branches. In the anterior neck, the plane above the platysma is relatively safe, but in the posterior triangle (behind the sternocleidomastoid), the spinal accessory nerve (CN XI) is at risk even with superficial dissection because it runs extremely close to the skin surface at Erb's point.