The lips, which form between the 4th and 8th week of gestation, consist of four basic components: the skin and subcutaneous tissue, the muscle, the mucosa, and the vermilion. Each of these structures has unique characteristics that must be considered when planning the reconstruction.
The skin of the lip is typical of facial skin. It is hair bearing. In women and children this is mostly vellus hair. The skin is rich in sebaceous and sweat glands. Deep to the skin is a significant amount of subcutaneous fat, that makes up the bulk of lip thickness.
The external landmarks of the lip are: The philtral columns, which are the bilateral verical bulges created by dermal insertion of the contralateral orbicularis oris fibers. The philtral dimple is a concavity between the comlums. The white roll is the ridge just above the cutaneous vermilion border. The vermilion is the red portion of the lip, it is divided into the keritanized dry vermilion, and nonkeritanized wet vermilion. The red line is the junction between the dry and wet vermilion. The cupids bow is the curvature of the central white roll. The tubercle is the vermilion fullness below cupid’s bow.
Mucosa and Vermilion
The inside of the lip is lined by mucosa, which is nonkeratinized epithelium. The mucosa is distinct from vermilion in its color, texture and appearance.
Vermilion, on the other hand, is the visible portion of the lip inside the white roll. It is duller than mucosa in its appearance. It gets its unique color and spongy nature from the underlying dense capillary network. It has a unique light reflection and is nearly impossible to duplicate. The red line is the junction of the wet vermilion and dry vermilion.
A vermilion mismatch of 1 mm during repair can be noticed readily at conversational distance. Note that it is impossible to accurately identify the vermilion or the white roll after injection of local anesthetic. The mere presence of injected volume or vasoconstriction of epinephrine distorts the color and obscures the white roll. The vermilion can be marked with methylene blue prior to injection.
The blood supply to the lips is redundant. The main supply comes through the labial arteries, which are branches from the facial artery, and form a complete loop around the upper and lower lips. This is useful for allowing various flap designs. The arteries lie just deep to the orbicularis oris muscle and can be found in a cross sectional plane approximately at the wet-dry line.
The venous drainage does not follow the arterial blood supply. Instead there is a subdermal and submucous venous plexus of smaller diameter vessels.
The lymphatic drainage of the lips is important for oncologic considerations. Both lips drain primarily into the ipsilateral submandibular nodes, with the commissures also drainaing to the periparotid nodes.
The primary muscle of the lips is the paired orbicularis oris. The orbicularis oris fibers cross the midline and insert into the opposite philtral column in the upper lip. The deep portion of the muscle functions as a sphincter, with fibers passing from commissure to commissure. The superficial portion of the muscle functions in speech and facial expression.
The paired mentalis muscles are the main elevators of the lower lip. They originate from the mandible, and insert inferiorly into the chin pad below the labiomental fold. Contraction of the mentalis muscle works to strongly coapt the lower and upper lips and to push contents out of the gingivobuccal sulcus.
The upper lip receives its sensibility from the infraorbital nerve, which is a branch of the maxillary division of the trigeminal nerve. It exits the skull through the foramen rotundum, passes through the inferior orbital fissure and travels along the orbital floor before diving into the maxillary sinus and emerging from the bone through the infraorbital foramen. The infraorbita nerve provides sensation to the upper lip, cheek, ala, and nasal sidewall.
Local anesthesia can be established by injecting lidocaine around the infraorbital foramen, ehich opens inferomedialy about 5-7 mm from the infraorbital rim, and is in line with the medial limbus. This can be done through a skin injection or intraoraly.
The sensory innervation of the lower lip is provided by the mental nerve. The mental nerve is the terminal branch of the inferior alveolar nerve, which in turn is a branch of the mandibular division of the trigeminal nerve. The mental nerve exits the mandible through the mental foramen, which can consistently be found halfway between the superior and inferior borders of the mandible in the plane between the premolars.
Knowledge of nerve anatomy is important because it allows the plastic surgeon to quickly and easily establish local anesthesia for lip procedures.