The lips are muscular flaps which are lined with skin superficially and mucous membrane on their deep surface. The point of transition is known as the vermillion zone – the pigmented part of the lip. This zone has superficial vessels, giving the lips their reddish hue. The lips are continuous with the cheeks laterally and the soft tissue of the face superiorly and inferiorly. They form the anterior part of the oral vestibule.
Posteriorly, with the teeth, alveolar process, oral vestibule and maxillary sinus.
Laterally with the cheeks
Superiorly with the ala of the nose and the nares
Inferiorly with the chin and mandible.
The outer aspect of the lips, distant from oral orifice, is covered with skin of similar pigment to the rest of the body. The superior lip is characterised by the philtrum, a verticle ridge that extends from the median part to the columella of the nose.
The vermillion zone of the upper lip displays a ‘Cupid’s Bow’, which extends distant from the lip as it progresses medially before bowing in between the two ridges on each side of the philtrum.
On the inner aspect of the lip, numerous small salivary glands may appear as small dots of altered pigment.
The skin of the outer lips is of the normal keratinising stratified squamous epithelium.
The mucosa of the vermillion zone is also keratinising stratified squamous, but is more transparent then elsewhere. There are numerous dermal papillae which are highly vascularised, giving the lips their characteristic red hue. The mucosa is typically thinnest on the external surface and thickens rapidly just prior to the junction with the inner lip mucous membrane.
The mucous membrane of the inner lip is lined with thick, non-keratinising stratified squamous epithelium. It is highly vascular and contains small salivary glands.
The arterial supply to the lower lip is derived from the inferior labial branch of the facial artery and the mental branch of the inferior alveolar artery (itself a branch of the maxillary artery). The inferior labial artery follows a course between the orbicularis oris and the mucosa of the vermillion zone to anastamose with the contralateral equivalent.
The upper lip is supplied by the superior labial branch of the facial artery. This travels in a similar course to the inferior labial artery and also anastamoses with the contralateral artery.
Superior and inferior labial veins are relatively constant and empty into the facial vein. This in turn returns to the internal jugular vein.
The lymphatics of the upper lip and lateral lower lip drain to submandibular nodes, whereas those of the medial lower lip flow to submental nodes. The submental nodes usually drain to submandibular nodes before all lymph arrives in the upper deep cervical nodes (level 2).
The upper lip is supplied by descending branches of the infraorbital nerve (V2). The lower lip is supplied by mental branches of the inferior alveolar nerve (V3).
Routes of Cancer Spread
Cancer of the lip may ulcerate through either the internal or external part of the lip. Local invasion beyond the lip is unusual, but may occur to the skin of the face, the bones of the mandible or maxilla.
The upper lip and the lateral parts of the lower lip drain to submandibular nodes. The central lower lip drains first to submental nodes and then to the submandibular nodes.
The lower lip is supplied by the mental nerve, a terminal branch of the inferior alveolar nerve. This is a branch of the mandibular nerve (V3).
The upper lip is supplied by descending branches of the infraorbital nerve after it emerges from the infraorbital foramen of the maxilla. The infraorbital is a branch of the maxillary nerve (V2).
Tumours may spread back along these nerves and their courses need to be followed carefully to ensure there is no disease along their path.
If metastatic, squamous cell carcinomas of the lip typically travel to lung, followed by bone and then the liver.