The cervical region of the human body has a great deal of anatomical, functional and clinical relevance. The cervical spine is the most mobile and therefore most unstable part of the human spine. The vertebral bodies stack on top of one another and the region has a great deal of surrounding fascia and musculature to support. The roots of the brachial plexus emerge between the anterior and middle scalene muscles and pass inferiorly into the axilla. The neck is divided into the anterior and posterior triangles. The anterior triangle is divided into the carotid, submental, visceral (muscular), and submandibular triangles. This article will discuss the anatomical, surgical and clinical relevance of submandibular space. This will be concluded with some review questions.
The boundaries of the anterior triangle of the neck are:
- Lateral: sternocleidomastoid muscle
- Superior: inferior border of the mandible
- Medial: anterior midline of the neck
The submandibular triangle is also known as digastric triangle; it is bordered superiorly by the inferior border of the mandible and the mastoid process, posteriorly by the posterior belly of digastric and stylohyoid, and anteriorly by the anterior belly of digastric. The roof of the triangle is the skin, superficial fascia, the platysma and the deep fascia. The branches of the facial nerve and transverse cutaneous cervical nerves also pass over the roof of the triangle. The digastric is a double-bellied muscle that depresses the mandible i.e. opens the mouth. The anterior belly arises from the digastric fossa found in the inner aspect of the anterior mandible, and the posterior belly arises from the mastoid notch. Both are joined by a tendinous sheath, and attach to the hyoid bone.
The anterior part of the triangle contains the submandibular salivary gland (innervated by the facial nerve). Ascending within the substance of the parotid gland in the posterior portion of the triangle is the external carotid artery. The facial artery emerges from behind the posterior belly of the digastric muscle, as does the lingual artery further inferiorly. In terms of surgical practice, the submandibular triangle is best visualized as having four layers. These layers start from the skin and are progressively deeper.
The roof of the submandibular triangle i.e. the first plane encountered surgically comprises of the skin, the superficial fascia. These enclose the platysma muscle and the subcutaneous fat. Also enclosed are the cervical and mandibular branches of the facial nerve (cranial nerve VII).
If we study the second surgical plane of the submandibular triangle from superficial to deep, the following contents can be found. The first structures to be encountered will be the anterior and posterior facial vein, also the submental branch of the facial artery, as well as the superficial layer of the sub-maxillary fascia i.e. deep cervical fascia. Deeper still, we will find the lymph nodes, and the deep layer of the sub-maxillary fascia i.e. deep cervical fascia, as well as the hypoglossal nerve (cranial nerve XII).
Surgically, it is imperative to remember that the facial artery pierces the stylomandibular ligament (which connects the styloid process to the ramus of the mandible). The artery must therefore be ligated to prevent catastrophic bleeding after retraction. Also worth noting, is that within the fascial envelope of the submandibular fascia, are the lymph nodes.
Both the anterior and posterior facial veins cross the triangle anterior to the submandibular gland, and unite near to the angle of the mandible, to form the common facial vein. The common facial vein then drains into the internal jugular vein near the greater cornu of the hyoid bone.
The facial artery is the fourth branch of the external carotid artery, and also enters the submandibular triangle by passing beneath the posterior belly of the digastric muscle, as well as the stylohyoid muscle. Once it enters the triangle, it also lies beneath the submandibular gland. Once the artery has crossed the gland on its posterior aspect, it passes over the mandible, and is consistently found under the platysma.
Next is the third surgical layer. Once again the structures from superficial to deep are the mylohyoid muscle along with its nerve, the hyoglossus muscle, as well as the middle constrictor muscle which lies over the lower part of the superior constrictor, and a subsection of the styloglossus muscle.
The mylohyoid muscles are regarded as the true diaphragm of the floor of the mouth. These muscles arise from the mylohyoid line that is found on the inner surface of the mandible, and inserts into the body of the hyoid bone itself. The nerve that supplies the mylohyoid is a branch of the alveolar division of V3 (the mandibular division of the trigeminal nerve), and lies on the surface of the inferior aspect of the muscle. The superior surface of mylohyoid is in contact with the lingual nerve (division of V3) and hypoglossal nerve (cranial nerve XII).
Finally we have the deepest or fourth surgical plane. The structures within this plane, from superficial to deep are the deep portion of the submandibular gland, the duct of the submandibular gland (Wharton’s duct), the lingual nerve (division of V3), the sublingual artery, the sublingual vein, which lie superficial to the sublingual gland. Deeper still we find cranial nerve XII (hypoglossal nerve), as well as the submandibular ganglion. The submandibular duct is found inferior to the lingual nerve (except where the lingual nerve passes beneath it) as well as superior to the hypoglossal nerve.
This is said to be the worst pain ever felt by those who have it. It is caused usually by a harmless stimulus on the face e.g. shaving, which causes an electric shock like pain in the distribution of that division of the trigeminal, and sometimes the entire nerve. Theories as to its cause focus on small sites of demyelination, which when stimulated, cause an abnormal and excessive neural feedback to the brain.
This is a lower motor neurone lesion of the facial nerve, the commonest cause of which is a herpes infection (Ramsey-Hunt syndrome). Symptoms include hemiplegia of the face, pain on hearing sounds (hyperacusis) due to the lack of innervation by the nerve to stapedius. Consequently, the stapedius cannot pull on the stapes bone and dampen sown sounds. Lack of tears will result (facial nerve innervates the lacrimal glands), ptosis occurs (eyelid drooping) due to lack of innervation to orbicularis oculi that normally shut the eye. Therefore the eye can be abnormally dry, the corneal reflex will be impaired also. Taste to the anterior two thirds of the tongue will also be deranged, due to the lack of innervation by the chorda tympani.
Upper motor neurone lesions of the facial nerve can be differentiated from lower, by the presence of forehead sparing.
Mandibular nerve injury
Injury to the mandibular branch results in severe drooling at the corner of the mouth. Injury to the anterior cervical branch produces minimal drooling that will disappear in 4 to 6 months.
This is a potentially life threatening infection of the floor of the mouth resulting from dental infections, that spread into the submandibular triangle and beyond.