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Submandibular triangle
The neck, or cervical region, is perhaps one of the most anatomically complex regions of the body. Despite being a relatively small region, the contents within this region (and notably the interrelationships between them) hold a great deal of anatomical, functional and clinical relevance.
To deal with the complex anatomy involved, the cervical region can be organised and divided into what are known as the surgical triangles of the neck. At the most basic division, the neck is divided into the anterior and posterior triangles, which are separated by the sternocleidomastoid muscle.
Focusing specifically on the anterior triangle, it can be divided into four smaller triangles, which are the:
- carotid triangle
- submental triangle
- muscular (visceral) triangle
- submandibular triangle
This article will discuss the anatomical, surgical and clinical relevance of submandibular triangle.
Overview of the submandibular triangle
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As mentioned above, the submandibular triangle is a subsection of the larger anterior triangle of the neck, which is defined by the following boundaries:
- Lateral: sternocleidomastoid muscle
- Superior: inferior border of the mandible
- Medial: anterior midline of the neck
Boundaries
The submandibular triangle, also known as digastric triangle, is located superior to the hyoid bone. It is bordered:
- superiorly by the inferior border of the mandible and the mastoid process,
- posteriorly by the posterior belly of the digastric and stylohyoid muscles, and
- anteriorly by the anterior belly of digastric muscle.
The roof of the triangle is formed the skin, superficial cervical fascia, the platysma and deep cervical fascia. The branches of the facial nerve and transverse cutaneous cervical nerves also pass over the roof of the triangle.
Digastric muscle
The submandibular triangle is largely defined by the digastric muscle, which 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/internal aspect of the anterior mandible, and the posterior belly arises from the mastoid notch of temporal bone. Both are joined by a tendinous sheath, and attach to the hyoid bone.
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Submandibular gland
A major landmark of the submandibular triangle is the submandibular gland (innervated by the facial nerve). This salivary gland can be described as having two lobes, which are divided by the posterior border of the mylohyoid muscle. The superificial lobe is the larger of the two, located superficial to the inferior surface of the mylohyoid muscle. The smaller deep lobe wraps around the posterior border of the mylohyoid, which will be further elaborated on below.
Contents
In terms of surgical practice, the submandibular triangle is best visualized as having four layers. These layers start from the skin and continue progressively deeper.
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First layer (roof)
As previously mentioned, the roof of the submandibular triangle i.e. the first plane encountered surgically comprises of the skin and 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).
Second layer (submandubilar space)
If we study the second surgical plane of the submandibular triangle, the following contents can be found:
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- the submandibular lymph nodes
- the superficial portion/lobe of the submandibular gland,
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- the submental branch of the facial vein, which accompanies the submental branch of the facial artery.
- The vessels and nerves to mylohyoid muscle lie directly along the inferior surface of the same muscle.
- The superficial/investing layer of the deep cervical fascia is also located here.
Both the facial vein and anterior branch of the retromandicular vein cross the triangle anterior, or superficial 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.
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The facial artery (which is the fourth branch of the external carotid artery) 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 deep to the submandibular gland. Once the artery has crossed the gland over its posterior aspect, it curls around the inferior border of the mandible, and ascends superomedially across the facial region.
The inferior tip of the parotid gland can be found within in the posterior region of the digastric triangle. Ascending within the substance of the parotid gland is the external carotid artery.
Third layer (floor)
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.
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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 the mandibular division of the trigeminal nerve (CN V3), 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).
Fourth layer (basement/sublingual space)
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 & 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.
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Clinical notes
Trigeminal neuralgia
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.
Bell’s palsy
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This is a lower motor neuron 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 neuron 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.
Ludwig’s angina
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.
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