NBDHE

Study this section as applied anatomy. The exam rarely asks you to list structures for their own sake. It asks why the mesial surface of a maxillary first premolar traps calculus, which lymph node you would expect to be tender with an abscessed mandibular incisor, or which nerve is affected when a patient cannot close one eye after an inferior alveolar block. Every chapter ends with Key Takeaways you can review the week of the exam.

This chapter covers the bones and foramina of the skull, the muscle groups of the head and neck, the temporomandibular joint, and the salivary and thyroid glands. These structures are the landmarks for the extraoral examination and for local anesthetic injections.

The skull has 8 cranial bones (frontal, two parietal, two temporal, occipital, sphenoid and ethmoid) and 14 facial bones (two maxillae, two zygomatic, two nasal, two lacrimal, two palatine, two inferior nasal conchae, the vomer and the mandible). The mandible is the only freely movable skull bone. The hyoid bone sits in the neck and does not articulate with any other bone; it is suspended by muscles and ligaments.

The hard palate is formed by the palatine processes of the maxillae (anterior two-thirds) and the horizontal plates of the palatine bones (posterior third). The median palatine suture runs down the midline, and the incisive foramen sits just palatal to the maxillary central incisors. The maxillary tuberosity is the rounded bulge distal to the last maxillary molar; the posterior superior alveolar nerve enters small foramina on its surface.

Key landmarks of the mandible include the condyle (articulates in the TMJ), the coronoid process (insertion of the temporalis), the ramus, the mandibular notch, the lingula (guards the mandibular foramen), the mylohyoid line on the lingual surface (attachment of the mylohyoid muscle), the genial tubercles (attachment of the genioglossus and geniohyoid), the external oblique ridge, the retromolar triangle, and the mental foramen, usually located below or between the apices of the mandibular premolars.

Foramina are a favorite exam topic because they link bone, nerve and anesthesia.

All four muscles of mastication develop from the first pharyngeal arch and are innervated by the mandibular division of the trigeminal nerve (V3).

The muscles of facial expression come from the second pharyngeal arch and are innervated by the facial nerve (CN VII). They insert into skin, which is why they produce expressions. Key members include the orbicularis oris (closes and purses the lips), buccinator (keeps food on the occlusal table, pierced by the parotid duct), mentalis (raises the chin and protrudes the lower lip; hyperactive in some lip-incompetent patients), zygomaticus major (smiling), risorius, levator labii superioris, depressor anguli oris, orbicularis oculi (closes the eyelid), the frontal belly of the epicranius (raises the eyebrows) and the platysma.

Suprahyoid muscles (digastric, mylohyoid, stylohyoid, geniohyoid) elevate the hyoid during swallowing and help depress the mandible when the hyoid is fixed. Their innervation is mixed and testable: the mylohyoid and anterior belly of the digastric are innervated by V3 (via the mylohyoid nerve); the posterior belly of the digastric and the stylohyoid by CN VII; the geniohyoid by the first cervical nerve traveling with CN XII. The mylohyoid forms the muscular floor of the mouth.

Infrahyoid muscles (sternohyoid, sternothyroid, thyrohyoid, omohyoid) depress the hyoid and are innervated mainly by the ansa cervicalis (C1-C3).

The tongue has intrinsic muscles (change shape) and extrinsic muscles (change position): genioglossus (protrudes, the "safety muscle" that keeps the tongue from falling back), hyoglossus (depresses), styloglossus (retracts and elevates) and palatoglossus (elevates the back of the tongue). All tongue muscles are innervated by the hypoglossal nerve (CN XII) except the palatoglossus, which is supplied by the vagus (CN X) through the pharyngeal plexus. When CN XII is damaged on one side, the protruded tongue deviates toward the injured side.

Soft palate muscles are supplied by CN X except the tensor veli palatini (V3). The pharyngeal constrictors are supplied by CN X; the stylopharyngeus is the only muscle supplied by CN IX.

The TMJ is a ginglymoarthrodial joint, meaning it both hinges (ginglymus) and glides (arthrodial). The condyle of the mandible articulates with the articular fossa and articular eminence of the temporal bone. A fibrous articular disc divides the joint into two synovial cavities:

Supporting ligaments are the temporomandibular (lateral) ligament, the sphenomandibular ligament (a landmark near the IANB injection site) and the stylomandibular ligament. Dislocation occurs when the condyle moves anterior to the articular eminence and the patient cannot close. TMJ examination includes palpating just anterior to the tragus of the ear while the patient opens and closes, noting clicking, crepitus, deviation, pain and the maximum opening in millimeters.

The submandibular gland produces about 60-70% of unstimulated (resting) saliva, while the parotid contributes the largest share of stimulated saliva. Minor salivary glands are scattered throughout the lips, buccal mucosa and palate; von Ebner glands are serous minor glands that empty into the trenches around the circumvallate papillae. Salivary stones (sialoliths) form most often in the submandibular gland and duct because its secretion is more viscous and the duct travels uphill.

Parasympathetic secretomotor fibers reach the parotid from CN IX (via the otic ganglion and auriculotemporal nerve) and reach the submandibular and sublingual glands from CN VII (via the chorda tympani, lingual nerve and submandibular ganglion).

The thyroid gland has two lobes connected by an isthmus and lies inferior to the thyroid cartilage. It moves upward when the patient swallows, which helps distinguish it during palpation. Palpate it from behind or in front using both hands while the patient swallows, noting enlargement (goiter), nodules or tenderness.

• Foramen rotundum carries V2, foramen ovale carries V3, and the stylomastoid foramen carries CN VII out of the skull.
• Muscles of mastication are V3; muscles of facial expression are CN VII; tongue muscles are CN XII except the palatoglossus (CN X).
• The lateral pterygoid protrudes and opens; the masseter, temporalis and medial pterygoid elevate.
• TMJ rotation occurs in the lower synovial cavity and translation in the upper cavity.
• Parotid (serous) opens opposite the maxillary second molar; submandibular (mostly serous) produces most resting saliva and opens at the sublingual caruncle.