The oral cavity and salivary glands

Dental pantomography The dental pantomogram (see Fig. 1. 15) gives a panoramic image of both dental arches, as well as the mandible, temporomandibular joints and lower maxilla. This study is obtained using special equipment that moves around the patient's face as the radiograph is being taken, mapping out the lower face and jaw in a straight line.

Arthrography Arthrography of the temporomandibular joint may also be performed where radio-opaque contrast is injected directly into the synovial spaces under radiographic control. Contrast should not pass from one synovial compartment to the other.

THE ORAL CAVITY AND SALIVARY GLANDS

The oral cavity (Fig. 1. 20) This forms a passage from the lips to the oropharynx. It is largely filled by the tongue and teeth and is lined by a mucous membrane. The parotid gland opens on to its lateral wall, and the submandibular and sublingual glands open on to its floor. The roof is formed by the hard palate anteriorly and the soft palate posteriorly. The soft palate is a mobile flap that hangs posteroinferiorly at rest, separating the oro- from the nasopharynx. Two muscles insert into it from the lateral wall of the pharynx - the levator and the tensor veli palatini. These elevate the soft palate during swallowing to prevent reflux into the nose. The uvula hangs from the middle of the soft palate, and two pairs of muscles, the palatoglossus and the palatopharyngeus, run from its base to the tongue and pharynx. These muscles and their overlying mucosa form the anterior and posterior fauces, in whose concavity the palatine tonsils lie.

The muscles of the tongue form two groups. The intrinsic group are arranged in various planes and alter the shape of the tongue. The extrinsic group are paired muscles that move the tongue and have attachments outside it. The genioglossus arises from the inner surface of the symphysis menti and fans out to form the ventral surface of the tongue. Its inferior fibres form a tendon that attaches to the hyoid bone. The hyoglossus and chondroglossus are thin sheets of muscle that arise from the hyoid bone and insert into the side of the tongue. The styloglossus passes from the styloid process to the side of the tongue. A median raphe divides the tongue into two halves.

The floor of the mouth is formed by other muscles that also support the tongue. The most important is the mylohyoid muscle, which is slung from the mylohyoid line on the inner surface of the mandible to the hyoid bone on either side.

Fig. 1. 20 Floor of mouth: coronal section.

The inferior fibres of genioglossus pass from the hyoid to the symphysis above the mylohyoid, and the anterior belly of digastric passes from the hyoid to the symphysis below the mylohyoid. The stylohyoid is lateral, passing from the styloid process to the hyoid. The posterior belly of digastric runs from the mastoid process to the lateral aspect of the hyoid bone. The anterior belly runs from here to the base of the anterior part of the mandible.

Lymphatic drainage of the oral cavity is to the submental and submandibular nodes, and to the retropharyngeal and deep cervical nodes.

Radiology of the oral cavity (Fig. 1. 21) Because the oral cavity is amenable to direct vision, radiological assessment is not often required. However, in the case of infiltrating pathology such as tumours, crosssectional imaging using CT or magnetic resonance imaging (MRI) is very useful. The hard and soft palate, palatine fossa and extrinsic muscles of the tongue may be identified using both modalities, as may the maxilla, mandible, hyoid bone and surrounding structures.

MRI has inherently better soft-tissue contrast than CT and can image in coronal and sagittal as well as axial planes. There is no artefact from the mandible or dental amalgam, and so MR images are superior to CT in this area.

The salivary glands

These exocrine glands are situated symmetrically around the oral cavity and produce saliva.

The parotid gland (Figs 1. 22-1. 24) This is the largest of the salivary glands and lies behind the angle of the jaw and in front of the ear. It is moulded against the adjacent bones and muscles. The gland has a smaller deep part and a larger superficial part, both of which are continuous around the posterior aspect of the ramus of the mandible via the isthmus.

The deep part of the gland extends medially to the carotid sheath and lateral wall of the pharynx, separated from these by the styloid process and muscles. The superficial part lies anterior to the tragus of the ear and is moulded to the mastoid process and sternomastoid muscle posteriorly, and to the posterior ramus of mandible and masseter muscle anteriorly. It has an anteroinferior extension, or tail, which wraps around the angle of the mandible.

The terminal part of the external carotid artery runs through the isthmus, dividing into superficial temporal and maxillary branches within the substance of the gland, and the confluence of the veins of the same name form the posterior facial vein just superficial to the artery. The facial nerve, having emerged from the stylomastoid

Fig 1. 22 Parotid gland: (a) lateral view; (b) transverse section.

1. Medial pterygoid muscle 6. Parotid gland 2. Mandible 7. External carotid artery and 3. Masseter muscle posterior facial vein 4. Internal carotid artery 8. Mastoid process 5. Internal jugular vein 9. Sternocleidomastoid muscle foramen, runs through the deep part of the gland via the isthmus to the superficial part within which it branches into its five terminal divisions. It passes superficial to the internal carotid artery and retromandibular vein in the isthmus.

The parotid duct (Stensen's duct) begins as the confluence of two ducts in the superficial part of the gland and runs anteriorly deep to the gland. It arches over the masseter muscle before turning medially to pierce buccinator and drain into the mouth opposite the second upper molar. The duct is approximately 5 cm long. Small accessory parotid glands are common (20%), joining the duct along its length.

The submandibular gland (Fig. 1. 24) This gland lies in the floor of the mouth medial to the angle of the mandible. It is a mixed mucinous and serous gland, hence its tendency to form calculi. It has a lower superficial lobe continuous w i th a smaller deep lobe above around the posterior border of the mylohyoid muscle.

The submandibular (Wharton's) duct is about 5 cm long and commences as a confluence of several ducts in the superficial (lower) lobe. From here it runs superiorly through the deep (upper) lobe before running forward in the floor of the mouth to open at the side of the frenulum of the tongue.

Fig. 1. 24 Sialography: (a) AP view of parotid gland; (b) lateral view of parotid gland; (c) lateral view of submandibular gland. Note how the duct and its branches are moulded around the ramus of the mandible.

(a) 1. Cannula in parotid duct 2. Parotid duct (Stensen's) 3. Normal branching ductules 4. Condylar process of mandible 5. Angle of mandible 6. Body of mandible

(b) 1. Cannula in parotid orifice 2. Parotid duct 3. Secondary ductules 4. Contrast on surface of tongue 5. Condylar process of mandible 6. Angle of mandible

(c)

1. Cannula in orifice of submandibular duct 2. Submandibular duct 3. Secondary ductules 4. Contrast on superior surface of tongue 5. Condylar process of mandible 6. Coronoid process of mandible 7. Mandibular notch