The mandible and teeth

Radiology of the nasal cavity and paranasal sinuses

Plain films (Fig. 1. 8) The frontal sinuses are not visible on the skull radiograph until the age of 2 years and achieve adult proportions by the age of 14. Asymmetry is common, and one or both may fail to develop. Absence of both may be associated w i th persistence of the metopic suture between the two halves of the frontal bone. Development of the ethmoids occurs at a rate similar to that of the frontal sinuses.

Pneumatization of the sphenoid sinus commences at 3 years of age and may extend into the greater wings of sphenoid or clinoid processes. The degree of pneumatization is variable and relevant to transsphenoidal hypo¬ physectomy.

The maxillary sinuses are the first to appear and are visible radiologically from a few weeks after birth. They continue to grow and develop throughout childhood. The tooth-bearing alveolar process does not begin to develop until the age of 6 years. Full pneumatization of the maxillary sinus is not achieved until there has been complete eruption of the permanent dentition in early adulthood.

Computed tomography and MRI CT scanning in either axial or coronal planes provides excellent visualization of the paranasal sinuses (see Fig. 1. 12). Particular attention is paid to the region of the ostiomeatal complex, where the maxillary, frontal and anterior ethmoidal sinuses drain, and the sphenoethmoid recess and superior meatus, on to which the sphenoid and posterior ethmoid sinuses drain. The pneumatized sinuses should contain nothing but air.

MRI is surprisingly good at demonstrating the sinuses, as the bony septa, which have no signal themselves, are lined by high-signal mucosa. The bone is seen as a lowintensity structure sandwiched between high-intensity mucosal layers. Air is also of low signal intensity.

Embolization for epistaxis When cautery of the bleeding area and nasal packing and other surgical methods fail to control epistaxis, embolization may be successful. Angiographic assessment of the facial, sphenopalatine and greater palatine branches of the external carotid circulation is most likely to identify the source of bleeding. The ethmoidal branches of the ophthalmic artery may also need to be visualized and embolized. In all vessels, the microcatheter to be used for embolization must be advanced distal to branches w i th a high potential for dangerous anastomotic collaterals, such as the middle meningeal or ophthalmic arteries. Embolization of the superior labial branch of the facial artery is associated w i th necrosis of the nasal alae.

THE MANDIBLE AND TEETH

The mandible (Figs 1. 13 and 1. 18) The mandible is composed of two halves united at the symphysis menti. Each half comprises a horizontal body and a vertical ramus joined at the angle of the mandible. The ramus has two superior projections, the coronoid process anteriorly and the condylar process posteriorly, separated by the mandibular (or condylar) notch. The coronoid process gives attachment to the temporalis muscle, and the condylar process (or head of mandible) articulates w i th the base of the skull at the temporomandibular joint. The body of the mandible bears the alveolar border w i th its 16 tooth sockets.

The mandibular canal runs in the ramus and body of the bone, transmitting the inferior alveolar artery (branch of the maxillary artery) and nerve (branch of the mandibular division of the trigeminal nerve). The mandibular canal opens proximally as the mandibular foramen on the inner surface of the upper ramus, and its distal opening is the mental foramen on the external surface of the body below and between the two premolars.

The muscles of the floor of the mouth, including the medial pterygoid muscles, are attached to the inner surface of the mandible and the muscles of mastication are attached to its outer surface.

The temporomandibular joint

(Figs 1. 14-1. 16) This is a synovial joint between the condyle of the mandible and the temporal bone. The temporal articular surface consists of a fossa posteriorly, the temporomandibular fossa, and a prominence anteriorly, the articular tubercle. The head of the mandible sits in the fossa at rest and glides anteriorly on to the articular tubercle when fully open. The joint is least stable during occlusion.

The articular surfaces are covered w i th fibrous cartilage. In addition, a fibrocartilaginous disc divides the joint into separate smaller upper and larger lower compartments, each lined by a synovial membrane. The disc is described as having anterior and posterior bands w i th a thin zone in the middle, and is attached to the joint capsule. The anterior band is also attached to the lateral pterygoid muscle. The posterior band is attached to the temporal bone by bands of fibres called the translational zone. No communication between joint compartments is possible unless the disc is damaged. The upper compartment is involved in translational movements and the lower in rotational movements.

Fig. 1. 15 Radiographs of the temporomandibular joint: (a) closed mouth view; (b) open mouth view.

The teeth - nomenclature and anatomy (Fig. 1. 17) There are 20 deciduous or milk teeth; in the adult these are replaced by 32 permanent teeth. The complement of teeth in each quadrant is as follows: • In the child: two incisors, one canine, two molars; and • In the adult: two incisors, one canine, two premolars, three molars.

The teeth are referred to by their position in each of the four quadrants. The relevant quadrant is designated by two arms of a cross, and the tooth by its position relative to the midline. The permanent teeth are referred to by number and the milk teeth by capital letter. Thus the second right lower premolar in the adult is designated 5 and the second left upper molar in a child's milk dentition is designated E.

Each tooth has its own root embedded in a separate socket. The neck of the tooth is covered by the firm fibrous tissue of the gum, and this is covered by the mucous membrane of the mouth. The exposed intraoral part of the tooth is the crown, and this is covered by enamel, which is the hardest and most radio-opaque substance in the body. The remainder of the tooth is composed mostly of dentine, which is of a radiographic density similar to compact bone. A radiolucent pulp cavity occupies the middle of the tooth and is continuous w i th the root canal, which transmits the nerves and vessels from the supporting bone. The root and neck of the tooth are surrounded by the periodontal membrane, which forms a radiolucent line around them on the radiograph. A dense white line of bone surrounds this and is known as the lamina dura. This surrounds the root of each tooth and is continuous w i th the lamina dura of the adjacent teeth around the margin of the alveolar crest.

Radiology of the mandible and teeth

1. 13, 1. 16, 1. 18 and 1. 19)

Plain films

The mandible may be seen on the OF, OF20, OM, OM30 and lateral projections. Special oblique views may be required to show the rami if a fracture is suspected. There are also special views for the temporomandibular joints, and a f u ll radiographic study of these includes images of both joints w i th the mouth open and closed. The teeth can be radiographed on small films placed close up against them inside the mouth, which provide excellent detail.

The symphysis menti fuses at 2 years of age. Eruption of the milk dentition is normally complete by this time. The permanent dentition develops in the mandible and maxilla during childhood and can be identified radiographically. Most have calcified by 3 years of age. The roots of the milk teeth are resorbed as the permanent teeth erupt. The medial teeth erupt before the lateral teeth, and lower teeth erupt approximately 6-12 months before the upper teeth. The first permanent molar erupts at 6 years of age, and all the permanent dentition is present by the age of 12 or 13 years except the third molar (wisdom tooth), which does not appear until early adulthood.

The mandibular canal and mandibular and mental foramina may be identified on radiographs of the mandible. Cross-sectional imaging (see Fig. 1. 16) High-resolution CT is used in the assessment of fractures of the mandible. MRI is excellent for the demonstration of the internal temporomandibular joint anatomy. The anterior and posterior bands and the thin zone of the disc are identifiable, as are the disc attachments.

Fig. 1. 19 Occlusal radiograph of the teeth.

1. Lamina dura of bone 2. Periodontal membrane (radiolucent line between the lamina dura and tooth margin) 3. Dentine 4. Pulp chamber 5. Root canal 6. Enamel

1. Condylar process of mandible 5. Medial wall of maxillary sinus 2. Temporomandibular fossa 6. Floor of maxillary sinus 3. Hard palate 7. Condylar canal 4. Maxillary sinus 8. Ramus of mandible 9. Angle of mandible 12. Upper left incisor 10. Body of mandible 13. Hyoid bone (projected laterally) 11. Mandibular notch 14. Ear lobe