The nose, paranasal sinuses and the facial skeleton

 
Technique

Both conventional and cross sectional imaging is an important complement to the clinical examination of the nose, paranasal sinuses and the facial skeleton in malformations, as well as, conditions relating to, trauma, infections or tumors. Conventional plain radiographs of the paranasal sinuses can be used to verify or rule out involvement of the sinuses in both allergic and infectious conditions. Large destructive lesions associated with an underlying malignancy can also be detected. Plain films are also indicated in the primary evaluation of facial trauma.

The complex anatomy of the facial skeleton nessecitates the use of up to four projections to completely depict the paranasal sinuses. The four views are the frontal or Caldwell view, the semiaxial or Waters view, and the lateral and the axial view (Fig. 9). The examination should be done with the patient sitting up in order to be able to demonstrate airfluid levels in the paranasal sinuses.

Tomography using panorama technique, so called orthopantomogram or Panorex views, can be used for an overview of the jaw and the teeth. Temporomandibular joints, the maxilla and the pterygoid plates can also be studied by this technique. For close up study of the teeth and their roots regular dental radiographs are needed.

Computed tomography, and more recently magnetic resonance imaging, have become invaluable tools for the assessment of malignancies. Both methods can also be used to assess both traumatic and infectious conditions in this area. MRI is the primary study for internal derangements of the temporomandibular joints.

Anatomy

The ethmoid aircells are present at birth. The maxillary sinuses develop thereafter from small outbuddings in the nose underneath the middle turbinate. The frontal sinuses start to develop at approximate two years of age and the sphenoid sinuses at 3-4 years. The paranasal sinuses are

not fully developed until the early teens.

The lateral walls of the ethmoid air cells are formed by the lamina papyracea and medially by the base of the upper and middle turbinates and the lateral wall of the nose. The ethmoids drain through 2-3 openings underneath the middle turbinate and thereby dividing the aircells into the anterior, middle and posterior compartments; the posterior cells drain into the sphenoethmoidal recess.

The openings of the maxillary antra or sinuses can also be found underneath the middle turbinates. Fully developed the antra will extend out towards the zygomatic process of the maxilla, down into the alveolar ridge, and superiorly up against the ethmoids and the orbital floor with the inferior orbital nerve canal.

The frontal sinuses can form multiple lobulated recesses in the frontal bone. Each frontal sinuses therefore has a unique shape which can be used for identification purposes. These sinuses empty into the nose through the frontonasal duct anteriorly in the nose dose to the anterior ethmoids. Both the frontal sinuses and the anterior ethmoids can extend into the orbital roofs.

The sphenoid sinuses originate above and behind the middle turbinate and fully developed can reach back to the divus. Sometimes large lateral recesses will develop in underneath the foramen rotundum.

Pathology

Chantal atresia

Congenital malformations of the nasal cavity will lead to various forms of atresia of the posterior channel openings and can be both uni- or bilateral. Bilateral atresia will lead to life threatening breathing problems particularly when feeding. Unilateral atresia can sometimes be missed during childhood and be discovered later due to complaints of chronic, one-sided nasal congestion. The diagnosis can be made in the neonatal period by passing a feeding tube into the nose. Computed tomography using 2-3 mm slices can be used to confirm the diagnosis particularly if surgical intervention is contemplated. The exam is done preferably after suctioning the obstructed side dean. Computed tomography can display the thickness and shape of the nasal septum and the vomer and assess whether the atresia is bony or membranous (Fig. 10).

Acute sinusitis

The normal mucosal membrane cannot be separated from the underlying periosteum and bone on the plain radiograph. The mucosa will become visible first when it has become thickened by inflammation and can then be seen outlining the bony margins of the paranasal sinuses. The

	Figure 10. Choanal atresia - computed tomography Transverse section through the nasal cavities with bone window settings. A membrane is seen blocking the posterior choanal opening on the right side (arrow).
	Figure 11. Acute sinusitis Water's view demonstrating mucoperiosteal thickening in both maxillary antra (white arrow) and on the left side an air-fluid level can also be seen (open arrow).
	Figure 12. Ethmoiditis with preseptal cellulitis and early subperiosteal orbital abscess - computed tomography Transverse section through the ethmoid aircells. The anterior and middle ethmoids are opacified and beginning subperiosteal collection is also seen (arrow). Note also the swollen eyelid (preseptal soft tissues).
	Figure 13. Mucous retention cysts Water's view shows bilateral smoothly outlined retetion cysts in otherwise normally aerated maxillary sinuses (arrows).

inflamed mucosa will lead to impairment of the drainage causing stagnation of mucous and fluid in the sinuses. Air fluid levels can therefore be se en in the maxillary antra and sometimes also in the frontal, as well as, the sphenoid sinuses (Fig. 11). Mucosal thickening and fluid within the small ethmoid aircells will be seen as clouding of these sinuses on plain radiographs. Infectious processes in the paranasal sinuses can spread to the orbits or intracranially (Fig 12). Recurring unilateral maxillary sinusitis should lead to further assessment of the neighbouring teeth.

In allergic sinusitis all paranasal sinuses tends to be involved of varying degree, while air fluid levels are less common. There can also be coexisting nasal polyposis.

Mucous retention cysts

The drainage of a solitary mucous gland can become obstructed leading to the formation of a mucous retention cyst. These cyst are often discovered by accident since they usually do not cause any symptoms. On the plain radiograph they can be seen as a smoothly outlined soft tissue density without any surrounding mucosal thickening (Fig 13).

Mucoceles

When a sinus becomes permanently blocked, for example due to a previous fracture, chronic infection or nasal polyposis it will lead to mucocele formation. An isolated mucocele occurs most commonly in the frontal followed by the ethmoid and sphenoid sinuses. They form less

Figure 14. Mucocele - computed tomography Transverse section through the frontal sinuses. The right compartment is opacified and expanded. Erosion of the posterior wall is also noted (arrows).

often in the maxillary antra. The collection of normal desquamated epithelium in the blocked sinus will make the walls of the sinus become thinned as the sinus is taking on a more rounded expanded contour. With time a mucocele can erode the bony wall and empty into the orbit or intracranially (Fig. 14). In chronically allergic patient mucoceles occur commonly in the maxillo-ethmoidal complex because of nasal polyps obstructing the middle meatus.

Nasal polyps

Benign nasal polyps can be seen in allergic nasal conditions, but can also be of infectious origin. Nasal polyposis often involves both nasal cavities in a symmetric fashion leading to expansion of the nose and in turn leading to nasal obstruction and chronic sinusitis.
Inverting papilloma traces its name to the histologic appearance with squamous epithelium inverted in the polyps. This process is often unilateral originating from the lateral margin of the nasal cavity. As the papilloma grows it will lead to expansion of the involved nasal cavity and unilateral sinus obstruction. Inverting papilloma have a potential to become malignant and then behave in the same destructive way as any squamous cell carcinoma (Fig. 15); the incidence of associated malignancy is estimated at 10-15 % of cases.

Choanal polyps are solitary nasal polyps having more of the characteristics of a mucous retention cyst than a nasal polyp. They form near the ostium of the maxillary antrum and therefore hang out into the posterior choanal area, as well as, dumb belling into the maxillary antrum. Similar sphenochoanal polyps may extend out of the sphenoid sinus osteium.

	Figure 15. Inverting papilloma - computed tomography Transverse section through the nasal cavity. A spindle-shaped tumor fills in the nasal cavity. The tumor extends into the pterygopalatine fossa (arrow) and the posterior and medial wall of the maxillary sinus is destroyed being suspicious for amalignant component within the polypoid tumor.
	Figure 16. Squamous cell carcinoma - computed tomography Trarnsverse section through the sphenoid sinus. A tumor fills in the sinus with spread into the middle cranial fossa (black arrows) and the inferior orbital fissure (open arrow).

Malignant tumors

Squamous cell carcinoma, the most common cancer form in the paranasal sinuses, destroys the affected bony margins early in the disease process. In other malignancies like lymphoma and adenocarcinoma the tumor will first fill and expand the involved sinus and bone destruction may not be present. Localised bone destruction, being an important sign of a possible underlying malignancy in an opacified sinus, is difficult to detect on plain radiographs and computed tomography is needed to confirm such findings and to outline the tumor better (Fig. 16).

Fractures

The facial skeleton is built around the maxilla. Depending on the area of impact and the direction of the force, predictable fractures will occur in the facial skeleton.

Nasal fractures often being detected by clinical examination can be confirmed by plain radiographs of the nose and computed tomography is seldom needed.

The tripod fracture is the most common type of fracture in the rest of the facial skeleton. This fracture is unilateral and separates the lateral part of the maxilla. The fracture runs through the anterior wall of the maxillary antrum and the orbital rim at the level of the infraorbital foramen extending along the orbital floor and the lateral wall of the sinus. The fracture also separates the maxilla from the frontal bone at the frontozygomatic suture, as well as, fracturing the zygomatic arch. Plain radiographs using modified sinus views, including an axial view with exposure factors set to show the zygomatic arches, are often sufficient for diagnoses (Fig. 17). If open reduction and fixation of the orbital rim and floor is contemplated computed tomography in the coronal plane will better show the malalignment and possible loose fragments of the orbital floor.

Blow-out fractures of the orbit are caused by blunt direct trauma to one orbit. The orbital rim will remain undamaged while the force of the trauma will lead to a fracture of the more fragile orbital floor. The fracture fragment will become depressed leading to enophthalmus and double vision. Part of the orbital content including the inferior rectus muscle can also become trapped. Blow-out fractures of the lamina papyracea can also occur particularly if the maxillary antrum is hypoplastic, This type is however of less clinical concern.

Facial fractures due to severe midface trauma are classified according to the system of Le Fort. The Le Fort I fracture separates the alveolar process of the maxilla from the rest of the face. The Le fort II fracture extends from the nasal bridge down trough the maxilla separating a central pyramid like fragment from the face. The most complex fracture is the one of the Le Fort III type separating the whole facial skeleton from the skull base. This fracture extends from the nasal bridge through both orbits and the frontozygomatic sutures and down through the base of the pterygoid plates, also fracturing both zygomatic arches. Complex fractures of the midface can lead to leakage of cerebrospinal fluid, pneumocephalus and possible meningitis.

Fractures of the mandible are often bilateral because of the closed ring the mandible forms with the skull base. A fracture through the angle or foramen mentale area of the mandible is combined with a fracture through the neck on the contralateral side. Neighbouring teeth roots can also become fractured. Orthopantomography combined with plain radiographs of the mandible including axial views of the condyles is usually sufficient to diagnose these fractures. In more subtle fractures close up dental radiographs may be needed.

Sven G. Larsson and Anthony A. Mancuso