The maxillary antra

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Chapter 31

The maxillary antra

Introduction

The maxillary antra, because of their close proximity to the upper teeth, are the most important of the paranasal sinuses in dentistry. Nowadays the imaging modalities of choice to investigate possible disease within the antra are computed tomography (CT), cone beam CT (CBCT) and magnetic resonance (MR) (see Chapters 16 and 18). However, as the antra are often imaged on conventional dental radiographs, as well as on certain skull views, clinicians still need to know:

• The anatomy of the antra, including their shape, size, normal variations and related structures.

• How the antra are represented normally on conventional dental and skull radiographs and how these radiographs are assessed.

• The radiographic features of disease.

Similar radiographic changes can be seen in the other paranasal sinuses – frontal, ethmoidal and sphenoidal. They are of less clinical relevance in dentistry and are discussed only briefly.

Normal anatomy

The maxillary antrum or sinus is an approximately pyramidal cavity. It contains air, is lined by mucoperiosteum with a pseudostratified ciliated columnar epithelium and occupies most of the body of the maxillary bone. It is present at birth, but at that stage it is little more than a slit-like out-pouching of the nasal cavity. It grows rapidly by a process known as pneumatization during the eruption of the deciduous teeth and reaches about half its adult size by three years of age. The final size of the antra (like the other air sinuses) is very variable.

Pneumatization in adulthood causes further changes in antral shape and size. The cavity often enlarges downwards into the alveolar process or laterally into the body of the zygoma. The internal surface can be smooth or ridged with prominent bony septa. The lateral wall contains canals or grooves for the nerves and blood vessels supplying the upper posterior teeth.

The main anatomical parts of the antra (see Fig. 31.1) can be divided into:

Fig. 31.1 Diagrams of a left antrum showing the basic shape and various walls and margins: A from the front; B from the side.

• A central air-filled cavity

• A roof or upper border, bounded by the orbit

• A medial wall, bounded by the nasal cavity

• A posterior wall, related to the pterygopalatine fossa

• A lateral wall, related to the zygoma and cheek

• An anterior wall, related to the cheek

Normal appearance of the antra on conventional radiographs

An antrum appears radiographically as a radiolucent cavity in the maxilla, with well-defined, dense, corticated radiopaque margins or walls. In general, the larger the cavity the more radiolucent it will appear. The internal bony septa and blood vessel canals in the walls all produce their own shadows. The thin epithelial lining is not normally seen. The different parts of the antra shown on conventional dental and skull radiographs are summarized in Table 31.1. Typical normal radiographic appearances are shown in Figs 31.2–31.4. In addition, a suggested systematic approach to viewing the antra on the 0° OM is shown in Fig. 31.4.

Table 31.1

Summary of the different parts of the antra shown on conventional dental and skull radiographs

Area of antrum shown	Radiographic projection
Floor	Periapical
	Upper oblique occlusal
	Panoramic
Main antral cavity	0° OM
	True lateral skull
Lower aspect of antral cavity	Periapical
	Upper oblique occlusal
	Panoramic
Posterior wall	Panoramic
	True lateral skull
Anteromedial wall	Panoramic
	True lateral skull
Lateral wall	0° OM
Roof	0° OM
Relationship with upper posterior teeth	PeriapicalUpper oblique occlusalPanoramic
	PeriapicalUpper oblique occlusalPanoramic

Fig. 31.2 A Periapical of showing the usual appearance of the floor (arrowed) and base of the antral cavity in relation to the upper posterior teeth in a dentate adult. B Periapical of of a partially dentate adult showing various normal anatomical structures. These include the floor of the antrum (open white arrows), the hard palate/floor of the nasal cavity (open black arrows) and the zygomatic buttress (Z).

Fig. 31.3 Panoramic radiograph showing the usual appearance of the antral floor, medial and posterior walls (arrowed).

Fig. 31.4 Standard occipitomental (0° OM) showing the normal radiolucent appearance of the antral cavities together with a suggested systematic approach for examining the antra.

Antral disease

The major pathological conditions that can affect the antra, directly or indirectly, include:

• Infection/inflammation

– Acute sinusitis

– Chronic sinusitis.

• Trauma

– Oro-antral communication

– Fractures of the maxillofacial skeleton

– Foreign bodies within the antrum.

• Other bone abnormalities (see Ch. 28)

– Fibrous dysplasia

– Paget’s disease of bone

– Osteopetrosis.

These various disease entities can result in the following radiological changes:

• Total or partial opacity/obliteration of the main antral cavity

• Alteration in the integrity of the antral walls, including discontinuity as a result of a fracture or destruction by an intrinsic or extrinsic tumour

• Alteration in the antral outline, including expansion or compression caused by an intrinsic or extrinsic lesion or disease process

• The presence of a foreign body within the antrum.

Investigation and appearance of disease within the antra

Various imaging modalities can be used to investigate possible disease within the antra, depending on their availability. These include:

• Computed tomography (CT) – currently recommended by the Royal College of Radiologists in the UK

• Cone beam CT

• Magnetic resonance (MR)

• Multidirectional (spiral) tomography

• Panoramic radiography

• Intraoral periapical/occlusal radiography

• Skull radiography

– 0° occipitomental

– True lateral.

Appearances of antral diseases using various imaging modalities are shown in Figs 31.5–31.23.

Fig. 31.5 A Coronal and B sagittal CBCT images showing an oro-antral communication and a displaced root in the antrum (arrowed).

Fig. 31.6 Various appearances of the effects of chronic sinusitis shown on CT. A Contiguous coronal CT scans showing mucosal thickening in the base of the antrum (arrowed). B Contiguous axial CT scans showing mucosal thickening on the medial and lateral walls of the antrum (arrowed) (kindly provided by Drs J. Kabala and J. Luker). C Contiguous coronal CT scans showing mucosal thickening and total opacity of the antrum (arrowed).

Fig. 31.7 Various appearances of the effects of chronic sinusitis shown on MR. A Axial MR scan showing a round antral mucous retention cyst (arrowed) arising from the anterolateral wall. B T1-weighted and C T2-weighted coronal MR images of the same patient with bilateral mucosal retention cysts (arrowed) (kindly provided by Drs J. Kabala and J. Luker).

Fig. 31.8 Sagittal CBCT image showing re-modelling of the antral floor associated with non-vital infected molars producing the *antral halo* appearance (arrowed).

Fig. 31.9 Various appearances of chronic sinusitis evident on conventional occipitomental radiographs. A Totally opaque left antrum. B Bilateral mucosal thickening. C Fluid level in left antrum (arrowed). D Round, domed antral polyp in the base of the left antrum (arrowed). E Simplified line diagrams of the left antrum (as depicted on a 0° OM) illustrating the various appearances.

Fig. 31.10 Periapical of showing a discontinuity of the antral floor (arrowed) following the extraction of the molar tooth that resulted in a clinical oro-antral communication. Note this break in continuity of the floor is not always evident in cases of oro-antral communication.

Fig. 31.11 Axial MR scan showing a white right antrum (arrowed) that was filled with blood following a middle third facial fracture. (Kindly supplied by Mrs J. E. Brown.)

Fig. 31.12 Standard occipitomental showing fracture of the left zygomatic complex (arrowed) and resultant opacity of the left antrum.

Fig. 31.13 Coronal section tomograph of a patient with an orbital blow-out fracture of the right orbital floor/antral roof showing the downward herniated contents of the orbit into the antrum (arrowed), the so-called *hanging drop* appearance.

Fig. 31.14 Panoramic radiograph showing bilateral antral mucosal retention cysts (arrowed).

Fig. 31.15 Central part of an occipitomental showing a round, domed opacity on the left side (arrowed) caused by a radicular (dental) cyst arising from which has displaced the antral floor.

Fig. 31.16 Simplified line diagram illustrating the essential radiographic features of a small odontogenic cyst and its effects on the antrum (as depicted on an 0° OM).

Fig. 31.17 Large dentigerous cyst associated with A Periapical of the upper right posterior teeth. Note the lack of antral floor outline and absence of B Occipitomental of the same patient showing total opacity of the right antral region with no evidence of the lateral antral margin. The displaced upper wisdom tooth is evident underneath the orbit (outlined and arrowed).

Fig. 31.18 MR images of a dentigerous cyst associated with an unerupted displaced third molar expanding into the antrum. A T1-weighted coronal image showing the cyst and unerupted tooth (arrowed). B T2-weighted coronal image – the cyst fluid appears white (arrowed). C T2-weighted axial showing the cyst (arrowed) (kindly provided by Drs J. Kabala and J. Luker).

Fig. 31.19 Occipitomental showing a totally opaque right antrum caused by a large squamous cell carcinoma, with destruction of the lateral antral wall (large arrow) and zygoma (small arrow).

Fig. 31.20 Occipitomental showing destruction of the lateral wall of the right antrum (arrowed) by a squamous cell carcinoma. In this case, there is little evidence of increased opacity within the antrum.

Fig. 31.21 A Panoramic radiograph showing gross destruction of right maxilla (arrowed). B Axial CT and C coronal CT images of the same patient showing the large destructive mass of an adenocarcinoma (arrowed). (Kindly provided by Mr M Fardy.)

Fig. 31.22 Two axial CT slices showing a very large mass (arrowed) of a squamous cell carcinoma probably arising in the cheek showing extensive local destruction including the lateral wall of the antrum and extension into the antrum. (Kindly provided by Dr J. Kabala and Dr J. Luker.)

Fig. 31.23 Occipitomental showing the characteristic opacity of a benign ivory osteoma (arrowed) in the frontal sinus.

Infection/inflammation

Acute sinusitis

Acute sinusitis is most commonly caused by an upper respiratory tract infection, particularly the common cold. The effects on the antrum include:

• Thickening of the antral mucosal lining

• Increase in secretions

• Formation of pus.

Acute sinusitis can be diagnosed and treated clinically. As a result, the Royal College of Radiologists in the UK in their 2011 publication iRefer: Making the Best Use of Clinical Radiology recommend that radiological imaging is not necessary.

Chronic sinusitis (Figs 31.5–31.9)

Trauma

Oro-antral communication

Important causes

• Extraction of closely related upper posterior teeth can remove part of the antral floor or fracture the tuberosity

• Inappropriate or incorrect use of elevators during root or tooth removal – may also cause the root, or rarely the tooth, to be displaced into the antrum.

• Break in the continuity of the floor may be evident (see Fig. 31.10) – however, the diagnosis of an oro-antral communication is made clinically, not radiographically, since the defect in the floor of the antrum may not be evident on the two-dimensional radiograph

• Characteristic features of acute or chronic sinusitis owing to the ingress of bacteria

• Evidence of the displaced root or tooth – a second view of the antrum with the head in a different position or CBCT may be required to ascertain the exact location of the displaced object (see Fig. 31.5).

Fractures of the maxillofacial skeleton

Fractures are discussed in detail in Chapter 29 and only a brief summary is shown below.

Important sites possibly involving the antra

• Naso-ethmoidal complex

Main radiographic features

• Break in the continuity of one or more of the antral walls depending on the type of fracture.

• Total opacity or fluid level within the antral cavity caused by haemorrhage (see Figs 31.11 and 31.12).

• Features of sinusitis if subsequent infection develops (this is in fact surprisingly rare).

• The orbital blow-out fracture classically produces a tear-drop-shaped opacity in the upper part of the antrum, the hanging drop appearance, caused by herniation of the orbital contents downwards into the antrum following collapse of the antral roof (see Fig. 31.13). The infraorbital margin remains intact. See also Fig. 29.30.

Foreign bodies in the antrum

Important causes

• Displaced root fragments or teeth

• Excess root canal filling material forced through the apex of an upper posterior tooth during endodontics

• Antrolith – calcification within the antrum

• Foreign material pushed into the antrum through an existing oro-antral communication.

Main radiographic features

• The presence, position and often the nature of the foreign body.

• Occasionally, associated sinusitis.

Cysts

The more important cysts that can affect the antra include:

Mucosal retention cyst

Cause

The cause is unknown, but is presumably due to blockage of a mucus-secreting cell in the antral lining.

Main radiographic features (see Figs 31.7 and 31.9B)

• Incidental finding

• Well-defined, round, dome-shaped opacity within the antrum varying in size from a tiny lesion to one completely filling the antrum

• Usually no evidence of thickening of the remainder of the epithelial lining

• Usually no alteration of the antral outline

• Occasionally bilateral (see Fig. 31.14).

Odontogenic cysts

These cysts are extrinsic to the antra developing in the alveolar bone beneath the antral floor.

Main radiographic features of a small cyst

• Round, dome-shaped opacity in the base of the antrum with a well-defined, radiopaque corticated margin to the edge of the meniscus, i.e. the odontogenic cyst has a bony margin and so can be differentiated from the soft tissue mucosal retention cyst or antral polyp (see Figs 31.15 and 31.16)

• Lateral expansion of the alveolar bone

• Sometimes displacement of the associated tooth.

Main radiographic features of a large cyst

• Total opacity of the antral region owing to complete compression of the antral cavity

• Loss of antral outline

• Sometimes displacement of the associated tooth (see Figs 31.17 and 31.18).

Note: Subsequent marsupialization or decompression of the cyst will usually result in the reformation of the antral cavity.

Tumours

Benign intrinsic tumours

These are all rare but can include:

• Papilloma – this produces, radiographically, a well-defined, non-specific, soft tissue opacity within the antrum

• Osteoma – this produces, radiographically, a well-defined, round or lobulated, homogeneous, densely opaque mass within the antrum. The osteoma is most common in the frontal sinus.

Malignant intrinsic tumours

Squamous cell carcinoma and adenocarcinoma – these uncommon but important tumours produce a rapidly growing, aggressive soft tissue mass within the antrum causing destruction of one or more of the antral walls.

Main radiographic features of a small early lesion

• Non-specific, well-defined, round soft tissue opacity within the antrum

• Variable destruction of the bony antral walls.

Main radiographic features of a large well-established lesion

• Total opacity of the antral cavity (see Fig. 31.19) – in the absence of symptoms suggesting infection, or a history of trauma, a totally opaque antrum is a cause for serious concern and further investigation is necessary.

• Destruction of one or more of the antral walls (see Fig. 31.20) and invasion of surrounding hard and soft tissues – hence the need for CT

• Occasional displacement or resorption of teeth.