13 Oral and maxillofacial imaging for the anaesthetist

The oral and facial structures are a problematical area for both the anaesthetist and maxillofacial surgeon as the oral cavity and nasal passages provide a common portal for both airway access and surgical intervention. Obviously an understanding of the anatomy of these structures and their variations is essential. In addition, the jaws provide radiographic challenges by virtue of their unique anatomy.

Dental imaging is a specialist area, which utilizes radiographic techniques that are unusual in their acquisition. For example, the use of non-screen film for intraoral views that give high resolution images is the main way in which teeth and their supporting structures are demonstrated. Such techniques include bitewings, periapical, and occlusal views.

The dental panoramic tomogram (DPT), sometimes called the orthopantomogram (OPG) is the ‘work horse’ of jaw imaging. However, this view has unusual anomalies which are created by virtue of the way it is formed. These units produce an image of the dental arches using a variation on the technique of tomography that involves rotating the X-ray tube and film, producing blurring of the structures on either side of the centres of rotation.

In dental panoramic tomography, a horseshoe-shaped in-focus plane (focal trough) is created by moving the centre of rotation during the exposure. Care must be taken in patient positioning to ensure that the teeth lie within the trough otherwise, the resultant image will be distorted.

Advances in dental imaging include cross-sectional modalities such as modified dental panoramic views, conventional computed tomography (CT) and cone beam CT scans¹. Cone beam CT is a new process that, when compared with conventional CT, uses a lower radiation dose and a cross-sectional imaging technique which acquires a volume of data that can be presented in a number of formats, e.g. panoramic or as axial, coronal, or sagittal slices similar to those seen using conventional CT.

The use of magnetic resonance imaging (MRI) is becoming invaluable in helping to characterize neoplasia presenting in the head and neck. Cysts, inflammatory lesions, and bone pathology can also be assessed using this modality. MRI is now the gold standard for investigation of temporomandibular joint (TMJ) pathologies owing to its ability to resolve and demonstrate the complexities of the internal features of this atypical joint.

Cross-sectional imaging has simplified much of the clinical problem-solving carried out in the oral and maxillofacial region. The remainder of this chapter will address some of the key aspects of oral and maxillofacial imaging now available.

As in all areas of radiology, in order to recognize the abnormal/pathological, the normal must first be appreciated. The DPT is a commonly used view and consequently the anatomy it demonstrates is shown in Figures 13.1 and 13.2.

Pathology that was once only assessable from conventional images is now more frequently assessed from cross-sectional images such as CT (Figures 13.3a and b; see also Plates 3 and 4). Yet more recently, imaging modalities allowing three-dimensional reformatting of the initial ‘raw data’ can be viewed from any angle. This data can be linked to a computer-aided design/computer-aided manufacturing (CAD/CAM) process to allow the build up of accurate resin models of the structures imaged (Figure 13.4). These types of models can allow preplanning for surgery such as preforming of titanium fixation plates and for dentofacial implant planning.

A detailed discussion of the physics of such scanning equipment is beyond the scope of this chapter; however, examples of current imaging modalities will be shown.

Many excellent texts are available covering the more intricate details of oral and maxillofacial radiology^{2  ,3}. The emphasis in this chapter will therefore be on applied radiological imaging anatomy with some relevant pathological examples.

The face has special importance, as personality, emotion, and the basic necessities of respiration and digestion are all facilitated at least initially in and around the craniofacial area. Direct exposure to the external environment of neural tissue (the eyes) happens in the face. The eyes and their supporting structures are all too often victims of such contact!

Regarding the mouth, the anaesthetist must give special consideration to the potential hazards of the teeth when securing an airway, as dental disease and the presence of overt or occult inflammatory disease can easily be overlooked. Invariably, cross-sectional scans of the head and neck will contain images of the cervical spine and the convolutions of the upper respiratory tract, which can be helpful when carrying out preoperative anaesthetic risk assessments.

The facial skeleton is a delicate, lightweight, scaffolding with more rigid bony buttresses, which allow for a ‘protective crumple zone’ in the event of trauma. Dissipation of forces that may potentially injure the brain in its protective calvarial ‘box’ is thus facilitated. These thin, bony buttresses are aligned vertically and support horizontal bony platforms that run across the face. Two examples of these horizontal planes run from the right supraorbital and infraorbital ridges across the midline to the left. Within this scaffolding, the paranasal sinuses provide mucosal lined, air-filled spaces. The paranasal sinuses allow for an overall reduction in the weight of the skull, give resonance for the voice and, some would argue, are a means of humidifying and warming air on its way to the lungs. Radiologically, these are low density features.

Changes in the appearances of the paranasal sinuses and mastoid air cells on imaging may give vital clues to the clinician who may suspect infection, tumour, or trauma.

The three main vertical facial buttresses to be appreciated are:

These dense struts of bone provide the thickened bony pillars around which the rest of the face is supported (Figure 13.5). Normal suture lines in the oral–maxillofacial and base of skull should be appreciated so that they are not confused with pathology (Figures 13.6 and 13.7).

There is a degree of clinical myth and mystery around the subject of the dentition.

Apart from an obvious source of intense pain, the teeth provide a special hazard to the anaesthetist. The accidental fracturing of an anterior tooth or dislodging of a crown or post-retained restoration can be cause for litigation. Patients may wear small partial, dental prostheses that, if not noted at the time of admission, may become dislodged at operation with obvious potential for medical complications and legal ramifications. Dentures are usually made from acrylic, which is radiolucent and may be difficult to locate if no metal clasps (fastener) are present to give away its location on a radiograph. Crowns form a special hazard, especially if the underlying tooth is diseased as this may lead to fracture or dislodgement during intubation (Chapter 12). The anaesthetist therefore requires a broad appreciation of the teeth and dental structures so that risk can be assessed and effective communication between clinicians can be made.

Primary (deciduous), mixed, and secondary (adult) dentitions may be encountered depending on the maturity of the patient. The Federation Dentate International (FDI) system of dental nomenclature is now accepted to denote the position and type (class) of tooth. This method is computer-friendly in that each tooth is given a two-digit number (Figure 13.8).

In an adult arch there are usually two incisors, a canine, two premolars, and three molars in each quadrant. The first digit in the system denotes the quadrant 1–4, each quadrant being numbered in a clockwise fashion. The adult quadrants are denoted by 1, 2, 3, and 4. Where present, each of the eight teeth in each quadrant are then ascribed a number 1–8 running posteriorly, thus giving a second digit. For example, an upper first incisor on the patient's right side is given the quadrant digit 1 and the arch location 1, denoting tooth 11. Similarly, a lower left third molar will be denoted 38.

In the primary dentition any teeth present are given the quadrant prefix, again in a clockwise fashion, 5 (upper right), 6 (upper left) 7 (lower left) and 8 (lower right). This time the dentition is made up of two incisors, a canine, and a first and second molar tooth. These are denoted in the arch as 1 through 5, again running posteriorly.

Alveolar bone is specialist bone that supports the teeth. In it are located Sharpey's fibres, specialized collagen bundles that make up the periodontal ligament. This is represented as a radiolucent line around the tooth root on imaging. If intact and unaffected by physiological or pathological processes, it should be 1 mm thick (Figure 13.9). If this is widened by infection/cyst or trauma, then the integrity of the tooth support may be compromised. In the traumatized dentition, a partially extruded upper anterior tooth may give rise to a risk of foreign body inhalation.

In the past, investigation of the facial skeleton was limited to using the following conventional radiographic views.

Posteroanterior (PA) or occipitofrontal (OF) skull view with 20° caudad angulation of the X-ray tube (OF 20 view). This mainly shows the upper third of the face and frontal sinuses with the skull vault.

Occipitomental (OM) view to show the mid third of the face. This is achieved by angling the patient's head so that the dense petrous ridge is projected inferior to the lower borders of the maxillary sinuses.

This used the same head position as the OM with a 30° caudad angle on the X-ray tube to give a further demonstration of the mid third of face, free of the dense petrous bone.

In the lateral facial view, both sides of the face are superimposed. This allows a crude assessment of the anteroposterior extent of any pathology.

The SMV view is a full axial conventional radiographic technique, designed to outline the zygomatic arches in profile from below. It is sometimes called a ‘jug handle’ view. This can be a very difficult technical feat for both patient and radiographer!

Although some centres still use these radiographic projections, the advent of low dose conventional CT and the arrival of cone beam CT has revolutionized the investigation of the maxillofacial skeleton.

MRI is a non-ionizing modality and a well established tool in the diagnostic armoury. MRI is particularly good at discriminating soft tissue structures and tissue/facial planes depending on the sequence requested. With this technique, tumour characterization and the extent of a lesion/mass can be appreciated fully.

In acute trauma, MRI does have limitations. Accessibility by the injured patient, comparatively lengthy scan times, and the necessity to respect the strength of the magnet can make the CT scanner a more practical option. Specialized equipment and training of the anaesthetic team are required before gaining entry to the MRI suite.

Specialist interpretative skills are now utilized by the viewing clinician to maximize the diagnostic yield from these cross-sectional modalities.

Imaging oral and maxillofacial trauma includes the spectrum of predominantly dentoalveolar trauma through to full scale craniofacial trauma.

A tooth may undergo a fracture of any of its component structures, including the root, dentine or enamel only. Prostheses (dentures) may be in situ at the time of injury which will result in forces being exerted in a non-classical distribution, so altering any expected fracture pattern. Fractured or dislodged dentures may have metallic components and so be easily appreciated on a radiograph/scan, or be made of acrylic plastic with a relatively low radio-opacity which is more difficult to see. Acrylic may be difficult to appreciate on the standard chest film if dislodged. Recognition of the fate of any lost dental fragment is important—‘count and account’. A fragment can potentially be inhaled, ingested or embedded into surrounding orofacial soft tissues (Figure 13.10).

Increasingly, patients are undergoing restorative treatments with titanium implants, which are radiodense and which, if dislodged, as with luxated teeth, can provide a potential for an inhaled foreign body. Significant cost implications can occur if these types of restorations undergo iatrogenic damage! Intraoral films with a low kVp technique may be useful in localization of these fragments if impaction in the immediate soft tissues of the face is suspected; alternatively, ultrasound may help in their location.

The mechanism of facial injury is important. It cannot be assumed that the oral and maxillofacial region is the only focus of trauma. Brain, cervical spine, and life-threatening trauma to the thorax or abdomen may take precedence with regard to imaging and patient management. For the purposes of classification, tissue trauma to the oral and maxillofacial structures will be dealt with in four sections—the upper third, mid third, lower third of face, and dentoalveolar fractures.

The upper third of the face is that skeleton above the supraorbital margin. The frontal sinuses are intimately related at their deep surface to the dura and frontal lobes of the brain. Soft tissue swelling will often accompany trauma to this area. The tendency for the young child to injure this area is greater than that of an adult owing to the comparative prominence of this region as the developing facial skeleton grows downwards and outwards.

In the developed frontal sinus, radiography can demonstrate the difference in density represented by an air–fluid level in a supine patient (using a horizontal beam lateral projection of the skull/facial bones). Blood from the trauma settles in the frontal sinus with air above it, and the horizontal rays of the X-ray beam run tangentially along this interface. Coincidentally, reviewing the cervical spine depicted in lateral projection is recommended in such an injury. A formal cervical imaging assessment may be required.

Before the advent of isocentric skull units, fronto-occipital views (also called AP views) were carried out if the patient was on a casualty trolley. More ideally, occipitofrontal projections (so-called PA) provide the view at 90° in assessing the frontal sinus. Increasingly, however, injury to the brain has been recognized as being of primary concern and so cross-sectional imaging using CT is employed as a first line modality. Any breach of the inner frontal sinus wall, with involvement of the dura or brain parenchyma, needs to be recognized.

The area of the mid third of the face brings a set of unique challenges in planning and executing adequate imaging. An understanding of the classical fracture distribution and the potential for anatomical superimposition when using conventional radiography are crucial to answering the clinical questions posed by facial trauma.

Rene Le Fort (1869–1951), a French surgeon, first described the patterns of facial bone injuries in 1901 and his name has become inextricably associated with their classification. All the Le Fort fractures involve the pterygoid processes of the sphenoid, and their classical descriptions are given below. It should be remembered that it is the adult patient, where facial maturation has resulted in forward growth of the face, that is more likely to sustain a mid third facial injury. In addition, the most common fractures of the facial skeleton are to the nasal bones and zygomatic complex.

In reality, facial fractures often do not obey the classical rules, and combinations of fracture patterns can present depending on the mechanism and direction of injury, especially if significant force has been experienced (Figure 13.11).

This is a transverse (horizontal) maxillary fracture caused by a blow to the premaxilla. The fracture line involves the alveolar ridge, lateral aperture of the nose, and inferior wall of the maxillary sinus. The Le Fort I fracture results in detachment of the dentoalveolar process of the maxilla with any teeth contained within this arch.

This can result clinically in an ‘anterior open bite’ (mouth gagged open) appearance owing to the unopposed action of the pterygoid muscles. Incidentally, often it will be accompanied by a midline palatal fracture. Clinically, haematoma will be appreciated in the upper buccal sulci (lateral to the upper teeth), with a loss of resonance on tooth percussion (sounds like a ‘cracked china cup’) and mobility of the lower maxilla.

This injury may compromise the airway by posterior bony displacement, giving rise to compression or through the massive haemorrhage that will result if the palatine arteries are transected.

The surgeon, when advancing or setting back the upper jaw in orthognathic surgery, will induce this fracture deliberately.

This is the so-called ‘pyramidal fracture’. The midface is in essence acting as a crumple zone to absorb the traumatic forces and so protects the calvarium (skullcap). This class of fracture may be unilateral, the fracture line passing through the posterior alveolar ridge, medial orbital rim and across the nasal bones (either just superior or just inferior to the nasal bones). The result is a separation of the mid portion of the face with the fracture line involving the floor of the orbit, hard palate, and the nasal cavity (Figure 13.12).

The fracture path follows an essentially horizontal course through the nasofrontal suture, maxillofrontal suture, the orbital wall, zygomatic arch, and separates the entire face from the base of the skull leading to craniofacial dysjunction. There is a very real risk of intracranial infection from this injury.

Le Fort II and III fractures will result in extensive periorbital bruising, the so-called ‘panda eye appearance’, with significant soft tissue response to this injury.

Imaging the above in 2D using OM, OM 30 and lateral facial views gives very limited information. CT at an early stage will give definitively more detailed information of soft and hard tissues.

The zygomatic bones are prominent and, as such, may be exposed to trauma readily.

Trauma to this area has been termed zygomatic complex, malar or trimalar fracture (Figures 13.13 and 13.14). Various classifications of injury to this bone have been offered.

Displacement or rotation of the zygoma is of prime concern to the surgeon.

The trimalar fracture is a bit of a misnomer in that the zygoma has four points of articulation. This bone has sutural union with the maxilla, temporal, frontal, and, often forgotten, the sphenoid bone. Any of the four sutural joints, the zygomatico-temporal, the zygomatico-frontal, the zygomatico-maxillary, and the spheno-zygomatic may be involved in its fracture.

Fractures of the zygomatic arch tend to occur at the weakest point along this rim of bone that bounds the infratemporal fossa laterally, specifically a point 1.5 cm posterior to the zygomatico-temporal suture. The arch can be bowed inwards or fully displaced with three distinct breaks into a W-type shape when viewed from below.

Significant fractures with displacement or rotation of the zygoma will present with predictable signs and symptoms. Prior to swelling, a depression in the cheek may be appreciated clinically. Soft tissue swelling may obscure this feature. Typical signs and symptoms may include the loss of sensation in the infraorbital nerve distribution. Zygomaticotemporal and zygomaticofacial nerves (branches of the maxillary division of trigeminal) that exit the lateral border of this bone may also be affected, giving cheek paraesthesia.

The fracture lines can follow a path seen on imaging to travel through the lateral wall of the maxillary sinus, orbital rim close to the infraorbital foramen. The fracture line will involve the floor of the orbit, and, if significant force is used, most if not all of the abovementioned sutural joints would be disrupted. Orbital involvement via the attachment of the suspensory ligament of Lockwood will lead to double vision and ophthalmoplegia. The traumatic impression of the zygomatic arch against the normally free-moving coronoid process will disrupt jaw opening. This will have obvious anaesthetic access implications.

OM, OM 30, SMV (submentovertex), and reverse Townes views have been the mainstay of conventional imaging of the zygoma. The ‘elephant trunk sign’, where an imaginary outline of an elephant head and trunk, which, if not continuous in its smooth outline, may denote a fracture of the zygoma to even the most junior casualty officer, is one of the more useful radiological aids to diagnosis in the OM view (Figure 13.15). Access to CT allows full appreciation of this type of injury and the potential impact on the base of the skull.

The involvement of the sphenoid bone in the so-called sphenotemporal buttress indicates that significant force has been experienced (Figure 13.16).

Nasal fractures are the third most common of all fractures to occur and the commonest facial fracture. Despite this, they are frequently underassessed with resultant functional and cosmetic sequelae. The latter include the typical flattened ‘boxer's nose’ to the malaligned nose. The former results in poor nasal airway patency, deviation of the nasal septum, and septal haematoma. Significant problems can arise if the adjacent ethmoidal complex is involved—nasoethmoidal injury (Figure 13.17). These can result in significant haemorrhage to the anterior ethmoidal arteries, CSF leakage, communication with intracranial tissues, and traumatic telecanthus if the medial canthal ligament becomes dislodged from its native position. As with all trauma, the first assessment is essentially clinical, but these injuries may also be apparent on the facial radiographs already discussed. It is unusual nowadays to take specific radiographs to asses a ‘simple’ fractured nose.

During trauma to the eye, the globe is protected at the time of impact as a result of the orbital floor or inner medial orbital surface giving way in preference. Classically, a squash ball or a ball with a similar dimension with sufficient velocity could cause this injury to the unprotected eye. This mechanism of injury is an example of a ‘fail safe’ which gives classical signs on imaging. Signs and symptoms include diplopia on upward gaze, as the eyeball struggles to rotate against its tethered inferior rectus and inferior oblique muscles.The infraorbital nerve is often involved because of its close proximity.

Historically, an undertilted OM view was used which allowed the central X-ray to pass tangentially just inferior to the lower orbital rim. This highlighted a ‘tear-shaped drop’ of radio-opacity (infraorbital fat) against the air-filled (radiolucent) background of the maxillary sinus. This represented the inferior orbital contents herniating into the maxillary sinus. It was a difficult projection to perfect and demanded a degree of image interpretation skill, especially if accompanying haemorrhage degraded the classical teardrop sign (Figure 13.18).

Often the maxillary sinus involved would completely opacify with blood.

The ethmoid sinuses are frequently involved given their close proximity to the thin medial orbital wall (lamina papyracea). Post-traumatic changes can bring about enophthalmos. Today CT, in coronal section, makes this injury more easily diagnosable.

The mandible is an unusual bone in that it spans the midline and articulates with the skull base via the TMJs. These are sliding hinge joints, with two joint compartments separated by a fibrocartilage disc. The TMJs are atypical joints both in their anatomy and the dynamics of their function. One joint's movement elicits movement on the contralateral joint.

The mandibular condylar neck is a particularly weak point, which will fracture in order to dissipate forces directed towards the cranial base. Other areas of potential weakness in the mandible include those spanned by teeth such as the lower canine or third molar regions. Unerupted teeth are common and can facilitate a fracture through the bone in these regions. The angle of the mandible is an especially common site of fracture as a result of both a change in direction of the bone and the frequent presence of unerupted third molars. Fractures are classified as displaced or undisplaced.

When inspecting the dental panoramic for mandibular trauma, remembering the fact that the mandible can fracture in two sites is essential. The ‘contra coup injury’ is often noted with a body/ramus and condyle fracture combination (Figure 13.19).

The danger to the airway is particularly acute if the fracture segments experience posterior or medial displacement by muscle action, or if swelling due to oedema and haemorrhage effaces the upper respiratory tract.

Infections of the oral and maxillofacial regions are common (Chapter 11).

The changes in the jaws noted on imaging can be non-specific. Dental infections can cause marrow changes and erosions of cortical bone often associated with more sinister pathology. Severe infections do occur on occasion and, in the immunocompromised, may spread into the surrounding tissues (Figure 13.20).

There are numerous types of neoplasia that can affect the dentofacial region⁴.

Those affecting the jaws are usually classified as odontogenic (those arising from toothbearing tissues) and non-odontogenic. Both groups of neoplasia are again subdivided into benign and malignant. Odontogenic tumours are rare, with the most likely encountered being the ameloblastoma.

Malignant neoplasms affecting the orofacial region are most commonly of primary origin; for example, the squamous cell carcinoma (Chapter 16), lymphomas or, more rarely, osteosarcomas. In addition, the mandible can be the site for metastatic deposits, particularly from malignancies of the breast, thyroid, lung, brochi, renal, bowel, ovary and prostate.

Other common tumours affecting the oromaxillofacial region include salivary tumours, about which the reader would be advised to consult one of the more specialist texts^{2  ,3}.

Dental and maxillofacial radiology is a specialist complex area of radiology which can often be problematical for the non-radiologist. An appreciation of some of the complexities in obtaining and interpreting the radiological images is useful for those anaesthetists treating oral and maxillofacial surgical patients.