The introduction of cone beam computed tomography (CBCT) has dramatically changed how an oral and maxillofacial surgeon conducts his or her practice. This technology has improved the efficiency of oral and maxillofacial surgeons in private offices, where access to cross-sectional imaging has now become quicker and easier than in a hospital-based practice. Prior to the introduction of CBCT, panoramic radiography was the most common imaging tool in private oral and maxillofacial surgery (OMS) offices. Only limited cases were evaluated with cross-sectional imaging. While oral and maxillofacial surgeons have successfully practised using panoramic radiography, the limitations of this imaging technique include variable magnification, distortion, superimposition of structures, and suboptimal imaging of structures not located in the focal trough. CBCT has overcome these limitations. Depending on the field of view, CBCT scans show a large area of the facial skeleton beyond the limits of a panoramic radiograph or a small area of focused clinical interest. As the CBCT slices can be reformatted and viewed in multiple possible orientations (multiplanar views), anatomic structures are not superimposed (Fig. 1).1 Within the last decade, the technology and design of CBCT scanning machines has made the placement of the machines both physically and financially possible. Greater access to interoffice scanners allows for a greater ease of patient acceptance and use for the treating surgeon.
Prior to the introduction of CBCT, multiplanar views were obtained primarily with multi-detector CTs (MDCT) and magnetic resonance imaging (MRI). Physical dimensions and cost of MDCT and MRI equipment are prohibitive for installation in a typical OMS office. Smaller physical dimension, lower cost and easier operation have led to rapid acceptance of CBCT units. There are many instances where an oral and maxillofacial surgeon may reliably use a CBCT scan where an MDCT may otherwise have been chosen to provide diagnostic information. However, the need of MDCT and MRI examinations in oral surgery is not obsolete, even though the quality of CBCT images may be better than MDCT scans.2 A study that evaluated the image quality of bone structures acquired by five different CBCT machines and one MDCT machine showed that the image quality of one CBCT machine was superior to that from the tested MDCT machine while images from other CBCT units were comparable to the test MDCT images.2 However, soft tissues are better displayed on MRI and soft-tissue window CTs. Currently, neither MDCT nor CBCT can replace the MRI where soft tissue diagnosis is the primary aim. These situations include analysis of soft tissue tumours, extension of intraosseous tumours into surrounding soft tissue and position of the disc in temporomandibular joints.
Major uses of CBCT examination in oral surgery practice include surgical extraction of third molars and impacted teeth, tracing of the inferior alveolar canals, implant planning, evaluation of cysts and tumours, fracture diagnosis, orthognathic surgical planning and follow-up, inflammatory conditions of the jaws and the sinuses, evaluation of the temporomandibular joints, and as an aid in diagnosing unexplained symptoms of pain. The following subsections provide utility of CBCT in different surgical situations.
Evaluation of impacted teeth
Dentoalveolar surgery for impacted teeth is a common procedure in an OMS office. Location of the inferior alveolar canal and its close contact to the third molar root structures are risk factors in dentoalveolar surgery. Therefore, image analysis principles were developed for panoramic or periapical radiographs to identify the canal location.3 However, the inferior alveolar canal may follow a tortuous path, and may not be reliably interpreted on a 2-D image. Multiplanar views from a CBCT are useful not only in tracing the canal, but also in assessing a bifurcated or trifurcated canal (Fig. 2).4 In addition, knowledge of the location of the canal allows the surgeon to develop a safer surgical plan related to the access to the tooth and root elevation. Ankylosis of impacted teeth adds another layer of complication in dentoalveolar surgery. Plain films are not reliable in revealing ankylosis of teeth.5 Compared to panoramic radiography, CBCT images allow better risk assessment of third molar removal.6 Panoramic or periapical radiographs are often inadequate to locate impacted maxillary canines and to identify their relationship to the roots of the lateral incisors. Surgical exposure of the canine crowns for orthodontic bracket placement may require multiple periapical radiographs obtained at differing horizontal or vertical angles. Application of image shift principles is complicated and time consuming. A surgeon can eliminate the complication of guess-work when CBCT scans are available (Fig. 3). The use of 3-D reconstructions also allow for a more complete visual picture for the treating orthodontist to provide proper vectors of tooth movement.
Use of CBCT for benign lesions and cysts
In evaluating cysts or benign tumours, intraoral or panoramic radiographs show only the two dimensions of the lesion. Observation of the third dimension, i.e. bucco-lingual extension of a lesion, requires additional radiographs obtained at 90 degrees from the original view. In contrast, all three dimensions are recorded by the multiplanar (axial, coronal and sagittal planes) imaging of CBCT (Fig. 4). Such multiplanar views provide important information on the presence and extent of bone resorption, sclerosis of neighbouring bone, cortical expansion and internal or external calcifications, and proximity to other vital anatomy (Fig. 5).7 Multiplanar sections are preferred when examining cysts or tumours deep in the tissues.8,9 If the lesion borders can be clearly seen, then multiple extraoral plain film radiographs, oriented at 90 degrees to each other, can provide adequate information of the size of a lesion. Information on the spatial relationship of the lesion with other anatomic landmarks on such images is limited, and often difficult to interpret. Because of superimposition of large tissue volume, extraoral plain film radiographs often cannot provide reliable information on the internal structure of a lesion.
Newer CBCT units allow slice thickness to be as low as 0.1 mm. These thin slices allow better visualization of the bony margins of a lesion. Oral and maxillofacial surgeons may depend on panoramic radiography if the margins of cystic or benign lesions are well defined.10 If the margins are ill-defined, CBCT is a better option for diagnosis.11 Apart from presurgical evaluation of aggressive benign cysts or tumours, CBCT is also helpful in post-surgical follow-up of the margins of lesions that may have a high recurrence rate (Fig. 6). A surgeon may find CBCT scans acquired in their own OMS office more convenient and diagnostically sufficient compared to MDCT scans (Fig. 7).
For surgical planning, a lesion may need to be measured from different angles. For osseous components, when compared to the gold standard dry skull, the measurements on CBCT images are acceptably accurate with less than 1% error.12,13 In comparison, panoramic radiographs are not reliable for size measurement due to variable magnification error.14
Use of CBCT for malignant lesions
The limitation of plain films in depicting the margins of a benign lesion is also encountered in diagnosing malignant lesions. A lesion that may have a ‘benign’ appearance on a panoramic radiograph could reveal ominous features in thin slices of CBCT scan (Fig. 8). Compared to smooth margins of cysts and benign tumours, the margins of malignant tumours are irregular. CT images can identify such irregular margins and provide information in the early stages of a malignant lesion (Fig. 9). The advantage of CBCT over MDCT lies in the lower radiation dose and low cost.15 Whenever a malignancy is suspected to involve osseous components, cross-sectional imaging with CT or CBCT must be obtained (Fig. 10). CBCT images are as reliable as MDCT images in predicting bone invasion by malignant lesions.16 CBCT images are not useful in analysing soft tissue tumours, rather MRI or soft tissue window MDCT is a better diagnostic tool. Multiple examinations using CBCT, MDCT, MRI or nuclear medicine may be needed for a complete diagnostic work-up of a patient with a malignant lesion.
Use of CBCT for inflammatory changes in the bone
As mentioned in the previous paragraph, irregular margins are a common radiographic feature of malignancy. Interestingly, osteomyelitis has similar irregular margins. However, a malignant lesion is less likely to develop a new layer of periosteal bone, while chronic infection frequently results in such layering. Periosteal reaction and cortical destruction, as viewed on multiplanar images, can be useful in differentiating these radiographically similar lesions of widely different prognosis (Fig. 11).17,18 If the infection is acute, neither plain film radiography nor CBCT scan is useful, as early infection does not cause enough bony change to be radiographically detectable. If an aggressive infection persists for two weeks or more, the primary finding on a radiograph is a lytic lesion with irregular margins. If the infection is chronic or moderate to low grade, the bone appears of mixed density. The margin of a chronic infection is often sclerotic and can be adequately viewed on plain film radiographs. To identify periosteal bony reactions, oral and maxillofacial surgeons traditionally used occlusal radiographs. However, wrong exposure factors or angulation can limit the utility of an occlusal radiograph to demonstrate a thin periosteal bony layer. With CBCT images, where multiplanar slices are easy to adjust, thin layers of periosteal bones are better viewed compared to occlusal radiographs. In addition, small bony sequestra associated with osteomyelitis are better identified with cross-sectional imaging.
Features of osteomyelitis are also seen in bisphosphonate related osteonecrosis of the jaws (BRONJ). Although BRONJ is a debilitating condition, fortunately the incidence of this disease is low. In 2004 and 2005, a survey of Australian oral and maxillofacial surgeons identified 158 cases of BRONJ.19 Since that time, the prescriptions of bisphosphonate have increased. In evaluating BRONJ, CBCT images are better than panoramic radiography.20 Currently, all these imaging modalities have limited values in detecting early stages of the disease.21,22 BRONJ may also be associated with failing dental implants. In South Australia, seven BRONJ-related implant failures were reported in a population of 16 000 patients.23 In implant cases, MDCT is likely to produce image artefacts arising from metal implants. CBCT can be used to evaluate the status of alveolar bone adjacent to the implants and also as a follow-up examination (Fig. 11C and 11D).
Orthognathic surgical planning and follow-up studies
For orthognathic surgery, DICOM data from CBCT can be used to fabricate physical stereolithographic models or to generate virtual 3-D models.24–26 Such 3-D reconstructions are most useful for morphological analysis and spatial relationship of the neighbouring structures as well as for growth and developmental anomalies, gross tumour development or fracture displacement.8,27 These 3-D surface models generated from CBCT data may be slightly inferior to that from MDCT, but are usually of acceptable quality.28 The 3-D reconstructions are extremely useful in the diagnosing and treatment planning of facial asymmetry cases. Airway measurement techniques are improving with newer software options.29,30 These data are being used for surgical orthodontic cases as well as for sleep apnoea patients.31 Follow-up CBCT imaging is useful in evaluating the success of orthognathic surgery (Fig. 12), as well as to measure the displacement of the surgical segments in all three orientations.25
Fracture of dentomaxillofacial structures
The diagnosis of a simple dental or jaw fracture can be achieved with periapical or panoramic radiographs. Initial assessment of a complex jaw fracture may also be performed with plain films. However, vertical root fracture or multiple jaw fractures with bone displacement may be better evaluated with CBCT images. Compared to periapical radiographs, CBCT images are significantly better for diagnosing root fractures.32,33 For complex jaw fractures, CBCT may be a valid alternative imaging tool to MDCT, considering radiation dose and image quality.34 Non-displaced fractures of the mandibular condyle can be very difficult to diagnose with conventional radiographs. Multiplanar views of CBCT scans allow much better assessment of interarticular fractures of the condylar head. Currently, most dental CBCT units require the patient to be in an upright sitting or standing position during image acquisition. Therefore, a CBCT unit in its current configuration may not be appropriate where trauma to the cervical vertebra is also suspected and the neck is stabilized. In addition, involvement of cranium and leakage of cerebro-spinal fluid cannot be studied with CBCT. The role of CBCT in fracture diagnosis, therefore, appears to be limited to fracture of teeth and jaw fractures from fall, sports-related injury (Fig. 13) or minor assault. MDCT with or without MRI is a better imaging choice in automobile or industrial accidents involving jaws and other parts of the body.
Use of CBCT for diseases of paranasal sinuses
In addition to dental offices, ENT practitioners are also using CBCT units as an efficient in-house examination tool. Likewise for oral and maxillofacial surgeons, identifying the condition of the maxillary sinuses is important for implant planning and to rule out sinus disease as a cause for orofacial pain (Fig. 14). Sinusitis, a common inflammatory disease involving the maxillofacial skeleton, is often of odontogenic origin.35,36 In some cases with sinusitis, endodontic therapy of the offending tooth may fail, requiring a surgical intervention.37 CBCT not only provides diagnostic information of the status of extension of periapical lesions into the maxillary sinuses,38 but also provides reliable information on the septa of the sinus and presence of exostoses, useful presurgical information when planning sinus floor augmentation in preparation for implant placement.39
Waters’ sinus view, a traditional sinus examination, is now considered inadequate in detecting maxillary sinus opacification and ‘very poor’ in detecting masses in the ethmoid, frontal and sphenoid sinuses.40,41 CBCT images are helpful in identifying mucous retention phenomena, antral polyps, sinonasal polyposis and malignant tumours of the sinuses. In addition, an oral and maxillofacial surgeon should consider a CBCT scan if there is a suspicion of oro-antral fistula formation or if an implant is displaced into the sinus (Fig. 15).
A limitation of CBCT is its poor resolution of soft tissues.42 Sinus masses can be composed of different types of soft tissues with or without fluid accumulation. In addition, the fluid may be thin watery secretion blood or a mix with pus. On a CBCT scan, a mass in the sinus usually has a uniform density. Therefore, differentiation of the density into a fluid or soft tissue mass is often not reliable. CBCT data can be relied on for the size and margin of the sinus mass, status of the sinus wall, and blockage of the ostium. Some software allows measurement of the air space, which can be accurate.43,44 Fungal sinusitis often accumulates calcified materials. On a CBCT scan, these calcified materials can be easily differentiated from the soft tissue component of the sinusitis.
Frequently, patients with developmental disturbances require surgical treatment. CBCT images are invaluable in patient education, treatment planning and as a follow-up study to evaluate growth, development and function. For a cleft palate patient, use of a panoramic radiograph is limited to identifying an alveolar cleft only. Cross-sectional imaging, such as with CBCT, assists in the assessment of the width of the cleft, tooth proximity to the cleft, deviation of the nasal septum and its degree of fusion to the palate, as well as the location of supernumerary teeth and the visualization of the entire osseous defect. Thorough evaluation of the maxillofacial structures with cleft or other developmental defects and syndromes can be achieved with cross-sectional imaging and 3-D reconstruction (Fig. 16).
Use of CBCT in detecting foreign bodies in the maxillofacial complex
One of the limitations of using MDCT scans in the maxillofacial area is artefacts arising from metal restorations. Extensive bridgework or metal restorations can make a MDCT scan virtually non-diagnostic. Such artefacts from metal objects are lower on CBCT images (Fig. 17A and 17B).45,46 Therefore, CBCT is a better imaging modality to assess metal objects in the face, such as fragments embedded from a gunshot,45,47 following automobile or industrial accidents and for localizing retained broken dental needles or surgical wires (Fig. 17C, 17D and 17E).
Use of CBCT scans in soft tissue calcifications
Although CBCT images have low contrast (soft tissue) resolution, they can be better than MDCT in depicting soft tissue calcifications, such as carotid atherosclerosis.42 Other calcifications, such as tonsilloliths and sialoliths (Fig. 18A and 18B), are adequately viewed on CBCT images.48 Ossification of the stylohyoid ligament can impinge the cranial nerves (classic Eagle syndrome) or the carotid artery (carotid artery syndrome). Surgical correction of the ossified ligaments can provide relief of the symptoms.49 Although an ossified stylohyoid ligament can easily be diagnosed on a panoramic radiograph, the relationship of the ligament to other structures is better evaluated by 3-D reconstruction of CBCT (Fig. 18C).