Temporomandibular joint dysfunction (TMD) can affect a significant portion of the population.1 Clinical symptoms may include one or more of the following: pain in the region of the temporomandibular joint (TMJ), headaches, earaches, muscle tenderness, joint noises such as clicking, popping or grating, limited opening or deviation of the mandible on opening/closing, locking, and occlusal changes due to alteration in mandibular positioning. Evaluation of TMD begins with a thorough patient history and clinical examination. In some cases, the clinical examination findings are sufficient to allow the dentist to arrive at a preliminary diagnosis and begin conservative treatment. However, other patients will require diagnostic imaging of the TMJs in order to provide information, which is not available from the clinical examination.
Indications for TMJ imaging include the following: conservative treatment that has failed or symptoms are worsening, patients with a history of trauma, significant dysfunction, sensory or motor abnormalities, significant changes in occlusion, or if an osseous abnormality or infection is suspected.2 Some practitioners order TMJ imaging if there is a history of TMD and the treatment plan includes extensive reconstructive work or orthodontia, since these types of treatment can significantly alter the occlusion and predispose the patient to a recurrence of their TMD symptoms. Imaging allows the practitioner to evaluate the integrity and relationships of the TMJ osseous components, confirm the extent or progression of joint disease and evaluate effects of treatment.3 The results of imaging studies must be correlated with the patient history and clinical findings in order to arrive at a diagnosis and plan treatment. The purpose of this article is to review current TMJ imaging techniques so that the dental practitioner understands the contribution imaging can make to the diagnosis of TMD.
The choice of imaging technique will depend on the specific clinical problem, whether hard or soft tissues will be imaged, radiation dose, cost, availability of the imaging technique, and the amount of diagnostic information provided by the technique.3 There have been considerable advances in imaging technology to reduce radiation dose and availability of imaging continues to improve. Usually the hard tissues are imaged first to evaluate osseous contours, positional relationship of the condyle and glenoid fossa, and range of motion. Soft tissue imaging is indicated when information about disk position or morphology is needed or to image abnormalities in the surrounding muscles or soft tissues.
Images should depict the entire joint and surrounding structures. Ideally, images should be available in a minimum of two planes perpendicular to each other, such as the lateral and frontal planes. Views at additional orientations may also be useful, allowing three-dimensional assessment of the joint. Consideration should also be given to imaging structures further removed from the TMJ, particularly if the TMJ findings are normal, since the etiology for the patient's symptoms may in fact be from a source remote from the TMJ.
HARD TISSUE IMAGING
A panoramic radiograph is considered a "screening" projection and is often used in combination with other hard tissue imaging techniques to image the TMJs.4 (Fig 1a). It gives an overview of the jaws and teeth, allowing evaluation of mandibular symmetry, the maxillary sinuses and the dentition. Mandibular asymmetries may not be clinically apparent and a discrepancy in size of one condyle or one side of the mandible may be a contributing factor in the development of TMD.5-7 Maxillary sinus disease or odontogenic inflammatory disease, particularly of the posterior maxillary teeth, may refer pain to the TMJ, simulating TMD. Although the TMJ clinical examination should also include examination of the teeth, occult inflammatory lesions may occasionally be overlooked. Any abnormality of the teeth noted in the panoramic view should be imaged with one or more intraoral views in order to provide maximum bony detail, which aids in arriving at an accurate diagnosis. The panoramic view also depicts the mandibular condyles but caution must be exercised in interpreting the radiographic appearance of the TMJs in a panoramic view. Condylar position cannot be evaluated because the patient is placed in a protrusive and slightly open position. Furthermore, the glenoid fossa does not image clearly and the articulating surfaces of the condyles are distorted due to the angle of the projection so osseous components of the joints cannot be accurately assessed.
Cephalometric plain film radiographs are occasionally indicated as an adjunct to the TMJ imaging study, particularly in patients with developmental abnormalities, some neoplasms, fracture of the jaw or condylar necks, or facial asymmetries2 (Figs. 1b & c).
Imaging of the TMJ itself is best accomplished by the use of tomography. Traditional plain film techniques, such as transcranial and transpharyngeal (Parma) views have largely been replaced by tomography, which has the advantage of depicting the TMJs in thin layers or slice increments. The film and x-ray source are in motion, which blurs structures that are not in a predetermined plane of focus. The joints can be imaged in different orientations, achieving the aim of producing views perpendicular to each other. Tomography may be carried out using conventional tomography or by computed tomography.
With conventional tomography, several exposures are made with the area of interest moving through the plane of focus. A submentovertex (basal) view may be used to determine the angulations of the condylar headlong axes in order to "correct" the angulation of the tomographic images. This produces an undistorted view of joint morphology and allows accurate assessment of condylar position. Normally, several image slices in the sagittal (lateral) and coronal (frontal) plane are made. Sagittal images provide information on condylar position with respect to the glenoid fossa and can be taken at various mandibular positions. Most commonly, sagittal views are exposed with the teeth in the closed (maximum intercuspation) and maximum open positions but additional views with a splint or bite registration in place may also be taken (Fig. 2). Some practitioners also request views with the mandible in the rest or centric relation position, depending on diagnostic needs and treatment goals. Frontal images allow assessment of condylar and glenoid fossa morphology in the medial-lateral orientation and are particularly useful for identifying erosive changes of the articular surfaces. A disadvantage of conventional tomography is that there is some superimposition of neighboring structures on the image in the plane of interest, so the images still have some inherent blurring. Additionally, the technique is more time-consuming than computed tomography, discussed below.
Computed tomography (CT) is a more sophisticated digital tomographic technique where the patient is exposed to a fan-shaped x-ray beam directed to a series of detectors. The detectors and/or the x-ray beam move around the patient, usually in the axial plane, to acquire numerous projections at various angles. The transmission data from these projections is used to reconstruct the image, which is viewed on a computer monitor. Further manipulation can be done to reformat images in various orientations for viewing (Fig. 3). CT has several advantages over conventional tomography: there is no superimposition of structures outside the area of interest, contrast resolution is improved so that tissues with small differences in density can be distinguished, data from one imaging study can be viewed in various planes and three-dimensional images can be constructed. If the scan includes the rest of the skull, the need for additional cephalometric plain film views may be eliminated.
An exciting advance in CT technology is cone beam computed tomography (CBCT), which is particularly suited to imaging hard tissues of the skull and jaws. The patient is exposed to a round or rectangular cone-shaped x-ray beam, which scans the patient's head in one 360-degree rotation.8 The transmission data is captured by a two-dimensional sensor. During the scan, which may take from 17 seconds to over a minute to perform, 360 exposures or projections are made, one for each degree of rotation. The raw data is then reconstructed and reformatting of this reconstruction allows for two-dimensional or three-dimensional images in any selected plane to be made (Fig. 4). A major advantage of this technique is that one CBCT scan only delivers between 3-20 percent of the radiation dose of a conventional CT scan.9 Furthermore, the dose from a single CBCT scan may be the equivalent of as few as four film-based panoramic radiographs and may be less than the effective dose of a 19-film full mouth intraoral film series.10
CT techniques have a distinct advantage over conventional tomography in that large areas can be imaged in one scan and reformatting can be made in multiple planes chosen by the clinician, providing three-dimensional information about the osseous structures. This is particularly valuable for TMJ imaging, since in addition to the TMJs, the remainder of the jaws as well as the skull base can be evaluated. The scan can be done at various mandibular positions, as with conventional tomography. CBCT is rapidly growing in popularity and is starting to replace conventional tomography for many dental imaging applications. It is currently used for TMJ imaging, pre-surgical implant imaging, orthodontic imaging including localization and orientation of impacted teeth, airway analysis and for a wide range of oral surgery applications. This technique is useful for visualizing osseous detail of the TMJs, including evaluation of osseous ankylosis, neoplasms, heterotopic bone growth and other abnormalities in and around the joints which may not be as well visualized with conventional tomography.2 CBCT is not suitable for patients unable to remain motionless for the duration of the scan. A disadvantage of the technique is volume averaging, which results in artifacts that may simulate erosions on small curved cortical bone surfaces.3
Interpretation of the hard tissue imaging study includes evaluation of condylar and temporal component morphology and integrity of bony articulating surfaces. The TMJs are assessed for signs of remodeling, degenerative joint disease or morphological variations affecting the TMJs, jaws or skull. Condylar position in intercuspation and at maximum opening is evaluated and structures further removed from the TMJs are evaluated if they are included in the study.
SOFT TISSUE IMAGING
Conventional radiographic techniques do not demonstrate the disk so disk position, function and integrity cannot be evaluated. Normally the hard tissues are evaluated first; soft tissue imaging is indicated when symptoms are unresponsive to conservative treatment, when significant TMJ pain and dysfunction are present or when the clinical findings suggest a disk displacement.2,3 Soft tissue imaging techniques include arthrography or magnetic resonance imaging (MRI).
In arthrography, a radiopaque iodine-based contrast agent is injected into one or both joint spaces and radiographs are taken. The disk is visible as an indirect image between the outline of one or both joint spaces (Fig. 5). The flow of contrast agent as well as disk function on opening and closing is monitored using fluoroscopy. Small perforations of the disk or its attachments can be detected, as both joint spaces fill with contrast agent simultaneously. Disadvantages of arthrography include post-operative discomfort and inability to detect medial disk displacements. There is also risk of allergy to the contrast agent and risk of post-operative infection.4
MAGNETIC RESONANCE IMAGING
Magnetic resonance imaging (MRI) uses a magnetic field and radiofrequency pulses rather than ionizing radiation to produce multiple digital image slices. The TMJs are imaged in the closed and open positions. Images can be constructed in either the sagittal or coronal planes and therefore this technique is especially useful for diagnosis of medial disk displacements (Fig. 6). MRI has the advantage of being non-invasive and enables evaluation of the disk, surrounding muscles, and can image joint effusions.11 Contraindications to MRI include pregnancy, pacemakers, intracranial vascular clips, the presence of metal particles in vital structures, patient claustrophobia, obesity or inability to remain motionless for the examination, which may take several minutes to complete.3 The technique is also relatively expensive and is not readily available in some centres.
Arthrography and MRI have been shown to have similar degrees of diagnostic accuracy11 and both techniques can provide excellent diagnostic information. The choice of soft tissue imaging technique depends on factors such as cost, availability, and patient factors described above which may contraindicate a particular technique.
TMJ imaging is an adjunct to the clinical examination and provides useful information about the joint components. When selecting a TMJ imaging technique, the clinician must determine what type of information is needed from the imaging study and whether that information will affect patient management. Consultation with an oral and maxillofacial radiologist is encouraged, particularly if the clinician does not have knowledge and experience in interpreting the imaging study. This article reviewed various hard and soft tissue imaging techniques and this information will enable the clinician to choose the most appropriate imaging modality for their patients.
Dr. Petrikowski is an oral and maxillofacial radiologist and is Associate Professor in Oral Radiology at the Faculty of Dentistry at the University of Toronto. She maintains an active private practice and consulting service in oral and maxillofacial radiology at CMI Centres in Toronto.
Oral Health welcomes this original article.
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