We present a pictorial review of the spectrum of temporomandibular joint (TMJ) pathology diagnosed with CT. Although MRI is the modality of choice for most TMJ pathology, CT is useful when MRI is contraindicated or not accessible. With attention to technique and viewing conditions, CT is capable of showing internal disc derangement, arthritis, neoplasms and non-TMJ regional pathology at a relatively low radiation dose.
Temporomandibular disorders (TMDs) are common, affecting up to 28% of the population at some time in their lives. They are a heterogeneous group of disorders, usually classified into myogenous and arthrogenous groups, although there is often overlap. Articular causes of TMD include internal derangement of the articular disc (meniscus), inflammatory and degenerative arthropathies, and less commonly ankylosis and neoplastic conditions.
Magnetic resonance imaging is the imaging technique of choice for the evaluation of suspected temporomandibular joint (TMJ) disorders. However, it is contraindicated in some patients and is not tolerated because of claustrophobia in others, and its accessibility is relatively limited. Multidetector CT (MDCT) is more widely available and better tolerated. Computed tomography has been used to detect bony abnormalities of the TMJ and in rare conditions such as synovial osteochondromatosis. It has also previously been used for the diagnosis of internal disc derangement. We have found MDCT to be a useful imaging investigation in the reliable detection of internal disc derangement, arthritis and other miscellaneous conditions of the TMJ. We present a review of the technique and of the range of imaging abnormalities encountered.
Computed tomography technique and anatomy
MDCT (16- to 64-detector row) is performed in the closed- and open-mouth positions, without the use of intravenous or intra-articular contrast medium. Multiplanar reconstructions are performed in the coronal oblique (i.e., parallel to the long axis of the mandibular condyle) and in the sagittal oblique (i.e., perpendicular to the long axis of the mandibular condyle) planes using both bone and soft-tissue reconstruction algorithms. Using currently available low-dose iterative reconstruction algorithms, the dose-length product (DLP) of the examination can be reduced to as low as 540 mGy cm, resulting in an effective dose (ED) of approximately 1.2 mSv (if the ED/DLP ratio of 2.2 for CT of the head is used). The multiplanar reconstructions are viewed by the radiologist on a DICOM viewer, which allows window width and window level to be adjusted, to optimise visualisation of the articular disc. The source axial images are also reviewed to detect any abnormalities in the imaged volume, which may be incidental or which may be the source of symptoms mimicking TMJ dysfunction. Disc displacements may also be visible on the axial images.
The TMJ is a synovial joint that is divided into superior and inferior compartments by the fibrocartilaginous biconcave disc (meniscus) (Fig. 1). The disc has thick anterior and posterior bands, and a thinner intermediate zone: the latter is normally positioned between the anterosuperior aspect of the mandibular condyle and the articular surface of the squamous temporal bone. The articular disc has higher attenuation than the surrounding soft tissues but lower attenuation than the adjacent tendon of the lateral pterygoid muscle. The anterior and posterior bands can be seen, although the thinner intermediate and bilaminar zones are not generally visible on CT.
Internal disc derangement
Internal derangement is interference with a joint's smooth function, and in the TMJ this is usually due to displacement of the disc. Disc displacement is common, especially in women, and has been described in asymptomatic volunteers. Postulated causes include ligamentous laxity (there is frequently an antecedent history of trauma or prolonged mouth opening for dental procedures), bruxism and abnormal activity of the lateral pterygoid muscle. A displaced disc may be reduced (‘recaptured’) with mouth opening (Fig. 2), and reduction is accompanied by an audible and palpable click. In more advanced cases, a displaced disc may remain displaced in opening the mouth (the so-called ‘closed lock’) (Fig. 3), in which case there is usually restriction of mouth opening, and absence of a click. The disc may be deformed, and there may be associated synovitis (predominantly in the inferior joint compartment) and osseous erosions (usually affecting the condyle). Remodelling and osteoarthritis may follow (Fig. 4).
Disc displacement is most commonly anterior (see Figs 2, 3) or anterolateral (Fig. 5). The disc may also be displaced anteromedially. Pure medial (Fig. 6) and pure lateral displacement are uncommon, and posterior disc displacement is rare. Associated disc deformity, joint effusions, synovitis and erosions are visible on CT (see Fig. 3).
Osseous erosions are frequently seen in association with disc displacement, usually affecting the mandibular condyle (see Fig. 3c). Less commonly, TMJ erosions are seen as part of a systemic inflammatory arthropathy, such as juvenile chronic arthritis, rheumatoid arthritis, ankylosing spondylitis and psoriatic arthritis (Fig. 7).
Idiopathic condylar resorption
A more severe form of condylar erosion associated with high grade internal derangement is recognised in teenage girls who have a relatively small mandible (retrognathism). It has been referred to as ‘cheerleader's syndrome’ because of the demographic primarily affected. On imaging, there is a fairly rapid progression from a non-reducible displaced meniscus to progressive condylar erosion and resorption (Fig. 8). The glenoid fossa is normal, the joint space progressively widens and eventually only the ‘stump’ of the resorbed condyle remains. It is usually bilateral but may be asymmetric. There is significant debate as to whether orthodontic treatment is an aetiological or exacerbating factor in this condition. Several authors have proposed that avascular necrosis of the condyle is the likely pathogenesis. Treatment is difficult and some patients will need corrective jaw (orthognathic) surgery to correct their skeletal deformity with or without debridement of the TMJs once the condition has stabilised, usually at the end of puberty.
Osteoarthritis is usually seen in an older age group than internal derangement. Most patients with TMJ osteoarthritis have a history of preceding internal derangement, and available evidence suggests that patients with severe internal derangement can progress to osteoarthritis after a variable interval, often several years. The changes of osteoarthritis seen in the TMJ are identical to those seen in other affected joints: joint space loss, articular surface remodelling and flattening, cortical sclerosis and thickening, subcortical cysts, osteophytes and calcified intra-articular bodies (Fig. 9) As is typical of this condition, the changes seen on imaging do not always correlate with symptoms. Many patients may be pain-free despite advanced osteoarthritis and the only complaint is of joint noises or grating.
Marked restriction of mouth opening and condylar movement is uncommonly due to ankylosis of the TMJ, which may be intra- or extra-articular in location and fibrous or bony in nature. Intra-articular ankylosis usually results from prior trauma (such as intra-capsular condylar fracture), causing haemarthrosis or prior septic arthritis (Fig. 10). Osseous union may be partial or total. In fibrous ankylosis, there is marked joint space narrowing with reciprocal articular surface irregularity often giving a ‘saw-tooth’ appearance.
Extra-articular ankylosis is rare. Fibrous ankylosis can follow haemorrhage or infection in the muscles of mastication, some of these cases representing typical myositis ossificans. Bony ankylosis or development of a pseudo-arthrosis can occur in patients with developmental coronoid hyperplasia or post-traumatic depression of the zygomatic arch. In both cases, there is ankylosis or a pseudo-arthrosis between the coronoid process and the medial margin of the arch (Fig. 11).
Fractures of the mandibular condyle are common and usually occur following a blow to the contralateral side of the mandible. They may be the sole injury as the condylar neck is relatively thin but also occur in conjunction with fractures of the contralateral mandibular angle or body. A blow to the symphysis may result in bilateral condylar fractures, especially in elderly patients with a resorbed mandible.
Most condylar fractures are extra-capsular, running obliquely through the thin neck. The pull of the lateral pterygoid muscle that attaches to both the condyle and disc results in anteromedial displacement of the condyle from the glenoid fossa with the disc maintaining a normal anatomical relationship to the displaced condyle (Fig. 12a). If these fractures occur before growth is complete, growth of a neo-condyle may occur, replacing the displaced condyle within the glenoid fossa and producing a ‘double’ condyle appearance (Fig. 12b). Intra-capsular fractures are much less common and may be undisplaced and occult on plain radiographs. A variant of this injury is a tympanic plate fracture involving the posterior wall of the glenoid fossa (i.e., the anterior wall of the bony external auditory canal) (see Fig. 12a).
Miscellaneous neoplastic and hyperplastic TMJ conditions
This is an idiopathic developmental condition more common in males and generally manifesting in the teenage years and stabilising at skeletal maturity. Unilateral overgrowth of a mandibular condyle results in mandibular asymmetry; the condylar neck may also be enlarged, and there may be an ipsilateral posterior open bite. There is commensurate enlargement of the glenoid fossa (Fig. 13).
This may affect the temporal surfaces of the TMJ or the condyle itself. Its appearances are characteristic, with expansion of affected bone, ground glass texture and loss of the normal corticomedullary differentiation.
This benign bone tumour (cartilage-capped exostosis) is the most common neoplasm affecting the TMJ. It can arise from the condylar neck or head and can interfere with the normal joint movements of the TMJ (Fig. 14a). Other neoplasms of chondral origin are far less common (Fig. 14b,c).
Although the synovial proliferation associated with advanced internal derangement occurs in the lower joint space, other rare synovial proliferative disorders predominate in the upper joint space. Of these, synovial chrondromatosis is the most common. The upper joint space is distended by synovial proliferation and small intra-articular bodies, either chondral or ossified (Fig. 15). Chondral bodies may be missed on CT, being more clearly demonstrated by MRI. Nevertheless, the diagnosis of synovial osteochondromatosis should be suggested in a patient with symptoms of internal derangement, a normally positioned disc and significantly distended upper joint space, with or without articular surface erosions.
Metastases and other aggressive neoplastic lesions may affect the condyle and may present clinically as TMJ pain (Fig. 16).
Otalgia can be difficult to differentiate from pain of TMJ origin. The anterior wall of the bony segment of the external auditory canal (tympanic plate) also forms the posterior bony margin of the glenoid fossa of the TMJ. In addition, the auriculotemporal nerve, a branch of V3, provides the main innervation to the TMJ as well as a portion of the external auditory canal, the lateral aspect of the tympanic membrane, auricle and skin in the temporal region. Otomastoid inflammatory disease and destructive processes are optimally assessed on the volumetric data set provided by MDCT of the TMJ.
Parotitis, malignant salivary gland tumours and metastatic carcinomas to the intra-parotid lymph nodes may all cause pain referred to or within the TMJ. The auriculotemporal nerve provides secretomotor supply to the parotid gland, thus providing a ‘link’ between the Vth and VIIth cranial nerves. It may be involved by the perineural tumour spread that can sometimes be visualised on non-contrast MDCT in the fatty parotid glands viewed on a soft-tissue window (Fig. 17a).
Inflammatory or neoplastic processes in the masticator space can produce pain in the region of the TMJ as well as restricted mouth opening, mimicking TMJ disorders (Fig. 17b).
Neoplastic involvement of the V3 from Meckel's cave to the inferior alveolar canal in the mandible could potentially produce pain within or close to the TMJ. Asymmetric expansion of the foramen ovale and thickening of the V3 nerve can be appreciated on non-contrast MDCT. Uncommonly, primary tumours or metastases involving bone or the meninges of the floor of the middle cranial fossa may cause pain in the vicinity of the TMJ. Furthermore, even if not responsible for the presenting symptoms, significant incidental lesions may be seen within the scanned volume, including posterior fossa and pituitary neoplasms, cholesteatoma, otosclerosis and other temporal bone pathology.
Although CT has been traditionally viewed as a modality for assessing only bony pathology and calcification of the TMJs, with attention to technique and interactive reading with a DICOM viewer, it is an excellent modality for viewing the full range of TMJ pathology and can be used instead of MRI when MRI is contraindicated or not accessible. In the setting of trauma and primary bone lesions, it is the modality of choice.