The temporomandibular joint (TMJ) has three basic components, the condylar head (Figure 12.1), joint space, (Figure 12.2), and glenoid fossa and articular eminence of the temporal bone (Figure 12.1). On conventional radiography, including panoramic radiography, conventional tomography, bone window helical computed tomography (HCT) (Figure 12.3a), and cone-beam computed tomography (CBCT) the joint space is visualized as a radiolucent structure by virtue of its wholly soft-tissue content. The only radiopacities that may be observed occasionally within this space by the aforementioned modalities are articular loose bodies (Figure 12.2). These radiopacities range from innocent joint mice (isolated bone fragments from the condyle or temporal bone), synovial chondromatosis,¹ and pseudogout (chondrocalcinosis).² The last two diseases can erode through the skull base. Yokota et al. had included chondrosarcoma and osteosarcoma in the differential diagnosis of a case of synovial chondromatosis.³

The anatomical components and disease characteristics of the soft tissue of the joint space are displayed by soft-tissue window HCT and especially by magnetic resonance imaging (MRI) (see Figure 12.3b, c). Pereira et al. pioneered an inconclusive study using ultrasound.⁴

The first image usually taken of a patient presenting with symptoms or signs indicating a TMJ problem is the panoramic radiograph. This provides the clinician with a lateral view of the condylar head and neck. Although the width of the focal plane of the panoramic radiograph is likely to include the whole width of the condylar head, the shape of the head can vary between patients. Nevertheless, if the condyles are symmetrical in shape and size, it is reasonable to assume they are normal, particularly in the absence of symptoms. The significance of flattened condyles, erosions, and osteophytes (Figure 12.4) are considered later.

The size of the condylar heads can be assessed first by ensuring that the patient has been properly positioned within the panoramic radiographic unit prior to exposure. This may be readily assessed by comparing the width of the vertical rami and the molar teeth of both sides. Any difference should then be compared to the patient. If the patient displays no difference, the image has been distorted due to incorrect positioning.

After it has been determined that one side is indeed larger, the clinician needs to determine which side is abnormal, because the other, smaller side could be hypoplastic. This can be appreciated by an abnormally shaped vertical ramus and an obtuse gonial angle (the angle formed by the lower border of the mandible and the posterior margin of the vertical ramus) (see Figure 10.30). Hypoplasia of one side can result from a developmental accident such as hemorrhage of the stapedial artery in utero with disruption of adjacent tissues including the condylar growth center. It can also occur in infancy due to radiotherapy. The midline of the mandible is skewed toward the hypoplastic side (Figure 12.5b). An increase in size could be due to hyperplasia, neoplasia, or dysplasia.

Lesions affecting the condyle may arise either primarily within it, such as the osteoma (Figure 12.5) and chondrosarcoma, or arise elsewhere in the mandible and subsequently involve it, such as osteogenic sarcoma (Figure 12.6) and fibrous dysplasia (FD) (Figure 12.7).