Temporomandibular joint

Throughout life the TMJ is subject to remodelling, the most important reason being its functional relationship with the dentition. Functional stress may, however, be so severe that it leads to a pathological response, resulting in a break down of the joint tissues analogous to that found when other joints are overloaded. In addition to these alterations, ranging from physiological remodelling to degenerative osteoarthrosis, the

	Figure 50. Myeloma of the posterior mandible with punched out appearance.
	Figure 51. Metastatic breast carcinoma to the mandible. Irregular ill-defined osteolytic and sclerotic areas. Several teeth endodontically treated since symptoms and radiographic findings were thought to be caused by periapical inflammatory lesions.
a	Figure 52. Metastatic Ewing sarcoma to the right mandible. a) Panoramic radiographs obtained three months apart b) CT, and c) MRl of a patient with numbness of the lower lip. Pathological changes can only be seen in the MR image as a low signal intensity of the marrow spaces on the right side (arrows).
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	Figure 53. Oblique transcranial radiograph of the TMJ. The contours of the lateral parts of the joint are seen (arrows). The medial parts of the condyle are superimposed on the condylar process (open arrow). The medial parts of the fossa and articular eminence are not depicted
	Figure 54. Sagittal tomogram of the TMJ. Normal bony linings and structures. Normal position of the condyle at central occlusion.
	Figure 55. Panoramic radiograph showing degenerative changes of the left condyle and eminence (arrows).
a	Figure 56. MR images of the TMJ: a) T1-weighted sagittal section demonstrating disc at partial opening. b) Frontal section displaying capsule and disc c) T2-weighted sagittal section showing high signal intensity of the synovial and retrodiscal spaces.
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TMJ can be affected by trauma and the same diseases as the other synovial joints, such as septic arthritis and various tumours.

The TMJ has three main components enclosed by a capsule; the temporal with the glenoid fossa and the articular eminence, the mandibular comprising the condyle, and, interposed between these components, the disc. The glenoid fossa is a depression in the cranial base, posteriorly separated from the external auditory meatus and the middle ear by the tympanic plate and parts of the petrous portion of the temporal bone. The anterior aspect of the glenoid fossa forms the posterior slope of the articular eminence. The mandibular condyle is shaped like a half cylinder, convex in both lateromedial and anteroposterior directions. When the teeth are held together in central occlusion, the condyle is situated in the glenoid fossa. Projected on a horizontal plane its long axis forms an average angle of 25° with the frontal plane. Seen in the sagittal plane the normal disc is biconcave. It consists of collagen fibres condensed to a fibrous cartilage. Posteriorly the disc attaches to the posterior ligament and anteriorly to the capsule and the superior belly of the lateral pterygoid muscle. The capsule attaches to the tympanic plate, neck of condyle, cranial base and the disc. The movements of the mandible upon opening, closing, protrusion, and latero- and mediotrusion, are governed by a complex interaction between the masticatory muscles.

Earlier, radiographic examinations of the TMJ were mainly focused on determining the topographical relationship of its osseous components and on its morphology and structure. Today, the soft tissue components, disc and capsule, have come into focus as a result of an increasing awareness of the high prevalence of internal derangement of the joint.

Remodelling and degenerative changes of the osseous components of the joint can be studied by means of plain films, tomography, CT, and MRI. The oblique transcanial radiograph produces a limited and distorted view (Fig. 53). The panoramic radiograph can demonstrate marked changes in shape and structure, in particular in the condyle (Fig. 55). Sagittal tomography is an even better technique for examination of structural changes and condylar position (Fig. 54). For internal derangement, such as anterior and lateromedial disc displacement, with and without reduction, and deformation of the disc, single- and double contrast arthrography and MRI, are the methods of choice. Many consider MRI to provide the gold standard for examination of internal derangement. The depiction of the disc and distinguishing it from the posterior

	Figure 57. Fibrous scar following surgery including extraction of maxillary lateral incisor. Note sclerotic borders of the through and through defects (arrows).
a	Figure 58. Recurrence of a keratocyst: a) Prior to initial removal. b) Recurrence three years later. One year after surgery, apparent healing had taken place.
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attachment can, however, be difficult. Even MR images can in this respect be ambiguous, i.e. if the posterior attachment has become fibrous and yields a low signal intensity similar to that of the disc. One advantage of arthrography is that fluoroscopic studies can be made during function, facilitating the identification of the disc and posterior attachment and the differential diagnosis between disc displacement with and without reduction. MRI can provide similar information if a series of stationary positions of the lower jaw during opening is combined into a motion sequence. In the future, dynamic MRI will in all likelihood allow real time studies of the joint function. MRI provides other important information about the tissues in and around the TMJ (Fig. 56). Clinical evaluation in combination with a less sophisticated examination procedure may, however, produce similar diagnostic results.

Lars Hollender and Karl-Åke Omnell