What we really see in a TMJ radiograph

Abstract

1. 1. The zygomatic arch has been ruled out as forming the image of the glenoid fossa on TMJ radiographs. 2. 2. The three-dimensional head position that is required for a TMJ radiograph accommodates for condylar asymmetry and aligns that condyle almost perpendicular to the film. 3. 3. Due to the 75 degree x-ray angulation that is required, the lateral third of the condyle is outlined on the radiograph almost in profile, or as a cross-sectional view, and not in a “composite” view as commonly thought. 4. 4. The central and medial portions of the condyle are projected inferiorly on the radiograph and are obliterated by the profile view of the lateral third of the condyle. 5. 5. The corresponding central and medial portions of the fossa are projected inferiorly on the radiograph and not usually observable. 6. 6. The TMJ space varies in relative width from the lateral to the medial portions. It is generally widest laterally, which may be associated with providing space for condylar rotation in lateral mandibular movements. 7. 7. Although the joint space is not constant in various sagittal locations, the relative anterior and posterior joint spaces remain proportional so that the classification of the condyle position in the fossa (i.e., retruded, concentric, or protruded) remains the same. 8. 8. Variations in head position, in all three planes of space, do not effectively alter the resultant TMJ radiograph. 9. 9. Variations in x-ray angulation and position do not effectively permit the recording of the midline or medial portions of the fossa and condyle. 10. 10. The floor of the midcranial fossa in the TMJ area is not seen in cross section on the TMJ radiograph. 11. 11. Variations in the anatomy of the fossa (bony protruberances) may be seen as projections into the joint space on the radiograph. Corresponding alterations in condylar anatomy (which maintain a relatively proportional joint space) are projected inferiorly on the radiograph and are hidden by the profile or cross-sectional image of the lateral third of the condyle. 12. 12. The definite radiopaque outline of the fossa on the radiograph represents the true shape of the fossa at the lateral third. All superimpositions into the joint space are bony projections of the fossa that are more medial to the lateral third area. 13. 13. Most TMJ radiographs are profile or cross-sectional images of the lateral third of the fossa and condyle, and it is extremely difficult to record the image of any other portion.