To determine the position of the anteriorly displaced articular disc, we examined temporomandibular joints by magnetic resonance imaging (MRI) in 54 patients with unilateral temporomandibular joint dysfunction. MRI was done before conservative therapy in the occlusal and maximal open-mouth position. We drew a straight line connecting the lowest point of the articular eminence (point A) and the highest point of the external acoustic pore (point
E
) by tracing the sagittal view on MRI. On the standard A-
E
line, points
B
', C', and D' were sought by dropping perpendicular lines from the deepest point of the glenoid fossa (point
B
), the highest point of the condylar head (point C), and the most posterior point of the articular disc (point D). We then determined the sagittal position of the articular disc in the glenoid fossa by measuring the distances
AD
' and C'D' and sought the correlation between these distances and clinical findings. The results were as follows. 1. The distance
AD
' in 33 patients with MRI-defined irreversible displacement of the disc was shorter than that in 10 patients with reversible displacement (3.
7
±1.9mm vs 6.8±
2
.0mm, p<0.0001). In contrast, the distance C'D' in the former was longer than that in the latter (6.0±1.
7
mm vs 3.4±1.
5
mm, P<0.0001). Of 29 discs displaced anteriorly less than
5
mm in distance C'D', 19 (66%) had reversible or no displacement. However, 23 (
92
%) of 25 discs displaced more than
5
mm in distance C'D' had irreversible displacement.
2
. Disc deformity and abnormal signal intensity bands in the condylar head closely correlated with distances
AD
' and C'D'. In 17 patients without disc deformity, mean
AD
' and C'D' distances were 9.8±
2
.
7
mm and 0.
2
±
2
.8mm, respectively. In 19 and 18 patients with mild to moderate and severe deformity, the mean
AD
'(C'D') distances were
5
.4±1.8mm (4.
5
±1.6mm) and
2
.
7
±1.8mm (6.8±1.
5
mm), respectively. 3. Although distance
AD
' and C'D' did not correlate with clinical symptoms, the degree of anterior disc displacement correlated with the therapeutic effect of conservative therapy. The mean
AD
' distance (
7
.3±3.
7
mm) in 15 patients with a complete response to conservative therapy was longer than that (3.4±1.4mm) in
5
patients with no improvement. These results indicate that the sagittal position of the articular disc can be determined by MRI and that successful conservative therapy is unlikely if anterior disc displacement extends beyond
5
mm from the top of the condylar head, often resulting in irreversible displacement.
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