ANATOMIC BASIS FOR DISK DISPLACEMENT IN TEMPOROMANDIBULAR-JOINT (TMJ) DYSFUNCTION

This study was designed to investigate the site of lateral pterygoid muscle insertion into the temporomandibular joint (TMJ) disk, and the relationship between that attachment and the disk displacement. One hundred and ten TM joints from 78 cadavers were collected and sectioned in the sagittal plane. Sixty-two unilateral specimens were available and 16 additional specimens were selected randomly from the remaining cadavers in which both condyles were available. Thus statistical comparisons were not confounded by autocorrelation and were based on 78 independent specimens. The sites of muscle attachment in relation to the disk were identified histologically. Of the 78 joints 49 had anterior disk displacement. Of the 42 cases with insertion directly into the disk, 30 had anterior disk displacement. In contrast, only 19 of the 36 disks without direct muscle insertion were displaced. This finding indicated a statistically significant relationship between functional muscle attachment and disk displacement (chi2 = 9.28, df = 1, p = 0.006). However, the superior head of the lateral pterygoid muscle (SLP) may not be the precipitating factor for anterior disk displacement even though it does coordinate disk movement on closure. Since other factors (e.g., trauma) can result in disk displacement, the SLP can maintain disk displacement only when it inserts directly into the disk. in cases of normal disk arrangement and condylar attachment, the muscle may not play a clinically significant role in disk displacement because disk attachment at the medial and lateral poles of the condyle allows the disk to move freely with the condyle. This movement of the condyle and the disk may overcome the pull of the SLP when normal disk attachment is maintained.