A steep articular eminence is reported to be a predisposing factor for the development of disc displacement. The purpose of this study was to evaluate the biomechanics of the temporomandibular joint (TMJ) in internal derangements and, additionally, to investigate whether a relationship exists between the steepness of the articular eminence and disc displacement, with (DDR) and without reduction (DDWR). The material consisted of the sagittal TMJ magnetic resonance images (MRIs) of 39 joints (26 DDR and 13 DDWR). Sagittal and coronal TMJ MRIs were obtained at maximum intercuspation and in the maximum mouth-opening position. The steepness of the articular eminence, disc, and condyle rotation, and condyle translation were investigated. A Student's t-test was performed and correlation coefficients calculated. The results revealed an increase in disc rotation in the DDR group and in condyle translation in the DDWR group (P < 0.01). Condyle rotation and the steepness of the articular eminence were similar in both groups. Disc rotation was positively correlated with condyle rotation and negatively correlated with condyle translation in the DDR group (P < 0.05, r = 0.44). Condyle translation was positively correlated with steepness of the articular eminence in the DDWR group (P < 0.01, r = 0.74). There was no correlation between movements of the disc-condyle assembly and the steepness of the articular eminence in the DDR group. Nevertheless, a relationship between condyle translation and the steepness of the articular eminence was found in the DDWR subjects.
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