Spectral analyses were performed on sounds recorded from TMJ's that had previously been classified into different intracapsular categories, in an attempt to determine whether the inherent properties of these sounds were unique for each different disorder. A total of 55 joints was studied: 32 were diagnosed as displaced disc with reduction [DDR], 10 were diagnosed as displaced disc without reduction [DDN], and 13 were diagnosed as degenerative joint disease [DJD]. The spectral analysis for each recorded joint sound was performed using a Fast Fourier transform routine, the results of which were plotted as a frequency vs. amplitude envelope. These analyses showed that different intracapsular TMJ disorders were characterized by sounds whose energy distribution patterns, while showing certain across-group differences, usually shared significant common spectral properties. The joint-propagated noises associated with DDR, DDN, and DJD were each characterized by a spectral envelope whose primary band of energy was centered around a peak at approximately 1 kHz, and which dropped off from that point to background levels. These patterns presumably reflect the resonance characteristics of the disordered joint as defined by the mass and stiffness of its articulating surfaces. Based on the findings of this study, it would appear that comparisons of the spectral envelopes of joint-propagated sounds would have only limited application in the differential diagnosis of intracapsular TMJ disorders.
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