Video imaging analysis of movements of the condyle articular disk and its control by the lateral pterygoid muscle activity and sesory information from the periodontal ligament in rabbits

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The purpose of the present study is to analyze the movements of the articular disk of the mandibular condyle of rabbits during fictive mastication induced by repetitive electrical stimulation of the masticatory cortical area (CMA). Stimulation of the lateral portion of the CMA is well known to induce grind type of rhythmical jaw movements on the side contralateral to the cortex stimulated. It would be possible to compare the movements of the disk on the working and balancing sides by means of video imaging of the disk, when we stimulated the CMAs on both sides. To take video images of the disk, the zygomatic process of squamous part of the temporal bone, which makes the roof of upper articular cavity, was removed in anesthetized rabbits. The movements of the disk were recorded with high-speed video camera (120 images/sec ) in lateral view. One masticatory cycle was divided into 9 stages on the bases of the incisor movements and the EMG activities of the masseter. The movements of the disk in a cycle were represented as a series of 9 line drawings of the disk That were taken by tracing the video images at these stages. The trajectories of the disk movements on the sagittal plane were far from circular arc that was particularly observed in the chopper type of jaw movements. The pathway of the disk movements were widely different between on the balancing and working sides. The range of the movements of the disk was about 3.5 mm in antero-posterior and dorso-ventral directions. Absolute positions of the anterior- and inferior-most deviated disk on the working side were as much as 1 mm posterior and inferior relative to those on the balancing side, respectively. The period of the anterior-most position of the disk was coincident with the period during which the EMG burst activities of the masseter muscle ipsilateral to the condyle studied was present. It was revealed that the movements of the articular disk are widely different between on the working and balancing sides.