title: Unusual postero-inferior condylar movements that depend on the position of occlusal contact during fictive mastication in rabbits.
abstract: OBJECTIVE
  The mandible can be modelled as a triangular plate supported at two joints and the point of occlusion. The mandible is stable if the vector of the jaw-closing muscle forces lies within the triangle of support. If this vector lies outside of the triangle of support, one of the three contact points will tend to separate as the mandible rotates around a line connecting the other two points. Here, we examined whether postero-inferior condylar movements (Pi-Cm) due to mandibular rotation may occur during fictive mastication in anaesthetized rabbits.
  
  
  METHODS
  EMG activities of the masseter (MS) and lateral pterygoid (LP) muscles and movements of the condyle and incisal points were recorded. Condylar movements in the sagittal plane were recorded using a high speed CCD camera.
  
  
  RESULTS
  Pi-Cm were observed on the working side during occlusal phase in half of the rabbits (altered-movement group), if the biting point was restricted at the posterior most tooth (M3) on the working side using a metal biting plate. Pi-Cm appeared in the period between the estimated maximum force of the MS and the LP during late occlusal phase. The MS EMG ratio between the working and balancing sides in the altered-movement group was significantly less than that in the unaltered-movement group.
  
  
  CONCLUSIONS
  Since the space lying between the condyle and the articular eminence expanded during the Pi-Cm, it is likely that the posterior band of the articular disc tended to slip anteriorly. The clinical significance of the Pi-Cm is discussed concerning the anterior dislocation of the disc in patients with TMJ disorder.
authors.affiliation: Department of Physiology, School of Dentistry, Aichi-Gakuin University, Kusumoto-cho 1-100, Chikusa-ku, Nagoya 464-8650, Japan. Electronic address: takumi@dpc.agu.ac.jp.
authors.forename: Takumi
authors.initials: T
authors.lastname: Morita
authors.affiliation: Department of Physiology, School of Dentistry, Aichi-Gakuin University, Kusumoto-cho 1-100, Chikusa-ku, Nagoya 464-8650, Japan.
authors.forename: Katsunari
authors.initials: K
authors.lastname: Hiraba
authors.affiliation: Department of Physiology, School of Dentistry, Aichi-Gakuin University, Kusumoto-cho 1-100, Chikusa-ku, Nagoya 464-8650, Japan.
authors.forename: Tomoko
authors.initials: T
authors.lastname: Matsunaga
authors.affiliation: Department of Oral and Maxillofacial Surgery, School of Dentistry, Aichi-Gakuin University, Suemori-dohri 2-11, Chikusa-ku, Nagoya 464-8651, Japan.
authors.forename: Yu
authors.initials: Y
authors.lastname: Ito
authors.affiliation: Department of Oral and Maxillofacial Surgery, School of Dentistry, Aichi-Gakuin University, Suemori-dohri 2-11, Chikusa-ku, Nagoya 464-8651, Japan.
authors.forename: Hisanobu
authors.initials: H
authors.lastname: Maruo
authors.affiliation: Department of Oral and Maxillofacial Surgery, School of Dentistry, Aichi-Gakuin University, Suemori-dohri 2-11, Chikusa-ku, Nagoya 464-8651, Japan.
authors.forename: Kenichi
authors.initials: K
authors.lastname: Kurita
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D001732:Bite Force; D003766:Dental Occlusion; D004558:Electric Stimulation; D004576:Electromyography; D008335:Mandibular Condyle / Q000502:physiology*; D008406:Masseter Muscle / Q000502:physiology*; D008409:Mastication / Q000502:physiology*; D019736:Prostheses and Implants; D011626:Pterygoid Muscles / Q000502:physiology*; D011817:Rabbits; D014886:Weaning
keywords: Anterior dislocation of the disc; Balance of bilateral masseter muscle activities; Fictive mastication; Position of occlusal contact; Postero-inferior condylar movements; TMJ disorder