title: Relationship between facial asymmetry and masseter reflex activity.
abstract: OBJECTIVE
  In this study, we evaluated the tonic vibration reflex (TVR) of the masseter muscles in patients with facial asymmetry.
  
  
  METHODS
  The study was conducted with 10 volunteers without facial asymmetry and 12 orthognathic patients with facial asymmetry. Subjects were seated in a chair and held a stimulator composed of an electric motor and an acrylic bite block between the upper and lower dentitions at facial midline, to elicit TVR. Electromyographic activity was recorded using a pair of silver electrodes affixed bilaterally with adhesive tape to the skin over the superficial masseter. The amount of mandibular deviation was measured on the frontal cephalogram. The reflex response was evaluated with the following: TVR index (%) = integral TVR x 100/ integral maximum voluntary contraction (MVC), and with the TVR ratio: integral TVR on the high MVC side/ integral TVR on the low MVC side.
  
  
  RESULTS
  In the patient group, the average TVR index on the deviated side was significantly higher than that on the nondeviated side. In all subjects, including the control and patient groups, a negative correlation between the amount of mandibular deviation and the side-to-side difference in TVR index was seen (r = -0.536, P <.05, n = 22). In addition, patients with a lower MVC on the deviated side than on the nondeviated side showed a significantly higher TVR ratio than did the control group.
  
  
  CONCLUSIONS
  These results suggest that the difference between the right and left reflex responses elicited by TVR might be related to frontal craniofacial morphology.
authors.affiliation: Division of Orthodontics, Department of Oral Biological Science, Course for Oral Life Science, Niigata University Graduate School of Medical and Dental Sciences, Niigata City, Japan. orthomac@pop02.odn.ne.jp
authors.forename: Naoki
authors.initials: N
authors.lastname: Machida
authors.forename: Kazuhiro
authors.initials: K
authors.lastname: Yamada
authors.forename: Yoshiyuki
authors.initials: Y
authors.lastname: Takata
authors.forename: Yoshiaki
authors.initials: Y
authors.lastname: Yamada
mesh_terms: D000328:Adult; D016022:Case-Control Studies; D002508:Cephalometry; D004558:Electric Stimulation; D004576:Electromyography; D005146:Facial Asymmetry / Q000503:physiopathology*; D006801:Humans; D008334:Mandible / Q000002:abnormalities; D008406:Masseter Muscle / Q000503:physiopathology*; D009119:Muscle Contraction; D009470:Muscle Spindles / Q000503:physiopathology; D011999:Recruitment, Neurophysiological; D012026:Reflex, Stretch; D014732:Vibration