title: Masseter muscular weakness affects temporomandibular synovitis induced by jaw opening in growing rats.
abstract: OBJECTIVE
  To evaluate the influence of impaired masseter function during growth on the development of temporomandibular synovitis.
  
  
  METHODS
  Sixteen 3-week-old male Wistar rats were classified into four groups. The first group served as control; and in the second group, jaw opening was forced for 3 hours when the rats were 9 weeks old. In the third and fourth groups, the masseter muscles were bilaterally resected at 3 weeks of age, and the rats in the fourth group were additionally forced to open their jaw at 9 weeks of age. All rats were sacrificed at 9 weeks. Temporomandibular joint (TMJ) tissue samples were processed for histology, and evaluated for cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS) expressions by immunohistochemistry to examine the inflammatory changes in the synovial membrane.
  
  
  RESULTS
  The control group showed noninflammatory changes. In the jaw-opening group, vascular dilation and weak COX-2 immunoreactivity were induced by jaw opening in the synovium. In the masseter-resection group, the masseter-resected rats exhibited moderate synovial changes while in the resection with opening group, the masseter-resected rats revealed more significant inflammatory changes including synovial hyperplasia, dilated vasculature, fibrin deposits, and intense immunoreactivity for COX-2 and iNOS, all caused by jaw opening.
  
  
  CONCLUSIONS
  These results suggest that masseter activity in the growth period is an important factor in the induction of temporomandibular synovitis.
authors.affiliation: Department of Orofacial Development and Function, Tokyo Medical and Dental University, Tokyo, Japan. mihk.orts@tmd.ac.jp
authors.forename: Miho
authors.initials: M
authors.lastname: Ozaki
authors.forename: Sawa
authors.initials: S
authors.lastname: Kaneko
authors.forename: Kunimichi
authors.initials: K
authors.lastname: Soma
mesh_terms: D000818:Animals; D051546:Cyclooxygenase 2 / Q000032:analysis; D004108:Dilatation, Pathologic / Q000209:etiology; D005337:Fibrin / Q000032:analysis; D006965:Hyperplasia; D007150:Immunohistochemistry; D008297:Male; D008406:Masseter Muscle / Q000473:pathology* / Q000601:surgery; D018908:Muscle Weakness / Q000150:complications*; D052247:Nitric Oxide Synthase Type II / Q000032:analysis; D016059:Range of Motion, Articular / Q000502:physiology*; D051381:Rats; D017208:Rats, Wistar; D013314:Stress, Mechanical; D013583:Synovial Membrane / Q000098:blood supply / Q000473:pathology; D013585:Synovitis / Q000209:etiology* / Q000473:pathology; D013704:Temporomandibular Joint / Q000201:enzymology / Q000473:pathology; D013705:Temporomandibular Joint Disorders / Q000209:etiology*; D014652:Vascular Diseases / Q000209:etiology
chemical_list: D005337:Fibrin; D052247:Nitric Oxide Synthase Type II; D051546:Cyclooxygenase 2