title: Individual variations in numerically modeled human muscle and temporomandibular joint forces during static biting.
abstract: OBJECTIVE
  To test the effects of occlusal force (OF) angle on the variations in predicted muscle and temporomandibular joint (TMJ) forces during unilateral molar bites.
  
  
  METHODS
  The craniomandibular (CM) geometries of 21 individuals were determined from lateral and posteroanterior cephalometric radiographs. These geometries were used in a numerical model based on minimization of muscle effort. This model was previously validated for this subject group through the use of jaw tracking and electromyographic data. The model predicted muscle and TMJ forces associated with static OFs on the right mandibular first molar. OF angle was varied from vertical to 40 degrees in the buccal and lingual directions, in increments of 10 degrees.
  
  
  RESULTS
  Intra- and intersubject variations in predicted muscle and TMJ forces for unilateral molar biting were dependent on OF angle and CM geometry. Nonvertical OFs were associated with either large anterior temporalis muscle forces (> 100% of applied OF in 3 subjects) or large inferior lateral pterygoid muscle forces (> 90% of applied OF in 3 subjects). On average, vertically and buccally directed OFs were associated with higher mean contralateral TMJ forces (60% of applied OF, SD 12%). Two subjects had large ipsilateral or contralateral TMJ forces (> 90% of applied OF).
  
  
  CONCLUSIONS
  In a group of healthy subjects, depending on the individual CM geometry, large muscle and/or TMJ forces were predicted to be associated with specific unilateral molar OF angles. Propensities to increased muscle or joint forces may be predisposing factors in the development of myofascial pain or intracapsular disease. The results may explain, in part, the variation in location of symptoms in individuals who first present with temporomandibular disorders.
authors.affiliation: Department of Growth and Development, Orthodontic Section, University of Nebraska Medical Center, College of Dentistry, Lincoln, NE 68583-0755, USA. liwasaki@unmc.edu
authors.forename: Laura R
authors.initials: LR
authors.lastname: Iwasaki
authors.forename: Benjamin R
authors.initials: BR
authors.lastname: Thornton
authors.forename: Willard D
authors.initials: WD
authors.lastname: McCall
authors.forename: Jeffrey C
authors.initials: JC
authors.lastname: Nickel
mesh_terms: D000328:Adult; D001732:Bite Force; D002508:Cephalometry; D003766:Dental Occlusion; D004576:Electromyography; D005260:Female; D005544:Forecasting; D006801:Humans; D008297:Male; D008334:Mandible / Q000033:anatomy & histology; D008335:Mandibular Condyle / Q000033:anatomy & histology; D008406:Masseter Muscle / Q000502:physiology; D008410:Masticatory Muscles / Q000502:physiology*; D008954:Models, Biological; D008963:Molar / Q000502:physiology; D009119:Muscle Contraction / Q000502:physiology; D009334:Neck Muscles / Q000502:physiology; D011626:Pterygoid Muscles / Q000502:physiology; D013703:Temporal Muscle / Q000502:physiology; D013704:Temporomandibular Joint / Q000033:anatomy & histology / Q000502:physiology*