Mandibular rest position and electrical activity of the masticatory muscles.
BACKGROUND
The determination of a correct vertical dimension of occlusion is a critical procedure in clinical dentistry.


OBJECTIVE
The objectives of this study were to analyze the relation between mandibular rest position and electrical activity of masticatory muscles and to compare clinical and electromyographic rest position in subjects with different vertical facial morphologic features.


METHODS
Clinical rest position and electromyographic rest position were investigated in 40 subjects. Electromyographic rest position ranged from 0.4 to 12.7 mm (average 7.7 +/- 2.7 mm). Clinical rest position ranged from 0.1 to 4.4 mm (average 1.4 +/- 1.1 mm). The average difference between electromyographic rest position and clinical rest position was 6.3 +/- 2.5 mm (range 0.3 to 10.3 mm). Sixteen subjects were selected according to the Frankfort mandibular plane angle and separated in two groups having a mandibular plane angle > or = 28 degrees.


RESULTS
Rest position was significantly greater (p < 0.05) in the low-angle group (2 +/- 1.3 mm) than in the high angle group (0.8 +/- 0.8 mm). Electromyographic rest position did not differ between subjects with different facial morphologic features (8.1 +/- 1.7 mm low-angle group; 7.6 +/- 4.1 mm high angle group). By varying the vertical dimension millimeter by millimeter, masseter and anterior temporal electromyographic activity demonstrated a considerable decrease over an interocclusal distance of 3 to 4 mm. Further mandibular opening up to 18 mm corresponded to small changes in postural activity.


CONCLUSIONS
This study suggests that a jaw posture with a few millimeters of interocclusal distance involves a great reduction of masticatory muscle activity.
Michelotti
A
A
Department of Orthodontics, University of Naples Federico II, Italy.
Farella
M
M
Vollaro
S
S
Martina
R
R
D000328:Adult; D002508:Cephalometry; D003768:Dental Occlusion, Centric; D004576:Electromyography; D005145:Face / Q000033:anatomy & histology; D005260:Female; D006801:Humans; D007574:Jaw Relation Record; D008297:Male; D008334:Mandible / Q000033:anatomy & histology* / Q000502:physiology; D008406:Masseter Muscle / Q000502:physiology; D008410:Masticatory Muscles / Q000502:physiology*; D009068:Movement; D011187:Posture; D012044:Regression Analysis; D013703:Temporal Muscle / Q000502:physiology; D014716:Vertical Dimension