Face-bow testing on a dynamic extraoral force analyzer

Abstract

T o use all the possibilities of the extraoral appliance, and to determine the right force and direction, we need a good knowledge of the mechanical principles as described by Gould,* Greenspan and Kuhn5 The maxillary first molars are commonly used as anchorage teeth for the extraoral face-bow. They may be compared to wheels on a central axis (the center of resistance = the fulcrum) which are affected by forces of different directions. When force is applied on the wheel from the right, with the line of action above the fulcrum, the wheel will rotate counterclockwise; it will rotate in a clockwise direction if the force is applied below the fulcrum. If the direction of force goes through the fulcrum, the wheel will move without any rotation (translation). The fulcrum of the maxillary first molars is considered to lie somewhere between the apical and the median thirds of the root. The direction of force (the line of action) is determined by the position of the extraoral anchorage in relation to the ends of the outer arms and to the fulcrum of the maxillary first molars. A line of action obliquely from below is applied by t,he cervical traction, the straight-pull headgear gives a line of action in the distal prolongation of the occlusal plane, and the high-pull headgear gives a line of action obliquely upward and backward (occipital anchorage). Theoretically, we can predict how the maxillary first molars will react on different lines of action in relation to the fulcrum of the tooth. There will be distal tipping of the crown if the force is directed below the fulcrum, m&al tipping if the force is directed above the fulcrum, and bodily translation if the force passes through the fulcrum. In addition to the distal translating force, there will be an extruding force with a direction of pull from an anchorage below the occlusal plane (cervical traction j and an intruding force from an anchorage above the occlusal plane (high-pull headgear). Unilateral movement by means of extraoral force is a specific problem. Drenker’ and Haack and Weinstein4 have shown mathematically that this movement can be achieved by making the outer arm longer on the side where the