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Fig. 4 | Progress in Orthodontics

Fig. 4

From: Removable thermoplastic appliances modified by incisal cuts show altered biomechanical properties during tipping of a maxillary central incisor

Fig. 4

When a tooth is tipped, depending on the degree of misfit between tooth and appliance, a local elastic deformation of the appliance material at the point of contact with the respective restoring force results. The appliance is lifted off at the anterior part and is held in place at the posterior end depending on retention. Thus, a whole-body deformation of the appliance also creates a restoring force. Both restoring forces are acting vestibular and palatal on differently inclined inner surfaces of the appliance (a, b). This results in different Fx and Fz values. When the appliance is cut at the incisal edge, the inclined inner surfaces are partly removed and the local deformation of the appliance is located more apically at more vertically orientated appliance surfaces (c, d). This means, if the incisal link between the vestibular and the palatal appliance wall is detached, during tipping, the appliance walls can be moved more horizontally with less restoring force, depending on the extension of the cut. Thus, the appliance is lifted off much lesser in the anterior region which, in consequence, reduces the vertical force component again significantly (c, d). Depending on the inclination of the inner wall of the aligner at the contact area between appliance and tooth with incisor-side of cut aligners, even a reversal of the force component Fz (extrusion) may be observed

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