Kinematics of Mandibular Motion in the Sagittal Plane

Mandibular kinematics has been a constant concern and matter for study for a long time. Of all the mandibular movements the opening-closing movement in the sagittal plane is the most simple to observe and study. Among the various analyzation methods applicable to mandibular kinematics are the following: photographic, photoelectrical, radiographic, cinematographic, cinefluorographic, graphical, optoelectronic, and electromagnetometrical. Such methods allow kinematics to identify the main movements of rotation and translation. In motion, the mandible makes all the movements of a tridimensional body with connections. The connections are mainly represented by the temporomand ibular joints and the tooth contact. Of the symmetrical, primary mandibular movements, we often stop at the movement in the sagittal plane, the result of combined rotational (opening-closing) and translatory (propulsion-retropulsion with the teeth in contact) movements, this being a general planar movement. Pure rotation occurs at the beginning of the lowering movement, when there is a parting of about 2 mm at the dental arch level by the collective action of the following muscles: the external pterygoid muscle, the forward fibers of the digastric muscles, the mylohyoid muscles, and the geniohyoid muscles. 2· 10 The theory of pure rotation, namely of the terminal hinge axis, the prevalent feature of the gnathologic school, was the main concern of some researchers during the development of the clinical-technical capabilities of investigating mandibular kinematics. The results of such investigations have been used for the construction of articulators.