ARTICULATION

ARTICULATION (from Latin articulus-joint). In odontology, A. was formerly understood only as the relationship of the upper and lower teeth when the mouth is closed at rest, but now this concept also includes the nature of the movement of the lower jaw, while the relationship of the teeth is called occlusion or closure. In predatory animals, the lower jaw moves only up and down, in rodents also forward, in ruminants there are lateral movements; in humans all these movements occur, they are performed during chewing and with a smaller amplitude during speech. The nature of the movements is determined by: the height of the occlusion of the front teeth, the height and inclination of the tubercles of the molars, the degree of curvature of the line of closure of the lateral teeth, which forms part of the arc of a circle, the center of which lies in the orbit, the shape of the joint fossa of the temporal bone (the inclination of the articular tubercle), the shape of the joint head of the lower jaw, the joint ligaments and muscles that move the jaw. All these parts of the chewing apparatus are closely interconnected—for example, the height and inclination of the tubercles of the molars are related to the degree of inclination of the surface of the articular tubercle of the temporal bone and the degree of curvature of the line of closure of the teeth. A. does not remain unchanged throughout life: it changes due to the loss of teeth, the wearing down of tooth tubercles, and due to forces acting on the teeth, even if with little force, but prolonged and in the same direction; the latter is especially clearly manifested in childhood: the habit of sucking fingers and other similar forces obviously displace the teeth and change the normal A. In rickets, the action of the chewing muscles and the mentioned habits sometimes so much curve the arc of closure of the teeth that the contact of the upper teeth with the lower ones occurs only at one or two large molars.