Occlusion

Occlusion, the relationship of the teeth of the upper and lower jaws during the act of jaw closure. The correctness of occlusion determines the performance of the masticatory function. A correct occlusion is considered to be one in which certain dental points come into contact both in a state of rest, i.e., in central occlusion (see Teeth, eruption of teeth and their statics), as well as during jaw movements. Among the variants of occlusion that may fall within the limits of the norm, there are certain physiological fluctuations, for example, a greater or lesser depth of occlusion or the degree of prognathism, etc. These variants usually have a connection with the shape of the skull, which in turn depends on heredity, constitution, and a number of other factors. The occlusal surfaces of the teeth in man are adapted for chewing mixed food, and the movements produced by the human jaw are very diverse (see Articulation and Mastication). The dentist has to take into account, mainly, two moments during jaw movements: 1) during the forward movement of the jaw (as in rodents) the moment when the cutting surfaces of the anterior teeth come into contact, i.e., anterior occlusion, and 2) during movements to the side (as in herbivores) the moment of lateral occlusion, expressed in the fact that during movements to the left the buccal cusps of the lower molars close with the buccal cusps of the upper teeth, and the lingual cusps with the lingual ones, this is the so-called "working side." On the right side at this time the buccal cusps of the lower jaw rest against the lingual cusps of the upper jaw, this is the so-called "balancing side." During movements to the right the same phenomena are observed in reverse order. During all movements of the lower jaw the leading factors are: 1) the action of the masticatory muscles, 2) the deviation of the articular path, 3) the overlap of the incisors, 4) the height of the cusps of the molars, and 5) the inclined planes of the cusps. The indicated factors are interdependent, and the dominant role is played by one or the other of them depending on age and on the condition of the dental arches. The motor apparatus, consisting of muscles and ligaments, differs in its basic character as most constant, whereas the deviation of the articular path and the last of the three indicated factors, i.e., the factors that are elements of occlusion, change throughout life. In the period before the eruption of teeth, the main factor in the development of the masticatory apparatus is the action of the muscles. Later, when teeth are present, the movements of the jaws depend mainly on the shape of the occlusal surface of the dental arches, i.e., the shape of the joint is determined secondarily. After the loss of teeth, at an age when the joints are already fully formed, the movements of the jaw depend mainly on the shape of the joints. With age, the form of occlusion undergoes a series of physiological modifications. During the period of growth of the organism, the teeth change their position in the alveolar process. Together with the growth of the jaw, its angle, its shape, the shape of its joints change, eruption, shedding of teeth, their displacement occur, such as distal displacement of the milk incisors, medial displacement of the six-year molars, etc. In mature age, physiological wear of teeth occurs, which also reflects on the form of occlusion. With a significant degree of wear, the shape of the joint changes, and consequently the articular path. The action of the muscles also adapts to the changing conditions. In old age, atrophy of the articular tubercle and menisci and a change in the shape of the jaw occur. These phenomena, especially if they are associated with senile shedding of teeth, completely change the form of occlusion. The mentioned biological conditions of occlusion can be violated by the most diverse moments, therefore the restoration of occlusion is accomplished by all kinds of dental aid. Incorrectness of occlusion can be a consequence of a developmental anomaly of the teeth themselves or a developmental anomaly of the jaws (see Teeth-pathology). In the latter case, the correctness of occlusion is restored by orthodontic treatment (see Orthodontics). Dental caries, destroying the contact surfaces, very often serves as a cause of occlusion disturbances. Filling, restoring the original shape of the tooth, contributes to the correction of occlusion. With extensive destruction of molars by caries, occlusion becomes deep, and the upper incisors are pushed forward by the pressure of the lower ones. Here, preventive and therapeutic measures are prosthetics. Performed extractions, involving the displacement of adjacent teeth and antagonists, also cause a change in occlusion. Prosthetics serves as a preventive measure against such changes. Alveolar pyorrhea also leads to disorders of occlusion. Normally, teeth stably located in the alveoli undergo uniform wear. With pyorrhea, due to the mobility of the teeth, wear occurs unevenly, hence the overload and trauma of individual teeth and the periodontium. With occlusion anomalies, congenital or occurring after extractions due to uneven distribution of masticatory pressure on the teeth, the same trauma occurs. The pericemental tissues initially adapt—compensatory growths of elements of bone and periosteum occur, as a result of which the functions are more or less equalized. In cases where the limits of physiological adaptability of tissues are exceeded, such traumatic occlusion leads to local disturbances of circulation, destruction of the tooth ligament, formation of pockets, invasion of infection, and resorption of the alveoli. Thus, traumatic occlusion can be both a cause and a consequence of periodontal disease. Preventive measures against the loss of teeth due to traumatic occlusion [besides therapeutic measures for alveolar pyorrhea (see.)] consist of grinding the cusps not subjected to physiological wear with a corundum stone, correcting incorrectly placed fillings, mechanically strengthening loose teeth by means of special splints, and in case of need, prosthetics. From what has been said it becomes clear that the preparation of the mouth for prosthetics must include the correction of the occlusion of natural teeth. In prosthetics, in turn, it is necessary to provide for the uniform distribution of masticatory forces over the occlusal surfaces of natural and artificial teeth (see Teeth, Dental prostheses) not only in a state of rest, but also during movements, for which anatomical articulators serve, which reproduce the individual movements of the jaw according to a record of the articular path or according to the anterior and lateral occlusions printed on occlusal templates (see Articulator). In case of fractures of the jaw, occlusion is restored by the application of a splint, and after resection of the jaw—by splinting and complex prosthetics. Various defects of occlusion are to a certain extent equalized by the great adaptability of the joint, but in such a case the masticatory apparatus tires faster, and its function is lowered. The restoration of occlusion by various kinds of dental aid has the aim of raising the functional usefulness of the masticatory apparatus. Lit.-see lit. to art. Dental prostheses. G. Berkovich.