Occlusal Cycle in the Molar Areas During Right or Left Lateral Occlusal Relations
In lateral movements during mastication, the mandible drops downward and to the right or left of centric occlusion. As it continues the cycle of movement and returns toward centric occlusion, the bucco-occlusal portions of mandibular molars come into contact with the occlusal portions of the maxillary molars lingual to the summits of buccal cusps and in contact with the triangular ridges of the slopes on each side of them, continuing the sliding contact until centric occlusion is accomplished (Figure 16-49; see also Figure 16-39).
From these first contacts the mandibular molars slide into centric occlusion with maxillary molars and then come to a momentary rest. The movement continues with occlu-sal surfaces in sliding contact until the linguo-occlusal slopes of the buccal portions of the mandibular molars pass the final points of contact with the linguo-occlusal slopes of the lingual portions of the maxillary molars. When the molars lose contact, the mandible drops away in a circular movement to begin another cycle of lateral jaw movement (see Figure 16-49).
The actual distance traveled by mandibular molars in contact across the occlusal surfaces of maxillary molars, from first contacts to final contacts at separation, is very short. When measured at the incisors, it is only 2.8 mm in Australian aborigines and only half or less of this in Euro-peans.47 The lower molars, which are the moving antagonists, are taken out of contact before the first contact location on their buccal cusps reaches the final points of contact on the maxillary molars (compare A and C in Figure 16-39).
Biomechanics of Chewing Function
During the masticatory process, the individual generally chews on one side only at any one chewing stroke. Material is shifted from one side to the other when convenient; the shifting is generally confined to the molar and premolar regions, which do most of the work. Occasionally, for specific reasons the shift of mastication may be directed anteriorly. Nevertheless, the major portion of the work of mastication is done by the posterior teeth of the right or left side. The posterior teeth are aided in various ways by the canines, but the latter do not possess the broad occlusal surfaces required for chewing efficiency overall.
Figure 16-49 Schematic of mandibular movements at the mesial aspect of the first molars. Heavy outline represents the molar in intercuspal position/centric occlusion. Shadow outlines represent mandibular molar in various relations in the movement cycle during mastication. The short arrows (A and B) at right angles to the occlusal surface of the maxillary molar measure the extent of movement between them over the occlusal surface from first contact of the mandibular molar to last contact in the cycle before starting the next cycle.
The tongue, lips, and cheeks manipulate the food so that it is thrown between the teeth continuously during the mandibular movements, which bring the teeth together in their various relations. In other words, the major portion of the work is accomplished in the premolar and molar regions while the mandible is making right lateral and left lateral movements, bringing the teeth into right lateral and left lateral occlusal relation and terminating the strokes in or near the intercuspal position and centric occlusion.
PROTRUSIVE OCCLUSAL RELATIONS
The anterior teeth in their protrusive occlusal relations negotiate the process of biting or shearing food material.
Although the mandible may be lowered considerably in producing a wide opening of the mouth, the occlusion of the anterior teeth is not concerned with any arrangement very far removed from centric relation.
When the jaw is opened and moved directly forward to the normal protrusive relation, the mandibular arch bears a forward, or anterior, relation of only l or 2 mm in most cases to its centric relation with the maxillary arch.
The protrusive occlusal relation places the labioincisal areas of the incisal ridges of the mandibular incisors in contact with the linguoincisal areas of the incisal portions of the maxillary incisors. The mesiolabial portion of the mesial cusp ridge of the mandibular canine should be in contact with the maxillary lateral incisors distolinguoincisally.
From the protrusive occlusal relation, the teeth glide over each other in a retrusive movement of the mandible, a movement that terminates in centric occlusion. During this final shearing action, the incisal ridges of the lower incisors are in continuous contact with the linguoincisal third portions of the maxillary incisors, from the position of protrusive occlusal relation to the return to centric occlusal relation.
The maxillary canines may assist by having their distal cusp ridges in contact with the mesial cusp of the mandibular first premolar. They cooperate with the incisors most of the time in one way or another. A slight movement to the right or left during protrusion will bring the canines together in a "biting" manner. In addition, at the end of the incisive cycle, the contact of the canines with each other in centric occlusion lends final effectiveness to the process.
Neurobehavioral Aspects of Occlusion
Up to this point, the emphasis has been on the structural, anatomical alignment of the teeth.In addition, although it would be impossible to do justice to the topic of the neuroscience of occlusion in a brief review, it is imperative to call attention to the meaning of occlusion in its broadest sense.
Recent ideas concerning the diagnosis and treatment of disturbances such as chronic orofacial pain, temporoman-dibular disorders, craniomandibular disorders, and bruxism, and the diagnosis and treatment of malocclusion involving orthognathic surgery require a greater knowledge of the neurobehavioral aspects of oral motor behavior than ever before in the practice of dentistry.
The neurobehavioral aspects of occlusion relate to function and parafunction of the stomatognathic system. Function includes a variety of actions or human behavior such as chewing, sucking, swallowing, speech, and respiration. Parafunction refers to action such as bruxism (e.g., clenching and grinding of the teeth). All these functions require highly developed sensorimotor mechanisms. The coordination of occlusal contacts, jaw motion, and tongue movement during mastication requires a very intricate control system involving a number of guiding influences from the teeth and their supporting structures, TMJs, masticatory muscles, and higher centers in the central nervous system. Frequent contact of the teeth during mastication without biting the tongue; closure of the jaw to facilitate swallowing (occurring about 600 times a day); remarkable tactile sensitivity in which threshold values for detecting foreign bodies between the teeth may be as little as 8 μm; and the presence of protective reflexes suggest the need for intricate mechanisms of control of jaw position and occlusal forces.
The presence of several classes of teeth, powerful musculature, and a most delicate positional control system indicates that it is important to understand the strategy underlying such sensitive control mechanisms. Although the ease with which these mechanisms may be disturbed at the periphery (i.e., the teeth, joints, periodontium, peripheral neural system) and centrally (brainstem and higher centers) is not well understood, the adaptive capacity of the stoma-tognathic system appears to be considerable. On an individual clinical basis, however, the responses of a patient to occlusal therapy may be reflected in oral behavior outside the range of normal. Inasmuch as function and parafunction share similar anatomical, physiological, and psychological substrates, it is necessary to review briefly the neurobehav-ioral correlates of the activities of the stomatognathic system.
Occlusal Stability
The stability of the occlusion and the maintenance of tooth position are dependent on all of the forces that act on the teeth. Occlusal forces, eruptive forces, lip and cheek pressure, periodontal support, and tongue pressure are all involved in maintaining the position of the teeth. As long as all of these forces are balanced, the teeth and the occlusion will remain stable. Should one or more of the influences change in magnitude, duration, or frequency, stability is lost and the teeth will shift, disrupting a previously stable occlusion. The loss of teeth, tooth structure, or occlusal supporting cusps, or a decrease in their support from periodontal disease or trauma is a factor in maintaining occlusal stability.
Occlusal stability refers to the tendency of the teeth, jaws, joints, and muscles to remain in an optimal functional state. A few of the mechanisms involved include mesial migration of teeth, eruption of teeth to compensate for occlusal wear or intrusion by occlusal forces, remodeling of bone, protective reflexes and control of occlusal forces, reparative processes, and a number of others even less understood than those just listed. Although the strategy for stability related to the functional level required for survival appears obvious, orchestration of such diverse mechanisms can be couched only in terms such as homeostasis. The influence on occlusal stability of such factors as disease, aging, and dysfunction has yet to be clarified.
From a clinical standpoint, several concepts of occlusal stability are used as goals for occlusal therapy, including maintenance of a stable jaw relation in centric occlusion and centric relation; direction of occlusal forces along the long axis of a tooth; maintenance of centric stops, supporting cusps, and contact vertical dimension; replacement of lost teeth; and control of tooth mobility. Discussion of these aspects of occlusal stability is more appropriately found in topics on occlusion.
Mesial migration is a term used to describe the migration of teeth in a mesial direction. The cause of this phenomenon has not been fully clarified, although a number of ideas have been advanced. There seems to be little doubt that mesial drift does occur, but no general agreement is evident on how much movement occurs, which teeth move, and how the movement is achieved. Suggested causes include traction of the transseptal fiber system,48 forces of mastication,49,50 and tongue pressure.51 The strategy behind the mesial migration appears to be related to closure of proximal tooth contacts. Although occlusal forces may be considered as a passive mechanism and traction of transseptal fibers as an active one, it is difficult to determine in what way occlusal stability is influenced. The contact relations of the teeth may promote occlusal stability, but if incorrect relations are present, opening of proximal contacts can occur (Figure 16-50).
A tendency exists to assume that a particular arrangement of teeth is unstable (Figure 16-51); however, such an occlu-sal relationship may have become stabilized, at least at a particular point in time. Whether an occlusion is fully stable can be determined only by periodic evaluation of that occlusion. Many factors (caries, periodontal disease, occlusal trauma, bruxism) may upset the delicate balance of an already marginally stable occlusion.
An ideal occlusion may be defined as one that has no structural, functional, or neurobehavioral characteristics that tend to interfere with occlusal stability. A response to bruxism may be loss of tooth structure, increased tooth mobility, root resorption (Figure 16-52), or decreased tooth mobility and increased density and thickness of the supporting tissues.
Figure 16-50 A, Opening of contact between mandibular molars due to a protrusive interference on the second molar (mirror view of left side). B, Outline of excessive restored mesial buccal cusp ridge, which involved protrusive bruxing by the patient.
Guidance of occlusion
Guidance of occlusion is usually discussed only in terms of tooth contact or anatomical or physical guidance, and more specifically in relation to canine and incisal guidance. Less specific is the term anterior guidance, which may refer to tooth guidance for all or any of the anterior teeth or to guidance involving the neuromuscular system. Yet another type of guidance is condylar (disk-condylar complex) guidance, which, like incisal guidance, may refer to so-called mechanical equivalents of guidance on an articulator. Again, as with anterior guidance, the neuromuscular aspects of condylar guidance are not well understood. The paths of the condyles in the mandibular fossae are not well represented in the mechanical equivalents of an articulator, especially in less than a "fully adjustable" articulator; and neuromuscular mechanisms are not represented at all.
Figure 16-51 Occlusal instability cannot be determined simply on the basis of occlusal contact relations and spacing of the teeth. A changing occlusion that restabilizes suggests adaptation.
Figure 16-52 Root resorption associated with high restoration and clenching habit. Top, Prior to placement of restoration. Bottom left, "Soreness" of the tooth began soon after the restoration and then rapidly some root resorption. High restoration was removed by selective grinding. Bottom right, Several months later, no evidence of soreness or additional root resorption.
It is of particular importance to the clinician to make certain that physical guidance of a restoration (or of the natural teeth in the treatment of dysfunction or malocclusion) is in harmony with the neuromuscular system and neurobehavioral attributes of the patient. Although some degree of compatibility may be assured through evaluation of occlusal relations and determination that smooth gliding movements are present in various excursions, the acceptance and adaptability of the neuromuscular system may not be apparent until an unfavorable response occurs (Figure 16-53).
