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Of extreme clinical significance for centric as well as for other occlusal problems is a patient’s adaptive capacity or tolerance of occlusal imperfections.

Centric relation is the only reference posi­tion that assures simultaneous harmonious alignment of both temporomandibular joints.

Both gnathological point centric” on the stationary hinge path, and freedom in centric, are treatment concepts; they are not usually seen in normal human occlusion.


Ramfjord and Ash

1995, W. B. Sanders, 34

Occlusion – 4th Ed, p 77, 83, 84

MANDIBULAR MOVEMENTS ANDOCCLUSAL MORPHOLOGY

Although cusps, fossae, grooves, and ridges should be compatible with functional and parafunctional mandibular movements, the absence of a specified relationship in the natural dentition may not or need not be corrected by the reconstruction of an entire occlusion in order to produce an ideal occlusion.. However, in indicated restorations, functional occlusion should be determined on the basis of mandibular movements of a degree necessary or possible for an individual patient.




Normal Versus Ideal Occlusion

Normal Occlusion

Normal implies a situation commonly found in the absence of disease, and normal values in a biologic system are given within an adaptive physiologic range. Normal occlusion, therefore, should imply more than a range of anatomically acceptable values; it should also indicate physiologic adaptability and the absence of recognizable pathologic manifestations. Such a concept of normal occlusion emphasizes the functional aspect of occlusion and the capability of the masticatory system to adapt to or compensate for some deviations within the range of tolerance of the system.

The functional adaptation of the dentition is well recognized, i.e., the occlusion undergoes certain changes with moderate wear that appear to be beneficial to the health of the entire masticatory system. Such adaptive changes in the temporomandibular joint, at least for adults, appear to be very unlikely


The neuromuscular mechanism seems to have great potential for adaptation. However, the adaptive capacity of the neuromuscular system depends to a great extent on the irritability threshold of the central nervous system, which is influenced by emotional and psychic tension.

The intimate relationship between the pe­ripheral and central nervous systems is, in the final analysis, one of the most significant factors in the study of occlusion. One may consider a person’s occlusion from two viewpoints: (1) the anatomic occlusion evident in an examination of the functional relationship of the masticatory system, and (2) how a person’s neuromuscular mechanism reacts to his or her occlusion.

Functional disturbances of the masticatory system may be caused by very severe occlusal interferences and mild psychic tension or by severe psychic tension and very slight occlusal interferences, the average tolerance level being between these extremes. Every analysis of occlusion, therefore, should include an evaluation of the patient’s reaction to occlusion and occlusal interferences.


However, functional occlusal therapy, if performed with great accuracy, has usually been found to eliminate dysfunctional manifestations in the masticatory system in spite of persistent nervous tension. The therapy introduces ideal occlusion, a state in which no (or minimal) neuromuscular adaptation is needed because no disturbing occlusal relationships are present. Ideal occlusion indicates a completely harmonious relationship of the masticatory system for mastication as well as for swallowing and speech.

Similarly, an occlusion may be considered clinically normal in the presence of occlusal interferences in lateral excursions, provided the interferences are bypassed by neuromuscular adaptation and there are no clinically apparent disturbances of masticatory function or pathologic periodontal changes.
Ideal Occlusion

The concept of an ideal or optimal occlusion refers both to an esthetic and a physiologic ideal. The emphasis has moved more and more from esthetic and anatomic standards to a current concern with function, health, and comfort. Extensive electromyographic research has confirmed the common clinical observation that esthetic ideals have a very limited relationship to optimum function and health of the dentition.



Ramfjord and Ash

1995, W. B. Sanders, 35

Occlusion – 4th Ed, p 85

NEUROMUSCULAR HARMONY

It is essential for functional comfort that neuromuscular harmony prevails in the mas­ticatory system. Fulfillment of certain requirements regarding the relationship between temporomandibular joint guidance and occlusal guidance assures such harmony. These requirements are as follows:

1. Stable jaw relationship is required when the teeth make contact in centric relation.

2. Centric occlusion must be slightly in front of centric relation and in the same sagittal plane as the path made by the mandible in a straight protrusive movement between centric relation and centric occlusion. Centric relation and centric occlusion contacts do not have to be in the same horizon­tal plane, but this arrangement has some practical advantages.




The distance between centric relation and centric occlusion is about 0.1 to 0.2 mm in the temporo-mandibular joints and about 0.2 to 0.5 mm at tooth level.

3. Ideal occlusion requires an unrestricted glide with maintained occlusal contacts between centric relation and centric occlusion.

4. The various excursions, both from centric occlusion and centric relation, need complete freedom for smooth gliding occlusal contact movements.

5. The occlusal guidance in various excursions must be on the working (functioning) side rather than on the balancing (nonfunctioning) side. Steepness of incisal or cuspal guidance is not important for neuromuscular harmony.




Functional Stability. Another equally important aspect of ideal occlusion is functional stability of the masticatory system. Stable occlusal relationship refers to self-perpetuating, solid, and harmonious relationships between teeth and temporomandibular joints throughout life.

1. The impact of full intercuspation closure must be in the long axis of all posterior teeth and against the central part of the meniscus of the temporomandibular joints.

2. An even degree of wear resistance must be present. Also, the cutting effectiveness of all functionally alike teeth should be the same.

3. No displacing impact may be present on anterior teeth in centric occlusal closure.

4. There should be no soft tissue contact in functional occlusion.

5. There should be an acceptable interocclusal space present.





Orthodontic classifications are related more to anatomic and esthetic standards than to neuromuscular harmony and functional stability.

On the basis of clinical and electromyo­graphic studies, it can be concluded that the prerequisites for an ideal occlusion are: (1) a stable and harmonious occlusal relationship in centric relation as well as in the range between centric relation and centric occlusion; (2) equal occlusal facility for bilateral and protrusive excursions; and (3) optimal direction of occlusal forces for stability of the teeth.

Although such a concept of ideal occlusion enables the clinician to help patients who have a low tolerance, level to occlusal imperfections or advanced loss of periodontal support for the teeth, it does not mean that such an “ideal” should be imposed upon every patient with a functionally normal occlusion and a healthy periodontium.


Ramfjord and Ash

1995, W. B. Sanders, 36

Occlusion – 4th Ed, p 85 -87

TACTILE SENSIBILITY AND OCCLUSION
Studies to determine the activity of oral sensations include those that discriminate dif­ferences in size and those that determine the threshold thicknesses that can be perceived. Factors influencing oral kinesthesia have been reviewed recently.
Sensory Receptors

Sensory receptors responsible for oral kinesthesia are located in the periodontium, temporomandibular joints (e.g., mechanoreceptors), and muscles (e.g., spindles). The contributions of each are estimated by an­esthetizing the receptors or their afferents. The tactile sensibility of the periodontium is assumed to be important in the regulation of occlusal forces and in the reflex opening of the mandible.




This sensibility has a potential relationship to bruxism, traumatic occlusion, and functional disturbances of the masticatory system where occlusion is a factor. In addition to determining the perception threshold in a static state (by placing thin foil of various dimensions between occluding chewing (by placing foreign bodies of various sizes in the food), interdental dimension discrimination (lDD) is considered to be important in detecting dimensional changes in food particles and in detecting efficiency from the degree of food grinding. It has been shown that metal foil as thin as 8p (0.008 mm) could be detected by some individuals and that the threshold ranged from 8 to 60 mu.

From these studies, it is quite apparent that receptors in the masticatory system are capable of detecting extremely small changes in the occlusion; they add significance to the clinical observation that minute discrepancies in the occlusion are capable of influencing the masticator), system. However, the role of periodontal receptors in kinesthesia or IDD has been questioned. Whether basic differences in periodontal sensibility account in part for some individuals’ failure to adapt to even minor occlusal interferences is not known. A significant difference between the sensory threshold for the normal periodontium and the periodontium in periodontal disease or bruxism has not yet been determined. Like the periodontium, it does not appear that TMJ receptors are involved significantly in IDD.

Muscle spindles have been suggested as being mainly responsible for IDD. Directional sensitivity (DS) is characteristic of IDD in that a test piece smaller than the reference comparison piece is discriminated better than a test piece larger than the reference piece. DS appears to be advantageous for detecting the dimensional change in food particles during chewing, which may relate to control of masticatory forces and the swallowing threshold.



Perception of Occlusal Forces
It is possible to detect a force on the teeth of as little as 1.5 gm however, the results of studies in our own laboratory show that forces of 600 mg or less can be detected by some individuals. Because of the possibility of adaptation of periodontal membrane pressoreceptors, a clear picture of the threshold for pressoreceptors has not been established. It appears likely that receptors do adapt to light, continuous forces, but they do not ap­pear to adapt to intermittent dynamic forces.


Ramfjord and Ash

1995, W. B. Sanders, 37

Occlusion – 4th Ed, p 87-88

Occlusal Forces
In a study9 where a crown was raised above the occlusal level by 0.5 mm, the load on the tooth was twice as great as normal. Since it appears unlikely that the receptors in the periodontal membrane adapt to such “high” restorations, a protective avoidance of the restoration or a force compensation must take place within the limits and tolerance of the masticator), system components.


Interocclusal forces during mastication vary from individual to individual. These studies suggest that the nature of the food controls occlusal forces to some extent and that such forces are greater when closer to an intercuspal rather than a lateral position.

However, when hard candy is eaten, there is a tendency to move it around until crushing can be accomplished without excessive force.




A force can be considered excessive when it acts as a painful stimulus or produces injury. It appears logical to assume that occlusal interferences constitute at least the potential for excessive forces, especially if protective mechanisms are bypassed.

Several earlier studies of maximum occlusal forces have demonstrated the very large forced-biting forces that are capable of being generated. Maximum biting force is related to facial morphology.



Strongest force has been recorded in cases of anterior inclination of the mandible, lesser anterior than posterior face height, smaller gonial angle, parallelism between the mandibular occlusal line and the lower border of the mandible, and broad maxillae. It has been suggested that the form of the face is influenced by the strength of the masticatory muscles.87

Biting force is increased with periodontal anesthesia. It is rational to assume that the periodontal membrane sensibility helps manage occlusal forces so that the tolerance level of the supporting structures of the teeth is not exceeded

Ramfjord and Ash

1995, W. B. Sanders, 38

Occlusion – 4th Ed, p 89-91

Occlusal Adaptation
Occlusal stability refers to a state of homeostasis in which ongoing functional or structural changes occur within an acceptable physiologic range for the masticatory system. Of particular interest are limitation of movement, avoidance of painful teeth and joints, and avoidance of occlusal interferences.

Avoidance may be a conscious movement, but it becomes automatic with success.

In most instances, there is a tendency to chew on the same side as a painful joint because movements are less there and forces less than when the condyle is in the balancing position.

Perhaps the most dramatic reflex avoidance occurs in some individuals who have an interference to guided closure in centric relation.




Occlusal Stability
The modern concept of a dynamic individual occlusion naturally includes an increased interest in stability of the occlusion before, during, and after dental and periodontal treatment. Adjustment of tooth position occurs throughout a person’s lifetime in response to naturally induced changes of occlusal forces associated with wear in response to pathologic changes in the support mechanisms or muscle tonicity, and following placement of restorations and other dental procedures.

The patterns of forces that act on the teeth are far more complex than usually conceived. Tooth movement and development of new interferences have been observed where occlusal adjustment did not include, in principle, the establishment and maintenance of occlusal stability.




A practical principle for tooth stabilization following occlusal adjustment or placement of dental restorations is to make centric stops in centric relation closure on the same horizontal level as the centric stops in centric occlusion. Vertical forces have less tendency than lateral forces to create excessive mobility of teeth

Another manifestation of occlusal stability is establishment and maintenance of a reproducible stationary hinge axis in centric relation. as determined by the temporomandibular joints. Centric relation may be altered by dysfunction and may change position during preliminary treatment.’65 Stability of occlusion can be established only after elimination of dysfunctional manifestation of pain or discomfort in the temporomandibular joints and relaxation of the jaw muscles.







Ramfjord and Ash

1995, W. B. Sanders, 39

Occlusion – 4th Ed, p 91 - 93

MASTICATION AND OCCLUSION

Mastication is considered to be an all-encompassing behavior pattern consisting of basic primary patterns of muscle activity that reflect a background of activity of the central nervous system as well as learning and adaptation of movement which occurs throughout life. Inasmuch as tooth guidance has an influence on muscle activity during chewing and swallowing function, it seems reasonable to accept that the neuromuscular system does adapt to changes in occlusion, including the position, alignment, and form of the teeth.

The terminal point of a chewing cycle is tooth contact with intercuspation of the teeth. In this respect, the duration and character of the chewing cycle may have a narrow teardrop shape when soft food is chewed, or the envelope may have a larger lateral component with hard foods.



Chewing Cycle

In a chewing cycle the approach to tooth contact appears to be relatively reproducible and based upon responses that have been learned and programmed for repetitive movements. The angle made by tooth guidance in and out of contact in the intercuspal position is called the functional angle of occlusion (FAO). A relationship between FAO and disc displacement has been suggested.

Each chewing cycle has a duration of about 700 ms and tooth contact of about 200 ms. Maximum interocclusal force begins about 90 ms after initial contact and lasts about 110 ms in a stable intercuspal position. After this power stroke or stationary phase, which varies in biting force according to type of food, the opening phase begins. The maximal force generated in the intercuspal position follows maximum EMO muscle activity.


The intercuspation of the teeth in lateral excursions on the working side is guided by the contact of the buccal aspects of the mandibular supporting buccal cusps with the slopes of the lingual aspects of the maxillary buccal cusps

Balancing side contacts may be made along the surfaces of the maxillary lingual cusp buccal inclines (including the cusp tips in wide excursion) and the buccal mandibular cusp lingual inclines (including the cusps in wide excursion.

Extensive studies made of the jaw movements, contact patterns, and occlusal facets of aboriginal Australians indicate that after heavy functional wear, the teeth on the balancing side do not make contact during mastication. However, if the wear has been produced mainly by bruxism, the contacting wear facets on the balancing side often interfere with masticatory movements to the other side.


Bilateral Mastication
Multidirectional, alternating, bilateral mastication is ideal for stimulation of the entire supporting structures, for stability of occlusion, and for cleansing of the teeth. Clinical studies and combined clinical and electromyographic studies show that bilateral function is assumed whenever a convenient, unrestricted, bilateral, occlusal relationship with equal bilateral cuspal guidance and functional capacity is provided.


Ramfjord and Ash

1995, W. B. Sanders, 40

Occlusion – 4th Ed, p 93 - 95

Unilateral Mastication
Habitual unilateral or protrusive preference patterns of mastication are often the result of adaptation to occlusal contacts that hamper or hinder smooth, gliding harmonious jaw movements with the teeth maintaining contact. Such patterns are commonly seen in persons who have been living on soft nonabrasive foods or whose normal occlusal pattern has been disturbed by dental and periodontal irregularities and disease or by loss of teeth.

A unilateral restricted pattern of mastica­tion may also be the result of a splinting or protective action of the jaw muscles in pa­tients with temporomandibular joint disturbances. If a sufficient number of teeth are present, such patients prefer to chew on the side of the painful joint, since there is more pressure on the balancing side condyle than on the working side condyle in the process of biting through food.




Masticatory Habits
The sequence and distribution of the activities of the jaw muscles during mastication normally depend on the type of food being chewed and on the individual’s habitual masticatory pattern. During the chewing of hard foods, such as a carrot, there is heavy masseter action on both sides coinciding with the action of the temporalis. During the last phase of chewing a carrot and during the chewing of soft food, the masseter muscle on the working side shows more activity than the balancing masseter.

In a study of persons with full complements of teeth, more than two thirds were shown to have an alternating bilateral masticatory pattern, about 10 per cent chewed bi­laterally simultaneously, and about 12 per cent had unilateral mastication restricted to one side (equal number right and left).




The rate of mastication is, to a large extent, governed by the texture of the food. Soft food is chewed at a slower movement rate than hard food.

The lack of abrasiveness in the modem diet, however, is probably conducive to the development of restricted masticatory movements. Tests on biting strength show that really heavy biting is more comfortably done close to centric occlusion than in lateral or protrusive positions of the jaw, so that the crushing of very hard food probably involves very limited lateral excursions.




Effect of Loss of Teeth
Muscle activity and chewing pattern may also be radically altered by loss of teeth. Normal muscle activity resumes following insertion of well-fitting dentures. It is worth noting that, besides the “masticatory muscles,” a number of head and neck muscles actively and passively participate in the act of mastication, and muscle activity is always guided toward the optimum functional result with the masticatory “tool” available.


Ramfjord and Ash

1995, W. B. Sanders, 41

Occlusion – 4th Ed, p 95 -

Mastication
Mastication is a complex three-dimensional movement involving the mandible, tongue, masticatory muscles, lips, and cheek muscles under central nervous system control and modulations of peripheral sensory inputs. 45

Stages of Mastication
Mastication is often described as occurring in three stages: (I) incision, (2) crushing and diminishing of the size of large particles, and (3) milling or trituration of the food preparatory to deglutition.

It appears that there may be gliding contact back over the working side in open­ing from centric occlusion. Occlusal stress pattern studies also provide some evidence concerning the contact measurements of the teeth in excentric function. From practically all of the findings of studies of jaw movements it is evident that some lateral, combined lateral and protrusive strokes end­ing in centric occlusion constitute the normal pattern of mastication, but that the strokes vary considerably from individual to individ­ual. In some instances, instead of ending in centric occlusion, the chewing stroke in the milling stage of mastication carries over slightly laterally or retrusively from centric occlusion. Observations with highly sophisti­cated recording devices indicate that occlusal surfaces of posterior teeth may participate in masticatory function, although they do not make contact in empty excentric movements.


Masticatory Adaptation
One study related masticatory performance or effectiveness to occlusal contacts recorded both by size of contact area and by number of contacts. It appeared from the study that the masticatory performance was well correlated (in a linear manner) with food platform areas, less well with molar imprint length, and poorly with tooth units. The food platform area, or total occlusal contact, is influenced by occlusal interferences, missing teeth, and irregular positions of the teeth. Attrition usually increases the food platform area, as does occlusal adjustment.

The entire dentition undergoes a continuous adaptation to functional wear. This manifests as compensatory eruption of teeth, mesial drift to compensate for interproximal wear, and changes in tooth position which attempt to compensate for pathologic tooth movements or loss of teeth. These changes signify an unceasing effort to maintain a proper physiologic balance of the masticatory system throughout a person’s lifetime.



Advanced attrition with loss of cusps leads, by uneven wear of enamel and dentin, to the formation of “inverted” cusps~ and fossae that are as efficient in masticatory function as the originals, thus maintaining the efficiency of the masticatory system.



DEGLUTITION AND OCCLUSION
Deglutition (swallowing) is a complex and innate behavior seen in the unborn fetus. There is a complex functional relationship between maintenance of the airway mandible posture, and tongue activity and opening of the alimentary canal for the passage of foods, fluids and saliva. Most of the activities involve reflexes that interact first to protect the patency of the airway and second to close the airway and pass food into the alimentary canal.
Stages of Deglutition
Mastication is based on a combined cyclic and learned reflex pattern and, like the initiation of swallowing, is partially under voluntary control; however, when the bolus (food) has reached the upper pharynx, the rest of the swallowing function is based on primary unlearned reflexes. The process of swallowing has been divided into four stages:” (I) the swallow-preparatory position of the bolus within the mouth, (2) passage from the mouth to the pharynx, (3) passage through the pharynx, and (4) passage through the hypopharyngeal sphincter. The process can be reduced to three phases by combining (I) and (2) so that there is an oral phase, a pharyngeal phase, and an oesopharyngeal phase.

The first stages (or oral phase), which are under voluntary control, involve placing the chewed food or liquid between the tongue and the anterior teeth and palate.

The swallowing action (pharyngeal phase) in humans is rapid and the bolus reaches the upper end of the esophagus less than I s after the initial act of swallowing.’

The exact mechanism for triggering the pharyngeal phase is not known. It appears that the complex swallow pattern is set in the brain stem.














































































































Ramfjord and Ash, 4th Ed

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