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Several reasons have been given to account for an accommodation that must be made if the mandible does grow forward. They include: distal displacement of the mandible with possible disturbances of the temporomandibular joints, disks, and associated muscles: the maxillary incisors moving labially (flaring) and leaving spaces between the anterior teeth; or the mandibular incisors becoming lingually displaced and crowded. Probably flaring of the maxillary incisors occurs the least, perhaps because of lip pressure. The most probable response to late mandibular growth is distal displacement of the mandible and crowding of the mandibular incisors4°6


Ramfjord and Ash

1995, W. B. Sanders, 24

Occlusion – 4th Ed, p 57

Distal displacement of the mandible as a result of an anterior interference in the intercuspal position is more likely to result in temporomandibular joint and muscle disorders in the adult where plasticity for change is less in the joints than in the occlusion. Although the possibility of TMJ and muscle disorders associated with ‘entrapment” due to distal displacement is controversial.

We have encountered symptoms of TMD in adults at times in relation to orthodontic retraction of incisors as well as to encroachment on centric by anterior restorative treatment



. Relief of symptoms may occur almost immediately with easement of the displacement. It is also true that some small amount of growth does occur into middle age, i.e., the size and shape of the craniofacial complex changes as does the pattern of growth that occurs in the maturation period of the dentition, but to a lesser extent. Rotation of both jaws occurs along with vertical change in the facial dimension

NORMAL OCCLUSION

A normal occlusion, or what is referred to orthodontically as an Angle Class I occlusion; the key teeth for the classification are the permanent first molars. The mesiobuccal cusp of the maxillary first molar should occlude in the mesiobuccal groove of the mandibular first permanent molar

However, even with this molar relationship, when the teeth are in full closure there may be a significant discrepancy between the relationships of the mandible or temporomandibular joints and the maxilla.


Orthodontic textbooks6’ often refer to normal occlusion as an “imaginary ideal occlusion” which rarely exists in our civilization. The concept of normal occlusion provides an ideal for treatment, though the optimum of 138 occlusal contacts in the closure of 32 teeth is very seldom, if ever, reached.” In this hook, the concept of ideal occlusion is based more on a neuromuscular than a morphologic evaluation of the position of molars.


Ramfjord and Ash

1995, W. B. Sanders, 25

Occlusion – 4th Ed, p 58 -60

FUNCTIONAL OCCLUSAL ANATOMY

Cusp Angles, Guiding Cusps, Guiding Inclines, and Condylar Guidance

In mandibular movements, tooth guidance influences muscle activity, e.g., the approach and departure angle of a cusp or point on a mandibular incisor. Functional movements require coordination of joints and muscles for appropriate sequential timing of the events via neuromuscular control mechanism s.





Imaginary Occlusal Planes and Curves
A plane of occlusion is an imaginary plane containing the incisal edges of the mandibular central incisors and the tips of the distobuccal cusps of the second mandibular molars

The curve of Spee is determined by the occlusal surfaces of the teeth, following the cusp tips of the anterior mandibular teeth to the buccal cusp tips of the posterior mandibular teeth. It is called the compensating curve for dentures.





The cusp tips of molars in a section through the frontal plane outline the curve of Wilson. This curve changes from the first to the third molars and with wear of the dentition. The curve of Wilson in the mandibular first molars is concave for the mandibular teeth in an unworn dentition, but becomes convex in a worn dentition.

Extending the curve of Spee and the curve of Wilson to all cusps and incisal edges reveals the curve of Monson.



These imaginary occlusal planes are not often used as references in modem dentistry, because such anatomic landmarks have only a very remote association with functional relations. (Emphasis mine – rgp)


Occlusion of the Anterior Teeth

In the intercuspal position, which is reached with maximal jaw closure, the anterior teeth should make either very light or no actual contact.

As a result of attrition, both overjet and overbite tend to decrease with age.

It has been suggested that the overbite in normal occlusion should not exceed one third of the length of the mandibular incisor. However, such “rules” are meaningless as standards for evaluation of occlusion. There are esthetic and functional considerations for overbite and related incisal guidance that have to be acknowledged in restorative treatment and orthodontics.



Ramfjord and Ash

1995, W. B. Sanders, 26

Occlusion – 4th Ed, p 60-62

Occlusion of the Supporting Cusps
In complete occlusal closure (centric occlusion or intercuspal position), the distobuccal cusps of the mandibular molar teeth are normally seated in the central fossae of the maxillary molars, and the lingual cusps of the maxillary molar teeth are normally seated in the central fossae of the mandibular molars. These cusps are called supporting cusps.

I


In complete closure of a normal, young, unworn dentition, the contacts are not usually at the very tip of the supporting cusps, nor are they at the deepest part. The actual contact points are called centric stops, or holding contacts, because they serve to hold the teeth in a stable position. The supporting cusps contact the opposing inclines leading to the fossae, ideally with a buccal and lingual contact for each cusp.



Furthermore, the supporting cusps have at least one mesial or distal contact with opposing marginal, triangular, transverse, or oblique ridges. Thus, a supporting cusp in closure is normally held in a precise firm position by at least three contacts (tripodization or tripodism).


These contact relations change with wear of the dentition. With advancing attrition, the supporting cusps seat closer and closer to the bottoms of the opposing fossae and the tripodization becomes less and less definite. This process continues until there are numerous flat surface contacts, and the cusps are seating in the bottoms of the opposing fossae, without definite locking of jaw position.


Ramfjord and Ash

1995, W. B. Sanders, 27

Occlusion – 4th Ed, p 62-64

Hanau’s and Thielemann’s Formulae
However, there does not seem to be any correlation in the natural dentition between condylar and incisal guidance. Thus, these principles of occlusal correlations are not useful for the natural dentition and also may be considered to be of questionable significance for complete denture occlusion.


KINESIOLOGY OF OCCLUSION

Kinesiology describes the movements of body parts on the basis of anatomy, physiology, and mechanics. The kinesiology of the functional relationship between mandible and maxilla is extremely complex, since it commonly involves a combination of movements in the sagittal, frontal, and horizontal planes.




Since some of the early works on mandibular kinesiology, many attempts have been made to explain jaw movements in simple terms. However, the complexity of the mechanical and neuromuscular principles involved in the various jaw movements defies all attempts at simple descriptions or explanations.

Mandibular movements have been studied with the help of a number of techniques, such as clinical and anatomic observations, engraving and graphic methods, tracing de­vices (mechanical, magnetic, and electronic), check bite registration, recording of facet patterns on the teeth, and roentgenographic and other photographic methods.




In recent times, electromyography and jaw-tracking devices have been used in the study of mandibular movements. All of these studies have been concerned with the movement patterns of the entire mandible, including the teeth and the temporomandibular joints.

In order to simplify the description, mandibular kinesiology will be discussed relative

to the sagittal plane, then relative to the horizontal and frontal planes.

Ramfjord and Ash

1995, W. B. Sanders, 28

Occlusion – 4th Ed, p 64-71

Border Movements and Positions of the Mandible Recorded in the Sagittal Plane

When the various parts of the mandible move perpendicular to the median or sagittal plane, they form a characteristic pattern. For example, the incisive point between the mesial edges of the two mandibular central incisors and similarly, the condyles and the other parts of the mandible, form such patterns.




Inasmuch as it has been shown that all mandibular movements take place within the framework of reproducible border movements, it appears logical to start the descrip­tion of mandibular movements with the border movements. The mandibular border movements recorded in the sagittal plane are shown in Figure 2—24. (Beyron’s or Posselt’s envelope of motion in which there is a definite vertically closing slide from CR to CO – rgp)

Centric Relation—Centric Occlusion and Freedom in Centric
Centric relation is the key reference position for analysis and reconstruction of the rnasticatory system. It is located by the dentist guiding both the patient’s condyles and disks against the posterior slope of the articulate tubercle for 1/2 to I in. of terminal closure, until the condyles are seated in an acceptable position as allowed by the disks.


The position is determined by the disks, not by the patient’s ligaments or muscles. It is very seldom (less than 10 per cent) that the jaw position reached by full closure guided by the teeth (CO) is the same as that of the position reached in centric relation (CR), i.e., CO equals CR is less than 10 per cent.



Ramfjord and Ash

1995, W. B. Sanders, 29

Occlusion – 4th Ed, p 70-72

How to Locate Centric Relation

The number of patients requiring a biteplane depends to a great extent on the ability of the operator to relax the patient, whether the patient is a bruxer, and how well the operator can manipulate the patient’s mandible.




The ultimate centric relation position in normal TMJ’s is determined by the condyles against the disks and in their uppermost position.(Fig. 2—32B).

Don’t try to locate centric relation by forceful manipulation of the mandible in a patient with tense muscles, and don’t try to locate CR in the presence of a painful joint.




Significance of Centric Relation

The significance of centric relation is that it provides a reference for evaluation and reconstruction of occlusion. It is not the optimal functional endpoint of occlusal closure with maximal occlusal contact (centric occlusion). The normal difference between centric relation and centric occlusion in dentitions with optimal function was well described almost 35 years ago.’2’ However, those findings contradicted the dogmas of the time, and numerous attempts have been made to support the old concept that centric relation and centric occlusion ideally should coincide.. However, it has never been proved that the patient’s optimal occlusal function will be performed in centric relation if the patient is given the opportunity of biting together comfortably slightly in front of centric relation.




All scientifically controlled tests with telemetry, electromyography, and clinical measurements have conclusively shown that if occlusal guidance to centric relation or centric occlusion is eliminated, the patient will naturally function, and be able to exert the heaviest force about 0.5 mm anteriorly to centric relation. Following elimination of all occlusal guidance on a flat biteplane, and testing when the patient could bite the hardest, this choice position invariably was about 0.5 mm in front of centric relation.


Ramfjord and Ash

1995, W. B. Sanders, 30

Occlusion – 4th Ed, p 72-73

Over the years we have successfully treated uncomfortable patients who have had oral rehabilitation to point centric by grinding away their anterior restriction. With freedom to function forward of point centric their symptoms abated. It has also been found that if you grind away occlusal guidance to establish a flat area between centric relation and centric occlusion, the patients still chew close to the previous centric occlusion.

Patients who have been reconstructed to have centric relation and centric occlusion coincide (according to old gnathological concepts), over the years tend to establish a distance between centric relation and centric occlusion. This proves that such reconstruction is neither stable nor physiologic.

That some patients with reconstructed occlusal guidance to centric relation may be or become comfortable does not by any means prove that centric relation and centric occlusion should coincide for all patients. The patients may also have been comfortable with other occlusal concepts.






Ramfjord and Ash

1995, W. B. Sanders, 31

Occlusion – 4th Ed, p 73-74

Freedom in Centric or Long Centric

It has been observed clinically and confirmed electromyographically that when the occlusion is adjusted in such a way that (when tested in centric relation) occlusal contacts are stable on a horizontal plane between centric relation and centric occlusion, the resultant occlusion is well tolerated by the neuro-mandibular system.



The term long centric has been used primarily for cases of full mouth reconstruction where lateral compo­nents of a slide could be eliminated in the restorations. In long centric, the patient is given the opportunity to move on a horizon­tal plane from centric relation to centric oc­clusion without any restrictions or changes in vertical dimension.

In partial restorative procedures and with an occlusal adjustment, there may be a lateral component to the slide in centric requiring a broad as well as long centric. Therefore, the term freedom in cen­tric is used and includes long centric. Correction of lateral slides to a flat area of impact on the vertical level of the centric occlusion may be done by occlusal adjustment when indicated.

It should be noted that the optimal occlu­sal force impact is at most 0.5 mm in front of centric relation, or as little as 0.2 to 0.3 mm but if the freedom in centric is extended beyond that level, no adverse effect is observed.




No method is available to locate the optimal point centric occlusion, and there does not seem to be any need for such definite position since both function and stability are well served by the freedom in centric con­cept. It should also be noted that patients in closure seldom hit centric occlusion; the jaw is guided to this maximum contact position on inclusive contact close to centric occlusion (I mm or less), but the power impact occurs when centric occlusion is reached. The initial contact in closure depends in part on head posture during the closing movements.



Ramfjord and Ash

1995, W. B. Sanders, 32

Occlusion – 4th Ed, p 74

Significance of Retrusion Facets
Retrusion facets distal to centric occlusion must be the result of frequent functional contacts. They have not received much attention for about 40 years’ and it has been implied that they developed as a sign of occlusal dysfunction.


However, numerous patients have retrusion facets corresponding both to a slide in centric and to lateral retrusive contact patterns without any evidence of occlusal dysfunction. It has been observed that retrusion facets may guide the final masticatory strokes into centric occlusion.

However, no controlled study has been reported where an accurate adjustment has been made on the retrusion facets eliminating all possible interferences. Such an adjustment would be technically difficult, but it appears from clinical experience that good results have been gained, in spite of some residual slide in centric, as long as all contact relationships were even bilaterally and unrestricted into centric occlusion.


It is also important that no interferences are left in the path between centric relation and centric occlusion. Centric relation is an abnormal position for mastication and is seldom contacted even after freedom in centric has been established. It has never been proven that patients will masticate into centric relation unless a definite occlusal guidance has been established artificially to point centric.


Ramfjord and Ash

1995, W. B. Sanders, 33

Occlusion – 4th Ed, p 74-75

Anterior Teeth and Concepts of Centric

In optimal natural occlusion, the anterior teeth may not quite make contact in centric occlusion or their contact is lighter than for the posterior teeth. The anterior teeth are kept in a stable position by functional contacts and by lip and tongue pressure. The anterior teeth in normal occlusion do not touch in centric relation. Even after occlusal adjustment to freedom in centric, there should be no contact between the maxillary and the mandibular anterior teeth in unguided closure. (Emphasis mine – rgp)




As has been emphasized it is essential for comfort and stability of occlusion that freedom in centric be provided to the extent needed by the individual patient—an average of 0.2 to 0.5 mm. Point centric in centric relation is not a defensible concept according to scientific evidence. It is about time to abandon this old dogma.

Also, be sure to allow freedom in centric including flat area about 0.5 mm both in front of centric relation and laterally before incisal and cuspid guidance is instituted.





SIGNIFICANCE OF CENTRIC

The word centric was originally used to indicate what was believed to be a stationary rotation center, or axis, of mandibular movements in mastication. No such center for all jaw movements has been established. However, the word centric, with various modifying terms, is used to designate an optimum functional relationship or contact position between the mandible and the maxilla. As such, it has become one of the most controversial words in dental terminology.




The controversy is, in part, semantic, but more significant is the serious difference in concepts. It is questionable whether there will ever be one word that can convey all the meanings attached to this word. An attempt will be made to summarize, without extensive documentation or discussion, the current status of centric.


Ramfjord and Ash

1995, W. B. Sanders, 33

Occlusion – 4th Ed, p 75 - 76

Summary of Centric

Centric relation is the key reference position for all occlusal contact evaluation.

Centric relation is not an optimal end point of masticatory function.

The optimal occlusal closure and power position of the mandible is normally in front of centric relation 0.2 to 0.5 mm (centric occlusion).

Empirically and experimentally it has been established that unrestricted mandibular free­dom in centric gives an optimal treatment of occlusion.

With freedom in centric, the main function will be close to centric occlusion, not to centric relation.





The teeth commonly make contact in centric occlusion during both chewing and swallowing.

Centric relation is a functional border position reached chiefly during swallowing, and occasionally during chewing.

Centric relation and centric occlusion do not coincide in the average healthy human dentition.

A stationary hinge axis recording or a gothic-arch tracing with a definite arrow point does not prove normal centric relation. A stationary hinge axis may change position following elimination of muscle dysfunction. A lateral slide from centric relation to centric occlusion is apt to create much more




A lateral slide from CR to CO is apt to create more neuromuscular disharmony than a straight forward slide.

A small, flat area between centric relation and centric occlusion, long centric or freedom in centric, is compatible with occlusal, temporomandibular joint, and muscle harmony.

Oral rehabilitation properly carried out according to gnathological principles, and in such a way that centric relation and centric occlusion coincide, may be well tolerated. Neuromuscular adaptation, however, is required, and there is a tendency for a slide to reoccur as the teeth move or the joints undergo changes. Thus, there are no demonstrated advantages over the more easily constructed long centric.


There is no valid reason to institute therapy just because a patient has a slide in centric if there axe no signs or symptoms to indicate any harmful effect from such a slide and the masticatory system is otherwise nor­mal.

For persons with bruxism and functional temporornandibular joint disturbances, the safest therapy is to produce freedom in centric by eliminating the slide in centric completely and replacing it with a horizontal long centric or, depending on the direction of the original slide, a wide centric.

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