Occlusal wear facets

Fixed partial dentures

Occlusal

wear

J. Marvin Virginia

facets

Reynolds,

D.D.S.*

Commonwealth

University,

School

of Dentistry,

Richmond,

Vu.

lh e purpose of this article is to report a study by a group of dentists** at the Virginia Commonwealth University, School of Dentistry, on how natural teeth should occlude. We were particularly interested in learning what effect various occlusions have upon the stamp (upper lingual and lower buccal) cusps, their coupled fossae, the blades of the incisors, and the shearing (upper buccal and lower lingual) cusps. Observations were made on natural teeth free of dental caries in healthy mouths. The main features existing in the mouths where the teeth showed the least wear are enumerated. The mechanical factors in the gnathic system that retard or hasten the “wearing in” of occlusal facets are analyzed. To some extent, notations were made concerning the effect of occlusion upon the mucous-membrane linings of the mouth and the linings around the necks of the teeth. SCOPE OF THE STUDIES At the start of these studies, the group knew that all branches of dentistry are concerned with the occlusion of teeth. Some of the specialists emphasize the importance of occlusion. Other specialists seem to de-emphasize occlusion. A larger number of general practitioners apparently are becoming less indifferent to occlusion. In continuing dental education courses, the group members are becoming familiar with the various occlusal concepts currently expounded by the literature and specialists who have differing opinions. It seems to be true that all experts on occlusion try to base their concepts upon the anatomy and physiology of the gnathic system, but each makes his own interpretations of the facts. Each devotee of a concept also has his own interpretations of the facts of the biomechanics of mandibular activities. Presented *Professor,

before

the Third

Department

International

of Crown

**Robert F. Harman, D.D.S., George R. McGuire, D.D.S.

Congress

and Bridge

Wallace

of Gnathology,

San Diego,

Calif.

Prosthodontics.

S. Edwards,

D.D.S.,

Marvin

F. West, D.D.S.,

and

367

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.I. Pros. Dent. October. 1970

Reynolds

The differences of opinion generally fall into three main areas: (1) Should the occlusal position coincide with terminal hinge relation? (2) What teeth should comr into contact in eccentric occlusion ? (3’) Should the molars and premolars 11nvtt distinct cusps and fossae in dentitions of adults? The group started to compare these several concepts of occlusion with what thus) found in clinic patients. In order to find reasonable ways to set up samples for the various phases of occlusion, the clinical observations were made on patients as they became available for examination. Each patient’s occlusion was viewed as he was encouraged to close in centric relation. He was then induced to close in pure protrusion so that he would show what teeth made closure contacts in that mandibular position. He was subsequently taught to close in the right and left lateral mandibular positions.

PRELIMINARY

FINDINGS

Clinical examinations of the patients, made at random, revealed that centric occlusion and centric relation were identical in only a small percentage (8 per cent 1 of the dentitions. In the larger percentage (92 per cent), centric occlusion was either lateral or anterior to centric relation. This was caused by cusps that deflected the closure of the mandible away from its centrically related position. Where deflective malocclusion occurs, the occlusal vertical dimension is increased when centric relation is established. This is caused by the interceptive occlusal contacts. In pure protrusive closure, most of the dentitions made only incisal surface contacts. However, in a smaller percentage of the dentitions, all or most of the plural cusped teeth also made occlusal contacts. In a still smaller percentage, only the plural cusped teeth made occlusal contacts when the mandible was protruded. In the lateral closures, more than 50 per cent of the dentitions made no molar or premolar occlusal contacts. In other words, on the working side, the only surfacr contacts made were by either the canines or the incisors. Usually, the canines only made the occlusal contacts. However, in some individuals, all or most of the posterior teeth (molars and premolars) on the working side and some of the incisors also made occlusal contacts. More often than not, no occlusal contacts were made on tht> “idling” (balancing) side of the dentition. When such contacts did exist, they were usually made by the molars. After four years of study, it became doubtful whether the kind of occlusions best suited natural teeth could be determined by our methods of observation. The group found all features of the various occlusions except those of the “long centric” slide. Quasi-bilateral balanced occlusion was found in a few individuals. If a Iarge number of patients in a sample could be found with such occlusions, impressive justifications for such a concept could be advanced.

CONCENTRATION ON THE CAR?&-FREE DENTITION The study group decided to concentrate their observations on dentitions that were free of dental caries. Orthodontically treated dentitions were not included, Clinical observations were made and tabulated for all basic jaw positions used in diagnosis. These included centric relation, centric occlusal position, protrusive relation, and both right and left lateral relations. In addition, casts of selected dentitions

Occlusal

wear facets

369

were mounted on adjustable articulators and were similarly studied. The casts were studied in detail for evidence of wear, and the facets were correlated with the kind of occlusal contacts made in the diagnostic jaw-closure positions for each dentition. Fifty such dentitions were studied. The ages of the people making up the caries-free sample ranged from 14 to 54 years. Sixteen were less than 20 years of age, 31 ranged between 20 and 40 years of age, and 3 subjects were over 40 years of age. Fifty-two per cent of the subjects were men.

FINDINGS IN THE CARIES-FREE SAMPLE In the three eccentric diagnostic jaw positions of the caries-free subjects, the findings, in regard to the number and kind of occlusal contacts made, were similar to those found in the individuals of the random sample. Twenty-four per cent (12) of the caries-free subjects had no detectable differences between the terminal hinge relation and maximum intercuspation when clinically examined. Slight discrepancies could be felt in the mounted casts of five subjects in this group, but they were difficult to see or measure. Physiologists say that the eye is not able to differentiate between differences much smaller than 0.01 inch (0.25 mm.). Thirty per cent (15) of the subjects had a difference between centric occlusion and centric relation not exceeding 1 mm. Forty-six per cent (23) had a slide greater than 1 mm. The dentitions which had the maximum intercuspation coincident with the terminal hinge jaw relation had the fewest and smallest facets of wear. When the molars and premolars were in a centric occlusion harmonious with centric relation and were disoccluded” immediately by the incisors in protrusive movement and also by the canines in sliding lateral movements, facets of wear were practically nil regardless of the patient’s age. The more the centric occlusal position of the molars and the premolars deviated from centric relation and the more it lacked immediate disocclusion by the incisors and canines in protrusive and lateral movements, respectively, the more numerous and the larger were the facets of wear. The molar and premolar wear facets appeared only on the surfaces of those teeth which made contacts between centric occlusion and centric relation or between centric occlusion and other eccentric positions. Anterior teeth also had more than usual wear when centric occlusion and centric relation were not identical. Incisors that make incisal contacts in lateral movements demonstrate the most wear. These lateral malocclusal relations occur when the canines are malposed or

*The term “disocclude” > as used in this article, means that certain parts of the opposing occlusal surfaces do not make contact with their opponents. The term “disclude” has been suggested to describe the situation wherein teeth are set and shaped so that the maxillary buccomarginal ridges and the mandibular linguomarginal ridges of the posterior teeth never close against the edges and wherein the “stamp cusps” never make contacts in eccentric closures. The common understanding of the word “disocclude” does not apply here, because it means “to separate from occlusion or occlusal contact.”

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J. Pros. Dent. October. 1970

Reynolds

malrelated, and thus the incisors fail to disocclude when the jaw is moved laterall) with the teeth in contact. Incisors that have too much vertical overlap unfortunatei? participate in the disocclusion of the posterior teeth in lateral movements. The number and extent of facets of wear on all teeth seemed to be more closely related to the length of the slide rather than the age of the individual.

MECHANiCAL

WEAR OF TEETH

The wear facets discussed in this article were seen on occlusal surfaces of the molars and the premolar-s and on edges of the canines and the incisors. It is presumed that wear from masticating while eating need not be facet producing but may produce a more uniform thinning of the enamel over all occlusal areas and a lessened tendency toward making flat surfaces. The facets studied and presented here are from mechanical rubbings of enamel surfaces against opposing enamel surfaces when little or no foods are between the here are on the surfaces surfaces-as in “empty chewing.” The facets demonstrated of those teeth which deflect the closure direction of the mandible and keep the muscles from closing the jaw into centric relation; or they are on surfaces that interfere with the cyclic chewing or that are rubbed by gritting maloccluded teeth; or they result from parafunctions used by individuals presumably testing their occlusal relations in emotional eccentric movements. The rubbing action of the enamel surfaces, of course, comes from the closing or lateralizing activities of the muscles. If the teeth simply stop the closure, they will not wear enough to form facets; but if they divert the direction of closure, horizontal skidding of the lower cusps occurs which develops the flat surfaces of the facets. It is, therefore, presumed that the retracting muscles of the mandible tend to put the jaw in its most posterior position in many maximum closures. If malrelated teeth limit this tendency of the muscles, they interfere with the closing directions of movement produced by these muscles. Teeth that do not stand in the way of the paths of closure wear the least. The canines and incisors are not at the mercies of the muscles when they ungear the molars and premolars in eccentric occlusions. Well-fitting anterior teeth slide only when a person wants to test the fit of his teeth, or when a dentist wants to test their fitness, or in parafunctions. Even in biting off chunks of food, the incisors or canines seldom bang together. The final act of incision is accomplished by reciprocal action between the head and the hand.

A NEAR-FRICTIONLESS OCCLUSION The principles of an ideal plan of occlusion that seems to promote tranquility in the gnathic system and retard the formation of wear facets caused by friction in horizontal rubbings are presented as follows : 1. The upper lingual cusps stamp into the fossae of the lower teeth with which they are coupled. The lower buccal cusps stamp into the fossae of the upper teeth with which they are coupled. 2. All teeth make occlusal contacts simultaneously when the jaws close in centric relation, and the centric occlusion of each plural cusped tooth coincides with centric relation. No one tooth guides the cyclic jaw closures.

Occlusal

wear facets

371

3. The lower lingual cusps and the upper buccal cusps overlap the stamp cusps with which they are coupled. 4. The upper canines overlap the lower canines, and the coupled canines on each side contact in centric occlusion but are so positioned and so sloped that in lateral excursions they immediately disocclude all other teeth. 5. The horizontal overlap of the upper canines conforms to the side shift of the working side condyle, and the slant of the lingual concavity is in harmony with the eminence of the balancing-side condyle. 6. The upper incisors overlap the lower incisors; the lower incisors stamp into the concavities of the palatal side of the upper incisors in centric occlusion, and the positions and the slopes of the upper incisors are such that they immediately disocclude all other teeth in protrusive excursions. 7. The lingual surface of upper incisors have slants (inclines) as steep as the eminences of the joints. 8. Horizontal rubbings of the molars and premolars do not occur, because their centrically related occlusion is reached by closure that is more vertical than cuspal inclines. The coupled anterior teeth immediately disocclude the premolars and molars in protrusive and lateral diagnostic movements. 9. In cyclic chewing, the edges and the slopes of the upper buccal cusps and lower lingual cusps move close by the stamp cusps with which they are coupled but do not make sliding contacts. Eccentric contact of anterior teeth occurs outside the chewing cycles and ordinary uses of the mandible. 10. Each stamp cusp reaches or leaves its fossae by three “runways” which are grooves in the coupled teeth, The grooves and coupled ridges are shaped and directed SO that no occlusal contact occurs in the entrances and the exits of these fossae. ORGANIZATION

OF OCCLUSION AND DISOCCLUSION

In such a frictionless occlusion, the teeth are organized to be used in groups, namely, ( 1) the incisors for gross cuttings, (2) the canines for tearing, and (3) the plural cusped teeth for multiple incision. The occlusion of all teeth is organized to afford group actions in eccentric chewing relations and collective action in centric relation. OCCLUSAL ANATOMY By the fusion of the developmental lobes, an uneven surface is made on the occlusal surface of a tooth. This surface has cusp tips, marginal ridges, triangular ridges, supplemental ridges, developmental grooves, supplemental grooves, and fossae-the elements of occlusion. The following facts are observed in the occlusal elements : ( 1) Definite cusps, ridges, and grooves exist on molars and premolars; (2) the buccolingual width of the occlusal table is about the same on molars as that on premolars; (3) each cusp has a primary ridge with a smaller supplementary ridge on each side; (4) each primary ridge is at least twice as wide as each supplementary ridge; (5) the primary and supplementary ridges are biconvex; (6) the lingual surfaces of anterior teeth have a definite concave surface incisogingivally; and (7) the in&al edges of anterior teeth are rounded. The elements of occlusion are common to all teeth, but they are more defined in some than they are in others.

372

Reynolds

J. Pros. Dent. October, 1970

CONCLUSIONS Facets of wear are evidence of malfitting teeth. This wear seems to begin in a dentition from two possible defects: ( 1) disagreement between centric relation and centric occlusion, and (2) lack of organized disocclusion of the multicusp teeth in eccentric occlusions. In order that the teeth can function in harmony with the temporomandibular articulations, the muscles, nerves, and joints must determine the occlusal position of the mandible. The functional paths of mandibular movement should not be altered biologically by conditioned reflexes or mechanically by deflective cuspal inclines. Whenever teeth dictate mandibular movement or compel eccentric closures to be made that do not fit the musculocondylar intent, either the cusps are worn off rapidly, or the the bearing teeth have their periodontiums traumatized, or the temporomandibular articulations are strained. Maximum intercuspation should occur at posterior terminal hinge (centric) relation and at a vertical level that ensures an adequate shortening of the jaw muscles in power closures. Eccentric occlusion should be limited to contacts of anterior teeth that are in front of the genial tubercle. Such contacts occur outside the normal functional use of the mandible. Cusps, fossae, and their associated elements are the basic units of occlusion. The cusps and their parts must be placed and graded in the interalveolar ridge space to suit the musculocondylar movements so that the teeth remain just outside the mandibular cyclic movements. In addition, the teeth should have suitable lip, cheek, and tongue relations for pleasing appearance, proper speech, and optimum comfort with the oral tissue.

SUMMARY This article reports the findings and conclusions of a study made by a group of dentists at the Virginia Commonwealth University. The data reported were obtained by studying a sample of 50 individuals who had no caries and no periodontal illnesses beyond mild gingivitis. Twenty-four per cent of the sample had maximum intercuspation agreeing with centric relation. Casts of the dentitions were mounted on articulators so that detailed studies of condyle-cusp relations could be made. Facets of wear were fewest in centrically related, occlusally organized dentitions. The smallest number of facets of wear were found in dentitions which had canine and incisor contacts that uncoupled the molars and premolars in the eccentric mandibular excursions. Certain conclusions were reached which are based partly on scientific fact and partly on clinical evidence and judgment. The author wishes to thank Dr. Harvey Stallard assistance and advice in the preparation of this article. VIRGINIA SCHOOL

COMMONWEALTH DENTISTRY

OF

521 N. 11 ST. RICHMOND,

VA.

23219

UNIVERSITY

and Dr. Alex

Koper

for their

editorial