CENTRIC AM) ECCENTRIC OCCLUSIONS

VICTOR H. SEARS, D.D.S. Vallejo, Calif.

0 CCLUSION IS ANY CONTACT of the lower with the upper occlusal surface. Occlu- sion may be balanced or unbalanced. Balanced occlusion in centric position

exists with simultaneous contact of the right and left occlusal surfaces when the teeth are in centric occlusion. Balanced occlusion in lateral position is present with simultaneous contact of right and left opposing occlusal surfaces when the lower teeth are moved sideways. Balanced occlusion in protrusive position exists with si- multaneous contact of the front and back opposing surfaces when the teeth are moved sagittally. These criteria of balanced occlusion apply to both natural and artificial teeth. For artificial teeth, they apply to both anatomic and nonanatomic forms.

APPLICATION ,ACCORDING TO THE SITUATION

The principles of tooth occlusions are the same in all branches of dentistry, but their application differs according to the situation. Perhaps the greatest difference of application exists between orthodontics and complete denture prosthodontics.

Essential differences between the two situations have been pointed out,l but they are given again as a basis for discussion :

1. Individual natural teeth are held by fibrous attachments in sockets of bone, whereas prosthetic restorations rest on slippery surfaces.

2. Individual natural teeth move slowly, whereas prosthetic restorations can be dislodged instantaneously.

3. Individual tooth movement is possible with natural teeth, whereas the entire prosthetic restoration moves as a unit.

4. Malocclusions of natural teeth, especially in young patients, may exist for years without apparent injury. The same malocclusions in complete dentures usually manifest themselves in a few hours.

5. Malocclusion of a natural tooth is detrimental to its immediate investing tissues. Malocclusion of a tooth on a complete denture is detrimental to the support- ing tissues at other sites than the one underlying the malocclusion.

6. Horizontal pressure from the lingua1 side of a natural upper crown has no effect on the teeth on the opposite side, whereas such pressure on a prosthetic res- toration tends to push the teeth on the opposite side horizontally.

7. Vertical pressure on the lingual cusp of a natural tooth has no effect on the teeth of the opposite side, whereas such pressure applied on the lingual side of the ridge crest on dentures tends to press the opposite side more firmly toward the

Read before the San Diego County Dental Society, San Diego, Calif.

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tissues. Vertical pressure applied on the buccal side of one ridge crest tends to teeter the dentures, lifting them from the supporting tissues of the opposite side.

8. Interceptive contact of incisors in protrusion has no immediate effect on the natural posterior teeth, whereas such contact of prosthetic restorations tends to lift the posterior teeth of both dentures from their supporting ridges.

9. Interceptive contact of second and third molars has no immediate dislodg- ing effect on the other natural teeth. However, such contact of prosthetic restora- tions tends to slide the lower denture anteriorly and to lift the anterior teeth from their supporting ridges.

10. Natural teeth in centric occlusion meet their antagonists throughout the entire extent of the dental arch, whereas in complete dentures only those teeth near- est the center of the denture foundation should meet their antagonists.

The problems of balanced occlusion should be treated from different stand- points because of these differences. An early appreciation of the mechanical ‘re- quirements for different oral situations better equips the dentist to meet the prob- lems of tooth form and tooth placement.

ANATOMIC OCCLUSION

The logical place to teach anatomic occlusion is in the department of ortho- dontics or, better still, in an autonomous department of dental occlusion. This is the logical place where the instructors are capable of teaching all phases of occlu-

sion with natural teeth. Such instruction would require the use of a highly adjusta- ble mechanism for controlling the movements of the lower cast of the teeth.

Before deciding the new positions of the individual teeth, the orthodontist should mount the casts on a suitable dental articulator and adjust the mechanism to the recorded or anticipated2 centric and eccentric jaw relations of the patient. Test- ing occlusions with dentures or casts of natural teeth merely held by hand is un- scientific. Indeed, the anatomic occlusal forms of natural teeth with which the ortho- dontist deals require more versatile and more exacting articulator adjustments than do artificial teeth designed especially for the edentulous situation.

The orthodontist is bound by certain anatomic features which have little per- tinence in denture work. He should align the teeth for pleasing appearance, chew- ing efficiency, and preservation of the periodontal structures with due regard for the integrity of the temporomandibular joint and associated structures.

PROSTHETIC OCCLUSION

Until the dental schools in the United States establish autonomous depart- ments of dental occlusion as has been done elsewhere, the departments of prostho- dontics must teach the essentials of both natural and nonanatomic tooth forms and tooth positions.

Some of the essentials of tooth occlusion in the making of complete dentures are (1) smooth-gliding opposing surfaces, (2) absence of interference in horizontal movement, (3) lever balance, (4) balanced occlusion in centric position, and (5) balanced occlusion in the eccentric positions. Tests in many hundreds of mouths leave little doubt that all five of these are important. However, only the require- ments of balanced occlusion in centric and eccentric positions will be discussed.

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MERITS OF I’ROSTHETIC BALANCED OCCLUSION

It is sometimes claimed that there is no need for eccentrically balanced occlu- sions for complete dentures. Some theoretic considerations are involved, but ,the best way to resolve this question is to make tests in the mouths of patients. The tests should be for comfort, stability, and efficiency of the dentures, and subse- quently for the condition of the tissues underlying the dentures.

With respect to bilateral balanced occlusion, it has been argued that because the teeth of the balancing side are held apart by food on the working side, the balancing surfaces have no effect. A phrase has been coined to bolster the argu- ment-“enter bolus, exit balance.”

Whatever validity there may be to the argument of “enter bolus, exit balance,” carefully conducted tests show that dentures can be made more satisfactory by bal- ancing the occlusion in all functional jaw relations. On inserting dentures with newly balanced occlusions, it is not uncommon for a patient to remark, “You tight- ened them, didn’t you ?” This reaction should not be surprising, because if every closure brings the right and left sides together simultaneously, the dentures are pressed evenly against their basal seats.

In many hundreds of tests, most patients reported increased comfort and sta- bility of dentures with balanced occlusions. A few patients noticed no difference, but none found the change detrimental. Part of the explanation for the lack of unanimity is that other variables were present, such as smooth gliding of opposing surfaces, absence of cuspal interference, and lever balance.

BALANCED OCCLUSION IN GENERAL

Centric occlusion is the contact position of the lower teeth against the upper teeth at which all horizontal centripetal movements end. Any horizontal deviation from this position results in eccentric occlusion. Centric occlusion of the teeth should coincide with centric jaw relation. Although the two positions can be ma.de to coincide exactly with each closure on a sturdy articulator, this is not true in the patient’s mouth.

The mandible is not a part of a precision machine and does not always close at exactly the same place .4 Leaning the head will cause the mandible to close some- what in the direction of the lean.5 Even without this tilt there is often a slight shift away from the “center” position. Posselt writes of his tests, “. . . In no case were the functional positions situated on the hinge movement paths.“B

A degree of freedom should be provided in the occlusion at centric jaw relation. The opposing teeth should meet in a restricted area rather than at a point. This pro- vision is especially important if cusp inclines are present to cause horizontal shifting of the dentures when the teeth are closed away from the exact centric occlusion.

NONCHEWING OCCLUSION

This horizontal freedom of movement near centric occlusion is especially im- portant when no food is present. There are two phases of balanced occlusion. The first and perhaps the more important phase pertains to tooth occlusion without in- terposed food. The second phase pertains to tooth occlusion with interposed food.

1032 SEARS J. Pros. Den. Nov.-Dec., 1960

Fig. l.-The anatomic scheme of occlusion maintains all of the upper lingual cusps in contact with the lower buccal cusps on the balancing side.

One reason that balanced occlusion is important in the absence of food is that the teeth are brought together so frequently between meals. Since closures are not always at the same horizontal position, it is necessary to give attention to the occlusions that occur “at and near” centric jaw relation. The beneficial effect of balanced occlusions “at and near” centric jaw relation in the absence of food may explain why some dentists who use flat plane teeth have questioned the value of balanced occlusion in eccentric positions. 7*8 When flat plane teeth are balanced at the exact centric position, they are almost of necessity also balanced in the slightly eccentric positions.

The same statement cannot be made for opposing teeth with cusps that mesh. The steeper the cuspal inclines and the more precise the meshing, the greater is the dislodging effect when the teeth occlude in eccentric occlusions. It is because of this

Fig. Z.-Not all of the posterjor teeth are necessary to maintain balanced occlusion on the balancing side. The removed teeth are not necessary to ensure balancing contacts.

CENTRIC AND ECCENTRIC OCCLUSIONS 1033

Fig. 3.- With flat plane teeth, the upper bicuspids are not needed for lateral balancing contacts and are arranged to assure greatest denture stability in the working position.

dislodging effect with precisely meshing teeth that the rotary grinder mounted on the articulator assures the patient more comfortable and more stable dentures.”

When the clearance produced by the rotary grinder is enough to accommo- date the slightly eccentric closures of the teeth in the absence of food, the harmful effects of meshing teeth are partly overcome. The improvement obtained with a rotary grinder is greatest in dentures with tightly meshed steep cusps.

LATERAL BALANCED OCCLUSION

In this discussion, it will be assumed that centric occlusion of the teeth is in harmony with centric jaw relation and that lateral jaw relations produced cor- responding lateral tooth occlusions.

Fig. 4.-The upper bicuspids and the upper second molar, as well as the lower bicuspids and Arat molar, have been removed to show that a protrusive balancing contact can be obtained with- out them.

J. Pros. Den. Nov.-Dec., 1960

Balanced occlusion in lateral positions is produced by causing some surface on the balancing side to maintain contact while the teeth of the working side are together. With cusp teeth, it has been standard practice to maintain the lingual cusps of all of the bicuspids and molars of the balancing side in contact (Fig. 1). However, this is not necessary. The lower second molar and the upper first molar are the only teeth needed to maintain lateral balancing contacts (Fig. 2).

With flat plane teeth, balanced occlusion in lateral positions is assured with the same two molars (Fig. 3). The lower first molar and all of the bicuspids can then be set to their best positions for chewing without having to tilt them at undesirable angles. It is clearly an advantage to set the working surfaces independ- ently of the balancing surfaces so that each tooth can best discharge its function.

Fig. 5.-Flat plane posterior teeth are set so that the lower second molar and the upper first molar maintain protrusive balancing contact. The other teeth are set independently to assure greatest denture stability when they are in the working positions.

The upper second molar should be out of contact in all or nearly all of its occlusal surface. Because occlusal pressure on the lower second molar is unfavor- able to denture stability except when this tooth is used to produce lateral and pro- trusive balancing contacts, the upper second molar should ordinarily be omitted or made subocclusal.10 Omitting the upper second molar makes it possible for the lower second molar to maintain a balancing contact in the protrusive and lateral positions without carrying any of the load at the centric position,

PROTRUSIVE OCCLUSION

The need for balanced occlusion in full protrusion is less imperative than that of the centric and lateral positions. However, this contact is easily obtained with either flat plane or cusp teeth.

Although a protrusive balancing contact in the position of incision is me- chanically advantageous, sometimes the full advantage cannot be attained because of’ conflicting requirements of appearance. When the anterior teeth are set with

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considerable vertical overlap for esthetic reasons, it is sometimes not advisable to produce balanced occlusion in full protrusion. The dentist must decide whether to give dominance to esthetics or mechanics. This holds true with flat plane teeth as well as with cusp teeth.

As with balanced occlusion in lateral positions, the key teeth for balanced occlusion in protrusive positions for both anatomic and nonanatomic schemes of occlusion are the lower second molar and the upper first molar. All of the other posterior teeth can be disregarded in this connection (Figs. 4 and 5).

The lower second molars, as they glide on the upper first molars, produce balanced occlusions in all eccentric positions. The lingual tilt of the lower second molar assures a lateral balancing contact, while the anterior tilt produces a pro- trusive balancing contact.11v*2

SUMMARY

The problems of occlusion extend into nearly all branches of dentistry. While the principles involved are the same, their application should be different according to the situation. The dentist should not fall into the error of trying to make the same application in all situations.

The examples of orthodontics and prosthodontics show the need for recogniz- ing the similarities and the differences in applying the same principles to different situations. Such recognition should bring about more realistic teaching in the dental schools and more rational procedures in dental practice.

The occlusion for complete dentures should be balanced “at and near” centric jaw relation for protection of the ridges when there is no interposed food. With cusp teeth, this balanced occlusion “at and near” centric relation is obtainable with a rotary grinder. With flat plane teeth, this mechanism is not needed. The occlusion should be balanced at habitual lateral relations used in chewing as well as some- times in the protrusive position.

Using only the lower second and the upper first molars for establishing lateral and protrusive balancing contacts solves the problem of maintaining balanced occlu- sion in eccentric positions. At the same time, this method permits the independent arrangement of the other teeth for better direction of the closing force during chewing.

REFERENCES

1. Sears, V. H. : Occlusion: The Common Meeting Ground in Dentistry, J. PROS. DEN. 2:15-21, 1952.

2. Sears, V. H. : Mandibular Condyle Migrations as Influenced by Tooth Occlusions, J.A.D.A. 45:179-192, 1952.

3. MacMillan, H. W.: Clinical Application of Unilateral Balance to Prosthetic Dentistry, J.A.D.A. 18:1029-1034Z 1931.

4. Moyers, R. E.: Some Physiologic Considerations of Centric and Other Jaw Relations, J. PROS. DEN. 6:183-194, 19.56.

5. Sears, lY9H.: Suggestions for Accurate Preparation for Plate Work, D. Digest 25:82-86,

6. Posselt, U:: Occlusal Relationship in Deglutition and Mastication, Transactions of the European Odontological Society, 1958.

7. DeVan, M. M.: An Analysis of Stress Counteraction on the Part of Alveolar Bone, With a View to Its Preservation, D. Cosmos 77:109-123, 1935.

1036 SEARS J. Pros. Den. Nov.-Dee., 1960

8. Kurth, L. E.: Methods of Obtaining Vertical Dimension and Centric Relation: A Prac- tical Evaluation of Various Methods, J.A.D.A. 59:669-673, 1959.

9. Upp, R. W. : U. S. Patent No. 1,180,745, April 25, 1916. 10. Sears, V. H.: Let’s Sink the Upper Second Molar, Cal, March, 1942, pp. 8-10. 11. Sauser, C. W.: Posterior Occlusion in Complete Denture Construction, J. PROS. DEN.

7:456-464, 1957. 12. Nepola, S. R.: Balancing Ramps in Prosthetic Occlusion, J. PROS. DEN. 8:776-780, 1958.

646 WASHINGTON ST. VALLEJO, CALIF.

MODERN EQUIPMENT HELPS DENTISTS TREAT MORE PATIENTS

High-speed cutting instruments are enabling dentists today to treat more patients in less time and in more comfort, both for dentist and patient, according to the American Dental Association,

Reporting on a newly released survey of dental practice, which was conducted for 1958, the Association said two of every three dentists reported having high- or super-speed cutting instruments.

The instruments are classified in four speeds: low (under 10,000 revolutions per minute), intermediate (10,000 to 30,000 rpm), high (30,000 to lOO,@O rpm), and super (over 100,000 rpm). Until after World War II, almost all dentists used low speed.

In the current study, nearly half of the dentists reported using super speeds while two-thirds reported either high or super speeds. Younger dentists tend to use high and super speeds most often.

Concerning the effects of the speeded-up instruments, more than half of the dentists who re- sponded found it enabled them to treat more patients, one-third found it reduced office hours, and a significant number found that it did both. It was also emphasized that patient comfort was greatly increased and dentist fatigue greatly lowered.