SELECTION OF POSTERIOR TEETH IN COMPLETE DENTURE TREATMENT

INTRODUCTION

Selection and placement of posterior teeth is based on knowledge of

anatomy, histology physiology, pathology and biomechanics. The nature of the

mucosa and submucosa, the form and relationship of the residual ridges, the

general systemic condidtion of the patient, the tone of mandibular musculature,

esthetic factors and post patient experiences can all relate to the size, form and

number of artificial posterior teeth that may be used on the denture bases. The

dentist is solely responsible for the selection of the posterior teeth as he alone

possesses the necessary information required in this regard.

An analysis of dentulous and edentuolous conditions

1. Individual tooth movement is possible in natural dentition, but not in the

artificial denture. Thus vertical / horizontal pressure on a natural tooth

would affect only the individual tooth and the local surrounding tissue,

while same type of pressure on an artificial tooth in a denture would affect

the entire denture, probably causing displacement.

2. Premature contact of a few natural teeth rarely has an immediate effect on

the whole dentition, but premature contact of artificial teeth could dislodge

denture.

3. More pressure can be developed between natural teeth than between

artificial teeth, and thus the efficiency of dentures is less. This may be due

to sensory nerve endings, working in conjunction with proprioceptor and

exteroceptic nerve mechanisms which are located in the soft tissue under

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the denture base protecting the patient from painful pressure. This is

achieved either by lessening the pressure or moving the mandible to a new

position of comfort if it is found possible.

4. Retention and stability are different for both natural and artificial dentitions.

The natural is able to withstand tolerable force in all directions whereas the

only force which will not tend to dislodge a denture base is one where the

direction is arranged in such a manner that tilting and gliding of the denture

base on the mucosa is avoided.

5. Tooth wear in natural dentitions occurs where it is permitted by retention of

abrasive elements in the food. No changes occur in the denture unless teeth

are made from materials which are too soft to be of service for long periods

of time, but changes do occur in the underlying tissues.

These differences between the edentulous and dentulous mouth indicates

that a new condition has arisen in which the edentulous condition requires a

new approach.

There are four major patient factors to be considered, along with their

influence on the selection for anatomic, semianatomic, nonanatomic and mold

combinations of teeth.

4 factors Neuromuscular control

Anteroposterior jaw relationship

Mediolateral jaw relationship

Esthetics

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Neuromuscular control

During the registering and transferring of entire jaw relation position

from the patient to the articulator, the dentist has the opportunity to assess the

muscle control capabilities of the patient. If little difficulty is encountered

during these procedures, use of the anatomic or semianatomic molds are

acceptable. If however the muscle control of the patient is questioned and the

dentist experience great difficulty in registering transferring and verifying the

several jaw registration records, then molds with precise interdigitation would

not be indicated.

Anteroposterior jaw relationships

Anatomic and semianatomic teeth are designed to interdigitate in the

normal class I skeletal and molar relationship. As long as the patient presents

clinically with an anteroposterior jaw relation position that is a skeletal class I,

the cusp teeth may be arranged and adjusted to provide the balanced occlusal

concept. Skeletal class II patients may position the mandible in several

anteroposterior jaw positions for such activities as rest, chewing and

swallowing and the range of this forward movement has been observed to be as

great as the full width of a bicuspid. Even though the recording and transfer of

the retruded centric jaw relation position may be reproducible, these patients

bring their mandible forward to some anterior position that usually is not

repeatable. The patient will experience multiple deflective occlusal contacts as

he or she moves to one of the anterior position.

Midiolateral jaw relationships

The patient with a skeletal class III jaw relations will present similar

problems in tooth selection and arrangement as those observed with class II

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skeletal relationship. Any attempt to arrange Anatomic and Semianatomic teeth

with a different mediolateral contacts other than class I relation will usually

cause multiple deflective occlusal contacts.

Esthetics: The cusp is an item that contributes to naturalness, especially the

buccal cusp. The “buccal corridor”, the region from the maxillary premolars

posteriorly enhances a patients smile. A tooth mold with a buccal cusp form

most assuredly offers the greatest potential for a pleasing natural appearance.

REVIEW OF LITERATURE

Development of the artificial posterior tooth

Until 1913 artificial posterior teeth were only approximately anatomic in

form

In 1913, Gysi carried his “normal-bite” teeth which originated from

averages of his jaw recordings. They were carried to harmonize with a condylar

inclinatin of 33 degrees from the horizontal and similar incisal guidance.

The normal bite teeth did not function to his satisfaction in cases where

alveolar resorption had so narrowed the maxilla in relation to the mandible that

a cross bite arrangement was necessary to prevent dislodgement of denture

bases.

Gysi then introduced the “Cross-bite teeth”. The mandibular teeth were

33% and maxillary teeth 40% smaller than the normal bite teeth.

Anatomic teeth: Teeth that have prominent cusps on the masticating surfaces

and that are designed to articulate with the teeth of the opposing natural or

prosthetics.

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In 1928, Sears presented his “Channel teeth”, the first truly

nonanatomically designed teeth. The purpose was to provide efficient

mastication and eliminate the shifting of dentures caused by anatomic forms.

The maxillary teeth were shaped anatomically on the buccal aspect, but

occlusally they were nonanatomic. Sears maxillary teeth presented both a

buccal and lingual plane, following away from a central horizontal fossa,

because without such inclination, he said it would be impossible to obtain

balanced occlusion during function. The lower teeth were buccolingually less

then one half as wide as anatomic molds and the working occlusal portion, the

only part making contact with the opposing upper teeth was an unbroken ridge

of porcelain 1mm to 1.5 mm in width, running mesiodistally the entire length

of the occlusal surface.

Nearly 10 years after the introducing of channel teeth Sears presented

the “platform posterior”.

In 1929 Hall presented his non anatomic “inverted cusp” teeth. Their

design was based on the principle of a cutting machine with two cusped disks

taking in opposite directions. The efficiency of these were generally low. They

tend to along the food.

A short time later, Myerson marketed his “trucusp teeth”. These teeth

had no cuspal inclinations, but had cutting blades and crushing tables curved in

the occlusal surface, along with Slevice ways for escape of the food.

In 1935, French presented his non anatomic teeth. They consisted of

upper bicuspids and molars having buccolingual cuspal pitch, the width of the

teeth being about the same as anatomically designed teeth. The lower teeth

were much narrower than the conventional pattern but at the same time much

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wider than Sears channel teeth. French provided a food table on his mandibular

teeth, but the food table was comparatively narrow consisting of an area which

on an anatomic tooth would be the lingual cusps. The area normally occupied

by the buccal cusps was entirely eliminated, so the tooth was really about one

half a tooth in width. French reasoned that by eliminating the buccal half of the

occlusal surface, greater stability of the denture bases was assessed.

Swenson presented a tooth to improve upon Hall’s theory. The teeth

were provided with slevice ways for clearing food from the depressions

between the cutting edges and they had buccolingual pitch to provide balance.

It was called the “non-lock” teeth.

Arury Brothers (1930) designed teeth which sucked to solve the problem

of efficiency in artificial teeth by having the teeth definitely interlock

anteroposteriorly, but free them of any cuspal inclines buccolingually. This was

done to promote the shearing principle.

In 1936, McGrane discribed a “curved cusp” tooth constructed with

cervical grooves and curved cusps forming segments of an arc, concentrically

arranged from a radical center corresponding to the right and left rotation

centers in the heads of the condyles in the grenoid fossae. These teeth lak very

definitely in anteroposterior relationship.

In 1946 Idardy introduced his “metal insert teeth”. These cure plastic

teeth with ribbons of metal inserted on the occlusal surface. It was suggested

that they might make an efficient food shearing tooth.

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Radical changes in tooth form

1. Sears chewing members: Sears was of the openion that derivations from

anatomic forms are justifiable if the substitute produces efficient

mastication with a minimum of damage or change in appearance with this

in mind he derived nonanatomic teeth built in blocks rather than single

units. An upper and lower artificial posterior dentition of this design

contains four blocks, a block being composed of two bicuspids and two

molars Sears claims that these teeth are 1. Efficient, having fairly steep

inclined planes, 2. Less traumatic in nature as the antagenists guide

smoothly over each other instead of interfering as seen in protrusive and

lateral movements with cuspid teeth. 3. Easier to set up, 4. Adequate in

appearance.

Hardy’s metal insert teeth

He devised his teeth basically for obtaining efficiency in mastication.

This tooth form in occlusal outline in similar to the other types except that it is

composed of one block of plastic with embedded underlating strips of metal.

Efficiency is very high. These teeth are useful with patients who have heavy

bites that fracture porcelain teeth or wear plastic teeth.

Types of posterior teeth according to cusp inclines

Posterior artificial teeth are manufactured with cusp inclines that vary

from relatively steep to flat planes. The commonly used posterior teeth are

those with cuspal inclination of 33°, 20° and 0°. The cuspal inclination is

measured as the angle formed by the incline of the mesiobuccal cusp of the

lower first molar with the horizontal plane.

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The 33° posterior teeth offer the maximum opportunity for a fully

balanced occlusion. However the final effective height of the cusp for a given

patient depends on the way in which the teeth are inclined (tipped) and the

interelation of the other factors of occlusion, that is the incisal guidance, the

height of the occlusal plane and the compensating curve maintaining a shallow

incisal guidance compatible with esthetics allows a balanced occlusion to be

developed with as little cusp height on the posterior teeth as possible, thus

reducing lateral forces on the residual ridges.

The 20° posterior tooth is semianatomic in form and wider

buccolingually than corresponding 33° tooth. The 20° tooth provides less cusp

height with which to develop balancing contacts in eccentric jaw positions that

the 33° tooth does. Non anatomic teeth are advisable when only a centric

relation record is transferred from the patient to the articulator and no effort is

directed to establish a cross arch balanced occlusion. Non anatomic teeth are

also effective when it is difficult/impossible to precisely record centric jaw

relation from the patient or there are abnormal jaw relationships.

Factors influencing function of anatomic and non anatomic teeth

1. Efficiency: Efficiency has been defined as the ability to produce results.

This is applicable to denture functions the aim of mastication being the

comminution of food.

Factors influencing efficiency

a. Type of patient

b. Condition of the mucosa and bony ridge.

c. Type of denture

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d. The biting force

e. The character of food and size of the bolus

f. The arrangement of the teeth

2. Directional forces

Forces on dentures must be in a direction which will give the greatest

stability.

This is accomplished by

a. The form of the tooth

b. Placement of the tooth

c. The arrangement of the teeth.

a) Form of the tooth: A flat occlusal surface makes it relatively simple not

only to direct the force of mastification where it is desired, but to eliminate any

resultant horizontal stresses. The proponents of the anatomic teeth, however

claim that it is not the presence of the cusp that influences direction of force but

the mismanagement of these cusps.

One of the greatest objections to the use of anatomic teeth is the

difficulty to maintain them in occlusal balance after dentures are inserted in the

mouth. This is due to dentures “settling because of alterations of tissues under

the denture base. A new denture position is then produced with a resulting

change in the prearranged directional force. Cusps tend to cause anteroposterior

locking which prevents the dentures from moving to their new position of

directional force, thus resulting in a loss of stability. The forces are also

redistributed, sometimes causing excessive pressure in certain areas with a

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resulting resorption of portions of the ridge. This in turn causes further

malalignment of the dentures.

The nonanatomic tooth is easier to arrange and maintain in proper line

of force under similar conditions.

b) Placement of the tooth: Due to the greater retention of the upper denture,

any mismanagement of directional forces tends to occur at the expense of the

lower, producing an unstable lower replacement. Forces directed to fall inside

the mandibular ridge create increased stability. This can be accomplished by

placing the lower tooth lingual to the lower ridge and / or tilting the tooth so

that force is directed lingually.

3. Horizontal stress: When a vertical force is applied to an inclined plane

nonvertical forces or horizontal stresses appear. It is essential to minimize

horizontal stress or dentures for the maintenance of oral comfort, health and

denture service, the proponents of flat teeth claim cusps cause this particular

stress, whereas Schuyler maintains that the inclination of the eminentia

articularis portion of the mandibular fossa along with the incisal guidance, and

not the tooth form governs horizontal stress. In other words cuspid posterior

tooth, as long as it confirms to these factors will not cause denture movement.

4. Stability: It is the ability of the denture to remain in position during

masticatory and nonmasticatory movements. Factors

Selection of posterior teeth

The selection includes Shade Buccolingual width

Size Mesiodistal length

Number Vertical length

Form

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Shade:

It should harmonize with the shade of the anterior teeth

Bulk influences the shade of the teeth and for this reason it is advisable to

select a slightly lighter shade for the bicuspids if they are to be arranged for

esthetics. They may be slightly lighter than the other posterior teeth but not

lighter than anterior teeth.

Size and number of posterior teeth

The size and number of posterior teeth are closely related to usage. These

characteristics are dictated by the anatomy of the surrounding oral

environment and physiologic acceptance of supporting tissues. The

posterior teeth must support the cheeks and tongue and function in harmony

with the musculature in swallowing and speaking as well as in mastication.

Buccolingual width of posterior teeth

The buccolingual width of artificial teeth should be greatly reduced from

the width of natural teeth they replace.

Artificial posterior teeth that are narrow in buccolingual direction enhance

the development of the correct form of the polished surfaces of the denture

by allowing the buccal and lingual denture flanges to slope away from the

occlusal surfaces. This occlusal form permits forces from the cheeks and

tongue to maintain the dentures in position on their residual ridges.

Narrow occlusal surfaces with proper escapeways for food also reduce the

amount of stress applied on food during mastication to the supporting

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tissues of the basal seat. On the other hand, the posterior teeth should have

sufficient width to act as table upon which to hold food during trituration.

DIAGRAM

Mesiodistal width of posterior teeth

The mesiodistal width of the posterior teeth are determined by the

edentulous area between the distal of the mandibular cuspids and the

ascending area of the mandible. After the six mandibular anterior teeth have

been placed in their final position a point is marked on the crest of the

mandibular ridge at the anterior border of the retromolar pad. This is the

maximum extent posteriorly of any artificial teeth on the mandibular ridge.

In well formed ridge the apex of retromolar pad is taken as posterior level

and in resorbed ridges the point where retromolar pad turns upward.

DIAGRAM

Vertical length of buccal surfaces of posterior teeth

It is best to select posterior teeth corresponding to the interarch space and to

the length of the anterior teeth. The length of the maxillary first premolars

should be comparable to that of the maxillary canines to have the proper

esthetic effect.

Type of posterior teeth according to materials

Most artificial teeth are made of air fired or vacuum fired porcelain, acrylic

resin or a combination of acrylic resin and metal occlusal surfaces.

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Vacuum fired porcelain teeth are more resistant to abrasion and therefore

maintain their luster longer than acrylic resin teeth.

Acrylic resin teeth can be altered and will bond with the denture base for

retention in instances of limited inter arch space.

Acrylic resin teeth are preferred when the teeth in the opposing arch have

been restored with gold, as the porcelain has a higher coefficient of wear

than gold.

Acrylic posterior teeth should never be used in combination with porcelain

anterior teeth. The rate of wear of resin teeth in relation to that of porcelain

teeth is such that the excessive forces from opposing tooth contact will

eventually be developed in the anterior part of residual alveolar ridges. The

supporting structures for the dentures are unusually least able to withstand

increased stress in this region.

Advantages of cusp posterior teeth

1. They are considered more efficient in the cutting of food, thererby reducing

the forces that are directed at the support during masticatory movements.

2. They can be arranged in balanced occlusion in the eccentric jaw positions.

3. When the cusps are making contact in the fossae at the correct vertical

dimension of occlusion with the jaws in vertical relation, the position is

comfortable. This position is a definite point of return, as through

proprioception the jaws will return to this position.

4. They look more like natural teeth and are therefore acceptable esthetically.

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5. The contours are more like natural teeth; therfore they will be more

compatible with the surrounding oral environment.

6. An attempted occlusion without cusps is disorganized because occlusion

has depth; it is not a sudden closure of flat surfaces.

Advantages of monoplane teeth

1. When teeth are contacting in non masticatory mandibular movements as in

bruxism, the flat polished surfaces offer less resistance, therefore less force

is directed to the support.

2. When the monoplane teeth are arranged to provide even contacting

bilaterally with the vertical dimension of jaw separation in harmony with

the jaws in centric relation, this position is comfortable. Through

proprioceptive impulses the patient will return to this position reflexly.

3. In cases of resorbed ridges dislodgement by horizontal or torquing forces

can occur. Monoplane teeth offer less resistance to these forces.

4. These teeth will allow a greater range of movement which is necessary in

patients with malrelated jaws.

5. Where the neuromuscular controls are so uncoordinated that jaw relation

records are not repeatable, the cusp form tooth cannot be balanced.

Monoplane teeth are less damaging than cusp teeth.

6. In case of diabetic patients where the underlying bone is vulnerable to

damage less stress is transmitted by the use of monoplane teeth.

Indications for use of Anatomic and Nonanatomic artificial posterior teeth

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Payne indicates that flat teeth be used

1. Where ridges are flat, rendering dentures more susceptible to horizontal

stress.

2. In old age where the ridges are flat.

3. When the vertical dimension is great which would cause tipping forces to

develop.

4. If a maximum of vertical force and a minimum of horizontal stress is

desired.

Cusp teeth are to be used 1) where good ridges exist 2) with younger

people.

Shanahan suggests that the general rule, “High cusps for the young and

low or flat cusps for the aging” be the criteria of tooth selection.

Problems with anatomic tooth forms

1. The use of an adjustable articulator is mandatory.

2. Eccentric records must be made for articulator adjustments. Setting the

adjustments will vary from dentist to dentist with the same records.

3. Mesiodistal interlocking will not permit settling the base without horizontal

forces developing.

4. Harmonious balanced occlusion is lost when settling occurs.

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5. The bases need prompt and frequent refitting to keep the occlusion stable

and balanced.

6. The presence of cusps generates more horizontal force during function.

Problems with non anatomic teeth

1. Non anatomic teeth occlude in only two dimensions (length and width) but

the mandibular has an arcuate three dimensional movement due to its

condylar behaviour.

2. The vertical component present in mastication and non functional

movements is not provided for so that this form loses shearing efficiency.

3. Bilateral and protrusive balance is not possible with a purely flat occlusion.

Non anatomic teeth set on inclines for balance require as much concern as

anatomic teeth for jaw movements.

4. The flat teeth do not function efficiently unless the occlusion surface

provides cutting ridges and generous spillways (several ways).

5. They cannot be corrected by much occlusal grinding without impairing their

efficiency.

6. Non anatomic teeth appear dull and unnatural to some patients which may

create a psychological problem concerning function.

CONCLUSION

There are no definite guidelines for selection of posterior teeth. But it

mainly depends on the type of patient and the condition of the supporting

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tissues. And hence, the selection of teeth is the responsibility of the dentist

which he acquires through knowledge and experience.

References

1) Boucher : Prosthodontic treatment for edentulous patients. 334-343, 9th

Edition.

2) Briene R. Lange et al : A practical approach to restoring occlusion for

edentulous patients part I, guiding principles of tooth selection. J.

Prosthet. Dent., 50 : 455-458, 1983.

3) Charles W. Ellinger et al : Complete denture prosthodontics. Pg. 175-

190.

4) Charles H. Heartwel, Arthur O. Rahn : Syllabus for complete dentures.

Pg. 322-325.

5) Fenn, Liddelow and Geinsons : Clinical dental prosthetics. Pg. 104-105,

Third Edition.

6) Hardy I.R. : The developments in the occlusal patterns of artificial teeth.

J. Prosthet. Dent., 1 : 14-27, 1951.

7) Harold R. Ortman : Complete denture occlusion dental clinics of North

America. 21 : 299-319. Apr. 1977.

8) Moses C.H. : Biomechanics and artificial posterior teeth. J. Prosthet.

Dent., 4 : 782, 1954.

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9) Nasr M.F. et al : The relative efficiency of different types of posterior

teeth. J. Prosthet. Dent., 18 : 3, 1967.

10) Paul J. Leherner : Selection of artificial teeth. J. Prosthet. Dent., 18 :

229-232, 1967.

11) Pleasure M.A. : Anatomic versus non anatomic teeth. J. Prosthet. Dent.,

3 : 747, 1953.

12)Raymond J. Nagle, Victor H. Sears : Dental Prosthetics, 389-414.

13) Robert M. Morrow, John E. Rhoads, Kenneth D. Rudd : Complete

dentures, 2nd edition, Pg. 187.

14)Robert Rapp : the occlusion and occlusal patterns of artificial posterior

teeth. J. Prosthet. Dent., 4 : 461, 1954.

15)Sears V.H. : Selection and management of posterior teeth. J. Prosthet.

Dent., 7 : 723, 1957.

16) Sheldon Winkler : Essentials of complete denture prosthodontics. 2nd

edition, 223-238.

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