The effect of occlusal contact localization on the stress distribution in complete maxillary denture, By Tatiana Araya

M. ATES¸ *, A. C¸ ILINGIR*, T. SU¨ LU¨ N*, E. SU¨ NBU¨ LOG˘ LU† & E. BOZDAG˘ †

*Department of Removable Prosthodontics, Istanbul University, Istanbul, Turkey, †Faculty of Mechanical Engineering Istanbul Technical University, Istanbul, Turkey

SUMMARY The fracture of acrylic resin dentures is an

unresolved problem in removable prosthodontics

despite many efforts to determine its cause. Unfavourable

occlusion could be playing an important

role in the fracture of the denture. The aim of this

study was to investigate the effect of occlusal

contact localization on the stress distribution in

complete maxillary denture bases utilizing twodimensional

finite element analysis. The results of

this study have shown that maximum compressive

stresses in a complete maxillary denture under

functional masticatory forces concentrates always

on the artificial tooth/denture base junction irrespective

to the occlusal contact localization. Tensile

stresses were observed in areas toward the midline,

although the midline itself usually had lower

stresses. Shifting the occlusal contacts to a more

buccal localization resulted in an increase of the

calculated stresses. As a conclusion, it can be

speculated that the buccal placement of the occlusal

contacts may play a role in the fatigue fracture

of the complete maxillary denture.

KEYWORDS: artificial teeth, complete maxillary denture,

finite element analysis, fracture, occlusal contacts

Accepted for publication 25 October 2005

Introduction

One of the most common problems seen in prosthetic

dentistry is the fracture of the acrylic complete dentures

under functional masticatory forces. It is well known

that an edentulous patient can only exert occlusal

forces at a level of 15–25% of a dentate patients and an

acrylic complete denture has a tensile strength of

48–62 MPa, a compressive strength of 75 MPa and an

elastic modulus of 3792 MPa. As a consequence the

denture should not be fractured under functional

masticatory forces (1). However, lack of integrity is a

frequently noted problem for both upper and lower full

dentures in the United States. In fact, over 15% of

upper and 9% of lower full dentures had problems with

fractures and missing/chipped teeth (2). It has been also

reported that 0Æ78 million denture repairs are carried

out annually in UK (3). Darbar et al. have shown that

the most common types of fracture are debonding/

fracture of denture teeth (33%) in both complete and

partial dentures followed by the midline fractures of

complete dentures (29%) (4).

Reasons for the fracture of the denture teeth are:

poor laboratory technique, the use of porcelain teeth,

and the increase in stress concentration at the tooth/

denture base interface, heavy or uneven masticatory

forces, unbalanced occlusion and patient related habits

(1, 4–7). Moreover; sharp notches, diastema and maxillary

tori are additional causes for midline fractures of

upper dentures (8–13).

It is important to note that fractures are more

frequently seen in upper single dentures (12). It is mostly

difficult to establish a harmonious occlusion in a maxillary

complete denture opposing natural dentition. The

reason for this are: (i) the mandibular teeth may be

malpositioned and may have assumed positions that

present excessively steep cuspal inclinations, (ii) the

bucco-lingual width of the natural teeth may be too

ª 2006 Blackwell Publishing Ltd doi: 10.1111/j.1365-2842.2006.01603.x

Journal of Oral Rehabilitation 2006 33; 509–513

wide. Failure to alter these conditions will often prevent

to achieve a favourable occlusion (8). Ellinger et al.

recommended recontouring the natural teeth to obtain

an occlusal plane that is suitable for a maxillary denture

(14). They stated that the most common error in single

denture construction is failure to modify the occlusal

plane. Several techniques for making single dentures

have been described up to date (15, 16). Regardless of the

technique, if attention is not paid to detail, fracture and

failure of the denture may occur.

Fractures in dentures result mainly from flexural

fatigue, which occurs after repeated flexing of the

denture base. This type of failure can be explained by

the development of microscopic cracks in areas of stress

concentration (12). Many methods of experimental

stress analysis such as; brittle coating (17), photoelastic

models (6), strain gauges (18), holography (19) and finite

element modelling (7, 11) have been used to examine

the stress distribution in complete maxillary dentures. It

has been shown that the stress concentrates in the

anterior palatal areas of maxillary dentures rather than

the posterior regions (18). Moreover in a three-dimensional

photoelastic stress analysis study, Craig et al. have

shown that the maximum stresses are developed in areas

right beneath the artificial porcelain teeth (6). In a finite

element analysis (FEA) study, Darbar et al. have stated

that the critical area of peak stress concentration is at the

beginning of the palatal aspect of tooth/denture base

interface (7). It has been also suggested that changes in

the loading conditions or modification of the occlusal

scheme will alter the pattern of stress distribution (5).

In this study, the effect of the bucco-lingual localization

of the occlusal contacts on the stress distribution in

upper denture base plate was investigated using a twodimensional

FEA.

Materials and methods

The FEA model reproduced a frontal section of edentulous

maxillary bone, mucosa, denture base and artificial

teeth. The contour of the denture was obtained

from a demonstration model of a maxillary complete

denture, which was sliced through the mesio-palatal

cusps of first molars. External contours were digitized in

AutoCAD 2000* and then transferred to finite element

program†. The geometric model was meshed with

eight-node quadrilateral plane strain elements via a

personal computer. The model was assumed to behave

like a slice on the symmetry axis of the denture, thus

enabling two-dimensional analysis with plane-strain

elements (Strain in direction of the slice normal is

forced to be zero). The FEA model of maxillary residual

ridge, mucosa, denture base and artificial teeth was

divided into 4444 elements and 13752 nodes (Fig. 1).

Several elements have been let to take place throughthickness

of thin parts such as the soft tissue to capture

the transverse stress distribution correctly. In the

absence of information concerning the precise material

properties of bone, like other materials of the model,

the bone has been assumed to be isotropic, homogeneous

and linearly elastic. The materials and their

properties used in the model are shown in Table 1.

These values were determined from the reports using

the FEA model of human jaws (20–22). The denture

base and artificial teeth were assumed to be acrylic

resin. The properties of cortical bone were set for the

bony structure. For representing the sutura palatina

media, two different modulus of elasticity were generated

(Table 1). An element-size sensitivity check has

Fig. 1. The FEA model divided into quadrilateral elements and

nodes.

Table 1. Materials’ properties used in the FEA analysis

Elasticity modulus (MPa) Poisson ratio

Bone 13 500 0Æ30

Mucosa 0Æ98/5* 0Æ30

Denture base 1960 0Æ30

Artificial teeth 2940 0Æ30

*0Æ98 is the first model and 5 is hard sutura palatina.

*Autodesk Inc., San Rafael, CA, USA.

†Ansys 8.0, ANSYS Inc., Canonsburg, PA, USA.

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ª 2006 Blackwell Publishing Ltd, Journal of Oral Rehabilitation 33; 509–513

been performed and the mesh represented here has

been found to be stable.

Chewing force for a single complete denture was

assumed to be 100 N. Three different loading points

were selected on the occlusal surface of the first molar:

buccal cusp, central fossa and palatinal cusp (Fig. 2).

Each loading point was loaded unilaterally and bilaterally

thus a sum of six analysis were performed. Another

set of six analysis, with different modulus of elasticity

for sutura palatina media, were also performed. All

lines of the outer contour of maxillary bone were

restrained in all directions. During post-processing,

nodes attached to the polished surface of the denture

base and the teeth-denture base interface were selected

for generating a path where the results of the finite

element stress analysis have been evaluated accordingly

(Fig. 3).

Results

The calculated von Mises stress values at three different

loading positions by unilateral and bilateral loading are

shown in Fig. 4a,b. The stress intensity increased

accordingly as the loading point moved to the buccal

side irrespective to the loading condition. According to

von Mises stress values obtained on the working side

for unilateral loading and on both sides for bilateral

loading from all three loading positions, maximum

stresses concentrate always at the teeth denture base

interface followed by the palatal incline of the base. But

signs of concentrated principal stresses at the artificial

teeth denture base interface and the palatal incline of

denture base were different. Stress values at the palatal

incline of the denture base were positive (tensile)

according to principal stress r1, whereas the values of

principal stress r2 were near to zero indicating the state

of stress to be two dimensional and values of principal

Fig. 2. Three types of the loading point on the occlusal plane. This

figure showed the occlusal table part at the right side of the model.

Fig. 3. A path was generated by selecting nodes attached to the

polished surface of the denture base and the teeth-denture base

interface.

The stress distribution in the finite element model

with a hard sutura palatina was similar to the first

model, but the stress values on the midline were

higher. The stress values of the finite element model

with hard sutura palatina were 18 000, 24 600 and

31 100 Pa and the values of the first model were 4000,

19 400 and 32 600 Pa for loading from the palatinal

cusp, central fossa and buccal cusp respectively.

In each of the 12 analysis, stress values at the midline

of the denture base were quiet low.

Discussion

The results of this study have shown that maximum

stresses in a complete maxillary denture under functional

masticatory forces concentrates always on the

buccal aspect of the artificial tooth/denture base junction

irrespective to the occlusal contact localization.

This result is in accordance with three dimensional

photoelastic stress analysis study of Craig et al. (6). They

concluded that the stresses in the bases were compressive

with maximum values in areas beneath the

artificial porcelain teeth. It is commonly agreed that

the stress at the teeth/denture base interface causes

fracture. For that reason, this result obtained in this

study also coincided well with the clinical finding that

most common types of fractures are debonding/fracture

of denture teeth in complete dentures (4).

However, as the stresses in this region are always

compressive and the values are too low to cause

fracture, it makes sense to conclude that not only the

fatigue stresses but also factors concerning laboratory

procedures should also be considered to be responsible

for the crack phenomenon.

In the present study, it has also been revealed that

shifting the occlusal contacts to a more buccal localization

resulted in an increase of the calculated stresses in

the buccal aspect of the tooth denture base junction but

a decrease in the palatinal aspect. In a FEA study,

Nishigawa et al. investigated the effect of the buccolingual

position of the artificial posterior teeth on the

denture supporting bone (22). As a result, they have

concluded that the stress observed at the buccal side of

the alveolar bone crest increased when the masticatory

forces applied from a more buccal loading point.

However they did not analyse the stress concentration

within the denture base, as the maxillary model was

not representing the correct shape of a complete

maxillary denture.

Prombonas and Vlissidis compared the effect of the

position of artificial teeth on the stress in the

complete maxillary denture by using two strain

gauges placed onto the midline of the intaglio surface

of the denture base (18). They reported that the outer

Stress (Pa) Stress (Pa) Stress (Pa)

Principle stress values of bilateral loading condition. (a)

Principle stress r1, (b) principle stress r2 and (c) principle stress r3.

TDB, tooth denture base interface; PI, palatal incline; ML, midline.

512 M. A T E S¸ et al.

ª 2006 Blackwell Publishing Ltd, Journal of Oral Rehabilitation 33; 509–513

placement of the posterior teeth resulted in an

increased principal stresses of the anterior stress field

and a decrease of the principal stresses in the

posterior stress field. Furthermore they suggested that

the midline fractures of the maxillary complete

denture start always from the anterior stress field.

The result of our study was not confirmed by these

findings. The findings of the present study revealed

that, the equivalent stresses toward the midline and

in the midline itself increased as the occlusal contacts

shifted to the buccal side.

In our study higher tensile stresses were observed in

areas near the midline, although the midline itself

usually had lower stresses. This finding is in accordance

with the results of the three dimensional photoelastic

analysis study of Craig et al. (6). Contrary to our

expectations stress distribution did not change by

increasing the elastic modulus of the mucosa over

the sutura palatina, but the stress values became

higher.

Within the limitation of this study, higher stresses

were observed in the buccal aspect of the artificial

tooth/denture base junction (compressive) and toward

the midline (tensile) irrespective to the occlusal contact

localization. Shifting the occlusal contacts to a more

buccal localization resulted in an increase of the

calculated stresses. As a conclusion, it can be speculated

that the buccal placement of the occlusal contacts may

play a role in the fatigue fracture of the complete

maxillary denture.

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Correspondence: Altug˘ C¸ ilingir, Research Assistant med dent Department

of Removable Prosthodontics, Istanbul University, Istanbul,

Turkey.

E-mail: caaltug@yahoo.com

OCCLUSAL CONTACT LO C A L IZAT I ON AND S T R ES S D I S T R I BUT ION 513

ª 2006 Blackwell Publishing Ltd, Journal of Oral Rehabilitation 33; 509–513