Bakhit et al.

Asian Pac J Dent 2017; 17: 41-47

The stress distribution within dentin upon the use of different
restoration materials
Mohammed Bakhit, BDS, Daizo Okada, DDS, PhD, Siyang Luo, DDS, PhD, Chiharu
Shin, DDS, PhD, Reiko Ogura, DDS, PhD, Wataru Komada, DDS, PhD, and Hiroyuki
Miura, DDS, PhD
Fixed Prosthodontics, Department of Restorative Science, Graduate School, Tokyo Medical and Dental
University, Tokyo, Japan
Purpose: High flexibility makes polyetheretherketone (PEEK) attractive for use as a restoration material. The
aim of this study was to evaluate the stress distribution within a root for five types of restoration materials.
Materials and Methods: A three-dimensional root canal-treated premolar finite element model was fabricated,
and the model was reconstructed with five restoration materials. A 100-N occlusal force was loaded 45˚ to thelong axis, and the stress was calculated.
Results: The magnitudes of the stress surrounding the cervical area for the palladium-silver-gold alloy, hybrid
resin composite, zirconia, all ceramic, and PEEK materials were 22.7, 21.0, 22.2, 22.2, and 18.4 MPa,
respectively. The magnitudes of the stress surrounding the base of the posts were 6.5, 8.8, 5.9, 6.4, and 14.6 MPa,
respectively.
Conclusion: The restoration fabricated using PEEK prevented stress from occurring at the marginal area of thedentin; however, PEEK increased the stress at the base of the post to a greater degree than the other restorationmaterials.
(Asian Pac J Dent 2017; 17: 41-47.)
Key Words: stress distribution, von Mises stress, Young’s modulus

Introduction
Various materials are used as restorations to restore decayed teeth. Metals or porcelain fused with metals areexample of materials used in restorations. Full metal restorations have often been used in the premolar and molar
regions due to their good strength, reliability, and adaptability; however, full metal restorations are not
esthetically pleasing. Therefore, porcelain fused to metal restorations or hybrid resin composites are used insteadof full metal restorations to satisfy aesthetic demands in the premolar and molar regions. Porcelain fused tometal restorations exhibit good mechanical strength, are abrasion resistant, and have good biocompatibility;
however, the equipment needed is expensive, and these restorations requires much skill to fabricate. Hybrid resin
composites are easier to fabricate than porcelain fused to metal restorations, but they are less abrasion resistantand have poorer mechanical strength. The Young's modulus of most of these materials is the same or higher thanthat of dentin. The differences between the value of a material and dentin may influence the stress distribution
around the marginal area of a restoration, which can lead to secondary caries. Wedge-shaped defects are often
found at the cervical area of natural teeth and restorations; this defect may also be caused by the concentration ofstress at the cervical area of dentin. Secondary caries often cause restoration dislocations and may be related tothe concentration of stress around the marginal area, the adaptability of a restoration and the elution of a luting
agent.
Polyetheretherketone (PEEK), which is derived from polyaryletherketone, has both good chemical stability
and biocompatibility. PEEK was developed three decades ago and is used in medicine, dentistry, and otherindustries [1-10]. PEEK is a crystalline thermoplastic polymer and an aromatic polyether that displays very goodbiological and mechanical properties. Thus, PEEK is a good candidate for the clasps used in removable partial
dentures, abutments, and implants because it exhibits high strength and fatigue tolerance and is impact resistant.
In addition, PEEK also has good thermostability and creep behavior, and it more naturally resembles normal

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tooth colors than do other restoration materials. In addition, PEEK can be used for patients suffering from metalallergies because it releases less metallic ions than other materials [2,4,5,7-12].
Moreover, PEEK is more flexible than other materials, and restorations made from PEEK may prevent stressfrom occurring in the marginal areas of dentin; however, PEEK restorations may also enhance the stress at theother part of root because of its higher flexibility [12]. The aim of this study was to evaluate the stress
distribution within a root induced by five different types of restoration materials, including PEEK.

Materials and Methods
A three-dimensional finite element model was fabricated on a personal computer using finite element structural
analysis software (MSC Marc Mentat 2003, MSC Software Corp., Santa Ana, CA, USA), as per previous reports[13-15]. The model was assumed to contain endodontically treated premolar teeth. The teeth in our model were
18 mm long, and the cervical area was 6 mm in diameter. The root of this model was 12 mm long and wassurrounded by periodontal ligaments and lamina dura, whose thicknesses were 0.2 and 0.3 mm, respectively.
The coronal part of this finite element model was assumed to be 6 mm tall. The finite tooth model was
surrounded by two types of bone: cortical bone and cancellous bone Fig. 1.

Fig. 1 Finite element model
Table 1 Mechanical properties of the materials
DentinPeriodontal ligament
Lamina dura
Cancellous bone
Cortical bone
Gutta-percha
Composite resin core
Glass fiber post
Luting agent
Palladium-silver-gold alloy
Hybrid resin composite
Polyetheretherketone
Zirconia
Ceramics

Young’s modulus (MPa)15,00010
13,700345
13,7000.69
12,00029,2008,000
86,00021,0004,100
205,000
95,000Poisson’s ratio
0.31
0.49
0.30
0.31
0.30
0.45
0.33
0.30
0.33
0.33
0.27
0.40
0.19
0.24

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Reference[15,16][17][15,18][15,16][15,18][15,19][15,20][15,21]
[22]
[23]
[17]
[24]
[22]
[25]

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Asian Pac J Dent 2017; 17: 41-47

Fig. 2 Three-dimensional occlusal force withbottom surface of mandibular restriction

Fig. 3 The three dentin analysis points

The post and core system was assumed to be that of a composite resin composed of a glass fiber post, which
was 8 mm long. The height of the ferule was assumed to be 2 mm, and five restoration materials(palladium-silver-gold alloy (PD), hybrid resin composite (HR), zirconia (ZR), all ceramic (AC), and PEEK(PK)) were applied to each model. The material properties of each element are shown in Table 1 [15-25].
In all models, an occlusal force of 100 N was applied at the center of the occlusal surface, and the direction ofthis force was 45° to the long axis. The bottom of the mandibular bone was restricted, as shown in Fig. 2. In thisstudy, three different analysis points were used, as shown in Fig. 3: the cervical area of the dentin, the base of thepost, and the end of the post. Following this analysis, the magnitude of the von Mises stress distribution was
calculated.

Results
Figures 4, 5, and 6 show the stress distribution from the mesiodistal direction and under the loading force of 100
N at the center of the occlusal surface, while Table 2 presents the magnitude of the stress distribution for eachmaterial at each analysis point. The magnitude of the stress concentration at the cervical area of the dentin was
calculated for each analysis point. PD had the highest stress concentration, whereas PK showed the lowest stressconcentration. However, at the base of the post, PK had the highest stress concentration, whereas ZR had thelowest stress concentration.
Meanwhile, at the end of the post, there were no differences in the magnitude of stress, suggesting that thetype of restoration material did not affect the magnitude of the stress concentration at the dentin around the endof the post. Table 2 shows that the Young’s modulus of PK is much lower than that of the other restorationmaterials, which means that PK is highly flexible and can readily deform during use.

Discussion
Various experimental methods are used in dentistry to analyze stress distribution in teeth. One of these methods
is finite element analysis, the application of which has grown considerably among clinicians, technicians, andother medical professionals [13,26]. Finite element analysis has many advantages over the conventional

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experimental methods, such as the tooth fracture test or photo elastic method. The advantages of this technique
include the following; a) ability to observe any cross-sectional plane, b) ability to evaluate the magnitude ofstress at any analysis point, and c) ability to observe or analyze the inside of the root. Additional advantages ofthis approach over conventional experimental methods include time, cost, possibility for modification, andrepetition [13,19,25-30].
In this study, a 3-dimensional finite element analysis of endodontically treated premolar teeth was fabricatedto evaluate the stress distribution within the dentin, particularly in the cervical area of the dentin. Secondary
caries can be caused by inadequate adaptation of the restoration, thickness of the luting agent, and dissolution of
the luting agent. Related to secondary caries is the stress in the marginal area, where the teeth perform several
functions. The occurrence of wedge-shaped defects, which can be observed in the cervical area of natural teeth
or restoration, is also related to the stress in the cervical area of the dentin. Therefore, this study focuses on stressconcentrations.

Fig. 4 The stress distribution (frontal plane)
Table 2 Magnitude of von Mises stresses for each material at each analysis point by finite element analysis (MPa)
Cervical area
Base of the post
End of the post

PD
22.7
6.5
6.8

HR
21.0
8.8
6.8

ZR
22.2
5.9
6.8

Fig. 5 The stress distribution within the dentin root (frontal plane)

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AC
22.2
6.4
6.8

PK
18.4
14.6
6.8

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Asian Pac J Dent 2017; 17: 41-47

Fig. 6 The stress distribution within the cervical area of the dentin (frontal plane)

During restoration, the post and core system influences the stress concentration in the cervical area of thedentin. When a cast post and core system is applied to the endodontically treated teeth, the stress is concentrated
at the end of the post, which can lead to a fatal vertical root fracture. In contrast, the composite resin core with aglass fiber post concentrates less stress at the end of the post. Therefore, the composite resin core with the glass
fiber post is applied to all models in the present study because this post and core system prevents the occurrenceof fatal vertical root fractures [14,31,32].
In the composite resin core with a glass fiber post, stress occurs in the cervical area of the dentin, causing ahorizontal root fracture in the cervical area or secondary caries to decrease the stress at the end of the post [33].
The type of restoration materials may influence the stress in the cervical area of the dentin. If the magnitude ofstress depends on the restoration material, the stress may be concentrated on other parts of the dentin to reduce
the stress in the cervical area of the restoration. Therefore, we evaluated the stress at the base of the post, the endof the post, and the cervical area of the dentin.
In all models, a 100 N occlusal force was applied at a 45˚ angle to the long axis of the tooth, and the bottomof the mandibular bone was restricted. The stress distribution was calculated at three different analysis points:
around the cervical area of the dentin, around the base of the post, and around the end of the post for fivedifferent types of restoration material. PD, HR, ZR, and AC showed higher stress in the cervical area in thedentin than PK.
In our study, a restoration material with a high Young’s modulus led to a high concentration of stress in themarginal area of the dentin. Therefore, a restoration material with a lower elastic modulus than that of the dentin
may largely reduce the stress in the marginal area. Most restoration materials have an elastic modulus either
higher or equal to that of dentin [30]; however, PEEK has the least elastic modulus among all traditional
restoration materials considered in this study. Thus, PK was more deformed than other restorations under thesame loading, and the stress was directly transmitted to the top of the composite resin core.

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Meanwhile, PD, ZR, and AC produced lower stress, whereas HR produced moderate stress at the base of thepost. PK resulted in the highest concentration of stress at the base of the post. Instead of reducing the stress inthe marginal area, PK exhibited the highest concentration of stress. This stress was also transmitted from the topof the restoration because of the deformation of the PK restoration. Therefore, in the case of PK, the composite
resin must adhere strongly to the dentin. The composite resin core and glass fiber post must exhibit high fracture
strength. If the post and core system in the case of PK exhibits no sufficient adhesive properties or fracture
strength, a horizontal root fracture in the cervical area may occur.
All five restoration materials displayed the same stress distribution value of 6.8 MPa at the end of the post.
This finding suggests that the restoration material does not influence the stress distribution around the end of thepost, which sometimes leads to a vertical root fracture. These results indicate that PD results in the highest
concentration of stress in the marginal area, whereas PK exhibits the least stress among all restoration materials
examined in this study. As a restoration material, PK may prevent or reduce the stress that leads to secondary
caries. PK can potentially reduce secondary caries when this material is applied to the restoration.
However, in the case of PK, we have to pay attention to the adhesion strength between the dentin, composite
resin, and the glass fiber post, considering the fracture strength of these materials at the base of the post. The
following conclusions were drawn; 1) The restoration fabricated by PEEK prevents stress in the marginal area ofdentin, and 2) PEEK enhances stress at the base of the post compared with other restoration materials, including
metal, composite resin, and ceramics.

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Correspondence to:
Dr. Daizo Okada
Fixed Prosthodontics, Department of Restorative Sciences,
Graduate School, Tokyo Medical and Dental University,
1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan
Fax: +81-3-5803-0201 E-mail: d.okada.fpro@tmd.ac.jp
Copyright ©2017 by the Asian Pacific Journal of Dentistry.

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Accepted July 7, 2017.
Online ISSN 2185-3487, Print ISSN 2185-3479