|Year : 2011 | Volume
| Issue : 2 | Page : 75-79
Three-dimensional finite element analysis of the stress distribution around the implant and tooth in tooth implant-supported fixed prosthesis designs
G Arun Kumar, Lin Cherian Kovoor, Vinni Mary Oommen
Department of Prosthodontics, Rajas Dental College, Tirunelveli, Tamil Nadu, India
|Date of Web Publication||30-Dec-2011|
G Arun Kumar
Madavam, TC 17/1092 (13), CG-193, Cherukara Road, Poojappura, Trivandrum, Kerala
| Abstract|| |
Aim: The study evaluates the stress formed around an implant and a natural tooth under occlusal forces, on different tooth implant-supported fixed prosthesis (TIFP) designs in order to suggest a design, which transmits less stress to the bone.
Materials and Methods: A distal extension situation was utilized in this study to evaluate stress distribution around a natural tooth and an implant in TIFP models with three connection designs (i.e., rigidly connected to an abutment tooth, connected to an abutment tooth with a nonrigid connector [NRC], and connected to an abutment implant with an NRC). The stress values of the three models loaded with vertical forces (300 N) were analyzed using three-dimensional finite element analysis.
Results: The highest level of stress around the implant and natural tooth was noted on the TIFP models with the RC. On the other hand, NRCs incorporated into the prostheses reduced the stress in the bone around the implant and natural tooth.
Conclusion: The present study recommends the use of NRCs on the implant abutment-supported site, if the tooth and implant abutment are to be used together as fixed prosthesis supports. The NRC placed on the implant abutment site reduces the stress around the implant and natural tooth in a fixed prosthesis supported by tooth and implant increasing the life span of both.
Keywords: Finite element, fixed prosthesis, tooth implant
|How to cite this article:|
Kumar G A, Kovoor LC, Oommen VM. Three-dimensional finite element analysis of the stress distribution around the implant and tooth in tooth implant-supported fixed prosthesis designs. J Dent Implant 2011;1:75-9
|How to cite this URL:|
Kumar G A, Kovoor LC, Oommen VM. Three-dimensional finite element analysis of the stress distribution around the implant and tooth in tooth implant-supported fixed prosthesis designs. J Dent Implant [serial online] 2011 [cited 2013 May 18];1:75-9. Available from: http://www.jdionline.org/text.asp?2011/1/2/75/91283
| Introduction|| |
Treatment with implant-supported fixed partial dentures (FPDs) has shown excellent long-term results, in the rehabilitation of partially edentulous patients.  However, because of anatomic limitations and reduced bone volume, in some cases it may be necessary to join an implant to a natural tooth. Combining teeth with implants provides support for a fixed partial prosthesis and may allow the restoration of a unilateral posterior edentulous segment when only one implant can be placed.
Rigid connections between natural teeth and osseointegrated implants present a biomechanical dilemma because of their difference in movement under masticatory forces. Teeth with a sound periodontal ligament have mobility characteristics between 50 and 200 μm, while osseointegrated implants demonstrate a mobility less than 10 μm.  The difference in mobility causes extensive torsion movements and could result in the fracture of the framework, loosening or fracture of screws in screw-retained prostheses, cement failure on the abutments, gradual loss of the crestal bone around the implant or breakdown of implant osseointegration. , Previous research has advocated the use of nonrigid FPD designs as a method of compensating for this differential movement. ,, Rigid partial denture designs are also advocated by many clinicians. , Misch and Ismail could not report any difference between models upon comparison of the tooth implant-supported fixed prosthesis (TIFP) design with nonrigid connectors (NRCs) placed on the tooth side and designs with an RC using a three-dimensional finite element model (3D FEM).  Melo et al. could not observe any reduction in stress in the surrounding bone when using an NRC in the TIFP model.  Even though individual research reports are published previously, the question whether rigid connections or nonrigid connections were more advantageous and the location of the NRC on the implant site or natural tooth site is more beneficial, when fixed prosthesis is supported by the implant and natural tooth, still remains unanswered.
The present study was aimed to investigate the stress formed around the implant and natural tooth under occlusal forces on TIFP designs by three-dimensional finite element analysis.
| Materials and Methods|| |
A 3D FEM of the mandibular section of the bone with the second premolar natural tooth and implant abutment in the second molar area to receive a fixed prosthesis was used in this study. The fixed prosthesis designs used in this study were divided into three models:
- Model 1: The second premolar and the implant were connected by a nonrigid attachment with an NRC positioned on the mesial side of the implant.
- Model 2: The second premolar and the implant were connected by a nonrigid attachment with an NRC positioned on the distal side of the second premolar.
- Model 3: The second premolar and the implant are connected rigidly.
The modeling was done using 3D software called Pro/Engineer, 2000i (Parametric Technology Corp., Needham, MA, USA), after which it is exported to an analysis package. The finite element software, ANSYS Workbench (Santa Monica, CA, USA), was used to analyze the models. The models were processed in ANSYS to generate the meshed structure. Meshing divided the entire model into smaller elements. The elements are interconnected at specific joints called nodes. Once meshing and contacts are defined, the next process is to define boundary conditions. After defining the boundary of the model, the loads to be applied were defined, and then the stress analysis was completed by the incorporation of material properties. The material properties were determined from values obtained from the literature ,,,, [Table 1].
A bone block with a height of 29 mm, width of 12 , and cortical bone thickness of 1.5 mm surrounding the cancellous bone was modeled. The height of the premolar crown was 8 mm, mesiodistal length (M-D) was 7.5 mm, buccolingual width (B-L) 7 mm, and the height of the root 16.5 mm. The height of the pontic was 8.75 mm, B-L width 10 mm, and M-D length 13 mm. The height of the implant abutment crown was 10 mm, B-L width 9.5 mm, and M-D length 13 mm. The periodontal membrane width was accepted as 0.2 mm.
A solid 3.5 × 13 mm screw-type, commercially pure titanium dental implant system was selected for this study. The simulated fixed partial denture consisted of the framework material and porcelain. A cobalt-chromium alloy was used as the crown framework material and feldspathic porcelain was used for the occlusal surface. The porcelain thickness used was 1.2 mm and the metal thickness was 0.8 mm.
The NRC selected for the study was the Beyeler intracoronal, nonadjustable, friction grip dovetail slide attachment (Cendres and Metaux, Switzerland).  It is indicated for fixed bridges in the posterior region. The vertical direction length of the NRC was fixed as 3.8 mm for all FE models [Figure 1].
The materials used for the models were presumed to be homogenous, isotropic, and linear, and the osseointegration of the implants was accepted as 100%. In the mathematical model while the implants were directly in contact with the bone, the natural teeth had primary mobility within the borders of the periodontal membrane. The NRCs of the fixed prosthesis designs were allowed to vertically move on each other under vertical loads. Linear static analysis was performed on the prepared 3D solid models with a total masticating force of 300 N at right angles to the long axis of support. The force (300 N) was selected based on the average occlusal force on the FPD.  The applied forces were static, and were given on the lingual inclines of the buccal cusp and distal fossa. The maximum equivalent von Mises stress values in each zone were recorded for each model on four planes.
The simulated bone surrounding tooth and implant models were divided into 12 zones to facilitate analysis of the stress pattern [Figure 2]. The zones were as follows: Zone 1, mesioalveolar crest of premolar; Zone 2, mesio middle third of premolar; Zone 3, mesioapical third of premolar; Zone 4, distoapical third of premolar; Zone 5, disto middle third of premolar; Zone 6, distoalveolar crest of premolar; Zone 7, mesioalveolar crest of implant; Zone 8, mesio middle third of implant; Zone 9, mesioapical third of implant; Zone 10, distoapical third of implant; Zone 11, disto middle third of implant; Zone 12, distoalveolar crest of implant.
|Figure 2: Maximum stress recorded in zones surrounding tooth and implant models|
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The maximum stress in each zone along four lines was recorded, added, and evaluated:
- Line 1 - Zone 1 + Zone 2 + Zone 3
- Line 2 - Zone 4 + Zone 5 + Zone 6
- Line 3 - Zone 7 + Zone 8 + Zone 9
- Line 4 - Zone 10 + Zone 11 + Zone 12.
| Results|| |
Stress distribution was represented numerically and was color coded. The maximum stress in each zone on the mesial and distal surface of the tooth and implant in the three models is shown in [Figure 3], [Figure 4] and [Figure 5]. The maximum stress along four lines of the three models is shown in [Table 2].
|Table 2: Maximum stress around the tooth and implant in three models along four lines|
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In Model 1 (with the NRC positioned on the mesial side of the implant), the highest equivalent von Mises values were 22.42 and 16.08 MPa and these values were located in the cortical region of the implant abutment along Lines 3 and 4. The highest stresses around the natural tooth were 4.19 and 6.08 MPa in Zones 1 and 6, respectively. The von Mises stress contours for Model 1 are shown in [Figure 3].
In Model 2 (with the NRC positioned on the distal side of the premolar), the highest von Mises stress values were obtained in the cortical bone region of both the mesial and distal sides of the implant along Lines 3 and 4 with values recorded as 26.19 and 26.18 MPa, respectively. The maximum stresses around the tooth were 4.27 and 6.13 MPa in Zones 1 and 6, respectively. The von Mises stress contours for Model 2 are shown in [Figure 4].
In Model 3 (where the second premolar and the implant are connected rigidly), the highest stress values recorded were located in the cortical bone region of the implant along Lines 3 and 4 with values recorded as 23.19 and 20.46 MPa, respectively. The maximum stress values generated around the natural tooth were 9.20 and 6.57 MPa in Zones 1 and 6, respectively. The von Mises stress contours for the rigid connection configuration are shown in [Figure 5].
In all the three models, the stress around the natural tooth and implant abutment was found to be gradually decreasing from the crestal region of the bone to the apical region. The stress around the natural tooth was found to be higher in Model 3 than in Models 1 and 2. The highest stress value around the natural tooth was recorded along Line 1 in Model 3. The lowest value of 9.49 was obtained on the mesial surface of the natural tooth along Line 1 in Model 2. The stress around the implant along Lines 3 and 4 was found to be the least in Model 1 when compared to Models 2 and 3. The lowest stress value of 32.57 was recorded along Line 4 in Model 1. The highest stress value of 40.83 was observed around the implant along Line 4 in Model 2.
| Discussion|| |
FEM analyses have been used extensively to study the biomechanics of stress transfer in dentistry. The FEM program used in this study has several limitations with respect to the unrealistic simulation of material properties of the structure. The program assumes that the bone, the tooth, and the periodontal ligament are homogenous, linear, elastic, and isotropic. Furthermore, all static mastication forces applied to FPDs were loaded axially in this study. However, the masticatory forces are dynamic and may be directed oblique to the occlusal surface of the TIFP model. Consequently, it is usually impossible to reproduce all the details of natural behavior. Due to these limitations, the values obtained in this study may not resemble actual values, but this may show the stress differences and advantages of various TIFP designs.
In the 3D FE study under a static load of 300 N, the stress was found to be more around the implant than the natural tooth in all the three models. The reason for this may be the cushioning effect of the periodontal ligament around the natural tooth. The viscoelastic properties of the periodontal ligament play a vital role in the good long-term performance of the tooth implant connected by fixed bridges. Menicucci et al. found that static loading is more harmful than the transitional loads and they concluded that the periodontal ligament plays the key role in the force distribution between a tooth and a rigidly connected implant. 
The stress values around the implant and natural tooth were found to be more in the compact bone region and decreased gradually toward the apical region. This is likely due to the difference in the modulus of elasticity in cortical and spongy bones. The cortical bone having a higher modulus of elasticity is more resistant to deformation and will bear more load than the cancellous bone.
The stress distribution around the natural tooth in Model 3 was found to be higher than that in Models 1 and 2. This may be due to the stress breaking effect of the NRC. The excess force on the natural tooth in Model 3 may cause intrusion of the tooth. The intrusion can be prevented by placing the keyway on the implant crown.  There was only minimal difference in stress values along Lines 1 and 2 between Models 1 and 2 [Table 2]. This shows that the stress around the natural tooth is not influenced by the position of the NRC either on the distal side of the tooth or on the mesial side of the implant in the TIFP design.
The stress values around the implant along Lines 3 and 4 in Model 1 were found to be lower than those in Models 2 and 3. This is because the NRC reduces the force that is transmitted to the implant when it is positioned on the mesial side of the implant. When a static load is applied to the TIFP as could occur in bruxism or clenching, the periodontal ligament deforms under the load, with the result that the tooth tends to sink into the alveoli. This causes the bridge to act as a cantilever on the implant and the stress concentrates in the bone around the implant.  A potential consequence of such overloading may be peri-implant marginal resorption which may eventually cause failure of osseointegration.  The NRC positioned on the mesial side of the implant was able to reduce the cantilever effect on the implant abutment. Bechelli suggested that the NRC should be placed on the implant abutment side with the TIFP designs to protect the implant from torque effects. He also indicated that this design has many advantages such as allowing the physiological movements of natural tooth, the equal distribution of forces on the natural tooth, and the protection of the implant from the torque effect.  These recommendations are consistent with our study in which a decrease in the stress formed around the implant was observed in the Model 1 TIFP design. In this study, the NRC placed adjacent to the tooth (Model 2) increased the stress around the implant along Lines 3and 4. The reason may be the additional movement of the pontic under occlusal forces which causes the bridge to produce a cantilever effect on the implant. Thus, the NRC placed on the tooth site has got more disadvantages than the fixed prosthesis that is rigidly connected.
| Conclusion|| |
The incorporation of an NRC on the distal side of the natural tooth or mesial side of the implant abutment reduces the stress around the natural tooth in a TIFP design. The NRC positioned on the mesial side of the implant reduces the stress transmitted to the implant, but increases the stress around the implant when the NRC is positioned on the distal side of the natural tooth. A greater level of stress was observed around natural teeth and the implant when the TIFP was rigidly connected. From the study, it could be suggested that if natural teeth and implants are used as support for fixed prosthesis, the NRC should be placed on the implant-supported site to reduce the load on the implant and natural teeth.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2]