| Abstract|| |
Aim: This study aims at evaluating stresses generated in a fixed tooth to implant connected fixed partial denture (FPD) by finite element method by varying implant design and position, in unilateral distal extension cases, i.e. for replacing mandibular 1 st and 2 nd molars.
Materials and Methods: Four models were created, Model 1-A finite element model (FEA) model of a crestal implant-tooth connected metal ceramic FPD with pontic in 36 region. The 35 region simulated the second premolar. Model 2-A FEA model of a basal implant-tooth connected metal ceramic FPD. Model 3-A FEA model of a crestal implant in 36 region connected to tooth 35 and cantilevered posteriorly with pontic in 37 region, made of metal ceramic. Model 4-A FEA model of a basal implant in 36 region connected to tooth 35 and cantilevered posteriorly with pontic in 37 region, made of metal ceramic. A vertical force of 100N directed evenly on the nodes on the occlusal surface of each of the three crowns. The models displayed stress both numerically and by color coding.
Results: On comparing models 1 and 2, the model 1 showed comparatively more stresses in crestal bone area of the implant. On comparing models 3 and 4, model 3 showed lesser von misses stress values.
Conclusion: In case of tooth implant connected FPDs without any cantilevers, basal implants show better stress distribution when compared to crestal implants. In case of cantilever designs, crestal implant design showed lower stress values, but the difference in stresses were less.
Keywords: Basal implant, crestal implant, finite element analysis, stress distribution, tooth to implant connection
|How to cite this article:|
Pratheep K V, Abraham A, Annapoorni H, Vigneshwaran S. Comparative evaluation of stresses in tooth implant connected fixed partial denture by varying the implant design and position: A 3D finite element study. Indian J Dent Res 2013;24:439-45
A variety of prosthetic techniques can be used to restore the dentition subsequent to loss of teeth. The method of rehabilitation depends on the number, arrangement, and status of residual teeth, cost, patient desires, and adequacy of the bone to support dental implants. Although, original protocol initially designed was for treatment of completely edentulous patients, the need arose to extrapolate treatment alternatives with osseointegerated implants to partially dentate patients. In such cases, a controversy has developed regarding whether implants should be connected to natural abutments or be self-supporting. 
|How to cite this URL:|
Pratheep K V, Abraham A, Annapoorni H, Vigneshwaran S. Comparative evaluation of stresses in tooth implant connected fixed partial denture by varying the implant design and position: A 3D finite element study. Indian J Dent Res [serial online] 2013 [cited 2019 Jul 19];24:439-45. Available from: http://www.ijdr.in/text.asp?2013/24/4/439/118388
A number of clinical situations and cost factors may necessitate connecting teeth to implant to replace missing teeth.  The dissimilar mobility patterns of the osseointegerated implants and natural teeth make the biomechanical behaviour of the entire system complicated.  An osseointegerated implant is rigidly fixed to bone and can move only 10 μm in the apical direction, whereas teeth with healthy periodontal ligaments can move 25-100 μm.  This movement disparity may cause relative motion of the implant-tooth superstructure when the splinted system is loaded by occlusal force. During loading, higher bending moment induced by the mismatch between the implant and tooth may result in abutment screw loosening or fracture of implants or prosthesis. Loss of osseointegration and increased marginal bone loss may also occur around the implant as a result of overload.  For this difference in mobility, non-rigid connectors were suggested, but designs with non-rigid connectors showed greater incidence of natural teeth abutment intrusion when compared to rigid connectors. 
If there is a difference in stress transmission pattern along the implant to better dissipate these bending forces on the tooth side of the bridge, problems like marginal bone loss, loss of osseointegration, implant fracture etc, can be overcome. Crestal and basal implants are endosseous, aid to create osseointegrated points of retention. These two types of implants are not only differentiated by the way they are inserted but also by the way the forces are transmitted.  The biomechanical aspect of Implant supported restorations are currently under investigations and a number of in vitro studies have been attempted to predict their behavior. Finite element model analysis (FEA) is one of the in vitro methods of analyzing stress distribution. 
FEA has been viewed as the most suitable stress analysis tool for analyzing and predicting stress distribution in the interface between the implant components and the bone.  The finite element method provides a unique way of determining stress and displacements because of its ability to model the geometrical complex structure. In FEA, the complex structure is split up into a number of smaller problems. Each smaller sample is called as an element and the whole collection of elements was known as mesh. With the incorporation of material properties to the elements physical characteristics of the materials are considered. This study aims at evaluating stresses generated in a fixed tooth to implant connected denture by finite element method by changing the implant design and position, i.e. for replacing mandibular 1 st and 2 nd molars.
| Materials and Methods|| |
Four models were created in this study using SOLIDWORKS software and processed with ANSYS11 software.
A mandibular D-2 (Misch classification) bone model was simulated. The shape of the bone was simplified to a cuboidal block. The bone block was 24 mm in height, 30 mm in mesiodistal length, and 12 mm in buccolingual width. It consisted of a spongy center surrounded by 2 mm of cortical bone. The interface between the implant and bone was to be an immovable junction since FEA models assume a state of optimal osseointegration, meaning that cortical bone and trabecular bone are assumed to be perfectly bonded to the implant.
The model of single tooth simulated the premolar. The root was 13.0 mm, and the crown was 8.0 mm according to Wheelers Atlas More Details of Tooth Form. The periodontal ligament was 0.25 mm in width. A layer of cortical bone, 0.4 mm in thickness, was added between periodontal ligament and spongy bone. For simplicity, cement thickness was not included in the models. The elastic modulus and poisson's ratio of titanium,  spongy and cortical bones,  periodontal ligament and dentin,  and feldspathic porcelain  were obtained from the literature and listed in [Table 1] and [Figure 1]. All materials used in the models were considered to be isotropic, homogenous, and linearly elastic.
The four models created were:
A FEA model of a crestal implant (ITI Straumann solid implant 4.1 × 10 mm) (in 37 region)-tooth connected fixed prosthesis with a metal ceramic superstructure which was rigidly connected was simulated. The 35 region simulated the second premolar. Conventional preparation techniques were applied for the preparation of natural teeth and creation of metal ceramic restorations.
A FEA model of a basal implant (BS 10 H10 BOI implant in 37 region) to tooth connected fixed prosthesis with metal ceramic superstructure which were splinted rigidly was simulated.
A FEA model of a crestal implant of 4.1 × 10 mm diameter in 36 region connected to tooth 35 cantilevered posteriorly.ie cantilevered pontic in 37 region, made of metal ceramic.
A FEA model of a basal implant BS 10 H10 BOI implant in 36 region connected to tooth 35 cantilevered posteriorly, i.e. cantilevered pontic in 37 region, made of metal ceramic.
Nickel-Chromium (Ni-Cr) alloy was used as a metal substructure material in all the models. The connector was designed elliptical and a dimension of 2.5 mm. Meshing is done using HYPERMESH by giving a meshing command to the software. The structure, design of implant, tooth, pontic, bone and superstructures was processed using ANSYS11 software. A force of 100 N was applied to the models along the vertical axis and directed evenly on the nodes of the occlusal surface of each of the 3 crowns [Figure 2]. The models displayed stress both numerically and by color coding. The results obtained were interpreted.
| Results|| |
An occlusal load of 100 N was applied to the nodes on each of the three occlusal surfaces simultaneously for all the four models. [Table 2] illustrates the maximum von misses stresses (Vm) in the models and their locations and [Table 3] shows the maximum stresses and maximum displacement in various locations of the models. In Model 1, the crestal bone area of the implant which was the area of interest was 20.87 MPa. The maximum stresses in this model were distributed around the crestal area of the implant and the abutment prosthesis interface. In the model 2, the maximum Vm stress of the assembly was 28.37 MPa. The crestal bone area of the basal implant had a Vm stress of 12.16 MPa. The maximum stresses were seen in the base of the bone block and the area where the basal plate of the basal implant integrates with the bone.
|Table 3: Von misses stress and maximum displacement in various components of the models|
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In the Model 3, the maximum von misses stress for the assembly was 28.40 MPa. The maximum stress was seen in the prosthesis. The crestal bone area of the implant was 10.12 MPa and the crestal bone area of tooth has 22.7 MPa. In Model 4, the maximum stress for the assembly was 30.56 MPa. The crestal bone area of implant was 13.76 MPa and the crestal bone area of tooth was 24.71 MPa.
| Discussion|| |
According to the literature, the connection between teeth and implants must not be considered as the first alternative for rehabilitation, and it is preferable to adopt planning of isolated implant supported dentures. Nevertheless, in case of anatomic limitations that may require advanced surgical techniques at high costs or if teeth already require restorative interventions and are favorably distributed in the arch, combination between teeth and implants may be adopted with success rates similar to those of fixed implant-supported prosthesis. Nickenig HJ et al., 2006 examined survival rates and complications arising for fixed implant-tooth-supported prostheses. They assessed eighty-three patients treated at different clinics: 37.1% were posterior and mandibular (second lower premolar); 85% of implants were Branemark/Straumann; a third of prostheses were cemented and 26% telescopic; 39.3% were of three pieces and approximately one-third were of five pieces or more; a third used non-rigid connection. Ten percent of these implant-tooth-supported prostheses were subjected to some modification after five years and 13% after eight years. Rigid connections caused fewer problems and so required fewer modifications to the prosthesis (three out of 56, amongst rigid connections and eight out of 28 amongst non-rigid). There were no significant differences arising from the choice of implant system used. Not a single implant was lost out of a total of a 142 but three teeth were lost out of a 132 due to periodontal problems. At 5 years, 8% of teeth needed some kind of treatment and less than 1% of implants showed complications. It was concluded that with rigid connection, success rates for tooth-implant supported and implant-supported prostheses are very similar.
The main complications associated with tooth implant connected fixed partial dentures (FPD) are intrusion of the natural teeth, crestal bone loss, and screw loosening etc. Also, the long term success of the implant depends on the maintenance of osseointegration and marginal bone height. Marginal bone height depends on proper distribution of occlusal loads. Celso Hita-Carrillo et al., in their study concluded that rigid connection achieves better outcomes with regard to avoiding dental intrusion, although it will produce greater marginal bone loss. This greater marginal bone loss around the implant is due to the difference in movement of tooth and implant. Lenz et al., suggested that greater the stress in the marginal bone the greater the bone resorption. Menicucci G et al.,  2002 studied some biomechanical aspects based on 2D and 3D finite element analysis. In two dimensions, a 50 kg load per piece was applied for 10 seconds, concentrated on the implant collar and tooth apex. In three dimensions (3D), forces of 50 kg were applied for 5 milliseconds, distributed over the whole implant surface, although more to the implant collar and the cervical area of the tooth's alveolar bone. It was concluded that a prolonged static load endangers peri-implant bone more than alveolar bone so that it would appear that periodontal ligament plays a key role in stress distribution in tooth-to-implant connection.
Naert I et al., 2001 made a comparative clinical study of implant-supported prostheses and tooth-implant- supported two-part prostheses. Marginal bone loss for tooth-implant-supported prostheses throughout the study period was 0.7 mm greater than for implant supported prostheses. There were no significant differences in bone loss between non-rigid tooth-implant and implant-implant prostheses, but there were significant differences in bone loss, bone loss being greater for tooth-implant prostheses than for implant-implant. In this situation, the implant tends to bend to the side of the tooth, there by exerting more force on the crestal bone area causing bone resorption. Ihde et al., suggested that crestal and basal implants differ in their stress distribution pattern. Basal implants are primarily anchored at the baseplates within the cortical areas of the bone. A considerable part of the load is transmitted via the base plates. With basal implants, the regions of load transmission and the place of bacterial attack do not coincide; no masticatory forces need to be transmitted to the bone via vertical aspects of the implant; the positive retention in the bone is created in the cortical bone region.
Thus, the main objective of this study was to quantitatively evaluate the stresses in the crestal bone area in tooth implant supported FPD by changing the implant design. Commercially available two different implant designs are taken into consideration. One is conventional screw type of implant. The other is a single disc basal implant. The height of the crestal implant and the height of the shaft of basal implant were same. Four models were created using solid works software. Model 1-tooth crestal implant supported FPD, Model 2-tooth basal implant supported FPD, Model 3-tooth crestal implant supported FPD with a distal molar cantilevered, and Model 4-tooth basal implant supported FPD with a distal molar cantilevered. The bone segment was considered to be a cuboidal block for simplicity.
A uniform axial load of 100 N was applied on the nodes on the occlusal surface of each crown of the assembly. Maximum stresses in various components of the model were studied. Stresses were analyzed in various positions of bone, tooth, implant, and superstructures.
On comparing models 1 and 2, [Figure 3] and [Figure 4], the crestal implant to tooth connected FPD model has a lesser stress value for the entire assembly. But the areas of stress concentration in the Model 1 are around the crestal bone area which is considered to be vulnerable to resorption on concentration of higher stresses, whereas in Model 2, the stresses were transmitted to the base of the bone which are more resistant to resorption On comparing Models 3 and 4 [Figure 4] and [Figure 5], the maximum von misses stresses were almost similar. In cantilever design, the model with crestal implant has relatively lower stress level.
On comparing all the four models, similar to a number of studies in the literature cantilever designs had higher stress values than the tooth to implant FPD. The stress around the crestal bone area of tooth was seen maximum in both cantilever designs when compared to first two models. Stresses were transferred to the basal bone area in models with basal implant [Figure 6], whereas stresses were concentrated in the neck of the implant, crestal bone area in the crestal implant design. For basal implants, according to Ihde et al.,  the load-transmitting interface areas are located in the cortical bone, which has to perform structural tasks and therefore has a more pronounced self-preservation tendency.
| Conclusion|| |
Within limitations of this study following conclusions were drawn:
- In case of tooth implant connected FPD without any cantilevers, basal implants show better stress distribution when compared to crestal implants.
- In case of cantilever designs, crestal implant design showed lower stress values, but the difference in stresses were less.
- Maximum stresses were concentrated in the crestal bone area and neck of the implants in case of crestal implants, whereas stresses were transferred to the basal bone area in case of basal implants.
- If the end abutments are implant and tooth basal implants can be preferred, if the cantilever design is the treatment plan crestal implants can be the first choice.
- For tooth implant connected FPD, the denture span should be as short as possible, preferably not more than a three unit FPD.
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K V Pratheep
Department of Prosthodontics and Crown and Bridge, Meenakshi Ammal Dental College and Hospital, Meenakshi Academy of Higher Education and Research University, Chennai, Tamil Nadu
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
[Table 1], [Table 2], [Table 3]