Year : 2010 | Volume
: 21 | Issue : 1 | Page : 59--62
Comparison of stress patterns and displacement in conventional cantilever fixed partial denture with resin bonded cantilever fixed partial denture: A finite element analysis
E Prashanti1, S Sajjan2, M Kumar3,
1 Department of Prosthodontics, Manipal College of Dental Sciences, Mangalore, Karnataka, India
2 Department of Prosthodontics, Vishnu Dental College, Vishnupuram, Bhimavaram, Karnataka, India
3 Department of Mechanical Engineering, Bapuji Institution of Engineering and Technology, Davangere, Karnataka, India
Department of Prosthodontics, Manipal College of Dental Sciences, Mangalore, Karnataka
Aim: This study aims to analyze the stress patterns and displacement in the cantilever resin bonded fixed partial denture (RBFPD) and compare it with the conventional cantilever fixed partial denture using 3-D finite element analysis. Also, the effect of cement on the displacement and stress patterns in conventional cantilever fixed partial denture was to be analyzed.
Materials and Methods: Three-dimensional models were prepared layer wise to depict the conventional cantilever and the cantilever RBFPD. Once the models were made, the material properties were assigned and divided into three groups. (2-conventional cantilever with resin cement, 1- conventional cantilever with GIC cement and 3-resin bonded cantilever with resin cement). Load was applied in vertical as well as lateral directions and the stress patterns along with displacement were analyzed.
Results: The results revealed that the von Mises stresses in all the three groups were found to be almost equal under vertical loading. Under lateral loading, the stress was more in cantilever RBFPD. Displacement in all the three axes was significantly less in the cantilever RBFPD.
Conclusion: Stress concentration in the lateral direction in cantilever RBFPD was found to be higher than the cantilever conventional group. Displacement in X, Y and Z axes was less in cantilever RBFPD.
|How to cite this article:|
Prashanti E, Sajjan S, Kumar M. Comparison of stress patterns and displacement in conventional cantilever fixed partial denture with resin bonded cantilever fixed partial denture: A finite element analysis.Indian J Dent Res 2010;21:59-62
|How to cite this URL:|
Prashanti E, Sajjan S, Kumar M. Comparison of stress patterns and displacement in conventional cantilever fixed partial denture with resin bonded cantilever fixed partial denture: A finite element analysis. Indian J Dent Res [serial online] 2010 [cited 2021 Mar 9 ];21:59-62
Available from: https://www.ijdr.in/text.asp?2010/21/1/59/62797
Resin bonded fixed partial dentures (RBFPD) have had a variable popularity since the technique for splinting mandibular anterior teeth with a perforated metal casting was described by Rochette in 1973.  Unquestionably, the major disadvantage of the conventional fixed partial denture i.e., the extensive preparation of the tooth structure, is overcome with the advent of conservative RBFPD.  Ever since the introduction of these minimal preparation fixed partial dentures, the major concern of the clinicians has been its longevity. De-bonding caused by complex multidirectional interabutment stresses was considered the major cause of failure of 3-unit RBFPD. ,, This failure led to the increase in popularity of the two-unit cantilever RBFPD which produce less complex forces on the prosthesis and single abutment, there by reducing the stresses, that leading to de-bonding. The purpose of the present study was to analyze the stress patterns and displacement in the cantilever RBFPD and compare it to the cantilever conventional fixed partial denture using 3-D finite element analysis (FEA). To analyze the role played by the cement, the conventional cantilever FPD was tested with GIC as well as resin cements.
Materials and Methods
Preparation of the FEA model was done using two typhodont teeth, which were selected to simulate a condition where the second premolar had to be replaced using the first molar as an abutment [Figure 1]. Tooth preparation for the conventional group (complete coverage) was done using the principles given by Schillingburg.  The design for the cantilever RBFPD (Partial coverage) was selected according to the design principles prepared by Botelho  [Figure 2]. The retainers, along with the pontics, were prepared using transparent self cure acrylic [Figure 3] and [Figure 4] and were embedded in pink self cure acrylic block 1 mm cross-sections were made using a microtome. The sections obtained were scanned, enlarged four times and plotted against a graph sheet. This data was fed into the NISA II software for the model preparation. Using nodes and elements, the 3-D models were built layer wise to depict the conventional cantilever and the cantilever RBFPD [Figure 5] a and b.
Once the models were made, the material properties were assigned so that the models closely simulate the natural structures. The material properties were assigned using the Young's modulous and Poisson's ratio of each material  [Table 1]. Three groups were formed. Group I: Conventional cantilever FPD with GIC cement; Group II: Conventional cantilever FPD with resin cement; Group III: Resin bonded cantilever with resin cement.
After assigning the material properties to the 3-D models, a load of 300 N was applied on the pontic. The load was applied in both vertical as well as lateral directions (with 30° inclination) and the results obtained in each of the models were compared.
The results were analyzed based on two parameters:
von- Mises stressesDisplacement in X, Y and Z-axesThe maximum values were obtained [Table 2]; the von Mises stresses in all three groups were found to be almost equal under vertical loading. Under lateral loading, the stresses were more in cantilever RBFPD. Stress concentration was below the connector region for all the three groups, under vertical as well as lateral loading. Displacement in all the three axes was significantly less in the cantilever RBFPD. The displacement in the vertical direction (i.e., direction of principal load application) was more in the distal aspect of the retainer for all the three groups.
With the advent of newer preparation designs, adhesive cements and surface treatment, the longevity of the RBFPD increased considerably.  In fact, according to a study conducted by Thompson et al. the longevity of RBFPDs is comparable to that of the conventional FPDs.  According to various reviews, two unit RBFPDs showed better longevity than its fixed counterparts. An interesting fact is that most unilaterally fractured 3-unit resin bonded restorations remained in function as cantilever RBFPDs for five or more years. 
This was attributed to the complex multidirectional interabutment stresses associated with the 3-unit design. When masticatory forces are applied on the 3-unit RBFPD, the abutments are subjected to twisting forces thus causing enormous stresses within the interface between the tooth and the FPD.  With every additional abutment the chance of de-bonding increases. With only one abutment present, theoretically the twisting forces are reduced to a minimum, thus preventing early debonding. , Evidence-based information supports the clinical performance of 2-unit cantilever RBFPDs and a success rate of 92.3% for five years has been reported. 
Among the different methods of stress analysis, the 3-D finite element stress analysis was selected, as this method allows close simulation of the components of the dental prosthesis under investigation.  The preparation design for the cantilever RBFPD was selected according to the design principles by Botelho who suggested a more rigid D-shaped design.  According to Botelho, the conventional C-shaped retainers were more flexible which led to their easy de-bonding. Connecting the free ends of the retainer with an occlusal isthmus, gave a more rigid D-shape design. Hence this design was selected for the study.
A vertical load of 300 N was applied as the masticatory forces on a premolar tooth are approximately 65 lb (300 N). The models were subjected to a vertical load to depict the forces perpendicular to the occlusal surface. The models were also subjected to an inclined load at 30° as the normal cuspal inclination of a premolar was found to be 30°. To analyze the role played by the cement the conventional cantilever FPD was tested with GIC as well as resin cements.
The stress concentration (vertical direction) in the cantilever RBFPD was almost equal to that of the conventional cantilever group irrespective of cement used [Table 2] [Figure 6]. This supports the principles of engineering quoted in theories of elasticity by Thimoshenko and Tata Magroil which states that when forces are applied on the free end of the cantilever i.e., the pontic, the stresses developing in the connector region will remain same whether the fixed end of the retainer is fully covering or partially covering the tooth structure. It was also observed that the von-Mises stresses were higher in the cantilever RBFPD in the lateral direction [Table 2]. This observation can be attributed to the less stress distribution due to the partial coverage of the retainer. The stress concentration can be controlled by minimizing the occlusal contacts on the pontic in such a way that centric contacts exist only on maximum biting force and eliminating the eccentric contacts completely.
Minimizing the occlusal contacts on the pontic automatically reduces the stresses in the connector and the retainer. The second observation was that the cantilever RBFPD showed significantly less displacement when compared to the conventional cantilever group [Table 2] [Figure 7]. This is an important factor for clinical longevity. Most RBFPDs de-bond due to development of the shear stresses in the retainers. When displacement is more, the shear stresses are more, which increases chances of de-bonding. But, as we observed in the cantilever RBFPD, displacement is less in all three axes; hence shear forces can be assumed to be less thereby reducing the chances for debonding.  The displacement might have been less due to various factors in favor of cantilever RBFPDs.
In the conventional cantilever design, the entire tooth is prepared and a significant taper is created. Whereas in the cantilever RBFPD, minimal tooth preparation with a minimal taper allows an intimate contact of the retainer to the tooth structure, exhibiting less displacement.
As we know, the bonding to enamel is much higher than the dentine bonding due to more inorganic content and homogenous nature of enamel. The presence of dentinal tubules with the dentinal fluid and the formation of smear layer make dentine bonding less reliable. As the bonding in cantilever RBFPD is to the enamel, displacement might have been less.
As the occlusal morphology is not much altered in the cantilever RBFPD, the contacts on the retainer are minimal. In the conventional group, however, the entire tooth is prepared and the occlusal morphology is simulated in the retainer. And due to the full coverage, contacts on the retainer are more, leading to possibility of more displacement.
The following conclusions were drawn from the study:
Stress concentration in the lateral direction was found to be higher in the cantilever RBFPD than the conventional cantilever group. Displacement in X, Y and Z axes was less in cantilever RBFPD, which is of significant value for increasing the clinical longevity. When forces are applied on the free end of the cantilever i.e., the pontic, the stresses developing in the connector region remain same, regardless of whether the fixed end in the retainer is fully or partially covering the tooth structure.
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