Compression of stress distribution in pull out and push out bond strength test set ups: A 3-D finite element stress analysis.

Transcription

1 ORIGINAL RESEARCH Compression of stress distribution in pull out and push out bond strength test set ups: A 3-D finite element stress analysis. Leila Ahmadian 1 DDS, MSc, Rasoul Arbabi 2 DDS, MSc, Jamal Kashani 3 1, 2 Assistant professor, 3 Mechanical engineer, Department of prosthodontics, Zahedan University of Medical Science, Zahedan, Iran Abstract Objectives: The purpose of this study was to compare the stress distribution in pull out and push out test set ups using finite element analysis, and to highlight some parameters that might have important influence on the results. Methods: Three dimensional finite element analyses were performed using Solid Works D CAD software. Model configurations were based on published experimental pull out and push out test set ups; therefore, in the push out test set up three different thicknesses of root slices (1, 2 and 3 mm) were modeled. Vertical tensile and compressive loads were applied in pull out and push out set up, respectively. COSMOS Works 2005 software was used to compute the local Von Mises stresses for each of the models at the interface area. Then the patterns of stress distribution in studied models were compared. Results: Stress distribution was non-uniform in pull out model and pronounced stress concentration was observed at cervical one-third of the root and cement layer. In 1mm push out model, stress was distributed uniformly but by increasing the thickness of the root segment, stress concentrated at the upper end of the interface that was near to the loading point. Significance: Highly non-uniform stress may develop at the adhesive interface when the pull-out test is performed on entire post or the push out test is performed on thick root segments. Therefore, 1 mm root segments were preferred to achieve true bond strength measurements. Keywords: Bond strength, fiber post, finite element analysis, stress distribution, pull out test, and push out test.. Leila Ahmadian, Rasoul Arbabi and Jamal Kashani.. Compression of stress distribution in pull out and push out bond strength test set ups: A 3-D finite element stress analysis. International Journal of Prosthetic Dentistry2013:4(1): International Journal of Prosthetic Dentistry. Published by Publishing Division, Celesta Software Private Limited. All Rights Reserved. Received on: 30/07/2012 Accepted on: 22/103/2013 Introduction Nowadays, adhesive restorative dentistry is advanced and new prefabricated posts are developed at a very rapid pace; therefore, in vitro mechanical tests are becoming of utmost importance to evaluate and to compare the bond strength of these posts to endodontically treated teeth. The in vitro methods used for evaluation of this bond strength vary from laboratory to laboratory. It is, therefore, often impossible to compare results of different researchers or manufactures 1. This has led to confusion and probably delayed the development of more effective systems. One reason for this contradiction is that there is no standard test set up and most of these tests can not often control the many variables that are present 2. The nature of stress distributions along with tested interfaces is an important variable that can cause a considerable scatter in the reported results. In addition, the other problems related to the validity of obtained measurements are cohesive failures in the substrate that are frequently observed with new adhesives that yield improved bond strengths, because the bond strength is the force per unit area required to break a bonded assembly with failure occurring in or near the adhesive interface 3,4. Although some authors believe that the bond strength of the interface in this situation had 2013 International Journal of Prosthetic Dentistry. Published by Publishing Division, Celesta Software Private Limited. All Rights Reserved Page 1

2 surpassed the cohesive strength of the substrate and no further need for improvement of it 5-7, the actual conclusion was that this test had turned out to be suitable to determine the true strength of a bonded interface 8, 9. According to Della Bona and van Noort 10, the real explanation for this fact was that stresses were mostly concentrated in the substrate, thus causing its premature failure prior to the interface itself. The most commonly employed test set-ups for this purpose are pull out and push out tests that are detailed in Figure 1. In the first method a tensile load dislodged a post from post space. It has been suggested that a highly non-uniform stress may develop at the adhesive interface when the pull-out test is performed on the whole post 21. But in the former method, tensile load is replaced by compressive load and a thin slice from root is prepared and tested. This test was first described to be used in dentistry in but in 1991, it was reintroduced by Haller et al. 23. This test would allow for testing small areas, thus permitting a regional mapping or depth profiling of different substrates, and preparing multiple specimens from a same tooth. In different studies 16-20, different thicknesses of specimens were used and there is no data about the best thickness. Considering the importance of knowing what a test is actually measuring to ensure proper interpretation of obtained data, finite element analysis becomes of great usefulness to elucidate the stress states found in bonded interfaces and provide information on the nature of stresses generated within the structure. The results of a FE analysis are expressed as stresses distributed in the structures under investigation. These stresses may be tensile, compressive, shear, or a combination known as equivalent Von Mises stresses. Von Mises stresses depend on the entire stress field and are a widely used indicator of the possibility of damage occurrence 24. FE analysis has been performed with 2- dimensional (2-D) and 3-dimensional (3-D) models, and the consensus is that the results obtained with 3-D models are more valid 25, 26 but also are more time-consuming and costly than 2-D models. Based on these considerations, the first objective of the present study was to comparatively analyze the stress distribution in pull out and push out bond strength test set-ups by 3-D models. The second objective of this study was to investigate the effect of thickness of the specimen in the push out test set up that may have an important influence on the results of the test. Materials and methods A three dimensional (3D) model of a maxillary second premolar was created from CT Scan data of a patient that had been made for another purpose. The images were obtained in 0.6 mm slice interval. The CT Scan images were converted to a 3D object by means of Mimics Software (Mimics, Materialise, Leuven, Belgium). Then the resulting object exported to a 3D parametric solid modeler (SolidWorks 2005, SolidWorks Corporation, USA) for simulation. The length of simulated root was 13.5 mm. The diameter of the root at the cervical end was 4.9 mm mesiodistally and 7.9 mm in bucoligually aspects. A carbon fiber post was modeled in post space. The simulated post in the post space was conical with a round tip. Its length was 9 mm that was equal to two thirds of the root. The end of the canal filled with Gutta perca. The diameter of the post was 1.1mm that was equal or less than one third of the radicular diameter. A 0.05mm thick adhesive layer (resin cement) was modeled for luting of the post in the post space. The model was surrounded with a cube that was extended 7mm beyond the 2013 International Journal of Prosthetic Dentistry. Published by Publishing Division, Celesta Software Private Limited. All Rights Reserved Page 2

3 root in all directions. The simulated material for mold was a self cure acrylic resin. The elastic modulus and Poisson s ratios were based on typical values that can be found in the literature (27-31), and can also be seen in Table 1. Material properties were assumed to be isotropic, homogeneous, and linear-elastic. In the pull out set up, a tensile load was applied to the coronal end of the post. It was parallel with the long axis of the root. In the push out set up, the root was cut horizontally to produce 1 and 2, 3 mm-thick segments. In this set up the post was loaded with a 0.9-mm diameter cylindrical plunger. The plunger tip was sized and positioned to touch only the post, without stressing the surrounding post space walls. Material Modulus Poisson s (GPa) ratio Dentin (27) Gutta-percha (27) Resin luting agent 8.0 (28) 0.30(29) Carbon fiber dowel (30) Acrylic resin (31) (Table I. List of elastic constants of investigated materials) The load was applied on the apical aspect of the root slice and in an apicalcoronal direction, to push the post toward the larger part of the root slice, thus avoiding any limitation to the post movement owing to the post space tapering (Figure 1A & 1B). 1A 1B (Figure1. 3D-FEM models and loading conditions. 1A Pull out test set up and 1B push out test set up) After finishing model creation, the created model was exported to a finite element analyzing software (Cosmos Works 2005, Solid Works Corporation) for analysis. The pull model was meshed by elements. In the push out set up, 1, 2 and 3 mm thick models were meshed by , and elements, respectively. Before starting analysis, boundary condition was defined. For all models, loading was arranged to achieve the same arbitrary nominal stress (shearing load/cross-sectional area) of 4MPa (32). These models were used as approximation for what actually specimens are that were used in the in vitro studies. Von Mises (equivalent stresses) energetic criterion was chosen to present the areas of highest stress concentration where possible failure is more likely to occur. Results In order to analyze stress location and distribution, the root and adhesive layer were isolated from the rest of the model. A cross section enabled stresses to be observed. Figures 2 5 show the stress distribution in the analyzed models. Regardless of the model, the greatest stress was observed in the adhesive layer. Analysis of the stress distribution reveals that the most intense stresses appeared in the peripheral wall of the post space (postdentin interface) International Journal of Prosthetic Dentistry. Published by Publishing Division, Celesta Software Private Limited. All Rights Reserved Page 3

4 International Journal of Prosthetic Dentistry.2013:4(1):1-8. Pull out: Figure 2 A and B shows stress distribution for pull out model at dentin and adhesive layer. A non-uniform stress distribution at dentin - adhesive interface was verified. Starting from the cervical end of the root, a decrease in stresses can be noticed, followed by a sharp decrease in middle third. Maximum Von Mises stresses occurs at the level of the cervical region of the root. No stress values are recorded in the apical third of the root. Stresses arising at interface area progressively decrease toward the peripheral. 2A a 2Ba (Figure. 2. Distribution of Von Mises stresses (MPa) in pull out test set up. (2A) Pattern of stress distribution in root dentine. (2B) Pattern of stress distribution in adhesive layer) Push out Figure 3 A and B shows stress distribution in the dentin and adhesive layer of 1mm root slice model. Pattern of stress distribution is uniform along the adhesive interface of this model. In this model, stresses progressively decrease from center toward upper and lower surfaces, but maximum Von Mises stresses occurred at ISSN no the most of the interface area. 3A a (Fig.3. Distribution of Von Mises stresses (MPa) in push out test set up with 1 mm root slice. (3A) Pattern of stress distribution in root dentine. 3B (3B. Pattern of stress distribution in adhesive layer.) Figure 4 A and B shows stress distribution in the dentin and adhesive layer of 2mm root slice model. The maximum Von Mises stresses are primarily located at the upper half of the root slice. Stress decreases progressively from this point to lower surface that is farther from loading area. Pattern of stress distribution is comparable to those recorded in 1 mm model, but in this model stress distribution is not as uniform as 1mm model. 4A a (Fig.4 Distribution of Von Mises stresses (MPa) in push out test set up with 2 mm root slice. 4A. Pattern of stress distribution in root dentine.) 2013 International Journal of Prosthetic Dentistry. Published by Publishing Division, Celesta Software Private Limited. All Rights Reserved Page 4

5 International Journal of Prosthetic Dentistry.2013:4(1):1-8. ISSN no B (4B. Pattern of stress distribution in adhesive layer.) Figure 5 A and B shows stress distribution in dentin and adhesive layer of 3mm root slice model. Starting from the load area, a decrease in stresses can be noticed, followed by a sharp decrease in lower third of specimen. A low stress concentration region is evident at the middle third of the model. No stress values are recorded in the lower third of the root slice. Pattern of stress distribution is comparable to those recorded in pull out model. 5A (Fig. 5 Distribution of Von Mises stresses (MPa) in push out test set up with 3 mm root slice. 5A. Pattern of stress distribution in root dentine) 5B (5B. Pattern of stress distribution in adhesive layer.) Discussion In the present work, the FEA method was used to investigate the stressdistribution resulting from different test set ups within the dowel-restored tooth. FE analysis is a powerful tool for calculating stress distributions in complex structures. As stated earlier, this method provides results without variation. The validity of this method, however, depends on the extent to which the model approaches reality. The models used in this study deviated from reality in several respects. In reality, the tooth is not axis symmetrical; the dimensions of naturally restored teeth deviated from CT scan of patient, and the same is true with the mechanical constants involved. In the present study, a three dimensional model was used, and thus a more accurate picture of stress distribution was observed. When a complex of materials such as dowel restored tooth is loaded, stress concentrates where a non-homogeneous material distribution is present, such as the interface regions. The interfaces of materials with different elastic modulus represent the weak point of a restorative system, as the toughness/ stiffness mismatch influences the stress distribution 33, 34. Thus, every effort should be made to reduce the stress that was concentrated in the interface area. Several in vitro studies have determined the bond strength of post to dowel restored teeth under static tensile or compressive loading and the conflicting results demonstrate the difficulty in standardizing this type of in vitro experiment. Therefore, uniform pattern of stress distribution in the interface is a property that may be highly beneficial to diminish the variance on the bond strength estimated from tests. Based on the result of this study the pattern of stress distribution in pull out bond strength test indicates that stress concentrates at the cervical end of the root International Journal of Prosthetic Dentistry. Published by Publishing Division, Celesta Software Private Limited. All Rights Reserved Page 5

6 Therefore, there are more possibilities of occurring cohesive failure in upper part of the root when load is applied and recoded bond strength grossly underestimates the real stress that specimen could tolerate. Some authors suggest that stress distribution in root can be improved; also, the extension of high stress region can be prevented by sectioning the root to thinner slices. In this manner, stress is distributed more evenly in root, high stress region extension increases and the incidence of underestimated bond strength would decrease. But, some authors believe that stress distribution of this set up is non-uniform 15 and this can be because of the different thickness of root slices was used in the different push out studies. Therefore, three thicknesses of root slices (1, 2 and 3 mm) used for analyzing of stress distribution in this study. According to the result of this study non-uniform stresses were created in 2 and 3 mm slices and only in 1-mm model, stress distributed uniformly; thus, the results are in agreement with both groups. Because, the loading of a thick specimen can concentrate stress in limited area and create the data variability. This limitation of the original push-out technique can be solved by slicing the root into 1-mm thick specimens. In this manner stress distributes more uniformly and the recorded data are more real. According to Van Noort et al. 32, in every bond strength test set up, in which stresses distributed in non-uniform pattern, the measured bond strength had little relationship to the actual bond strength. thus, it should be noted that the push out test using the 1mm specimens can reduce some differences between the measured bond strength and the actual bond strength. In another study, Goracci et al. 19 compared a micro-tensile technique with a push out test for the ability to accurately measure the bond strength of fiber posts luted inside post spaces. The authors concluded that a push-out test was more dependable than a micro-tensile technique and data variability was small. It should be noted that in that study, 1 mm root slices were used; therefore, the result of this study verify their in-vitro study. The results of our study confirm the importance of the bond strength test set up for assessment of the reliable bond of the post to dowel restored tooth. In the light of this study s findings it can be concluded that, when measuring the bond strength of fiber posts adhesively luted to root canal dentin, the push-out test done on the 1mm slice appears to be more efficient and dependable than pull out set up and push out set up done on the thicker slices. This set up provides a better estimation of the bond strength than the others, but further clinical studies should be performed to validate the experimental model presented in this study and to confirm the data obtained. Conclusions In conclusion, by using 3D FEA, bond strength values of post to dowel restored tooth may vary with test set up and not necessarily due to the actual adhesive bond strength at the interface. This excellent numerical model (FEA) showed the importance of the standardization of the bond strength test set-up. This study verified that thickness of root slice was one of the important parameters in push out tests that affected stress distribution. Acknowledgments This study was supported by the Research Fund of the Zahedan University of medical science. Project number is References 1. Retief DH, O'Brien JA, Smith LA, Marchman JL. In vitro investigation and evaluation of dentin bonding agents. Am J Dent Sep;1 Spec No: International Journal of Prosthetic Dentistry. Published by Publishing Division, Celesta Software Private Limited. All Rights Reserved Page 6

7 2. Oilo G. Bond strength testing--what does it mean? Int Dent J Oct;43(5): ISO/TC106/Subcommittee. CD TR Guidance on testing of adhesion to tooth structure. International organization for standardization, Geneva, Switzerland. 4. ISO 6345: 1982 Adhesion Vocabulary. International organization for standardization, Geneva, Switzerland. 5. Davidson CL, Abdalla AL, De Gee AJ. An investigation into the quality of dentin bonding systems for accomplishing a durable bond. J Oral Rehabil 1993; 20: Van Meerbeek B, De Munck J, Yoshida Y, Inoue S, Vargas M, Vijay P, et al. Adhesion to enamel and dentin: current status and future challenges. Oper Dent 2003; 28(3): Versluis A, Tantbirojn D, Douglas WH. Why do shear bond tests pull out dentin? J Dent Res 1997; 76(6): Tantbirojn D, Cheng Y-S, Versluis A, Hodges JS, Douglas WH. Nominal shear or fracture mechanics in the assessment of composite-dentin adhesion. J Dent Res 2000;79(1): Phrukkanon S, Burrow MF, Tyas MJ. Effect of cross-sectional surface area on bond strengths between resin and dentin. Dent Mater 1998; 14(2): Della Bona A, Van Noort R. Shear versus tensile bond strength of resin composite bonded to ceramic. J Dent Res 1995; 74(9): Jacqueline P. Duncan, Cornelis H. Pameijer. Retention of parallel-sided titanium posts cemented with six luting agents: An in vitro study. J Prosthet Dent 1998;80: K.W. Chapman, J.L. Worley, J.A. von Fraunhofer. Retention of prefabricated posts by cements and resins. J Prosthet Dent 1985;54: Jon P. Standlee, Angelo A. Caputo. Endodontic dowel retention with resinous cements. J Prosthet Dent 1992;68: Drummond JL. In vitro evaluation of endodontic posts. Am J Dent 2000; 13: Mitghell CA, Orr JF, Connor KN, Magill JPG, Maguire GR. Comparative study of four glass ionomer luting cements during post pull-out tests. Dent Mater 1994;10: Patierno JM, Rueggeberg FA, Anderson RW, Weller RN, Pashley DH. Push-out and SEM evaluation of resin composite bonded to internal cervical dentin. Endod Dent Traumatol 1996; 12: Boschian Pest L, Cavalli G, Bertani P, Gagliani M. Adhesive post-endodontic restoration with fiber posts: pushout tests and SEM observations. Dent Mater 2002; 18: Gallo JR, Miller T, Xu X, Burgess JO. In vitro evaluation of the retention of composite fiber and stainless steel posts. J Prosthodont 2002; 11: Goracci C, Tavares AU, Fabianelli A, Monticelli F, Raffaelli O, Cardoso PC, et al. The adhesion between fiber posts and root canal walls: comparison between microtensile and push-out bond strength measurements. Eur J Oral Sci 2004;112: International Journal of Prosthetic Dentistry. Published by Publishing Division, Celesta Software Private Limited. All Rights Reserved Page 7

8 20. Boff LL, Grossi ML, Prates LH, Burnett LH Jr, Shinkai RS. Effect of the activation mode of post adhesive cementation on pushout bond strength to root canal dentin. Quintessence Int. 2007;38(5): Gallo JR, Miller T, Xu X, Burgess JO. In vitro evaluation of the retention of composite fiber and stainless steel posts. J Prosthodont 2002;11: Roydhouse RH. Punch-shear test for dental purposes. J Dent Res 1970;49: Haller B, Thull R, Klaiber B, Teuber M. An extrusion test for determination of bond strength to dentine. Journal of Dental Research (Special Edition) 1991; 70: 525. (Abstract 2070). 24. Pegoretti A, Fambri L, Zappini G, Bianchetti M. Finite element analysis of a glass fibre reinforced composite endodontic post. Biomaterials 2002; 23: Ko CC, Chu CS, Chung KH, Lee MC. Effects of posts on dentin stress distributions in pulpless teeth. J Prosthet Dent 1992; 68: Holmes DC, Diaz-Arnold AM, Leary JM. Influence of post dimension on stress distribution in dentin. J Prosthet Dent 1996; 75: Ferrari M, Vichi A, Garcia-Godoy F. Clinical evaluation of fiber-reinforced epoxy resin posts and cast post and cores. Am J Dent 2000; 13:15B-8B. 28. Attar N, Tam LE, McComb D. Mechanical and physical properties of contemporary dental luting agents. J Prosthet Dent 2003; 89: Ausiello P, Apicella A, Davidson CL. Effect of adhesive layer properties on stress distribution in composite restorations a 3D finite element analysis. Dent Mater 2002;18: Necdet Adanir, Sema Belli. Stress Analysis of a Maxillary Central Incisor Restored with Different PostsEur J Dent 2007;2: T. Kawasaki, Y. Takayama, T. Yamada, K. Notani. Relationship between the stress distribution and the shape of the alveolar residual ridge three-dimensional behaviour of a lower complete denture.j Oral Rehab 2006; 33(10): Van Noort R, Noroozi S, Howard IC, Cardew G. A critique of bond strength measurements. J Dent 1989;17(2): Ausiello P, Gee AJ, Rengo S, Davidson CL. Fracture resistance of endodontically treated premolars adhesively restored. Am J Dent 1997; 10: Assif D, Gorfil C. Biomechanical considerations in restoring endodontically treated teeth. J Prosthet Dent 1994; 71: Address of correspondence Rasoul Arbabi, Department of Prosthodontics, Faculty of Dentistry, Zahedan University of Medical Sciences, Zahedan Iran. Phone: Fax: rasoularbabi@yahoo.com No conflict of interest reported 2013 International Journal of Prosthetic Dentistry. Published by Publishing Division, Celesta Software Private Limited. All Rights Reserved Page 8