Bovine dentin for dental implants: relationship between microstructure and failure mode
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https://www.eduzhai.net American Journal of Materials Science 2013, 3(6): 217-222 DOI: 10.5923/j.materials.20130306.04 Bovine Dentine for Dental Implants: Correlations between Microstructure and Failure Modes Karine T. A. Tavano1, Antonio F. Ávila2,*, Rudolf Huebner2 1Department of Dentistry, Universidade Federal do Vale do Mucuri e Jequitinhonha , Diamantina, 39100-000, Brazil 2Department of Mechanical Engineering, Universidade Federal de Minas Gerais, Belo Horizonte, 31270-901, Brazil Abstract The main objective of this paper is to discuss the mechanical behaviour and failure mechanisms of bio-posts made of bovine dentin for dental restoration. From bovine teeth, canine and incisors, posts were prepared. A set of twenty bio-posts were tested. Based on force-displacement diagrams and scanning electron microscopy (SEM) failure mechanisms were discussed at microstructure level. Two different failure mechanisms were identified, i.e. shear, and a mixed shear/ tensile, and correlated to the number of dentin tubules and age. The average bending strength (236 MPa) and stiffness (8.91 GPa) were close to the human dentin, which can be an excellent option for intraradicular dental post implants Keywords Biomaterials, Mechanical Properties, Microstructural Analysis, Failure Modes 1. Introduction According to Cheung et al., a biomaterial is a material used in implants or medical devices, designed to interact with biological systems. Among the various biological systems, one with special importance is the use of biomaterials for teeth restoration. In a healthy tooth, the distribution of occlusal forces occurs smoothly through the crown, root structure and periodontal tissue supporting the teeth (periodontal ligament fibers and bone). The structural modifications by endodontic treatment, as well as lateral forces, can lead to stress concentrations in a particular location of the tooth structure and can cause root or coronoradicular fracture. According to Knabe et al, titanium-based alloys were considered as biomaterial for bone implants, in special dental implants, due to their mechanical properties and tissue compatibility. As discussed by Aparicio et al., titanium-based dental implants are dependent on surface quality, which are directed related to mechanical and physicochemical properties. Moreover, the large difference on stiffness between titanium based alloys and bones and their adhesion were pointed as this biomaterial weakness. The work done by Qiu et al. tried to solve this problem applying electro-discharge compaction. This manufacturing process produced porous titanium posts, which lead to an increase on surface area and somehow control the alloy stiffness. Another important issue behind the porous surface is the bone growth stimulation and consequently improving * Corresponding author: email@example.com (Antonio F. Ávila) Published online at https://www.eduzhai.net Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved osseointegration. As discussed by Mendonça et al., osseointegration is considered as the key parameter to define the success of a dental implant. They even suggested that titanium-based alloys for dental implants with surface modification at nanoscale were a viable option for osseointegration. However, these materials retain their original stiffness, which is much higher than tissues surrounded. Another option of controlling stiffness was proposed by Watari et al.. To be able to reduce the titanium stiffness without losing the osseointegration property, they mixed different ratios of titanium and hydroxyapatite (Ti-HAp). A cold isostatic pressing followed by sintering under inert gas, i.e. argon, at 1300oC complete the Watari’s manufacturing process. Although, the biocompatibility was enhanced by the integration of hydroxyapatite, the hardness difference between the two components leads to weak regions and long term failure. This behaviour could be due to the difference on bone growth rate induced by these two materials. Lin et al also employed Ti-HAp, but a previous numerical simulation was employed to search for the best Ti/HAp composition. Although, their model predicted a good osseointegration, a practical design is not feasible now-a-days, and the problem remains unsolved. Moreover, the dental implant seems to be highly dependent on material properties and bone osseointegration. The implant is a viable option for a complete tooth replacement, where single tooth prosthesis is definitely placed into the patient’s mouth. When only the crown has to be replaced a different approach takes place. The use of pins or post into the root channel is the most common option. The placement of a pin within the channel is essential for crown retention, but it confers an additional risk. In this case, the occlusal load is transmitted directly 218 Karine T. A. Tavano et al.: Bovine Dentine for Dental Implants: Correlations between Microstructure and Failure Modes into the root, without the prior physiological damping performed by the structures of the teeth coronary healthy. Dallari and Rovatti believed that an endodontic pin, which had stiffness similar to that of dentin and secured to the root canal, would exert the function of a shock absorber. As a consequence the force would be assimilated by masticatory structures, which leads to a better stress distribution and thus decreasing the chance of fracture. Following the idea of equilibrated stiffness between posts and dentin, this study would like to investigate the mechanical behaviour of dental posts made of bio-composites (human and bovine dentin) when designed to support dental restorations. 2. Materials and Methods After the approval of the Research Ethics Committee and the animal Experimentation the bovine teeth were selected from a set of two years old bovines. Biocomposite posts were made from the roots of bovine and human incisors and canines. This research follows the standard protocol proposed by Attam et al., i.e. teeth were sterilized by seven days in 10% formalin solution, and kept in distilled water in all phases of research. From each bovine tooth, it was possible to obtain four biological posts. The crown portion was cut and discarded. The next step was cut the root portion with a carborundum disk at low speed refrigerated in its longitudinal axis, into four parts. Each root enables the bovine clipping four posts, but only two posts of each root were selected, one for each test (SEM and three point bending tests). A total of 20 posts with a diameter of 1.5 mm and a length of 15 mm were prepared for this investigation. The post diameter and length were based on commercial available post and the dimensions proposed by Seefeld et al. A mechanical characterization of composite posts was performed by Seefeld et al. by three point bending tests. Seefeld et al. tried to correlate the composite materials, in their case fiber reinforced plastics, with the failure mode. As the large majority of the posts were reinforced by unidirectional fiber glass (75%), the post shape played a key hole in their research. Although they were not able to identify the shape influence into their bending strength, this effect can be recognized if the critical cross sections are analysed. Kono et al. went further as they considered three different conditions, i.e. (i) Pre-fabricated fiber post alone, (ii) core composite resin with prefabricated fiber post, (iii) root reinforced with prefabricated fiber post. The work done by Schmage et al considered post with coronal retentive heads under bending test. They concluded that contrary to the expectations, the head addition did not improve the post yield stress, for both, titanium and fiber reinforced, post classes investigated. In all cases, the bending test seems to be the “standard” test for post mechanical characterization. In this investigation, the three point bending tests following ASTM D 790 were performed using a universal testing machine EMIC DL 10000 at constant head displacement of 0.5 mm/min. 3. Data Analysis and Discussion Dentin is considered a hard substrate, which has robustness to receive and dissipate masticatory forces. The restorative dentistry sciences have interest in such properties. Another key issue is the bio functionality, in other words, the material to replace dentin must have the ability of stress redistribution. A root post, which also retains the prosthetic crown, has the duty of receiving the masticatory loads; pass them to the remaining dentin, which dissipates them to the periodontal ligament and bone. Posts made of stiff materials are not able to equally redistribute the loads to dentin. Moreover, this load transfer is localized and it can cause large stress concentrations and consequently crack formation. As mentioned by Tang et al, stresses larger than the dentin ultimate strength will produce root fracture due to the localized stress concentration. Thus, it is important to know the pin stiffness and strength to anticipate possible problems after inserting the pin into the root canal. This is possible by knowing the stress-strain behaviour. Figure 1A shows the force-displacement curve for the post made of bovine teeth, while Table 1 summaries data obtained during the three point bending. To be able to have a basis for comparison a set of bio-posts made of human teeth was also tested and the results are shown in Fig. 1B and the results were also summarized in Table 1. Table 1. Mechanical tests summary Strength [MPa] Stiffness [GPa] Material Bovine Human Bovine Human Value 236.33±26.01 215.68±74.85 8.91±1.70 9.51±2.97 Median 232.56 220.52 8.95 9.71 (a) American Journal of Materials Science 2013, 3(6): 217-222 219 (b) Figure 1. Force-displacement curves The idea behind this comparison is to investigate if the two sets of bio-posts have any similarities, as the bovine bio-posts were designed to be applied into human teeth implants. By examining Figure 1A, a force-displacement plateau was observed. This behaviour can be explained by the chain porous accommodation inside the material. The mechanism can be explained as follows: (i) a compressive force is applied to the pin generates a stress distribution inside the bio-post; (ii) the micro channels/tubules under the compressive stress start to collapse leading to contact between walls. These collapses can be translated as an increase on displacement (or strain) at constant force (or stress); (iii) once the micro channels/tubules collapse (densification) process is over, the force (or stress) increases in a large rate. It is clear that mechanical behaviour of bio-posts is highly dependent on microstructure. Figure 2 represents a schematic description of a bovine tooth microstructure. A SEM analysis of these microstructures identified not only aligned porous, but it also different regions around them. The collapse mechanism is also represented in Figure 2. Once the large majority of porous is collapsed, non-linear force-displacement behaviour is observed, which can be due to the tooth anisotropy. When comparing the force-displacement plots (Figs. 1A-B) it can be observed that “bovine posts” presented a homogeneous behaviour. This behaviour is explained by the quality of the sample regarding age of the teeth used which has been standardized. Sample 10, however, presented a complete dissimilar behaviour from the others. This sample suffered a rapid densification (represented by the small plateau) and a very stiff behaviour up to the fracture. From SEM observations, it is possible to notice a different fracture mode (FIG 3 and 4). It can be observed on fracture line, circular structures of approximately 5.0 micrometres hypo calcification corresponding to areas of the dentin, which can be correlated with the incremental lines of Owen (FIG 4). As described by Rasmussen et al, the dentin growth is appositional with extracellular matrix deposition over the area delineated by forming regular layer of cells. Periods of activity and rest alternating at set intervals, so the lines are incremental Owen reflection of changes in structure and mineralization during dentin formation. Correspond to the incremental lines of von Ebner which are intensified due to disturbances in the mineralization process resulting hypo calcification areas. This type of fracture seems to be a combination of shear and tensile deformations, therefore a mixed mode. Figure 2. Diagram illustrating the cutting region of Biological posts on each pin is possible to check and tubules, each of these is called a unit cell 1 intertubular dentin, 2 - peritubular dentin, 3 - lumen of the dentinal tubule; Behaviour of dentinal tubules represented by a unit cell front by three points bending 220 Karine T. A. Tavano et al.: Bovine Dentine for Dental Implants: Correlations between Microstructure and Failure Modes Figure 3. Fracture of bovine dentine posts. 1-6 oblique fracture; 10 transverse fractures Figure 4. Microscopic scanning electron micrographs of bovine dentin fractured post so fragile (post 10). A - view of the fracture, B, C and D - the area of fracture with the presence of incremental lines hypomineralization dentin, E and F, this region is noted the rupture between the dentin structure low hypomineralization For all others post, fractures are typical of shear failure with fracture line around the π/4 slope from the horizontal axis (FIG 3). Figure 5 is a higher magnification SEM observation of a typical fractured region, where oblong microstructures into fractured layers are shown. These microstructures are the stretched the tubules’ mouth, where dentin are located. This dentin stretching mechanism allows a local stress redistribution, which leads to a larger rupture load. Another important issue that must be discussed is the differences between the two groups, human and bovine, regarding displacement and peak force. Posts made of bovine teeth present larger displacement and small peak forces. The explanation for such behaviour can be due to the “teeth age”. American Journal of Materials Science 2013, 3(6): 217-222 221 Figure 5. Type of fracture for the remaining bio-post bovine, showing fractured layers and elongated tubules According to Arola et al., young individuals have an immature dentin, which have a large amount of tubules for nutrition. This large number of tubules leads to small amount of minerals, less density and consequently a larger compliance. This observation is in accordance with Kinney et al.. Figure 6 shows the differences in terms of tubules for a human dentin (A and C) and the bovine ones (B and D). The human samples of dentin are clear more dense, with less tubules and smaller diameters as observed into the SEM analysis. Based on these observations, it is possible to identify four different sub-groups of individuals for the bovine set of posts. Specimen 10 seems to be made of the oldest individual, as its plateau is practically zero. This is an evidence of a denser dentin with small amount of tubules. Specimen 6 can be identified as the one made of the second older individual, while 3, 4 and 9 are the specimens made of teeth with intermediate age. The ones with larger force-displacement plateaus are the ones with smaller density and consequently with larger amount of tubules. According to Kinney et al, this is an evidence of a young dentin. For comparison purposes Figure 4 shows SEM observations of adult human and young bovine dentin. As it can be noticed, the number of tubules is much higher into the young bovine dentin. Based on these observations, it is possible to arrange the human dentin pins in accordance to the age of the donor. Smaller amount of dentin tubules means denser dentins and older individuals. Smaller force-displacement plateau also means denser materials as small numbers of tubules are deformed. The age and fracture mechanism seems to be related as three different failure modes were identified. The same pattern was observed by Tavano for the human dentin. Moreover, as discussed by Marquezan et al, these failures can also be related to the bone mineral density. As mentioned by Simonis et al, one consequence of aging into humans is the bone mineral content loss. Furthermore, this loss is the most probable cause of bone embrittlement. The brittle behaviour can cause the bone’s stiffness loss which can make the implant loose. Figure 6. Images of scanning electron microscopy of human and bovine dentin (2.500X magnification). A-Cross section of human dentin; B-Crosssection of the dentin bovine; C-Longitudinal section of human dentin; D Longitudinal section of bovine dentine 4. Conclusions Failure mechanism of bio-posts made of bovine dentin was investigated by three points bending tests and scanning electronic microscopy. Two different failure modes were observed, i.e. shear failure and a mixed mode composed by tensile and shear. These failure modes are related to dentin density, which is related to the number of tubules. A model based on micromechanical unit cell for the densification 222 Karine T. A. Tavano et al.: Bovine Dentine for Dental Implants: Correlations between Microstructure and Failure Modes process during the bending process is proposed. This densification process is the cause of the plateau shown into the force-displacement curve. Stiffness measured during the three point bending tests was 9.81±1.70 GPa, while the bending strength was around 236.33±26.01 MPa, values very close to the human dentin. This closeness is an excellent indication of possible mechanical compatibility between the bio-post made of bovine dentin and the human root canal. ACKNOWLEDGEMENTS This research was supported in part by the Brazilian Research Council (CNPq) under grants number 303447/ 2011-7 and 472583/2011-5, and the Minas Gerais State Research Foundation (FAPEMIG) grant TEC-PPM0019212. The authors are also grateful to the UFMG’s Centre of Microscopy and Microanalysis for the technical support.  Dallari, A.; Rovatti, L.; 1996. Six years of in vitro/in vivo experience with Composite. Compendium. 17(1):57-61.  Attam, K.; Talwar, S.; Yadav, S.; Miglani, S.; 2009. Comparative analysis of the effect of autoclaving and 10% formalin storage on extracted teeth: A micro leakage evaluation. J. Conserv. Dent. 12(1):26-30.  Seefeld, F.; Wenz, H-J.; Ludwig, K.; Kern, M.; 2007. Resistance to fracture and structural characteristics of different fiber reinforced post systems. Dent Mater. 23(1):265-271.  Kono, T.; Yoshinari, M.; Takemoto, S.; Hattori, M.; Kawada, E.; Oda, Y.; 2009. Mechanical properties of roots combined with prefabricated post. 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