Three-dimensional Finite Element Study Of Maxillary Canine Restored With Zirconia Post And Core

BackgroundSeveral factors have been discussed that influence the strength of endodontically treated teeth. Caries, fracture, abrasion and other idiopathic causes result in loss of tooth structure. The restoration of teeth with this condition becomes more complicated, choosing an appropriate prosthesis is very important for the duration of these teeth. In treating these teeth, intraradicular posts are recommended to aid in the retention of artificial crowns and support the teeth by distributing intraoral forces along the roots. The choice of an appropriate restoration for endodontically treated teeth is guided by strength and esthetics. Cast metal posts and cores have been used for many years; however, with the increasing demands on aesthetics, the metal post and core can not meet the needs of patients. As an alternative to metal post and core, fiber posts have good mechanical and biocompatibility properties and a desirable shade. However, most glass fiber posts need a composite resin core, and the weak interface between the resin core and fiber post may cause a higher restoration failure rate. These factors limit the use of fiber posts. Zirconia has become a popular material for all-ceramic restoration for its good mechanical strength, optical properties, chemical stability, and biocompatibility. The enhancement in toughness of the ceramic is based on the energy absorbing mechanism which is due to the martensiticlike transformation occurring at the crack tip. Zirconia posts were first introduced by Meyenberg at1995, which were only prefabricated zirconia posts used in clinic at the early stage. Owing to the diameter of prefabricated post is limited, it is more difficult to achieve the ideal border sealing when applied in maxillary canine with wide root canal. The loss combination of post and core will also increase the risk of restoration failure. With the progress of science and technology, the use of the computer aided design/computer aided manufacturing (CAD-CAM) technology of the yttrium-tetragonal zirconium polycrystals (Y-TZP) system to fabricate a custom-made ceramic post and core is introduced. Instead of investing and casting the post into metal, the pattern is scanned, milled, and sintered into Y-TZP. This technique creates a post and core with greater toughness and maximum adaptability to the canal and appropriate esthetic characteristics. However, the elastic modulus of zirconia is so high that it may cause root fracture. By locating in the dental arch turning point, maxillary canine should endure large occlusion stresses which increase the difficulty of restoration procedures. Further studies should investigate to evaluate whether zirconium post and core system is suitable to restore the maxillary canine.ObjectivesThrough three-dimensional-finite-element analysis, the aim of this article was to evaluate and establish three-dimensional-finite-element models of the maxillary canine restored with zirconia post system, analyze the influence of different restoration methods on distribution of stress of severely damaged maxillary canine as to provide some valuable information for clinical work.Materials and MethodsPart I:1. Obtainment of Primitive Data for Modeling: An intact maxillary right canine with an absence of cracks, fracture, and caries was used for the preparation of the three-dimensional tooth model. The sample which obtained from department of anatomy of southern medical university was selected in compliance with the standard criterion of Chinese human tooth. Scanned tooth from dental crown to root using Micro-CT, made the section vertical to long axis with layer distance of125μm, obtained dental3D dynamic images. Through three-dimensional dynamic were obtained235identifiable slices of Mimics (Materialise’s Interactive Medical Image Control System) software by Micro-CT own analysis software (bmp format). Then these slices were imported into the three-dimensional reconstruction software of Mimics. From this data, a three-dimensional image about the maxillary canine was reconstructed with the software. After that, this3D model was modified and smoothened with the software of Freeform.2. Establishment of Three-dimensional-finite-element Model:The3D solid modeling was imported into the ANSYS, universal finite-element analysis software. This finite element (FE) model was divided into different geometric structures including the dentine, gutta-percha, all-ceramic crown, post and core. Mesh command in the ANSYS software was used to perform a smart mesh division of the FE model. The top and2/3Mesiodistal of alveolar ridge was standard of the bound borders, root fixed in the alveolar socket. When load to the model, the sections had not mutual sliding, the unit had sufficient stability.3. Validated of Three-dimensional-finite-element Model:These study involved biological materials of mechanical properties were presumed to be homogeneous, continuous and isotropic. A maximum masticatory force was applied at45°3mm below the incisal edge of the palatinal surface. The loading type was static loading and the size was300N to simulate the largest occlusal force of the maxillary canine.Part IIThe severely damaged root with2.0mm ferrule height was restored with four different dowel systems including one milled zirconia post, cast gold post, prefabricated glass fiber post, and cast cochrome post. The observation of indexes was same to part II.Part IIITo simulate the severely damaged canine, the sound tooth was horizontally cut off from0.0mm,1.0mm,2.0mm,3.0mm of cemento-enamel junction. Four different ferrule heights of maxillary canine were restored with one milled zirconia post and core systems and all-ceramic crowns. The defined materials and bonder conditions were same to partⅠ. Von Mises stresses of each section of canine and apical1/3, middle1/3, cervical1/3of the root were calculated. A convenient way of reporting the stresses was in the form of a color representation of the stress distributions.Part ⅣTo simulate three different lengths and diameters of zirconia post, nine FE models were established by the model created in part Ⅰ. The observation of indexes was same to part Ⅰ.Results1. The three dimensional finite element model of the severely damaged maxillary canine restored with zirconia post and all-ceramic crown was established. Upon establishment,40949nodes and dots were produced and formed35463tetrahedrons each.2. The three dimensional finite element model of the maxillary canine rehabilitated with four different post and core systems and six dimensions of posts was simulated and established.3. With2.0mm ferrule height, the stress value and distribution of zirconia and Co-Cr alloy posts were similar, the concentration areas were located at cervical1/3of the root, occlusal fossa and post/dentine interface. The glass fiber and cast gold post was similar with stress distribution which was mainly located at cervical1/3of the root and occlusal fossa. The glass fiber post showed the lowest maximum von Mises stress in the dentinee at59.4MPa, while, Co-Cr alloy post group resulted in the highest stress concentration in the dentine at56.7MPa.4. With the same ferrule height, the Von Mises stress was varied from31.0MPa to32.1MPa at apical dentine by difference of post materials. The middle1/3root was varied from50.3MPa to52.4MPa. The cervical1/3was varied from56.7MPa to59.4MPa, and the post/dentine interface was varied from36.9MPa to149MPa. The variance ratios were3.4%,4%,4.8%and75%respectively. The variance ratio of post/dentine interface was the largest.5. Four kinds of ferrule height during stress loading:1.0mm,2.0mm,3.0mm of ferrule height groups were similar. Von Mises was mainly distributed on lingual of cervical root.3.0mm group showed the lowest maximum von Mises stress in the dentinee at55.7MPa, while,0.0mm group resulted in the highest stress concentration in the dentine at59.5MPa.6. With four different ferrule heights, the Von Mises stress values of different root sections were decreased by increasing the ferrule height. The maximum stress value in post/core interface was the largest in0.0mm group, while3.0mm group performed the least. The stress concentration of0.0mm group was located at the cervical of post, while the3.0mm group performs at apical1/3of the post.7. By six kinds of post systems, the12.9mm long and2.5mm diameter group showed the highest maximum von Mises stress in the dentinee at179MPa, while, the10.9mm length and2.0mm diameter group resulted in the lowest stress concentration in the dentine at161MPa.8. With the same diameter, the stress distributions were similar with different post length groups, the stress concentration areas were located at cervical1/3of the root, occlusal fossa and post/dentine interface. The maximum Von Mises stress was reduced with a decreasing of post length. The stress reduction was significant in post/dentine interface area.9. With the same post length, the stress distributions were similar with different post diameter groups, the stress concentration areas were shifted up to cervical1/3of root. The Von Mises stress was varied from148MPa to123MPa at the post/dentine interface, the variance ratio was16.9%, which was significantly different from other three sections.Conclusion1. Using Micro-CT scanning technology with Freeform, Mimics software reverse engineering, such as the establishment of severely damaged maxillary canine with different restorations of3D finite element models are structured clear, precise demarcation unit, and have excellent mechanics and similar geometry.2. The maximum Von Mises stress is reduced with decrease of elastic modulus of post materials. Compare to other post systems, one milled zirconia post performed slightly advantage. When glass fiber posts cannot be customized to adapt to individual post space preparations, one milled zirconia post would be a good alternative.3. The Von Mises stresses of an anatomically custom modified zirconia post are affected by the ferrule height. The maximum Von Mises stress is reduced with an increasing of ferrule height. The presence of ferrule has a positive effect on fracture resistance of severely damaged canines. The survival of severely damaged teeth depends on the condition of the tooth.4. Although the minimal length of the post performs the minimum stress value of the tooth structure, with the same diameter, different post lengths dose not significantly change the stress distribution of the overall model. With the reduction of the diameter, the stress concentration zone of coronal dentine decreases slightly. The mechanical properties of zirconia post may be appropriate to reduce the diameter and length of post and core in order to avoid the greater stress on the tooth structure.