Discussion
tooth dentin, failure of teeth, Fracture, fracture, incisors, rigidity, fractures, tooth, lower mean fracture
AIDS, ENDODONTICALLY TREATED TEETH, CAVITY
When the remaining tooth structure of endodontically treated teeth cannot provide sufficient support and retention for restoration; remaining less than 50% of tooth structure, post and core is beneficial. So the rehabilitation of pulpless teeth is crucial in order to restore them esthetically, functionally and structurally for a successful restorative outcome and prognosis [Fracture resistance of post-core restoration depends on the design of the post, post diameter, post length, the type of adhesive cement used along with material of the post and core system material that aids in dissipation of occlusal forces along to the post then to remaining radicular root [Mechanical properties vary between different types of posts. An ideal post-core foundation should have superior fracture resistance that is higher than mastication forces. While other mechanical properties such as elastic modulus, coefficient of thermal expansion and compressive strength must be same as those of radicular dentin [This study investigated the fracture resistance of endodontically treated maxillary central incisors, restored with two post-core systems in 1.3-mm (narrow) and 1.75-mm (wide) post space diametersThus in this study there was no statistically significant difference among the four groups. Although, mean fracture resistance was in its highest value in group GW (638.7 ± 285.1 N), followed by group ZW (598.5 ± 127.6 N), then GN group (442.8 ± 65.38 N). The lowest mean fracture resistance was recorded in group ZN (435.3 ± 117.3 N).The highest fracture resistance was found in subgroup GW; glass fiber posts with 1.75 mm diameter, with mean of 638.7 ± 285.1 N. This high value is attributed to that the modulus of elasticity of glass fiber posts is almost same as that of radicular dentin. This supported the presence of a monoblock that aided in dissipation of forces along the long axis of the post itself, thus high resistance of the roots against fracture when subjected to occlusal forces. This came in agreement with a study by Habibzadeh et al. [In accordance with the results of the current study, previous studies also reported that increasing the post diameter enhances its fracture resistance. They reported value of 514.2 ± 136.3 N, which is comparable to the findings of the present study. However, this leads to the excessive removal of sound tooth dentin and hence weakening of the residual tooth structure occurs [Kul et al. explained the most probable reason causing the variety in mode of failure. They reported that the low modulus of elasticity of glass-fiber post closely reaches that of dentin. While zirconia posts have high modulus of elasticity. The low modulus of elasticity aided in the absorption of forces along the length of fiber posts as well as its ability to bond to the tooth dentin; providing a monoblock effect, greatly reducing the force transmission to the tooth leading to lower risk of fracture. That also came in agreement with the results of the current study [Previous studies by Saritha et al., Kurthukoti et al. and Shukla et al. compared fracture resistance of zirconia posts and glass fiber posts of endodontically treated teeth. They proved that fracture resistance between tested groups was not significant (On the contrary, Habibzadeh at al., evaluated the fracture resistances of zirconia and fiber-composite post systems under all-ceramic crowns in endodontically treated teeth. Fiber-glass posts with composite cores showed the highest fracture resistance values (915.70 ± 323 N), while the zirconia post-system showed the lowest resistance (435.34 ± 220 N). The differences among the groups were statistically significant (Özarslan et al. investigated the fracture resistance and fracture mode of maxillary central incisors restored with two different diameters and three different post-core systems. Their study revealed that the highest fracture strength was found in Group F; teeth restored with glass fiber posts, followed by group Z; teeth restored with zirconia custom-made posts. Their findings showed statistically significant differences existed between the fracture strengths of the post materials, which came in contrary to the findings of the current study. That might be explained by the different types of zirconia used as well as the glass-fiber post, in addition to the minute difference in post space diameters in that study; 1.4-mm and 1.6-mm [Subgroup ZW; custom-made zirconia posts with 1.75 mm diameter, showed a slightly lower mean fracture resistance than subgroup GW. This is supported by a previous study by Alkhatri et al. and Abduljabbar et al., as they reported that CAD/CAM zirconia posts had high fracture resistance due to the chemical stability, high elastic modulus, and high toughness in conjugation with mechanical strength [Regarding zirconia post-core unit, Prettau Zirkonzahn was used in this study. Prettau Zirconia is partially stabilized with yttrium and enhanced with aluminum. This aided in increasing mechanical properties; high flexural strength up to 1200 MPa, along with stable shrinking feature allowing the optimum precision [All samples restored with zirconia post-core system showed non-favorable mode of fracture. This came in correlation with the variation between elastic modulus of zirconia post, which is high, and the radicular dentin that varies between 12 and 14 MPa [Previous studies evaluated flexural strength (900–1200 MPa) of zirconia posts and high modulus of elasticity of 200 MPa which prevent the plastic manner of the material to be presented; resisting high forces but without dissipation. This led to failure of teeth in a non-restorable manner; vertical root fracture that end by extraction of the restored teeth [Subgroup GN showed lower values of fracture resistance as they had a narrower diameter that couldn’t resist forces and failed at less exerted forces. However, failures were confined to the core foundation only, without any damage to the tooth itself. This came in agreement with a study by Özarslan et al. [A study by Beck at al. reported that fracture resistance of glass-fiber posts was not significantly different to that of custom-made zirconia one piece post and core. They also reported that catastrophic failures most often occur with zirconia posts and restorable fractures occurred with glass-fiber posts. That came in accordance to the findings of this study as all samples of group G showed favorable fractures that were confined to the core material only or barely reaching the post itself, while zirconia posts showed catastrophic failure that unfortunately end by tooth extraction [This study’s findings were also in agreement with a study by Habibzadeh et al., Gu et al. and Akkayan and Gülmez stated that fractures associated with zirconia posts were mostly non-restorable, while these associated with glass-fiber posts were more reparable. They came out with an explanation that the high rigidity as well as the high elastic modulus of zirconia posts transfer subjected forces directly to the tooth without any kind of dissipation or absorption by the post-core unit, and that was the main reason causing the fracture of restored teeth. That also came in agreement with the findings of the current study [Limitations of the present study is the lower sample size and the lack of thermal changes that are present in the oral environment and masticatory forces were not applied as it is an in-vitro study, which does not reflect the oral cavity conditions as well as the absence of cyclic loading of the specimens before testing which might alter the results. So, further in-vivo studies are required to evaluate fracture resistance of different esthetic posts in oral environment for successful prosthodontic procedures. So the use of chewing stimulator, crown restoration and cyclic loading of the samples is recommended to obtain more consistent results to the clinical situation. That’s because in the current study, a universal testing machine with a static manner was used to measure fracture resistance of specimens. Furthermore, the teeth collected for this study were selected according to the preferred post sizes. The different canal configurations, post drills and posts themselves may have affected the homogeneity of the cement thickness between the post and root canal wall. Also, a larger sample size is recommended in further in-vitro studies to obtain a more accurate significant difference among the groups in future studies.