Influence of remaining coronal thickness and height on biomechanical behavior of endodontically treated teeth: survival rates, load to fracture and finite element analysis

Abstract Objective To evaluate the effect of restorative strategy (fiber post vs cast post and core), coronal height (0 mm vs 2 mm) and thickness (higher than 1 mm vs lower than 1 mm) on survival rate, fracture resistance and stress distribution. Material and Methods Seventy-two bovine teeth were cleaned and allocated in six groups (n = 12). Twenty-four teeth were sectioned at 13 mm length (no remaining coronal structure) and forty-eight were sectioned at 15 mm (2 mm remaining coronal structure). Half of the forty-eight had remaining coronal thickness lower than 1 mm and the other half had thickness higher than 1 mm. All root canals were prepared at 10 mm (luting length), fiber posts were cemented in thirty-six specimens and cast post and core in other thirty-six. All teeth were restored with metallic crowns. Specimens were submitted to 1.5 million cycles (100 N, 45°, 10 Hz at 2 mm below incisal edge) and evaluated at each 500,000 cycles to detect failures. Specimens that survived were submitted to load to fracture test. Bidimensional (Rhinoceros® 4.0) models were obteined survival data submitted to Kaplan-Meier (α=0.05) analysis and load to fracture values submitted to ANOVA and Tukey tests (α=0.05). Results Groups without remaining coronal structure showed survival rates lower than other groups (p=0.001). ANOVA showed higher values of load to fracture for groups with coronal thickness higher than 1 mm (p=0.0043). Finite element analysis showed better stress distribution in groups with remaining coronal structure and restored with fiber post. Conclusion Specimens without remaining coronal structure have lower survival rates. Specimens with remaining structure lower than 1 mm and without coronal structure support the same load to fracture value independently of the restorative strategy.


Introduction
The reconstruction of endodontically treated teeth with reduced remaining coronal structure is usually associated with intracanal posts to provide retention and mechanical strength to coronal restoration 5,22 .
This can be related to the fact that teeth with no remaining coronal structure may have lower fracture resistance and increased risk of catastrophic fracture 10 .
In contrast, the presence of coronal structure could improve the biomechanical behavior of the system, reducing the stress transmission to the root and increasing the fracture strength of these teeth 14,25 .
Besides, some authors have reported that with remaining coronal structure the tensile stresses are Many post systems are available for restoration of endodontically treated teeth. Among them, the cast post and core have been used for many years and can be considered as gold standard strategy. However, it has some limitations, such as the need of at least two clinical sessions to be obtained, the need for greater dentin removal and a high elastic modulus (200 GPa) that increases the chance of catastrophic fractures 18,19 .
Currently, glass fiber posts have been used to restore endodontically treated teeth, mainly because they have an elastic modulus (30-50 GPa) very similar to dentin 19  It is very important to comprehend the biomechanical behavior of endodontically treated teeth to clarify the reasons/factors that lead to its failure. Thus, the use of non-destructive tests, such as finite element analysis, associated with laboratory testing, allows a better understanding of the phenomena that occur, in addition to the behavior of the tested system 15 in high speed with air-water irrigation. The drills were changed every 5 preparations. After coronal preparation of specimens, the remaining coronal tissue thickness was measured with a digital caliper and the specimens were allocated in different groups (remaining coronal dentin higher than 1 mm thickness; remaining coronal dentin lower than 1 mm thickness).
Finally, specimens were divided into 6 experimental groups, according to the remaining coronal height, thickness and restorative strategy ( Figure 1). Specimens with coronal structure greater than 1 mm, less than 1 mm and samples without coronal structure were numbered from 1 to 24 and three random number sequences of 24 numbers were generated by the computer program Random Allocator.

Restorative strategies
Cast post and core All canals were prepared with the custom bur of the

Load to fracture
The one-way ANOVA showed a statistical difference between groups (p=0.0043). Table 3 Figure 5-Representative fractures evaluated under stereomicroscope 7.5x. Roots A and B restored with fiber post and cast post and core, respectively without remaining coronal structure with displacement of the crown/core/post assembly and buccal fracture. Roots C and D restored with fiber post and cast post and core, respectively with remaining coronal structure lower than 1 mm with displacement of the crown/core/post assembly, cracks on the lingual surface of the coronal remaining and partial buccal fracture. Root E restored with fiber post with displacement of the crown/core/post assembly and total buccal fracture. Root F shows restoration with post and core with displacement of the crown/core/post assembly, both E and F roots do not have cracks on the lingual surface of the dental remaining. G and H roots restored with fiber post and cast post and core, respectively with remaining coronal structure higher than 1 mm with displacement of the crown/core/post assembly and partial (G) and total (H) buccal fracture

Data analysis by finite element method
The analysis showed a greater concentration of tensile stress in the groups with thickness of remaining coronal structure lower than 1 mm, compared with the groups with thickness higher than 1 mm ( Figure   6). Moreover, the specimens restored with CPC concentrated the greatest tensile value in the apical region of the root ( Figure 6) and inside root canal (Figure 7), while the teeth with glass fiber post had higher concentration of tensile values in the composite core ( Figure 7) and in the middle third of the root, at the outer portion ( Figure 6).

Discussion
Based on our findings, we can notice that all remaining coronal structure and thickness of this structure influenced on the outcomes. Thus, the assumed null hypothesis was rejected at all levels.
As highlighted finding, the absence of remaining tooth structure lead to statistically lower survival rate values, regardless of the restorative strategy used.   (Table 1), which also corroborates with some in vitro studies 2,6,11 . This behavior may be explained by the worst stress distribution to the root structure, concentrating higher tensile values and providing gaps into the assembly, as observed in the test of finite elements (Figures 5, 6 and 7).
Furthermore, in the groups with fiber post, the absence of remaining tooth structure leads to a larger quantity of restorative material in the core portion, which may generate defects and failures at the core.
Regarding the influence of the remaining dentin thickness in the coronal part on the load to fracture, statistical analysis showed that groups of remaining coronal thickness lower than 1 mm had loads to fracture lower than groups which had remaining coronal thickness greater than 1 mm and similar to specimens with no coronal structure (Table 1).
These results may be related to the fact that these specimens focus higher tensile values in the root region, as shown by finite element analysis ( Figure   5). This higher concentration may be due to the fact that these specimens have dimensions inferior than the other, thus having a higher risk of fracture.
Based on the survival analysis, both restorative options (cast post and fiber post) had similar behaviors (Table 1). This finding is consistent with clinical trial findings 18 , which concluded that both glass fiber and cast metal posts showed good and similar clinical performance. Additionally, we can notice that the failure mode observed was also consistent to clinical studies available, wherein the irreversible fracture was more frequent in tooth restored with cast metal core; while debonding of the retainer was the most common cause of failure in tooth restored with glass fiber posts 18 .
The load to fracture values were also similar between the strategies (cast post and core vs fiber post) (Table 1), agreeing with other studies that showed that both restorative strategies could be interesting regarding the biomechanical aspect 19 .
Although the similar values, the failure modes observed were different (Table 1), agreeing with other studies 6,7,16,23 , which may be due to different stress distribution in the specimens (Figure 6 and 7). Higher percentage of root fractures for the cast post and cores group may have been mainly caused by stress concentration in the inner portion of the root (Figure   7), as also observed by other study 19 , while lower tensile stress can be observed at the coronal portion of the tooth restored with fiber post (Figure 6). It can be explained considering that in a rigid body subjected to forces, the stress will concentrate on the structure created with higher stiffness and greater intensity will be transferred to the adjacent structure (Figure 7).
In groups restored with fiber posts, we observed a high concentration of tensile forces in the middle third of the root ( Figure 6). However, we could also observe high tensile values in the composite core portion (Figure 7). Thus, as already mentioned by other studies 13,19 , the risk of radicular fractures in this scenario is smaller, since the tendency is that the post or core composite fail before the root break. In fact, this study has depicted a higher number of "debonding crown" for groups restored with fiber post (Table 1) study. In the groups with remaining coronal structure, the presence of ferrule effect and its thickness had some variations in the internal failure pattern of the restorative assembly. In specimens with thickness lower than 1 mm, the displacement of the crown/core/ post assembly promoted firstly an adhesive failure in the lingual interface crown/root (due to tensile stress).
Subsequently, it evolved to a crack on the lingual surface of the remaining coronal structure (for some specimens - Figures 5C-D)  Even being a laboratory work, some care was taken aiming the quality and reliability of the study using procedures such as blinding the assessors, conducting sample calculation, randomization, among others 13   3. Specimens with remaining coronal structure with thickness lower than 1 mm support the same load to fracture to the specimens without remaining coronal structure, regardless of restorative strategy used; 4. Specimens restored with fiber posts tend to suffer less catastrophic fracture compared with the cast post and core in the specimens with 2 mm height of the remaining coronal structure and thickness higher than 1 mm, allowing restoration of the new set.