Abstract
Context:
Influence of luting agent, design of tooth preparation for cast posts on tooth resistance to fracture.
Aim:
To evaluate fracture resistance of teeth restored with cast posts and cores with or without cervical ferrule and cemented with zinc phosphate, glass ionomer, or resin cement.
Materials and Methods:
Sixty single-rooted maxillary first premolars of similar sizes were selected. Biomechanical preparation and post space preparation for cast post was done for all samples and then they were divided into two groups (n=30). Cervical ferrule preparation was done for Group A, and Group B was prepared without any cervical ferrule. Both groups were further divided into three subgroups (1, 2, and 3; n=10) Cast posts for subgroups 1, 2, and 3 were cemented with zinc phosphate, glass ionomer, and resin cement, respectively. A shearing load was applied to indented buccal cusp of specimens at an angle of 45° in universal testing machine at a cross-head speed of 1 mm/min until fracture.
Results:
Specimens with ferrule (Group A) had significantly higher shear bond strength values (mean 1503.37 N) than specimens without ferrule (Group B) (mean 1052.09 N).
Conclusion:
Inclusion of ferrule in tooth preparations for posts increased the fracture resistance regardless of the luting agent.
KEY WORDS: Cast posts and cores, ferrule, fracture resistance
The size and shape of the post and core, the final preparation design of the tooth, and the kind of luting agent used influence tooth resistance to fracture.[1,2] The custom-made post is the time-proven method for post and core construction. When properly designed, the custom-made post can conform to a canal of any shape to provide maximum retention and allow a more even distribution of stresses throughout the tooth structure.[3,4]
A dental ferrule is an encircling band of cast metal around the coronal surface of the tooth as part of the core or artificial crown and may be of benefit in reinforcing root filled teeth. A protective or “ferrule effect” could occur owing to the ferrule resisting stresses such as functional lever forces, the wedging effect of tapered posts, and the lateral forces exerted during the post insertion.[5]
All posts, to a greater or lesser extent, gain their final retention by cementation into the prepared root canal. Studies have provided conflicting results in relation to the adequacy of different types of cement to retain dental posts.[6–8] The mechanical properties of the cement, the bonding efficiency of the cement to the two surfaces being joined, the durability of the cement, and the configuration of the post and the prepared canal have a direct bearing of the ability of cements to retain posts.
The purpose of the present study was to evaluate fracture resistance in teeth restored with cast posts and cores with and without cervical ferrule and cemented with three types of cements, viz., zinc phosphate, glass ionomer, and resin cements.
Materials and Methods
Sixty single-rooted maxillary first premolars of similar sizes were selected which were removed for periodontal and orthodontic reasons free of caries, calculus, and fractures. The teeth were cleaned of debris, calculus, and any periodontal ligament remnants, and stored in normal saline at room temperature.
The crowns were horizontally sectioned using sectioning disks and the lengths of the teeth were standardized to 16 mm. The endodontic treatment followed the step back technique. The canals were instrumented with K-files and H-file under irrigation with 5.25% sodium hypochlorite, 17% ethylenediaminetetraacetic acid (EDTA), and normal saline, and then obturated using lateral condensation method with gutta percha cones combined with calcium hydroxide and resin-based cement sealer AH Plus. All samples after obturation were sealed with cavit and were kept in saline solution for 5 days at room temperature. Cavit was removed after 5 days and the palatal canals were prepared to receive the posts. The root diameter was enlarged using peeso reamer number 4 to a depth of 11 mm, with the control of rubber stoppers adapted to the reamers.
The canal orifices were sealed once again with cavit. The 60 roots were placed in square cylinders and filled with chemical-cured resin 4 mm apical to the cervical margin [Figure 1]. The 60 specimens were randomly divided into six groups of 10 teeth each and treated as follows:
Figure 1.
Teeth placed in square cylinders & filled with chemical cured resin
Group A1 – restored with cast posts and cores cemented with zinc phosphate cement, with cervical ferrule
Group A2 – restored with cast posts and cores cemented with glass ionomer cement, with cervical ferrule
Group A3 – restored with cast posts and cores cemented with resin cement, with cervical ferrule
Group B1 – restored with cast posts and cores cemented with zinc phosphate cement, without cervical ferrule
Group B2 – restored with cast posts and cores cemented with glass ionomer cement, without cervical ferrule
Group B3 – restored with cast posts and cores cemented with resin cement, without cervical ferrule.
All roots in groups A received a cervical ferrule preparation (2 mm height, 0.6 mm deep with 2°–5° convergence) [Figure 2]. The post and cores were fabricated by using direct technique and the metal casting was done with non-precious gold alloy (79.3% copper, 7.8% aluminum, 4.3% nickel).
Figure 2.
Diagrammatic representation of two restoration design tested
After casting, removal, and cleaning, the post and cores were sand blasted with aluminum oxide and adapted to the roots. Any areas impairing perfect adaptation were removed with a carbide bur.
The occlusobuccal angles of every core were indented with a carbide bur to receive the device adapted to universal testing machine so that the load was applied at a 45° angle [Figure 3].
Figure 3.
Diagrammatic representation of restoration and root embedded in acrylic resin in iron cubicle cylinders, and direction of load applied
The roots that received the posts and cores cemented with zinc phosphate cement had their canal walls cleaned with 17% EDTA solution by means of extra fine interdental brushes, rinsed with water, and dried by air blast. The zinc phosphate cement was handled according to the manufacturer's instructions and inserted into the canal with a lentulo spiral endo instrument. The post and cores were settled under hand vibration and kept under manual pressure for 30 sec.
The roots that received the posts and cores cemented with glass ionomer cement had their canal walls conditioned with 10% polyacrylic acid by means of extra fine interdental brushes, rinsed with water, and dried by air blast. The glass ionomer cement was handled according to the manufacturer's instructions and inserted into the canal with a lentula. The post and cores were settled under hand vibration and kept under manual pressure for 30 sec.
The roots that received the posts and cores cemented with resin cement had their canal walls etched with 32% phosphoric acid, rinsed with water, and dried with gentle air blast and absorbing points. The resin cement was handled according to the manufacturer's instructions and inserted into the canal with a lentulo spiral endodontic instrument. The post and cores were settled under hand vibration and kept under manual pressure for 30 sec.
Excess cement was removed with a scalpel blade. All specimens were stored in saline solution and kept at 37°C for 24 h. After this, a shearing load was applied to the specimens at an angle of 45° in a universal testing machine at a cross-head speed of 1 mm/ min, until fracture. The specimens were fixed to a cylindrical metal base with a 45° hole in its upper part. The indented buccal cusp was placed upward to fit the metal point coupled to the upper part of the universal testing machine [Figures 4 and 5]. Compression was applied at an angle of 45° to the cusp. All procedures were performed by the same operator. Findings were recorded and statistically analyzed.
Figure 4.
Specimen on placing the load using universal testing machine
Figure 5.
Universal testing machine
Results
Result shows that specimens with ferrule (Groups A1, A2, A3) had significantly higher shear bond strength values (mean 1503.37 N) than specimens without ferrule (Groups B1, B2, B3) (mean 1052.09 N) [Graph 2]. Among the non-ferruled groups (B1, B2, B3), the post and core cemented using glass ionomer cement and resin cement (i.e. Groups B2, B3) had a better fracture resistance than the post and core cemented with zinc phosphate cement (Group B1) [Graph 1]. The results were statistically analyzed using analysis of variance (ANOVA) at a level of significance of α = 0.05 and Bonferroni test.
Graph 2.
Mean shear strength recorded in the groups
Graph 1.
Comparison between ferruled and nonferruled groups
The ANOVA result [Table 1] shows that there was a significant difference among the six groups (P < 0.001). Higher mean shear strength was noticed in Group A3, followed by Group A2, Group A1, Group B3, Group B2, and Group B1.
Table 1.
ANOVA results among the group
Multiple comparisons (post hoc tests) done using Bonferroni test showed a significant difference between the groups.
The multiple comparisons done using Bonferroni test [Table 2] shows that there was a very high significant difference between Group A1 and Group B1, Group A1 and Group B2, and Group A1 and Group B3 (P < 0.001). There was no significant difference between Group A1 and Group A2 as well as between Group A1 and Group A3 (P > 0.05).
Table 2.
Multiple comparisons between the six groups
There was no significant difference between Group A2 and Group A3 (P > 0.05), but there was a very high significant difference between Group A2 and Group B1, Group A2 and Group B2, and Group A2 and Group B3 (P < 0.001). We observed that there was a very high significant difference between Group A3 and Group B1, Group A3 and Group B2, and Group A3 and Group B3 (P < 0.001). Significant difference was noticed between Group B1 and Group B2 (P < 0.05). Also, we observed a highly significant difference between Group B1 and Group B3 (P < 0.01). No significant difference was observed between Group B2 and Group B3 (P > 0.05). The difference in the fracture resistance of teeth restored using a ferrule design cemented using three different cements (Group A1, A2, A3) was statistically insignificant.
Discussion
Endodontically treated teeth often are treated with posts and cores to secure retention for a fixed restoration. Loss of retention of posts or root fractures of endodontically treated teeth restored with posts are common.[9–14] It is therefore important to use post and core techniques that minimize these risks.
Laboratory studies have shown that increasing the length of the post in teeth with a post and core increases the retention of the post[15–20] and results in a more favorable stress distribution along the post.[20–22] Encircling vertical tooth structure within the walls of a crown creates a ferrule effect, thereby decreasing the risk of root fracture.[23,24] The importance of a ferrule has also been observed in a clinical study regarding the survival rate of post and cores.[14]
The development of ionomer cements, modified ionomer cements, and resin cements led to an extensive review of restoration principles for badly mutilated teeth to provide retention and stability for the future artificial crown.[25,26] Materials with a modulus of elasticity similar to that of dentin seem to increase physical resistance of restored teeth to fracture.[27]
Since this study involved the use of resin cement as one of the cements for the cementation of the post, a resin-based (eugenol-free) sealer was used for the obturation of all the root canals using gutta percha cones.
The diameter of the root canal preparation for the post should be equal to one-third of the root diameter observed on the radiograph and the remaining dentine thickness should not be less than 1 mm. Despite the fact that these are old concepts, they are still valid as references for the lower limit of residual radicular dentine thickness.[28]
In the study, for the groups having ferrule, the height of the ferrule was standardized at 2 mm. some investigators have reported the effect of different ferrule heights (0.5-2.0 mm) on the load resistance of teeth restored with cast post and core. They have concluded that a ferrule height of at least 1.5 mm is required to ensure a favorable restoration prognosis.[29]
The cements used for luting the cast posts in the root canal were zinc phosphate, glass ionomer, and resin cement. It has been reported that the cement layer provides a buffer zone that contributes to uniform stress distribution between the post and the canal.[30]
The results of the current study show a higher fracture resistance in ferruled groups than in non-ferruled groups [Table 1 and Graph 2]. A 2.00-mm cervical ferrule is regarded as the key to restoration longevity because of its strengthening effect. Roots resist better the multiple stresses to which they are subjected when the tooth is in use, and even to the loads resulting from parafunctional habits. The role of ferrule is similar to that of the cast ring. Some authors suggest that the tooth should be extracted when its clinical placement is impossible.[31]
The results of the current study are in agreement with those of previous studies.[23,24,29,32,33] It is clear that teeth with cervical ferrule have greater physical resistance regardless of the cement employed in the cementation of morphological posts. Clinical guidelines prescribe that cervical ferrules should be used whenever clinically possible regardless of what material is used to make post and cores.[31]
The fracture resistance of posts cemented with resin cement and glass ionomer cement in the non-ferruled group was inferior when compared with the ferruled groups though there is formation of monoblock due to chemomechanical properties of these cements. This is because hydrodynamic forces present during placement of posts precisely adapted to the root walls may modify the structure and the adhesive properties of the resin cement and glass ionomer cement. As the gap between the post and the canal wall approached the cement film thickness, it becomes more difficult to produce a homogeneous cement layer.
Some investigators in their study found that a filtration phenomenon occurs in the cementation of crowns onto well-fitting teeth preparations using zinc phosphate cement. When the passage of cement is reduced and large grains of cement powder begin to jam together, cement liquid filtration occurred resulting in an uneven distribution of cement powder portion in the phosphate matrix. This “Filtration Phenomenon” results in cement with inferior properties. Similar modification to the other two cements tested in the study is possible.[34,35]
The small gap between the post and the wall of the well-fitting canal might make it difficult for air and excess cement to escape from the canal, thereby preventing the post from being coated evenly with cement in the canal.
Investigators have observed that the pull force of posts precisely adapted to the root walls is significantly lower than when there is a 0.5-mm free space, regardless of the type of cement used. In the comparative analysis between cements, resin cement showed a pull force more than twice as high as that of zinc phosphate cement. In thicker layers, the better physical and chemical properties of the resin cement became more evident. This thicker layer significantly reduces the action of hydrodynamic and filtration forces and does not affect the physicochemical properties of the cements. Moreover, a thicker cement layer acts as a shock absorber because the modulus of elasticity of cement is similar to that of dentin.[36]
The non-ferruled group that used glass ionomer cement (Group B2) and resin cement (Group B3) showed statistically higher results than those of the non-ferruled group with zinc phosphate cement (Group B1) [Graph 1, Tables 1 and 2]. These results can be analyzed in the light of physical and chemical properties of glass ionomer and resin cement, which present the chemical and micromechanical bond to metal and dentin that is not observed in the zinc phosphate cement. This may explain the earlier failure of zinc phosphate cement when under mechanical stress. The modulus of elasticity of glass ionomer and resin cement is much lower than that of the zinc phosphate cement, and glass ionomer and resin cement may consequently provide greater tensile and compression resistance. Its deformation properties make it more resistant to the process of mechanical fatigue, as demonstrated by some investigators.[27,37]
Young's modulus of dentin (18.6 GPa) is similar to that of glass ionomer cement and resin composite (16.6 GPa), while the elastic modulus of zinc phosphate is 22.4 GPa.[27] Stress is higher for resin composite than for resin cement; consequently, resin cement has an even lower elasticity modulus than resin composite. Thus, the deformation of resin cement prior to undergoing mechanical fatigue is even higher. Resin cement & glass ionomer cement account for statistically better fracture resistance in the unferruled group; this is because glass ionomer and resin cement absorb part of the load applied to the root, reducing the risk of fracture.
There is strong evidence inliterature that a more favorable physical behavior of restoration of teeth with great loss of mineralized structure is obtained when materials with an elasticity modulus similar to that of dentin are used. Some investigators by means of finite element analysis observed that load distribution was homogeneous when resin cement and flexible posts were employed. Flexible carbon fiber posts reduced to a minimum the risk of fracture of teeth restored with this material.[38] When an alloy with a high modulus of elasticity is used (NPG 150-200 GPa) as in this study, its low deformation capacity and low capacity to absorb loads caused all loads to be almost fully transmitted to dentin. A significantly lower force was consequently able to cause root fracture, regardless of what type of cement was employed or whether a cervical ferrule was used or not.[32]
No post and core system provides an ideal solution for all clinical cases.[39] The ideal post and core system would be the one that ideally meets all the mechanical requirements of a prosthesis and absorbs as much of the loads as possible without transmitting them to the root dentin. Maximal conservation of dentin remains a key factor in the choice of restoration procedure for endodontically treated teeth. The cervical ferrule prevents the wedge effect and improves load distribution. The physical resistance of a tooth is improved significantly when it is restored with materials that have a low modulus of elasticity rather than with materials having high modulus of elasticity.
Conclusions
Based on the results and experimental conditions of this study, it was concluded that:
A 2-mm cervical ferrule provided higher fracture resistance regardless of the luting agent used;
The type of luting cement used was not a determinant factor for the physical resistance to fracture of teeth restored with cast post and cores; and
Glass ionomer and resin cement were better than zinc phosphate cement in the non-ferruled group.
Footnotes
Source of Support: Nil
Conflict of Interest: None declared.
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