Fracture resistance of endodontically treated single rooted premolars restored with Sharonlay: An in vitro study.

AIMS: The aim of this study is to compare in vitro the fracture resistance of the endodontically treated tooth restored with a novel design Sharonlay, with the two component restorative method, i.e., post with separate onlay, and onlay without post. SUBJECTS AND METHODS: 45 single-rooted mandibular second premolars were taken, and root canal treatment was performed. Teeth were then randomly divided into three groups (n = 15) based on the type of restoration given: Group I - metal onlay with cast post extension (Sharonlay), Group II - metal onlay with prefabricated metal post (Mani post system), Group III - metal onlay over endodontically treated tooth obturated with Gutta-percha (control group). Fracture resistance was checked using the Instron universal testing machine and the fracture patterns were analyzed.
RESULTS: According to the values recorded, Group I showed maximum mean fracture resistance followed by Groups II and III. Statistically significant difference was found between Groups I and II and Groups I and III and statistically significant difference was found between Groups II and III.
CONCLUSION: A single unit component Sharonlay, gives higher fracture resistance to a premolar as compared to (a) metal onlay with prefabricated metal post and (b) metal onlay over endodontically treated tooth.

RCT Entities:   Population Interventions Outcomes

INTRODUCTION

Many changes occur to a tooth after root canal treatment, including changes in the physical and chemical properties of dentin, its elasticity, resistance to fatigue, changes in the morphology, biomechanical behavior, and loss of proprioception.[1] The root canal treatment procedures, i.e., access cavity preparation, straight line access to the root apex, shaping of the canal, use of various irrigants including the chelating agents, etc., further adds up to the removal of sound tooth structure influencing the fracture resistance of the teeth.[234] Since a direct relationship exists between the amount of remaining tooth structure and the ability to resist occlusal forces, it is, therefore, important to provide a suitable post endodontic restoration as soon as possible, after completion of the root canal treatment.[5]

Studies have clearly shown that oblique loads generate much more intense cervical stress than loads directed along the long axis of the tooth.[67] This is usually seen in single rooted teeth due to nonaxial loading.[8] A restoration must protect the tooth against these forces. The restoration must minimize further loss of coronal and radicular tooth substance during preparation[5] and the restoration must provide adequate retention and stability.

Many treatment alternatives for corono-radicular reconstruction for cervical reinforcement of endodontically treated teeth have been studied and successfully being used, like custom cast post and core, prefabricated post with direct core, fiber post with the composite core, prefabricated post/custom made post (prophylactic post) all finally restored with an onlay or a crown.[910111213] A full coverage restoration crown is usually advocated when the remaining tooth structure is weak. Richmond crowns have been advocated to provide occlusal and cervical reinforcement when there is minimal coronal tooth structure.[14]

As preservation of tooth structure is the main aim of minimally invasive dentistry, onlay could be a more conservative alternative to full coverage restorations especially when the remaining coronal structure is adequate. Yamada et al. reported that endodontically treated premolars with mesio-occluso-distal cavities could be successfully restored with cast metallic onlay and inlay. According to Marzouk, an onlay is indicated when the cuspal width to length ratio equals or exceed 1:2 and when the ratio is <1:2, a crown is indicated.

Sharonlay[15] is a new corono-radicular design, which is a single component cast restoration carrying an onlay with a post extending into the radicular portion of the tooth. It is hypothesized that the onlay protects the coronal tooth structure, and the post protects the cervical region of the tooth against the oblique loads. Hence, the purpose of this study is to compare in vitro the fracture resistance of the endodontically treated tooth restored with this novel design Sharonlay, with the two component restorative method, i.e., post with separate onlay, onlay without post, thus studying the reinforcing ability of these three treatment options for premolars.

SUBJECTS AND METHODS

A total of 45 single-rooted mandibular second premolars of similar dimensions were collected within 6 months which were extracted for orthodontic treatment and were stored in formalin until testing. Teeth with open apices, curved apices, oval canals, preexisting restorations, cracks, fractures, etc., were not included in the study. Root canal treatment was performed using circumferential filing till 50 K file and were obturated using lateral condensation technique using zinc oxide-eugenol sealer. The teeth were then randomly divided into three groups based on the type of restoration given:

Group I - Fifteen teeth with metal onlay with cast post extension (Sharonlay)

Group II - Fifteen teeth with metal onlay with prefabricated metal post (Mani post System) extending 1 mm above the CEJ - separating from the onlay by 2 mm cement base

Group III - Fifteen teeth with metal onlay over endodontically treated tooth obturated with Gutta-percha (control group).

In Group I and II, root canal filling was removed with peeso drill #1 to a depth of 7 mm (from CEJ on lingual side). The canals were enlarged from the canal orifice up to peeso #4 and the canal orifice enlarged using peeso #5.

An onlay preparation was then prepared for all the three groups, connecting the access preparation with the proximal extension of 3.5 mm on mesial and distal walls of the cavity from their respective marginal ridges. The width of the buccal and the lingual cusp was kept 2.5 mm each. The internal walls of the coronal cavity were finished with 5° taper on each wall. Cusp reduction was then done 1 mm of buccal cusp and 1.5 mm of the lingual cusp. The reverse bevel was placed on the buccal (0.5 mm) and the lingual (1 mm). Reverse bevel was joined to the bevel given on proximal aspect of the preparations. In Group I, a wax pattern of the post space with the onlay was made (Sharonlay design). The pattern was then casted and cemented in prepared teeth with ZnPO4 cement. For Group II, prefabricated metal post (Mani post system) corresponding to peeso #4, were used. The posts were cut at a length of 8 mm so that it extends 1 mm above the lingual CEJ. Posts were cemented using ZnPO4 cement. ZnPO4 base was then placed over the metal post till the orifice, to maintain standard onlay dimensions. Wax pattern was prepared for the onlay cavity and casting was done and cemented using ZnPO4 cement. For Group III, ZnPO4 base was placed over Gutta-percha at the level of CEJ and onlay was prepared as in Group II, and was cemented using ZnPO4 cement.

Prepared samples were then mounted in the stainless steel holders with the help of the self-cure acrylic resin to embed the root portion 2 mm below the cement enamel junction to simulate the oral conditions. These stainless steel holders were color coded according to the groups:

Group I - Red

Group II - Blue

Group III - Yellow

After the samples had been mounted, fracture resistance was checked using the Instron universal testing machine. Samples were loaded one by one on a custom jig which was made at an angle of 45°, to simulate the angular loading on the premolar teeth in the oral cavity, and the load required to fracture the samples were recorded in Newtons and the fracture patterns were analyzed.

RESULTS

According to the values recorded, Group I showed maximum resistance to fracture as compared to the other two groups with a mean fracture resistance of 514.67 N as compared to 358.67 N and 265.33 N for Groups II and III, respectively [Graph 1]. Statistical analysis was done using ANOVA test and Bonferroni test.

Graph 1

Mean fracture resistance in the three groups

According to the statistical analysis, a significant difference was found among Groups I and II, Groups I and III but insignificant difference between Groups II and III [Table 1].

Table 1

Intergroup comparison

For the fracture patterns, maximum samples in Group I, showed fracture at the apical extent of the post indicating the reinforcement provided by the single component Sharonlay [Table 2]. However, maximum fractures in Groups II and III, showed oblique fractures splitting the cusps uptil the neck of the tooth, indicating no effect on the cervical region of the tooth.

Table 2

Fracture patterns

DISCUSSION

Studies have shown that the greatest stresses are generated at the cervical region of the endodontically treated teeth, especially in single rooted teeth due to angular forces acting on them and also,[1617] due to less surface area for dissipation of forces. It is controversial whether the post reinforces the tooth or weakens the tooth. Irrespective of this controversy post sometimes are required for additional retention. However, in this study, we are trying to study the reinforcing ability of a prophylactic prefabricated post and an onlay with a post extension (Sharonlay).

Results showed a statistically significant difference between the groups, i.e., Groups I, II, and III. Group I showed highest mean fracture resistance among all the groups. This difference with previous studies may be because in their study, finally the groups have been restored with complete cast crowns with ferrule, which definitely reduces the cervical stress by distributing the stress along the entire root.[161819] In our study, since onlay is given with the aim of preservation of natural tooth structure, no crown ferrule has been given but the presence of reverse bevel act as a core ferrule in the preparations which also helps in dissipating the stresses and reinforces the tooth. However, onlay does not reinforce the tooth at the neck as a crown does, so cervical reinforcement becomes a major concern with onlay.

Sharonlay being a single component have a greater surface area for dissipation of stresses as compared to a two unit component, thereby taking more loads before fracture. Another possible explanation to this difference may be the postadaptation to the radicular dentin. Where custom cast post adapts perfectly to the tooth, prefabricated post do not. According to study by Sorenson et al., post adaptation significantly increases the fracture resistance of the restored teeth.[20]

The lower values of fracture resistance in Group II can also be attributed to the angulation of external loading of the groups is such that location of prefabricated post in such cases receives only minimal stress, thereby doing little to reinforce the tooth.[21] Thus, the prefabricated post with onlay in Group II did little to reinforce the tooth. Although Group II shows slightly better results than Group III, the results are comparable (statistically insignificant) among Groups II and III, which is in accordance with a study conducted by Heydecke et al. and Gary et al.[2122] However, the reason for such difference could be that control group (without post reinforcement but also without additional tooth structure loss due to post space preparation) exhibited a comparable fracture load to Group II.[22] Hence comparable results are observed in this study.

Zinc phosphate cement has been used in this study as a luting agent for the post to avoid the bias that bonding luting agents such as glass ionomer cement (GIC), adhesive resin may reinforce the tooth by bonding to the radicular dentin thereby, influencing the results.[23] Studies otherwise have also shown that zinc phosphate efficacy in luting is similar to that of GIC.[24]

According to a study by Sathyanarayana et al., the maximum voluntary bite force in the premolar region in males is 422.22 N and in females is 349.24 N.[25] Since the Sharonlay group took the maximum load of an average of 514 N, Sharonlay could be used as a post endodontic restoration and preserve the integrity of the endodontically treated teeth. Moreover, the fracture patterns of specimens are in congruence with the previous studies. Our in-vitro study showed that maximum fracture lines in Sharonlay group were deep horizontal fracture apical to the post, rendering the tooth nonrestorable. However, this is not much of clinical significance as the loads to fracture Group I specimens were even higher to the maximum voluntary bite force recorded in the oral cavity, which is not practical clinically. In Groups II and III, the load the specimens could take was much lower than the average masticatory load and maximum fractures were oblique fractures, splitting the two cusps and separating the segments till the cervical area which are favorable indicating future restorability. This is among the initial studies undertaken on this novel design. Further studies have to be conducted on this in future to compare its efficacy in cervical reinforcement to various other esthetic restorative alternatives.

CONCLUSION

Within the limitations of this study, it can be concluded that a single unit component Sharonlay, gives higher fracture resistance to a premolar as compared to (a) metal onlay with prefabricated metal post and (b) metal onlay over endodontically treated tooth. Therefore, it can be suggested that Sharonlay is a possible alternative as a post endodontic restoration for upper and lower premolars.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest