Ex vivo fracture resistance of endodontically treated maxillary central incisors restored with fiber-reinforced composite posts and experimental dentin posts.

AIM: To compare the fracture resistance of teeth restored with fiber-reinforced composite (FRC) posts and experimental dentin posts milled from human root dentin.
MATERIALS AND METHODS: Thirty maxillary central incisors were divided into three groups of ten each. Twenty teeth were restored with FRC posts and solid dentin posts and numbered as Groups 2 and 3 respectively while Group 1 acted as the control, without any post. The teeth were loaded at 135° angle to their long axes after core build-up and the failure loads were recorded.
RESULTS: One-way Analysis of Variance (ANOVA) and Bonferroni multiple comparisons revealed a significant difference among test groups with the control group showing the highest fracture resistance, followed by the dentin post group and lastly the FRC post group.
CONCLUSIONS: Teeth restored with dentin posts exhibited better fracture resistance than those restored with FRC posts.

INTRODUCTION

Endodontically treated teeth are known to present a higher risk of biomechanical failure than vital teeth.[1] Posts are needed for restoring teeth with insufficient coronal tooth structure to retain a core for definitive restoration. Creugers et al, reported that survival rates vary largely in endodontically treated teeth restored with different post and core systems.[2] An ongoing debate regarding the post restorations and their increasing usage still exists.

Post material plays a crucial role in the biomechanical performance of endodontically treated teeth. Ideally, the post material should have physical properties such as modulus of elasticity, compressive strength and thermal expansion, as well as esthetics similar to those of dentin and it should bond predictably to root dentin.[3] But the only material that can substantiate all these properties can be none other than dentin itself. A few reported cases utilizing dentin as post material have shown successful outcomes.[45] Hence, the probability of human dentin to serve as a post material needs to be investigated.

The aim of this ex vivo study is to evaluate and compare the fracture resistance of endodontically treated maxillary central incisors restored with prefabricated fiber-reinforced composite (FRC) posts and experimental dentin posts milled from human root dentin using Computer Aided Designing- Computer Aided Manufacturing (CAD- CAM).

MATERIALS AND METHODS

Thirty freshly extracted maxillary central incisors with completely formed apices were selected on the basis of similar root sizes and absence of caries, visible fracture lines or cracks. The root dimensions were measured with digital caliper for standardization (mean root length being 13± 1.0 mm). Teeth were then stored at 37°C in distilled water.

Specimen preparation

The coronal portions of ten incisors were sectioned perpendicular to the long axis, at a level 5 mm incisal to the labial cementoenamel junction (CEJ) with the use of a water-cooled diamond rotary cutting instrument (40,000 cycles/min). These comprised the control group. The clinical crowns of remaining 20 teeth were sectioned 2 mm incisal to the labial CEJ. In all 30 teeth, access openings were done, working lengths established 1.0 mm short of apical foramina and canals were prepared using a step-back technique to a master apical file size of 60 (Dentsply, Maillefer); 5.25% sodium hypochlorite and 17% ethylene diaminetetra acetic acid were alternatively used for 1 min each during biomechanical preparation. This was followed by a final rinse with distilled water before obturation with gutta percha and AH plus endodontic sealer (Dentsply, DeTrey, Konstanz, Germany) using cold lateral compaction method.

A thin coat of polyvinylsiloxane (Aquasil ultra LV, Dentsply, Germany) was painted on the root surfaces of all 30 teeth to within 1 mm of the CEJ, to simulate the effect of periodontal ligament.[6] Samples were embedded vertically in acrylic resin blocks 3 cm in height and 2 cm in diameter to a level 2 mm apical to the CEJ. During this, samples were kept in a moist environment to avoid dehydration of the dental tissues. The prepared teeth were divided into two experimental groups of ten specimens each. In these groups, post spaces of 10 mm length were prepared using special calibrated drills supplied from the manufacturer of FRC post, leaving 4 mm of root filling intact to preserve the apical seal. Study groups hence comprised:

Group 1 Teeth without any post space preparation and restoration of access openings done with glass ionomer cement (GC Fuji II - GC Corporation, Tokyo) (Control group)

Group 2 Teeth restored with prefabricated glass fiber-reinforced composite tapered posts (Snow post, Danville, Germany)

Group 3 Teeth restored with solid dentin posts

Preparation of experimental dentin posts

Five healthy maxillary canines freshly extracted for periodontal reasons were selected and decoronated. Each tooth was sectioned longitudinally (mesiodistally) into two halves along the root canal. Cylindrical dentin blocks were prepared out of each section using diamond drills under intense water cooling and were then subjected to CAD-CAM using advanced software at the Central Institute of Plastics and Engineering Technology (CIPET), Chennai, India. The milling was accomplished through manual part programming, using three-axes CNC (Computer Numerical Control) milling machine [Figure 1] to generate ten dentin posts of standardized shape and dimensions (12 mm length, 1.6 mm diameter) similar to FRC posts.

Post cementation

The prepared post spaces were irrigated with 0.2% chlorhexidine,[7] rinsed with water for 10 sec and dried with paper points. Posts (FRC posts and dentin posts) were cemented using dual-cure Rely X U100 self-adhesive universal resin cement (3M ESPE, St. Paul, Germany) according to the manufacturer's instructions. Base and catalyst cement pastes were dispensed on the mixing pad (1:1 ratio) and mixed within 20 sec. Cement was spread to the post and placed in the canals. Posts were held under moderate finger pressure. After 2 sec of light curing, excess cement was removed using cotton pellet. Light curing was further done for 20 sec with the curing unit (Cu 100, Unicorn).

Core fabrication

Custom-made standardized transparent polytetra-fluoroethylene matrices were used for core buildup using dual-cure composite resin (Biscore, Bisco Dental Products, Schaumburg, USA). Height of cores was adjusted to 5.0 ± 0.2 mm in the experimental groups. Also, tooth preparation was done in all ten teeth of the control group to a height of 5 ± 0.2 mm.

Mechanical loading

The samples were subjected to thermocycling (5000 cycles between 5°C to 55°C with a dwell time of 30 sec at each temperature)[8] and stored in distilled water for 24 h at 37°C in a humidor (100% relative humidity) to simulate conditions in the oral cavity prior to the fracture test. The samples were then mounted on a specifically designed inclined test block made of steel and tested on Universal Testing Machine (Instron 3369, UKAS Corporation, USA). A compressive load was applied 2 mm cervical to the incisal edge on the palatal aspect, at an angle of 135° to the long axis of the tooth at a crosshead speed of 2.5 mm/min[9] [Figure 2]. The failure threshold was defined as the point at which the loading force reached the maximum value for fracturing the root, post or core.

Mean failure load values were calculated for all groups [Table 1]. Data was analyzed using SPSS 14 software. One-way ANOVA showed significant difference (P < 0.001) among the test groups [Table 2].

Table 1

Mean failure load values for the three groups (in kg)

Table 2

One-way analysis of variance

RESULTS

Control group (Group 1) showed the maximum mean failure load value at 139.44 Kg, followed by the dentin post group (Group 3) and FRC post group (Group 2) at 100.88 Kg and 81.51 Kg respectively (depicted by the graph).

The intergroup and intra-group comparisons were done using Bonferroni multiple comparison test [Table 3], that revealed a significantly higher fracture resistance for the control group. All the groups differed significantly from each other. Group 2 had a significantly lower fracture resistance than Groups 1 and 3.

Table 3

Bonferroni multiple comparisons post hoc test

DISCUSSION

Numerous studies have been conducted till date regarding the potential of a post to reinforce endodontically treated teeth with conflicting results.[11011] The results of the present ex-vivo study showed a statistically significant increase in the fracture resistance of teeth restored with experimental dentin posts compared to the teeth restored with FRC posts which suggests the possible reinforcing potential of the dentin post.

The intergroup and intra-group variance in this study was reduced to a great extent by standardization of the test samples and restorative procedures. The sample dimensions were standardized using a digital caliper and the Analysis of Variance (ANOVA) was used to determine any significant difference across the groups. Dimensions of the dentin posts were standardized according to those of the fiber posts using CAD-CAM. Post space dimensions were standardized by use of calibrated low-speed drill provided by the manufacturer of FRC post. A custom transparent matrix was used to standardize the dimensions of core buildups.

Rely X U100 self -adhesive universal resin cement (dual-cure) used in the study allows a single-step luting process thereby eliminating any procedural technique sensitivity. Dual-cure resin cements are expected to combine the favorable properties of both photo curing (sufficient time and control for proper seating of the post into the canal) and auto curing (polymerization without the influence of post space depth) systems.[12] Moreover, this cement does not require any surface pre-treatment of the substrate such as silanation or etching.[12]

The study simulated a clinical scenario by creating an artificial periodontal ligament using polyvinylsiloxane impression material coating on the root, which has a modulus of elasticity very similar to that of natural periodontal ligament.[6] Cores were not restored with crowns to exclude any external strengthening influence on the post and core[13] and to eliminate parameters such as material structure, shape, length and thickness provided by crown restorations. In this manner, the structural integrity and fracture resistance of a post and core foundation could be tested more precisely.[14] The static compressive load was applied at a speed of 2.5 mm/min at an angle of 135°. Guzy and Nicolls[9] reported that for incisors, a loading angle of 130-135° is chosen to simulate a contact angle found in Class I occlusion between maxillary and mandibular anterior teeth. A higher speed of load application was used because at lower speed more fracture deformation takes place so that higher values of fracture resistance may be obtained.[14]

The mean failure load value of the control group in our study was more than the experimental dentin post group because of more amount of remaining tooth structure. It has been suggested that remaining dentin thickness is a critical factor in the resistance of the dentin/root restorative complex during function.[1]

Barjao-Escribano et al., showed that posts possessing elastic modulus similar to that of dentin and core have a better biomechanical performance.[15] A major objective in dentistry is the restoration of endodontically treated teeth with metal-free, physio-chemically homogeneous materials possessing these properties.[16]

The failure of the FRC post group in this study may be attributed to the difference in the biomechanical properties between the FRC post and the root dentin. The modulus of elasticity of glass fiber posts is ~ 40 GPa whereas the modulus of elasticity of root dentin is ~ 14.2 GPa and of core material is ~ 13.5 GPa. This difference might create stresses at different interfaces and the possibility of post separation and failure.[17] An added reason for failure at the post cement interface is the presence of interfacial gaps. Moreover, since the resin chemistry of the epoxy resin-based posts and methacrylate-based adhesive resin differs completely, the adhesion achieved may not be reliable.[18]

In the present study, teeth restored with solid dentin posts exhibited higher fracture resistance than those restored with FRC posts. This can probably be explained on the basis of the following factors:

Physiomechanical properties of a dentin post

Uniform stress distribution

Shock-absorbing potential of a dentin post.

Dentin has a complex microstructure and a modulus of elasticity 13-18 GPa, varying in different locations and directions, which may provide a mechanism that inhibits crack propagation in dentin.[19] The dentin post closely resembles root dentin in all the physical properties like modulus of elasticity, viscoelastic behavior,[20] compressive strength,[21] thermal expansion[22] etc. Furthermore the fracture toughness of dentin has been found to be better than most of the current restorative materials.[23] A dentin post forms a micromechanical homogenous unit with the root dentin that results in uniform stress distribution.[2425] The similarity in elasticity of a dentin post to root dentin may allow post flexion to mimic tooth flexion so that the post acts as a shock absorber, transmitting only a fraction of the stresses placed upon the tooth to the dentinal walls.-[26]

This study hence substantiates the notion that a post which is almost similar in all physical properties to the remaining root dentin along with good adhesion may increase the fracture resistance of the tooth. Since dentin is a dynamic substance with a range of physical properties, its biomechanical behavior cannot be predicted accurately. Hence future research is needed to highlight the outcomes of the study. Moreover, further studies inclusive of cyclic loading of the dentin post are anticipated to provide a better insight into its properties.

CONCLUSION

Within the limitations of this study, it can be concluded that:

Teeth restored with dentin posts exhibit better fracture resistance than those restored with FRC posts.

This article opens an introductory gate in support of the clinical implications of dentin posts.

Dentin may emerge as a successful alternative to currently available post materials. However, further in vitro and in vivo trials are required in this direction.