An in vitro comparative evaluation of physical properties of four different types of core materials.

INTRODUCTION: Compressive and tensile stresses of core materials are important properties because cores usually replace a large bulk of tooth structure and must resist multidirectional masticatory forces for many years.
MATERIAL AND METHODS: The present study was undertaken to find out the best core build up material with respect to their physical properties among resin-based composites. Individual compressive, tensile, and flexural strength of fiber-reinforced dual cure resin core build up material, silorane-based composite resin, and dual curing composite for core build up with silver amalgam core was used as control were evaluated and compared using universal testing machine. Data were statistical analysed using Kruskal-Wallis test to determine whether statistically significant differences (P < 0.05) existed among core materials. Both dual cure composite materials with nanofillers were found superior to amalgam core. The silorane-based material showed the highest flexural strength, but other mechanical properties were inferior to dual cure composite materials with nanofillers.

INTRODUCTION

The restoration of endodontically treated teeth offers many challenges for the restorative dentist because of the large percentage of failures and this high incidence of failure has led to the development of a magnitude of restorative alternatives for endodontically treated teeth.[1] A foundation restoration or core is used to build a badly broken down tooth to restore the bulk of the coronal portion of the tooth to an ideal anatomic form before the full coverage crown is placed. It should provide the patient a long-lasting restoration with adequate function. Subsequent tooth preparation is greatly simplified if the tooth is built up to an ideal contour.[2] Because the core becomes an integral part of the structure of the tooth, it should provide strength to resist intraoral compressive and tensile forces.[3]

Compressive and tensile stresses of core materials are thought to be important because cores usually replace a large bulk of tooth structure and must resist multidirectional masticatory forces for many years.[456] Flexural strength of core materials is considered to be sensitive to surface imperfections such as cracks, voids, and related flaws, which can influence the fracture strength of brittle materials.[7]

Development and advances in non-gamma-2 amalgams and the new concepts of bonding dental amalgam to tooth structure have helped to ensure that amalgam remains one of the material widely used for core build up procedures in posterior teeth.[89] However, this material has disadvantages like poor color, low-initial strength, lack of inherent bond to tooth structure, and high coefficient of thermal diffusibility. Patients are increasingly requesting biocompatible metal-free restorative systems, which are readily available in the market.[1011]

When considering resin-based composites, one should keep in mind that this in fact represents a composite family consisting of, particulate filler composites and fiber-reinforced composites, being the subject of the present study.[12]

LuxaCore dual (DMG, Germany) can be automatically mixed and dispensed with intraoral tips, has ideal flow properties allowing tooth substance, and posts to be totally surrounded, while avoiding gaps or air pockets and is available in different shades. LuxaCore dual (DMG) not only possesses similar strength, flexibility, and insulation properties to that of dentin but it also cuts and trims like dentin and is not too hard as many other core or general restorative composites tend to be. It has thorough and even distribution of nanoparticles throughout the resin matrix, resulting the virtual elimination of particle agglomeration.[13]

ParaPost ParaCore Automix 5 ml (Coltene Whaledent, USA) is a fiber-reinforced, dual core, and radiopaque core build up material. It exhibits a stackable, non-slumping consistency and is formulated to cut similar to dentin, allowing the bur to move smoothly between natural tooth structure and the material without creating troughs and grooves. It incorporates glass particles that impart high strength.[14]

Despite innovative improvements during the years and the excellent acceptance of methacrylate-based restorative materials, polymerization shrinkage stress is still considered as being their main drawback.[15] Several in vitro studies found a significant correlation between marginal adaptation of dental composites or microleakage and reduced shrinkage stress.[161718]

One of this new low-polymerization shrinkage materials currently introduced in the European market is based on innovative monomer system — silorane — obtained from the reaction of oxirane and siloxane molecules.[19] The novel resin is considered to have combined the two key advantages of the individual components: low-polymerization shrinkage due to the ring opening oxirane monomer and increased hydrophobicity due to the presence of the silorane species. The mechanism of compensating stress in this new system is achieved by the opening of the oxirane ring during polymerization.[20] FiltekTM P90 is the first composite to shrink less than 1% with low-shrinkage posterior restorative system based on 3M ESPE's latest product innovation: silorane chemistry. It combines the lowest shrinkage silorane-based composites with a dedicated two step self-etching bonding system. This silorane-based composite shows the lowest volumetric shrinkage to date. It has excellent marginal integrity, easy to handle, non-sticky, and excellent ability to hold shape. It is strong, durable and has low water sorption for substantially decreased exogenic staining.[21]

Hence, the present study was undertaken to find out the best core build up material with respect to their physical properties among resin-based composites.

Objectives were

To evaluate and to compare the individual compressive, tensile, and flexural strength of fiber-reinforced dual cure resin core build up material (ParaCore, Coltene Whaledent), silorane-based composite resin (P-90 FiltekTM) and dual curing composite for core build up (Luxacore, DMG, Germany) with silver amalgam core was used as control.

MATERIALS AND METHODS

All the four core build up materials were manipulated according to manufacturers’ instructions. Cylindrical specimens measuring 6-mm high and 4 mm in diameter were made in Teflon and aluminium molds, covered with polyethylene strips and held within clamps for 30 s at room temperature (23 ± 10°C) and relative humidity of 50 ± 5%. Photopolymerization was initiated by illuminating two surfaces of the specimen for 40 s. When the specimen's height was greater than 3 mm, the samples were taken out of molds and illuminated for 40 additional seconds to ensure a good polymerization depth. The samples were then left to set in the molds at room temperature for 10 min before being stored at 37°C in a humid environment. All properties were measured at end of 24 hrs. The dimensions of the samples were measured by use of a vernier caliper. Test specimens were mounted vertically between disks of blotting paper on the platens of a universal testing machine (Instron Machine 3366, made in USA). Specimens were loaded at a cross-head speed of 0.5 mm/min. Failure load was recorded. Ten specimens were made and tested for each core material.

Diametral tensile strengths were determined in a manner similar to the compressive test. Specimens 6 mm in diameter and 2-mm thick were mounted diametrically between disks of blotting paper on the platens of universal testing machine. Load was plotted against time so that any measurable permanent deformation could be noted. Ten specimens were made and tested for each group. A three-point bending test was carried out using a universal testing machine to evaluate the flexural strength. The cross-head speed was set at 0.1 mm/min. The mean specimen dimensions were 25 × 2 × 2 mm. Ten samples were measured for each group.

Statistical analysis for the strength of core materials was performed, and the mean value with its standard deviation was calculated for each core material. Kruskal–Wallis test was performed to determine whether statistically significant differences (P < 0.05) existed among core materials.

RESULTS

According to the results of factorial variance analysis done for compressive strength, dimetral tensile strength, and flexural strengths of core materials and the interactions between the factors were statistically significant.

Data were analyzed using Statistical Package for Social Sciences (SPSS) version 11.5 (SPSS Inc, Chicago IL). Descriptive statistics were calculated. Kruskal–Wallis test was applied to assess the difference between flexural, compressive, and diametral strength between groups. Using this method, variables selected will be those that show the significant difference at the 95% level (P < 0.05).

The compressive strength values for the materials were not found to be statistically significant. Diametral tensile strength of materials was found to be statistically significant with the values for ParaCore were significantly higher than those for all the other materials investigated. It was determined that amalgam is the weakest of all regarding tensile strength. Amalgam and P90 showed small changes compared with other materials. Significant differences were observed in the diametral tensile strength values for ParaCore and LuxaCore resin composite materials [Table 1].

Table 1

Strength values of all the experimental materials

Flexural strength values were significantly higher for P90 composite resin at humid temperature than those for all the materials tested with. Amalgam and ParaCore composite resin material showed the lowest flexural strength values compared with other materials.

DISCUSSION

According to the results of the study, resin composite core build up materials showed better mechanical properties than silver amalgam core, which is similar to the results of study done by Bonilla et al.[22] This could be due to the micromechanical bonding (monoblock effect) of resins to the tooth structure and resin composites behaving like stress breakers as well as complete curing of material with dual cure technology.[232425] Results of our study were consistent with the results of study done by Choi and others who found that some resin composites exhibited compressive strengths more than that of amalgam and could be used as alternatives to amalgam.[17] In our study, ParaCore composite resin material showed excellent physical properties because it is reinforced with glass fibers; it is a dual cure material that will ensure complete cure, thereby improve the strength of the material. The macroscopic size of the unidirectional fiber bundles used in fiber reinforces the resins and improves their mechanical properties. The presence of fibers affects the fracture process that results in interrupting crack growth progression and thus enhances the fracture toughness of the fiber-reinforced composite material.[14] The present study is in agreement with the study done by Peterson et al.[23]

LuxaCore showed comparative results with the ParaCore composites in all three properties, as both are dual cure materials. The nanotechnology used in LuxaCore dual eliminates particle agglomeration by incorporating a proprietary coating process during particle manufacture. LuxaCore dual (DMG) possesses strength, flexibility, and insulation properties similar to that of dentin, according to manufacturers. Type of fillers in LuxaCore is aluminoborosilicate glass, fumed silica, and titanium oxide, which could be the reason for their high strength. Similar results are found by Seung-Geun Ahn, and John A. Sorensen, in a study.[26]

In the present study, FiltekTM P90 (3M ESPE) composite resin showed the best flexural strength because of their the flexural fatigue limit is 80 MPa, indicating that under clinical conditions may likely to withstand mastication forces without fracturing even after many years in service.[21] In the present study, FiltekTM P90 composite resin was found to have lesser compressive strength compared with ParaCore and LuxaCore without statistical significant difference, whereas the diametral tensile strength was statistically low.

The results of our study indicate that, on the basis of their mechanical properties, dual cure core build up resin composites with nanofillers may be used as alternatives to amalgam core. When choosing the core material, the amount and mode of stress must be considered because it affects the stress transmission of the post. As the firmness increases, the stress goes more directly to the root and less to the post. Metal cores are known to cause great stress in the coronal part and to send the stress directly to the root. It is also important to choose a core that has similar physical properties with the post because of the favorable strong interface and lower risk of microleakage and failure.

Further studies concerning about fatigue load, insufficient adhesive strength between the resin cement and the crown, the effect of deformation of the resin core due to masticatory or shearing force on failure of glass fiber post are needed to be conducted.

CONCLUSIONS

With the constraints of the test design, the following conclusions were drawn from this in vitro study:

Both dual cure composite materials with nanofillers were found superior to amalgam core.

The silorane-based material showed the highest flexural strength, but other mechanical properties were inferior to dual cure composite materials with nanofillers.

Referring to the values of maximum fracture load and mean compressive fracture load, dual cure resin composites have high values and are recommended as tooth-colored resin core build up materials.