INTRODUCTION

The different properties of core materials and their various applications have been extensively reported in the dental literature. In particular, a technique of coronal-radicular stabilization of endodontically-treated posterior teeth with amalgam has been widely reported. Using this technique, Nayyar and others reported 400 successful restorations when cast crowns were placed as the final restoration.1 Amalgam has many favorable properties, which include high compressive strength and dimensional stability in the oral environment. Amalgam has been used successfully as a final restoration to replace tooth tissue in bulk in root-treated posterior teeth.2 However, this material does have disadvantages—poor color, low initial strength, lack of inherent bond to tooth structure and a high coefficient of thermal diffusivity.3 In addition, there has been much controversy regarding its alleged harmful effects on systemic health, although these effects have never been scientifically proven.4 Alternatives to amalgam, with different useful attributes, are now available and include fluoride release, pleasing aesthetics, adhesion to tooth structure, command cure, choice of curing methods and desirable handling properties. Against this backdrop, patients are increasingly requesting bio-compatible metal-free restorative systems.

Both resin composite and glass-ionomer cements have been advocated as alternative core build-up materials to amalgam. Resin composites offer an advantage over amalgam in that the material can be rapidly polymerized at the chairside and, therefore, prepared at the same visit. In addition, resin composites possess the property of adherence to tooth substance via the use of dentin adhesive systems. Disadvantages associated with the use of resin composites include polymerization shrinkage, microleakage and inferior mechanical strength. Glass ionomers, too, have many favorable characteristics, including self-adherence to tooth tissue, fluoride release and a favorable coefficient of thermal expansion. As a group of materials, glass ionomers have disadvantages that limit their use in core buildup applications; these include a lack of inherent strength, brittleness and unfavorable setting and maturation characteristics. Some improvement is possible with the incorporation of stainless steel powders, silver/tin alloy fibers or flakes and by high temperature sintering of silver and glass powders, although enhancement of physical properties by these means is marginal.5–6 In a study comparing the mechanical properties of direct core build-up materials, Combe and others concluded that cermets were not particularly suitable for this application, although they noted that cermets appeared to have a good clinical success rate.7

Glass ionomers incorporating hydrophilic resins are known as resin-modified glass ionomer cements (RMGIC) or hybrid ionomers. The methacrylate components are polymerizable by light curing, allowing command set of the material in addition to the acid-base reaction of a conventional glass ionomer. These materials benefit from having fluoride release and recharge characteristics similar to that of glass ionomer.8 These are chemically complex materials and are not fully understood. It has also been shown that the physical properties of resin-modified glass ionomers are affected by the presence of moisture.9

Various post-core systems have been compared: Gelfand and others10 and Hoag and Dwyer11 demonstrated no significant difference in the fracture force between cast post and cores and amalgam corono-radicular restorations treated with cast crowns. A study conducted by Howdle and others showed that different bonding agents could be utilized to improve the success rate of corono-radicular cores by preventing microleakage around the restoration.12 However, conflicting results, published by Kern and others, revealed that corono-radicular restorations had significantly lower mean fracture loads compared to post-retained amalgam cores when used as a core for a cast crown.13

The strength of core materials is an important feature that has received much attention. A core is usually required to replace a large bulk of tooth structure and, therefore, must resist multidirectional masticatory forces for many years. Bonilla and others compared the fracture toughness of several core materials and found that amalgam and composite were able to withstand stresses generated during mastication.14 These findings were consistent with the results of Cho and others15 and Gateau and others,16 both of whom found that some composites exhibited compressive strengths equal to that of amalgam and could be used as alternatives to amalgam. In contrast, glass ionomer cements are not regarded favorably as suitable core materials in areas of the oral cavity subjected to stresses. Cho and others found that GIC materials were markedly weaker than resin composites and amalgams. Similarly, Engelman also concluded that GICs should be used cautiously as core build-up materials.17 Combinations of these core materials with different post-retention systems have displayed similar results.

In addition to strength, there are many other characteristics that influence the choice of one core material over another. Oliva and others investigated the dimensional stability of silver amalgam and a conventional composite. These researchers found that amalgam used as a core material is more dimensionally stable than composite when exposed to moisture.18–19 In these studies, the dimensional instability of composite cores due to water sorption was reported to have significantly affected the seating of cast restorations. In contrast, a study by Vermilyea and others found no significant difference between the seating of cast restorations on natural tooth cores and composite cores that had been exposed to moisture. In addition, this article suggests that well-fitting provisional restorations may minimize this problem.20 The effect of thermal changes on core materials has also been studied. Yang and others found that non-metallic dowel and core materials generated greater thermal stresses than their metallic equivalents. These stresses may cause fracture of the dental structure or leakage of the restoration, ultimately resulting in restoration failure.21

This study determined the suitability of three core materials for coronal-radicular build-up by measurement of fracture resistance of these restorations in root-filled premolars in vitro under compressive load and compared these to a group of unrestored teeth.

METHODS AND MATERIALS

Sixty sound, intact, extracted maxillary first premolars were collected and stored in distilled water. They were then randomly divided into four groups, each comprising 15 teeth:

• Group I Amalgam core

• Group II Resin composite core

• Group III Resin-modified glass ionomer core

• Group IV Sound tooth (control group)

Restoration (Groups I—III)

Gutta-percha was removed to a depth of 3 mm from each canal with a size IV Gates-Glidden bur in a slow speed handpiece. Natural undercuts in the pulp chamber wall were retained in order to assist with core retention. A stainless steel Siqveland matrix band was custom fitted to each prepared tooth prior to core buildup. After core build-up, all the teeth were then stored in distilled water for 24 hours prior to testing.

The following core build-up materials used were:

• Group I Amalgam: Tytin FC fast-set silver amalgam (Kerr/Sybron, Orange, CA, USA)

• Group II Resin composite: Concise resin composite (3M ESPE, St Paul, MN, USA)

• Group III Resin-modified glass ionomer cement (RMGIC): Vitremer resin-modified glass ionomer cement (3M ESPE, St Paul, MN, USA)

All materials were manipulated according to the manufacturers' specifications. The general form of tooth preparation and restoration used in Groups 1 through 3 are shown in Figure 1.

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Group 1: Amalgam Corono-radicular Core Technique

The amalgam was triturated according to the manufacturer's guidelines using an amalgamator. Condensation was then commenced immediately, with the amalgam packed first into the root canal space, then built to a height of 4 mm above the CEJ. Following this procedure, the matrix band was carefully removed and the occlusal surface of the core was carved to an anatomic form.

Group 2: Resin Composite Corono-radicular Core Technique

Both enamel and dentin were etched with 37% ortho-phosphoric acid for 30 and 15 seconds, respectively, then a bonding agent was applied according to the manufacturer's recommendations (Concise Enamel Bond, 3M ESPE). A chemically cured resin composite material was mixed, again according to the manufacturer's guidelines. The core material was initially injected into the root canals before incremental placement of subsequent material. Two minutes after placement, the matrix band was carefully removed and contouring commenced. The core was built to a height of 4 mm above the cemento-enamel junction.

Group 3: Resin-modified Glass Ionomer Cement Corono-radicular Core Technique

Primer was applied to the enamel and dentin for 30 seconds, after which it was air-dried and light cured for 20 seconds. RMGIC was then mixed according to the manufacturer's instructions by adding equal proportions of powder and liquid. The material was injected into the root canal preparations, and the core was subsequently built to a height of 4 mm above the CEJ. The material was light cured in bulk for 40 seconds. After removal of the matrix band, the core was contoured using a high-speed handpiece and composite finishing burs.

Testing

All the teeth were mounted in stainless steel molds with a self-curing acrylic resin to a depth of 2 mm apical to the CEJ. The teeth were then loaded to failure in a Howden Universal Testing Machine (RDP Howden Ltd, Leamington, UK) using a 4.5 mm diameter stainless steel rod placed in the midline fissure at a crosshead speed of 0.5 mm/minute. The fracture force (kN) was recorded for each specimen, and the mode of failure was recorded using a system previously described for teeth restored with dentin bonded crowns.22 The score assigned to each type of failure is shown in Figure 2. The results were analyzed using a one-way analysis of variance (ANOVA) and Bonferroni's test at a 5% level of significance (p=0.05). A non-parametric test was used to interpret the data representing the modes of failure of the different core materials.

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RESULTS

The collected data was compiled and analyzed using the SPSS 10.1 statistical package. One-way ANOVA and Bonferroni's test revealed significant differences in the fracture resistance of the core materials at the 5% level of significance. Values for the mean fracture resistance of each material are presented in Table 1.

Table 1 Mean Applied Failure Load and Standard Deviations

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The mean (sd) fracture resistance (kN) obtained for the amalgam cores was 1.94 (0.22), which was greater than that obtained for the cores constructed from resin composite 0.75 (0.11), resin-modified glass ionomer cement 1.05 (0.2) and the controls 0.79 (0.2). Furthermore, multiple comparisons performed using the Bonferroni's test revealed that significant differences in fracture resistance existed among all the materials except the composite and control groups, which were similar.

The mode of fracture of the core materials was analyzed using non-parametric statistical tests. Cross tabulation of the results revealed the proportion of teeth in each group which fell into each mode of failure subdivision. The data are presented in Table 2, where it can clearly be seen that a higher proportion of the amalgam core failures constituted minimal fractures (66.7%) in comparison to the other materials. Resin composite core materials exhibited more severe fractures, with a high proportion of failures involving greater than half of the core (53%). It can also be seen that all the control teeth failures occurred in a similar manner (Figure 2, Code 2 failure—fracture involving less than half the core). This finding was supported by the Mann-Whitney tests, which were used to compare the mode of failure of one core material against another. Significant differences in the mode of failure were found among all the materials except for amalgam and resin-modified glass ionomer cement (RMGIC), and RMGIC and the controls.

Table 2 Modes of Failure by Restoration Type

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DISCUSSION

This study compared the fracture resistance of three different core materials against the fracture resistance of a sound tooth. This is clinically relevant, because strength in terms of fracture resistance is regarded as a critical indicator of success due to the masticatory and parafunctional forces that a core is subjected to. Prior to testing, tooth preparation and core build-up techniques were largely influenced by the findings of previous studies. Nayyar and others recommended that gutta-percha be removed to a depth of 2 to 4 mm in each canal and that the remaining pulp chamber should be of an adequate width and depth to provide retention for the core material.1 Kane and others found that the extension of amalgam into the root canal space was beneficial only when the pulp chamber height was 2 mm.23 Therefore, this study attempted to conform to recommendations from previously acknowledged publications wherever feasible. Previous studies measuring fracture resistance have used 5 to 10 teeth in each group;10–111323–25 however, in the current study, 15 teeth were used in each group, because of the potential variability introduced by tooth anatomy, core geometry and surface finish.

Analysis of mean fractural resistance values revealed that amalgam was better able to resist fracture when subjected to a compressive load; whereas, composite was least able (Table 1). The cores constructed from resin-modified glass ionomer cement ranked second behind amalgam (mean fracture strength 1.05 kN). The significance of these findings was determined by a one-way ANOVA test, where p=0.05. Similarly, amalgam core failure proved to be much less severe in comparison to the resin composite material. However, Mann-Whitney statistical testing revealed no significant differences between amalgam and RMGIC in the mode of failure. The control group, comprising sound, unmodified teeth, exhibited a mean fracture strength of 0.79 kN, and these teeth were consistent in their mode of failure (Figure 2, code 2). Interestingly, all values for fractural strength exceeded the mean maximal clenching force of healthy patients with natural teeth, which is reported to average up to 162 pounds (equivalent to 721 N). Indeed, biting force is generally less between premolars, around 310N.26 However, this is highly variable between subjects of different ages and sex and is dependent upon the manner in which it is measured.27

Analysis of the results also revealed that the cores constructed from resin-modified glass ionomer cement had a significantly higher fracture resistance than composite cores and exhibited less severe failure. This result would appear anomalous from consideration of the headline mechanical properties of the two materials. Resin composite has been shown to have greater fracture toughness,14 compressive, flexural and diametral tensile strength than RMGIC.15 However, resin composite possesses several features that may have been a disadvantage in this setting. In a study of crack propagation, composite was shown to have high crack propagation rates arising from a notional 1 μm surface defect compared to other restorative materials. It has been shown that composite exhibits high stresses at the point of contact, and it is suggested that this may explain catastrophic bulk failure observed in clinical usage.28 In addition, the presence of air-voids, unbonded filler particles and any contaminants all act as sites for crack growth. This may explain the findings of the current study, where the method of force application would almost certainly have stressed small areas of the samples, the finishing technique may have left surface defects and the two-paste system may have predisposed to air void inclusion. In contrast, RMGIC has been shown to exhibit high compressive toughness.29 Water sorption has been shown to progress through this material rapidly in the first 24 hours; in addition, the mechanical properties of this material mature over a period of months.930 In the current study, samples were freshly made and stored in distilled water for 24 hours prior to testing. This material is plasticized by water sorption, increasing its viscoelastic and plastic properties. This leads to softening of the surface, which may have allowed reduced stress concentration by increasing the area of contact. In combination with a concomitant decrease in flexural modulus and an increase in the degree of deformation before fracture, this may have maximized the plastic properties of the material, giving a softer, more compliant material, allowing it to better resist fracture. Interestingly, amalgam also behaves in a similar viscoelastic manner. Stress relaxation properties in both these materials, aided by a relatively low strain rate in this study, may help to explain why both cores resisted fracture better than composite, yet failed in a similar, less catastrophic manner.

The variables that are inherent with research of this type are tremendous. Consequently, this study could be considered to suffer some limitations that merit further discussion. First, the acrylic resin supporting the teeth and the force of the Howden Universal testing machine do not fully simulate the clinical situation, as there is no allowance for an artificial periodontium. Each core reconstruction was standardized to a height of 4 mm above the CEJ and prepared with two inclined planes (facial and lingual) and a central parallel groove. Reproduction of the occlusal surface was largely performed freehand and, therefore, introduced geometric variation between cores; however, efforts were made to minimize variation. The control teeth were left unprepared and problems associated with this became apparent upon testing. Wedging forces directed against the cuspal inclines may have resulted in premature fracture and consequently have affected the results obtained. Furthermore, the teeth were not restored with full coverage cast restorations. Despite these limitations, the results of this study provide valuable information about the fracture resistance of the three core materials investigated. This is increasingly relevant as practitioners choose to delay, sometimes indefinitely, the provision of a cuspal-coverage type restoration. It is suggested that further discussion of the suitability of the materials tested for the restoration of root-treated posterior teeth is timely.

CONCLUSIONS

Within the limitations of this study, it may be concluded that there are significant differences in the fracture resistance of the three core materials tested. Endodontically-treated premolars restored with amalgam corono-radicular restorations exhibited greater fracture resistance compared to equivalent teeth restored with composite and resin-modified glass ionomer cement.