Fracture strength and fracture patterns of root filled teeth restored with direct resin restorations
Introduction
Direct tooth-coloured restorations are often used for root-filled teeth as a relatively low cost, aesthetic alternative to cuspal coverage restorations. Historically, both amalgam and resin composite restorations have been widely used even for posterior teeth,1, 2 with composite resin direct restorations showing good long term outcomes. More recent studies3, 4 have indicated less favourable but still reasonably high survival of teeth with direct restorations, and the prospective studies of Mannocci et al.5, 6 indicate good outcomes with both amalgam and resin composite restorations (plus a prefabricated post) over the medium term (3 years). Clearly the choice of restoration will depend on remaining tooth structure, with direct restorations limited to teeth with substantial coronal dentine. In addition to aesthetic considerations, an acceptable restoration must restore function and preserve the remaining tooth structure against fracture.
Root-filled teeth are at increased risk of fracture. Caries and excessive removal of dentine during root canal treatment, rather than low moisture content and increased brittleness7, 8 reduce tooth strength. Endodontic procedures reduce tooth strength modestly compared to extensive cavity preparations,9 but only as long as the endodontic access is confined to the occlusal floor of the cavity. Loss of axial dentine walls, which is common in teeth requiring root filling, greatly weakens teeth.8
Resin composite restorations have the advantage of bonding to tooth structure, which might strengthen the tooth and offer an alternative restorative technique to cuspal coverage. However, polymerization shrinkage is a serious drawback of these materials, resulting in cuspal strains with subsequent stress or disruption of the bond, microleakage and recurrent caries. Attempts at minimizing this problem have included the use of low shrink composites,10 incremental placement11 and the use of liners including glass ionomer, flowable composites and polyacid-modified resin composites.12, 13, 14 The performance of direct resin composites for the restoration of root filled teeth has been investigated experimentally ever since posterior resin composite materials were first introduced15, 16, 17 and clinically in both retrospective and prospective clinical studies. Despite the less favourable outcomes in comparison with cuspal coverage restorations reported in retrospective studies,3, 4 two randomised clinical trials found superior performance compared to amalgam restorations in terms of fracture resistance, but with a problem of recurrent caries. Similar survival to full coronal coverage was observed over a three year period.5, 6
In experimental studies, fracture resistance to static loading has been used as a measure of the effect of cavity preparation and/or restoration on tooth strength. Although the fracture load is typically much higher than functional occlusal loads, it is still a valid method for comparing restorative materials and different cavity designs.
Adhesive resin composite restorations have been reported to increase the fracture resistance of root filled teeth compared to non adhesive fillings.18, 19, 20, 21, 22 Fibre reinforced resin composite has also been studied as a conservative restoration, but was not found to improve the fracture strength compared to conventional resin composite.23 Different bonding systems18 and base materials including glass ionomer cement (GIC) and composites24 have also been investigated for their effect on fracture strength.
This experimental study was conducted to compare the fracture resistance of extracted root filled maxillary premolars with variable cavity design and direct restoration techniques using resin composite. In a previous study,25 preserving the proximal dentine walls of an endodontic access cavity and placement of a glass ionomer base beneath the resin composite restoration of root filled maxillary premolars significantly reduced cuspal deflection and microleakage but did not affect the strains within cusps. The null hypothesis of this study is that preserving the proximal dentine and placement of a GIC base will improve the fracture strength and result in a more favourable fracture pattern of root filled maxillary premolars restored with direct resin composites.
Section snippets
Materials and methods
Overview: Three different cavity preparations were tested: MOD, MOD plus endodontic access confined to the occlusal floor of the MOD cavity, and MOD plus extensive endodontic access with the axial walls removed between the proximal boxes and access preparation (Fig. 1). Teeth were then restored with direct resin composite material, plus an additional group in which a GIC core was placed in teeth with the extensive endodontic access before placing resin composite. All teeth were then subjected
Unrestored teeth
Intact teeth fractured at a load of 747 ± 130 N (Fig. 3). Teeth became progressively weaker with more extensive cavity preparations. MOD cavity preparation reduced fracture strength by 37.5% (467 ± 141 N, p = 0.001 compared with intact teeth). Endodontic access confined within the occlusal floor did not further reduce strength compared to an MOD cavity (442 ± 132 N, p = 0.99 compared with MOD). Removal of axial walls further weakened teeth considerably (mean load at fracture 292 ± 80 N, only 39.1% of intact
Discussion
Methods: Despite its limitations, fracture testing remains a common experimental method of evaluating restorative procedures for root filled teeth. Reeh et al. highlighted the shortcomings of destructive methods of testing, which include the non-physiologic loads required to cause fracture, the variation amongst teeth used in experimental studies, and differences in test conditions leading to fracture.17 Differences in tooth morphology (such as the difference in cross-sectional shape of the
Conclusion
Endodontic access with loss of axial walls weakened teeth by 60% compared to intact teeth. Direct resin composite restoration significantly increased fracture resistance of these teeth, with a GIC core resulting in fracture strength similar to that of intact teeth.
Acknowledgment
We would like to thank Mr. Geoff Adams from Melbourne Dental School, University of Melbourne for his assistance in the statistical analysis.
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