Influence of Preventing Fracture of Composite Resin in Inward-opening Cavities for Repair of Deciduous Molar Crowns

<p>Introduction</p><p>Caries adjacent to primary molars often spreads buccolingually near the dental neck. For thin primary teeth smaller than permanent teeth, based on the concept of minimal intervention, it is necessary to manipulate the side walls in order to protect the teeth, prevent loss of restoration and maintain them long-term. In the past, the literature has recommended inward-opening cavities, but no supporting evidence has been presented.</p><p>In this study, in order to help reduce the possibility of marginal fracture of composite resin (CR) and secondary caries in complex cavities extending to adjacent surfaces, we investigated cavity conditions and CR filling conditions while controlling the amount of cutting. We compared CR compressive strengths with respect to the profile of the inward-opening cavity, side wall width and CR type.</p><p>Method</p><p>1. Six flowable and three paste-type CRs were divided into three groups (GF, BK and UN). Their compressive strength, elastic modulus and indirect tensile strength were measured and compared.</p><p>2. Inward-opening cavities were formed using S1, F1 or F2 files and filled with CR. The CR compressive strengths were measured in situ and compared across the CR types and cavity types.</p><p>3. After the compressive strength tests, fractured surfaces of the CR-filled cavities were examined using a scanning electron microscope (SEM).</p><p>Results</p><p>1. As the cavities widened due to file type, the compressive strengths of the flowable types in groups GF and UN significantly increased.</p><p>2. The mechanical properties of conventional flowable CR were not as good as those of the paste type but, in this study, in the cavities the flowable CR had higher compressive strength than the paste type. This suggests that the flowable type could have clinical application for laminated fillings comprised of this material alone.</p><p>3.Because the compressive strength of the cavity increased in the order F2＞F1＞S1, the thickness of CR in the side wall and the volume of CR are thought to influence the prevention of fractures in the restored crown of deciduous teeth which are smaller than permanent teeth and are thinner.</p><p>Conclusions</p><p>1. As the cavities widened, the high value of the flowable types in groups GF and UN significantly increased.</p><p>2.In each CR group, the fracture load of CR in the cavity showed a tendency to increase as the cavity became larger, suggesting that the CR thickness and volume at the side wall affect fracture prevention in deciduous crown restoration.</p><p>3.The results of the indirect tensile tests and the within-cavity compression tests were similar, so selecting a CR with a large indirect tensile strength could help prevent fracture.</p>