Influence of cavity dimensions and their derivatives (volume and 'C' factor) on shrinkage stress development and microleakage of composite restorations.

OBJECTIVES: To determine, in vitro, the influence of cavity dimensions on shrinkage stress development and microleakage of composite restorations. METHODS: Cylindrical cavities with 2, 3, 4 or 6 mm diameter and 1 or 2 mm depth were prepared in bovine incisors (with enamel margins) and reproduced in photoelastic resin. Cavities were restored in bulk (Single Bond+Filtek Z250). Stress fringes were analyzed 10 min after photoactivation. Restored teeth were stored for 24 h, coated with nail polish and immersed in 50% AgNO3 for 2 h, followed by 6 h in developing solution. Specimens were sectioned twice and microleakage was measured under 20x magnification. Photoelastic data were analyzed descriptively only, due to lack of variability in some groups. Microleakage was analyzed through (generalized linear) regression models. Pearson correlation coefficients were computed to evaluate the relationship between microleakage and 'C' factor or volume. RESULTS: Fringe order was higher at the internal angles than at the margins of the restorations. There was a trend for higher fringe orders with increasing diameter and depths. Regression analysis revealed a significant influence of restoration depth for diameters above 2mm. The effect of diameter on microleakage is significantly higher (p < 0.0001) for 2-mm deep restorations, compared to those 1-mm deep. Pearson correlation coefficients suggest some relationship between microleakage and volume (r = 0.724, p < 0.0001), but not between microleakage and 'C' factor (r = 0.048, p = 0.6120). CONCLUSIONS: Shrinkage stress and microleakage were higher in restorations with larger diameters and depths. Microleakage seemed to be related to a restoration's volume, but not to its 'C' factor.

Effects of incremental curing on contraction stresses associated with various resin composite buildups.

OBJECTIVE: The purpose of this study was to photoelastically evaluate contraction stresses associated with various resin composite build-up procedures, including incremental curing and the use of flowable composite for pulpless molars. METHOD AND MATERIALS: Life-sized photoelastic models of an endodontically treated molar were fabricated. The cavity represented a conservative access preparation and included four lateral walls. The following materials were used for buildup: dual-cured hybrid composite (Cleafil DC Core [DC], BIS-CORE [BC]); light-cured flowable composite (AELITEFLO LV [ALV]); and chemical-cured flowable composite (CORE-FLO [CF]). The photoelastic models were built up with the following techniques: bulk-cured (BDd [dual-cured DC], BDc [chemically-cured DC], BBd [dual-cured BC], and BC [CF]); and incremental-cured (1 mm gingivally + 3.5 mm occlusally; IAB [ALV + BC], ICB [CF + BC], and IBB [BC + BC]). Isochromatic fringes developed in the models were recorded photographically in the field of a circular polariscope, and maximum fringe order was determined. Five specimens were tested for each condition. RESULTS: Stress intensity of the build-up methods fell into two categories: high (BDd, BBd, BC, and IAB) and low (BDc, ICB, and IBB). The difference between high and low groups was statistically significant, except between BC and ICB. The maximum fringe order was developed around the point angles at the cavity floor for all the conditions tested. CONCLUSIONS: Type and setting mechanism of resin composite build-up technique had considerable influence on contraction stress. Incremental buildup using composites with low elastic modulus did not reduce contraction stress intensity compared with bulk-cured techniques.

Biomechanical comparison of straight and staggered implant placement configurations.

The effect of buccolingual staggered implant placement on stress distribution within the supporting structure was examined photoelastically. Two photoelastic models of a human mandible, edentulous distal to the canine, were fabricated. Three screw-type implants were embedded into the edentulous region of each model. The implants were placed in a straight line in one model and in a buccolingual staggered configuration in the other. Vertical and lateral loads were applied to a fixed partial denture superstructure. No clear biomechanical advantage to a staggered 1.5 mm buccal and lingual offset placement configuration was observed.

Contraction stress of composite resin build-up procedures for pulpless molars.

PURPOSE: The purpose of this study was to photoelastically evaluate contraction stresses associated with various resin composite build-up procedures for pulpless molars. MATERIALS AND METHODS: Photoelastic models of endodontically treated mandibular molars were fabricated to simulate a preparation for a full-cast crown. The model configuration included three lateral walls, but no post space. The buildups were made with dual-curing resin composite using the following procedures: 1) bulk dual cure, 2) bulk chemical cure, 3) horizontal two increments, dual cure, and 4) indirect. Five specimens were fabricated for each condition. The stresses developed in the models were recorded photographically in the field of a circular polariscope. RESULTS: The build-up procedures tested generated widely different stress distributions and intensities. The highest stresses were seen with the bulk dual-cure method. The slower polymerization chemical-cure group developed distributions similar to the bulk dual-cure group with a significantly lower fringe order. The two horizontal increments dual-cure techniques developed an individual group of fringes for each layer. Compared with the bulk dual-cure group, stresses around the occlusal margin were reduced by incrementalization, while fringes were more closely spaced at the line angles with a slightly lower fringe order. The indirect method demonstrated the lowest stress which extended over the smallest area. CONCLUSION: Contraction stress in resin composite buildups varied significantly depending upon the procedures of fabrication. The bulk dual-cure method developed the most severe contraction stresses, while the indirect technique resulted in significantly lower contraction stresses than the other techniques tested.