Compressive strength and compressive fatigue limit of conventional and high viscosity posterior resin composites

Low-temperature flow-synthesis-assisted urethane-grafted zinc oxide-based dental composites: physical, mechanical, and antibacterial responses

A novel way was adopted to graft zinc oxide (ZnO) with urethane-modified dimethacrylate (UDMA) in order to utilize them as reinforcing agents in resin-based dental composites. Experimental novel composites were synthesized having UDMA-grafted and nongrafted ZnO, at a concentration of 0 wt.%, 5 wt.%, and 10 wt.%. The same concentrations of ZnO were also incorporated in Filtek Z250 XT (3 M ESPE, USA). The antibacterial behavior was evaluated against Streptococcus mutans by direct-contact test at one, three, and seven days of incubation. The compressive strength and Vickers microhardness were tested as per ISO 9917 and ISO/CD6507-1, respectively. For abrasive wear resistance, mass loss and roughness average after tooth-brushing cycles of 24,000 at custom-made tooth-brushing simulator were evaluated using noncontact profilometer. Data analysis was carried out using post hoc Tucky's test and nonparametric Kruskal-Wallis test. Direct contact test revealed that the antibacterial potential of novel and commercial composites was increased with an increase in the concentration of grafted ZnO as compared with nongrafted, whereby the potential was the highest at day seven. There was a significant decrease in compressive strength and Vickers hardness of commercial composites on addition of grafted ZnO while there was no significant difference in the strength of experimental novel composite. The abrasive wear of commercial and experimental composites was within clinical limits. Low-temperature flow-synthesis method was successfully employed to synthesize grafted and nongrafted ZnO. The UDMA-grafted ZnO can be incorporated into dental composites without decreasing their strength and these composites can be used to combat secondary caries.

Resistance of bonded premolars to four artificial ageing models post enamel conditioning with a novel calcium-phosphate paste

Background

This in vitro study compares a novel calcium-phosphate etchant paste to conventional 37% phosphoric acid gel for bonding metal and ceramic brackets by evaluating the shear bond strength, remnant adhesive and enamel damage following water storage, acid challenge and fatigue loading.

Material and Methods

Metal and ceramic brackets were bonded to 240 extracted human premolars using two enamel conditioning protocols: conventional 37% phosphoric acid (PA) gel (control), and an acidic calcium-phosphate (CaP) paste. The CaP paste was prepared from β-tricalcium phosphate and monocalcium phosphate monohydrate powders mixed with 37% phosphoric acid solution, and the resulting phase was confirmed using FTIR. The bonded premolars were exposed to four artificial ageing models to examine the shear bond strength (SBS), adhesive remnant index (ARI score), with stereomicroscopic evaluation of enamel damage.

Results

Metal and ceramic control subgroups yielded significantly higher (p < 0.05) SBS (17.1-31.8 MPa) than the CaP subgroups (11.4-23.8 MPa) post all artificial ageing protocols, coupled with higher ARI scores and evidence of enamel damage. In contrast, the CaP subgroups survived all artificial ageing tests by maintaining adequate SBS for clinical performance, with the advantages of leaving unblemished enamel surface and bracket failures at the enamel-adhesive interface.

Conclusions

Enamel conditioning with acidic CaP pastes attained adequate bond strengths with no or minimal adhesive residue and enamel damage, suggesting a suitable alternative to the conventional PA gel for orthodontic bonding.

Key words:Enamel etching, calcium phosphate, bracket bond strength, adhesive residue, enamel damage.

The influence of cyclic shear fatigue on the bracket-adhesive-enamel complex: an in vitro study

OBJECTIVES

To describe the effect of fatigue on the strength of the bracket-adhesive-enamel complex and characterize the fatigue behavior of the materials tested.

METHODS

Upper central incisor brackets (Discovery(®), Dentaurum) were bonded with a light-curing (Transbond XT™, 3M Unitek) and a chemically-curing adhesive (Concise™, 3M Unitek) on bovine teeth embedded in cylindrical resign bases and stored in water at 37(±2)°C for 24 (±2)h. The first 15 specimens were tested with a universal testing machine ZMART.PRO(®) (Zwick GmbH & Co. KG, Ulm, Germany) for ultimate shear bond strength according to the DIN-13990-2-standard. The remaining three groups of 20 specimens underwent fatigue staircase testing of 100, 1000 and 3000 cycles at 1Hz with a self-made testing machine. The survived specimens were subjected to shear strength testing.

RESULTS

The fatigued specimens showed decreased shear strength with both adhesives at all cycle levels. The shear strength after fatigue for 100, 1000 and 3000 cycles was in the Concise™-groups 34.8%, 59.0%, 47.3% and in the Transbond™ XT-groups 33.6%, 23.1%, 27.3% relative to the ultimate shear strength. The fatigue life of the Concise™-groups decreased with increasing stress and Transbond™ XT showed lower fatigue ratio with no obvious trend. The specimens bonded with Transbond™ XT showed typically favorable fracture modes in contrary to Concise™.

SIGNIFICANCE

Fatigue of the bracket-adhesive-enamel complex decreased its shear strength. The staircase method can provide a standardized experimental protocol for fatigue studies, however testing at various cycle numbers is recommended.

Fracture resistance of endodontically treated premolars with direct composite restorations

Purpose:

To determine the fatigue resistance and failure mode of endodontically treated premolars using direct composite resin restorations.

Materials and Methods:

Eighty-four human premolars were divided into seven groups of 12, prepared as follows: Intact teeth used in Group 1 as control, the second group covers the endodontically treated teeth, restored with direct onlays using Z250 composite resin, the next two groups (i.e. 3,4) were similar to the second group, but subjected to 1 and 2 million fatigue load cycles, respectively. Groups 5, 6, 7 were similar to groups 2, 3, 4, however, in these groups Tetric Ceram was used as the restorative material. All specimens were loaded using a Universal Testing Machine until fracture occurred. One-way Anova andTukey's HSD tests were used to analyze the data of onlay groups.

Results:

All specimens withstood the masticating simulation. The mean fracture strength for Goups 1 to 7 was: 1276.92, 1373.47, 1269/70, 486/35, 484/12, 1130/49, 1113/79 Newton, respectively. No statistically significant differences were found between the groups in fracture strength and failure mode.

Conclusions:

No statistically significant differences in fracture strength were found between sound teeth and composite onlays that were subjected to 1 and 2 million fatigue load cycles.