Visco-elastic properties of dental elastomeric impression materials.

The visco-elastic properties of a number of set elastomeric impression materials were determined using a torsional pendulum apparatus, i.e. the real (G1) and imaginary (G2) parts of the complex shear modulus and the mechanical loss tangent (Tan delta). Comparison with published data on tension and compression set for these materials showed that whilst there was no correlation between Tan delta and tension set, there was significant (greater than 99%) correlation with compression set. Treating a permanent set test as the application of a rectangular stress pulse of duration tau, and assuming (i) linear visco-elasticity theory and (ii) that G1 and G2 are independent of frequency. Fourier integral analysis predicts that Tan delta should be proportional to permanent set. Tension set does not conform to this prediction because the deformation is nonlinear. The compression set test is shown to involve shear strains and elastomers generally are Hookean in shear. Whilst the theoretical treatment applies qualitatively, there are major quantitative disparities, the reasons for which are discussed.

Clinical evaluation of an anterior hybrid composite resin over 8 years.

AIM: To evaluate the performance of an anterior hybrid composite resin restorative material (Opalux). DESIGN: Single-centre clinical trial. MATERIALS AND METHODS: Scores were obtained based on USPHS criteria for marginal adaptation, anatomic form, colour match, surface roughness and marginal discolouration. INTERVENTIONS: The restorative material was evaluated in suitable cavities in 24 patients (12 male, 12 female). Ethical permission and consent were obtained. MAIN OUTCOME MEASURES: Baseline data was collected on 44 restorations (25 Class III, 3 Class IV, 16 Class V) and reviewed independently by two of the three clinicians who contributed to the study who then reached a consensus score. Restorations were reviewed and scored in the same way at 3 months and 1, 2, 3 and 8 years. RESULTS: The total number of alpha ratings at baseline and review (3 months and 1, 2, 3 and 8 years) appointments for the following parameters were: marginal adaptation (33, 28, 29, 27, 12, 17), anatomic form (39, 30, 32, 26, 17, 15), surface roughness (34, 26, 24, 23, 11, 4), colour match (24, 25, 26, 24, 15, 12) and marginal discolouration (39, 31, 30, 25, 10, 9). The incidence of secondary caries was very low. Life table analysis suggests a 73% survival, with no scores below B, at 8 years. CONCLUSIONS: The majority of these restorations continued to perform satisfactorily. Although this particular material is no longer available, these results may be applicable to currently available similar materials.