Packing-stress behaviour of resin-composite restorative pastes

Objectives: Handling behaviour is important in both resin-composite selection and in governing aspects of their clinical performance. The aim was to investigate rapid in vitro characterisation of packing behaviour. Methods: A precision instrument was designed upon the penetrometer principle. Five representative resin-composite materials were selected to evaluate the instrument. A thin cylindrical rod [d = 3 mm] was pushed via a lever-arm, end-weighted by a 500 g mass, to a controlled depth into each un-set material. Two temperature-controlled ‘cavities' were utilised to contain the material, representing: (A) ‘maximal' constraint of flow [i.d. = 6.5 mm] and (B) ‘minimal' constraint [i.d. = 12.8 mm]. Results: Rheological parameters, principally packing-stress sustained [(σ(t), typically 2-3 MPa] were obtained by time-dependent measurements during the step-response packing-event, which ranged from 2 s to 20 s. A relatively constant stress during the packing process was followed by stretched-exponential stress-relaxation. As expected, response at 37 ^oC was more rapid than at 23 ^oC. Cavity A was considered of greater clinical relevance. Statistical analysis, by ANOVA and post hoc methods, clearly distinction between materials via flow and packing-stress parameters. Conclusions: This rheological instrument quantified the packing behaviour of resin-composites in terms of paste stiffness and viscosity during cavity placement. It also demonstrated the strong flow-dependences upon temperature and cavity size.