Composite Warming Effects on Mechanical Properties of Bulk Fills

Objectives: The formulation of composite resin continues to be changed in attempts to enhance their properties. Although improvement of the inorganic filler has led to improved mechanical properties, changes in the organic matrix remains problematic which has led to concerns for decreased long-term clinical success. Past studies have shown increased temperature can increase flow rate, higher degree of conversion and improved mechanical properties. The purpose of this study is to assess the effect pre-warming has on the mechanical properties of 5 bulk-fill composites.
Methods: 5 bulk-fills and 1 incremental composite was tested. Flexural strength/modulus was accomplished according to ISO 4049: Specimens (2x2x25mm) were created from each composite, both from room temperature composite (22C) and composite pre-warmed (52C) 10 minutes in a composite warming unit (Vista Therma-Flo). Specimens were light polymerized, stored in Prosphate Buffered Solution for 24hrs (37C, 98% RH) prior to testing on a universal testing machine (Criterion 43, MTS).
Hardness: Knoop hardness testing of the irradiated top and non-irradiated bottom surfaces on 4mm diameter specimens fabricated with heights based on manufactures advertised depth of cure; 300g/15second dwell (MicroMet 6040, Buehler). Test was performed on composite specimens (room temperature/warmed) after 24hrs storage.
Polymerization shrinkage: a video-imaging device (AcuVol, Bisco) was used to record shrinkage of each composite material (room temperature/warmed) following light polymerization.
Statistical Analysis: Kruskal-Wallis with Dunns Post-hoc (alpha=0.05)
Results: Warmed composite showed significantly greater flexural strength/modulus, hardness, and volumetric shrinkage for all materials. Irrespective of temperature, the incremental fill composite demonstrated greater flexural strength/modulus, micro-hardness ratio compared to bulk-fill composites. Knoop hardness ratio using bottom-to-top hardness ratio was less than 80% for bulk fill composites; materials were tested at manufacture advised depth of cure.
See attached table
Conclusions: From the statistical analysis performed, it can be concluded that heating dental composite improves the mechanical properties of the composite resin.