The aim of this study was to investigate the effect of variations in filler particle size and shape on the polymerization shrinkage-stress kinetics of resin-composites.
A model series of 12 VLC resin-composites were studied. The particulate dispersed phase volume fraction was 56.7%: these filler particles were systematically graded in size, and further were either spherical or irregular. A Bioman instrument (cantilever beam method) was employed to determine the shrinkage-stress kinetics following 40 s irradiation (600 mW/cm 2 ) at 23 °C ( n = 3). All data were captured for 60 min and the final shrinkage-stress calculated.
Shrinkage-stress varied between 3.86 MPa (SD 0.14) for S3 (spherical filler particles of 500 nm) and 8.44 MPa (SD 0.41) for I1 (irregular filler particles of 450 nm). The shrinkage-stress values were generally lower for those composites with spherical filler particles than those with irregular filler particles. The differences in shrinkage-stress with filler particle size and shape were statistically significant ( p < 0.001).
Composites with spherical filler particles exhibit lower shrinkage-stress values compared to those with irregular filler particles. Shrinkage-stress and shrinkage-stress rate vary in a complex manner with variations in the size of the dispersed phase particles: a hypothesized explanation for the effect of filler particle size and shape is presented.
Resin-composites used for dental restorative materials undergo volumetric contraction when polymerized due to molecular densification. This shrinkage-strain, if it is not occurring freely, leads to tensile forces within the material at the filler–matrix interface, or at the interface of the composite and cavity wall. The development of shrinkage-stress is complex, with many factors having an effect including degree of conversion , pre-gelation flow , configuration of the cavity to be restored ( C -factor) , filler load , and post-cure hygroscopic expansion . The deleterious clinical effects of shrinkage-stress essentially arise due to either: gap formation and microleakage due to adhesive bond failure as bond-strength development does not withstand the developing shrinkage-stress ; or cuspal movement where bonds hold, but compliant tooth structure is deformed under the applied stress . With advances in multi-purpose bonding agents, constraint is higher and therefore higher stresses are likely to occur .
The development of shrinkage-stress is a dynamic process: shrinkage-strain develops with time and, given that shrinkage-stress fundamentally follows Hooke’s Law, factors influencing the modulus development during polymerization will be likely to influence the rate of shrinkage-stress development and also the final magnitude of shrinkage-stress. Similarly, the ability of a material to exhibit internal flow at a microscopic/molecular level (as well as macroscopically) will have an influence on the shrinkage-stress kinetics. The small particle filler size in most commercially available composites presents a large surface area available for interaction with the matrix during the polymerization process, and the developing polymer may be subjected to constraint, especially given the role of silane coupling agents. Studies on model formulations where reduced or no coupling agent was present demonstrated a reduction in shrinkage-stress ; however, the mechanical properties of the material are reduced by such an approach. Similarly, internal voids/micro-porosities provide sites of stress-relief , but have obvious drawbacks, again with respect to less favorable mechanical properties . The role of filler particle size and shape has received little attention, and this aspect deserves further study. Although clinical service is the key outcome, greater understanding of shrinkage-stress kinetics will serve to develop materials that will exhibit improved clinical service.
The aim of this study was to investigate the effect of variations in filler particle morphology on the shrinkage-stress kinetics of polymerization of resin-composites. The specific objectives were to study a series of model resin-composites (with varying filler particle shape and size) using a cantilever beam method in order to:
compare the maximum rate of shrinkage-stress;
compare the maximum shrinkage-stress at 60 min.
The null hypothesis was that variations in filler particle morphology would have no effect on the shrinkage-stress kinetics.
Materials and methods
The resin-composites to be studied were all visible-light cured [VLC] and comprised seventeen experimental formulations (Ivoclar Vivadent, Schaan, Liechtenstein) together with an established commercially available formulation (Tetric Ceram [TC] – Ivoclar Vivadent, Schaan, Liechtenstein) used as a control.
The resin matrix for all materials was a combination of BisGMA, UDMA and TEGDMA, with 0.33% camphoroquinone. All of the model composites had a particulate dispersed phase of the same volume fraction (56.7%), which was treated with a silane coupling agent (methacryloxypropyltrimethoxysilane). The filler particles were systematically graded in size, and further were either spherical or irregular. The spherical particles were silica and made from solution, the irregular particles were ground glass melts (Ba–Al–B–silicate glass). Tetric Ceram contained heterogeneous, multimodal filler particles, comprising barium glass 1 μm, Ba–Al–FB–silicate 1 μm, SiO 2 40 nm, spherical mixed oxide 0.2 μm, and ytterbium trifluoride. The composition of the materials is given in Table 1 .
|Resin-composite||Filler Particles a||Matrix a|
|S1||Spherical||100||72.3||56.7||BisGMA, UDMA, TEGDMA|
|S2||Spherical||250||72.6||56.7||BisGMA, UDMA, TEGDMA|
|S3||Spherical||500||72.6||56.7||BisGMA, UDMA, TEGDMA|
|S4||Spherical||1000||72.5||56.7||BisGMA, UDMA, TEGDMA|
|S5||Spherical||100 and 1000 (1:3)||72.0||56.7||BisGMA, UDMA, TEGDMA|
|S6||Spherical||100, 250 and 1000 (1:1:2)||72.0||56.7||BisGMA, UDMA, TEGDMA|
|I1||Irregular||450||76.4||56.7||BisGMA, UDMA, TEGDMA|
|I2||Irregular||700||76.4||56.7||BisGMA, UDMA, TEGDMA|
|I3||Irregular||1000||76.4||56.7||BisGMA, UDMA, TEGDMA|
|I4||Irregular||1500||76.4||56.7||BisGMA, UDMA, TEGDMA|
|I5||Irregular||450 and 1000 (1:3)||76.4||56.7||BisGMA, UDMA, TEGDMA|
|I6||Irregular||450, 700 and 1500 (1:1:3)||76.4||56.7||BisGMA, UDMA, TEGDMA|
|TC Lot:E50727 (Shade A2)||Spherical and irregular||40, 200 and 1000||79.0||60.0||BisGMA, UDMA, TEGDMA|
The Bioman instrument as described by Watts et al. was used for stress measurements. Its main component is a cantilever load cell free at one end (i.e. compliant): this end has an integral clamp holding a circular steel rod (10 mm in diameter). The rod is orientated such that it is vertical and perpendicular to the load cell axis. Facing the bottom of the rod is a rigidly held glass slab. The glass slab is positioned in an aluminum housing that incorporates a large hollow cylindrical screw that allows for passage of a light curing guide to contact the glass.
The lower face of the rod and the glass slab form the surfaces of the specimen ‘chamber’, both these surfaces were lightly grit-blasted with 50 μm alumina powder to promote bonding of the composite specimen. The specimen chamber gap was set with the aid of a feeler gage to 0.8 mm , as the disk-shaped specimen had the upper and lower surfaces bonded to an area equal to the area of the circular rod (of diameter 2 r ) and an unbonded periphery of an area equal to the circumference multiplied by the height [ h ] of the specimen, the C -factor [ C f ] was 6.25, derived as follows:
C f = bonded surfaces unbonded surfaces = 2 π r 2 2 π r h = r h