A simple optical method for measuring free shrinkage is compared with a dilatometer.
Shrinkage values with optical method ranked the same but were higher than dilatometer.
Finite element analysis showed sample bonding reduces dilatometer shrinkage values.
Optical method with microscope and image analysis software is simple and accurate.
A simple optical method for measuring polymerization shrinkage of dental composites is compared with an established dilatometer.
Five restorative composites were used to test the methods: Filtek Supreme Ultra (3M ESPE), Filtek LS (3M ESPE), Premise (Kerr), Gradia Direct (GC), and GC Kalore (GC). Uncured composites were attached to sandblasted silane-treated glass slides. The slides were placed sample side inside a mercury-filled dilatometer (ADAF). The mercury levels were recorded as the materials were light-cured through the glass-slides (40 s). Mercury levels, which correlated with volumetric shrinkage, were recorded for 60 min ( N = 6). For the optical method, uncured composite was placed on a smooth silicone platform. A pre-polymerization image was captured under a stereomicroscope, and the specimen was light-cured (40 s). Post-polymerization images were captured at 2, 10, 60, and 90 min ( N = 10). Composite outlines were traced to obtain projected surface areas (ImageJ) and volumetric shrinkage was calculated. Results were analyzed using two-way ANOVA ( α = 0.05) and Pearson Correlation tests. Shrinkage deformation for both methods was modeled using finite element analysis.
Volumetric shrinkage at 60 min ranged between 1.24% and 2.24% for dilatometer and 1.35–2.68% for optical methods. Optical method shrinkage was consistently higher than the dilatometer ( P = .0001), but the ranking of the composites was the same (Pearson Correlation Coefficient 0.9997). Finite element analysis showed that lower shrinkage values of the dilatometer method could be attributed to bonding of its samples.
The optical method using a general-purpose stereomicroscope and public-domain software is a simple and accurate alternative to measure free shrinkage.
Volumetric shrinkage is a consequence of polymerization of resin-based materials. It happens when formation of a polymer network creates a denser material. Shrinkage causes dimensional changes that can cause residual stress when it is hindered. Polymerization shrinkage is a concern in dentistry ever since dental composites were first developed as a restorative material . Reduction of polymerization shrinkage remains one of the critical design properties in the development of new dental composites.
Various methods have been used to measure polymerization shrinkage. Volumetric shrinkage can be derived from changes in density between uncured and cured composites, measured using the buoyancy principle explained by Archimedes . Another approach is measuring the displacement of liquids, such as water or mercury, in so-called dilatometers . Shrinkage can also be determined by measuring changes in dimensions. They can be measured in one dimension or ‘linear’ (for example, linometer and strain gauges ), or spatial (for example, Accuvol and micro-computed tomography ).
Not all these methods measure the same shrinkage. Obviously, there are differences in which dimensions are measured and the type of shrinkage. But those differences are well recognized (post-gel versus total shrinkage) and conversions between dimensional expressions (linear versus volumetric) are well established . However, a more fundamental concern that is generally overlooked is that although most methods — except post-gel shrinkage — assume to measure total (free) shrinkage, they often require attachment of the samples to a substrate to keep them in place (for example, to prevent the sample floating away or falling of the stage) or for attaching targets (for example, to allow a displacement sensor to contact or detect the sample) . The results of such shrinkage methods may therefore not determine actual free shrinkage. Despite being a simple and well-defined property, measurement of shrinkage for dental materials has not been trivial. The majority of methods that are currently used require dedicated and sometimes costly equipment and/or devices that often are specifically designed for shrinkage measurements.
The objective of this study was to evaluate a simpler approach to measure free shrinkage . This method used pre- and post-polymerizing images captured by a general-purpose stereomicroscope and processed with public-domain image analysis software. To validate the optical method, shrinkage values of five restorative composites were compared with a well-accepted shrinkage measurement technique (dilatometer). Finite element analysis was added to further compare the outcomes of the two methods.
Materials and methods
Five restorative composites were used to test the shrinkage methods: universal composite (Filtek Supreme, 3M ESPE, St Paul, MN, USA), anterior composite (Gradia Direct, GC Corporation, Tokyo, Japan), and three low-shrink composites (Filtek LS, 3M ESPE; Premise, Kerr Corporation, Orange, CA, USA; GC Kalore, GC Corporation). Material information is listed in Table 1 .
|Filtek Supreme Ultra
Shade: A2 Body
|Resin system: bis-GMA, UDMA, TEGDMA, bis-EMA(6), PEGDMA
Fillers: non-agglomerated/non-aggregated 20 nm silica filler, non-agglomerated/non-aggregated 4–11 nm zirconia filler, and aggregated zirconia/silica cluster filler (comprised of 20 nm silica and 4–11 nm zirconia particles). Average cluster particle size of 0.6–10 μm
Filler loading: 78.5 wt% (63.3 vol%)
|N393610||3M ESPE Dental Products, St Paul, MN, USA|
|Resin system: Silorane resin, camphorquinone, iodonium salts
Fillers: Silane-modified fine quartz particles and radiopaque yttrium fluoride
Filler loading: 76 wt%
|N478060||3M ESPE Dental Products, St Paul, MN, USA|
Shade: A2 Body
|Resin system: bis-GMA, UDMA, TEGDMA, light-cure initiators and stabilizers
Fillers: Prepolymerized filler 30–50 μm, barium glass 0.4 μm, silica filler 0.02 μm polymerizable organophosphate dispersant
Filler loading: 84 wt%, 70 vol%
|4818821||Kerr Corporation, Orange, CA, USA|
|Composite filler (with Lanthanoid Fluoride) 30–35 wt%, strontium/barium glass 20–33 wt%, fluoro-alumino-silicate glass 20–30 wt%, urethanedimethacrylate 5–10 wt%, urethanedimethacrylate (DX-511) 5–10 wt%, dimethacrylate 1–5 wt%, silicon dioxide 1–5 wt%, photo initiator < 1%, pigment < 1%||1104121||GC Corporation, Tokyo, Japan|
|Gradia Direct Anterior
|Resin: methacrylate monomers 27 wt% Fillers: Silica (particle size 0.85 μm) 38 wt%, prepolymerized filler 35 wt%||1108081||GC Corporation, Tokyo, Japan|
|Express Light Body Regular Set||Base: Vinyl polydimethylsiloxane 40–50%, cristobalite 30–40%, dimethyl methyl hydrogen silicone fluid 10–15%, silane treated silica 1–10%, polyethylene glycol, siloxane terminated 1–5%
Catalyst: Vinyl polydimethylsiloxane 40–50%, cristobalite 40–50%, poly(dimethylsiloxane) 1–5%, silane treated silica 1–5%, chromium oxide < 1%
|20071113||3M ESPE Dental Products, St Paul, MN, USA|