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Introduction

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.

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Materials and methods