Polymerization shrinkage stress of resin-based dental materials: A systematic review and meta-analyses of composition strategies

Abstract

Purpose

A systematic review was conducted to determine whether there were composition strategies available to reduce and control polymerization shrinkage stress development in resin-based restorative dental materials.

Data sources

This report was reported in accordance with the PRISMA Statement. Two reviewers performed a literature search up to December 2016, without restriction of the year of publication, in seven databases: PubMed, Web of Science, Scopus, SciELO, LILACS, IBECS, and BBO.

Study selection

Only laboratory studies that evaluated polymerization shrinkage stress by direct testing were included. Pilot studies, reviews and in vitro studies that evaluated polymerization shrinkage stress by indirect methods (e.g., microleakage or cuspal deflection measurements), finite elemental analysis, or theoretical and mathematical models were excluded. Of the 6113 eligible articles, 62 studies were included in the qualitative analysis, and the meta-analysis was performed with 58 studies. The composition strategy was subdivided according to the modified part of the material: filler phase, coupling agent, or resin matrix. A global comparison was performed with random-effects models (α = 0.05). The only subgroup that did not show a statistical difference between the alternative strategy and the control was ‘the use of alternative photo-initiators’ (p = 0.29).

Conclusion

Modification of the resin matrix made the largest contribution to minimizing stress development. The technology used for decreasing stress in the formulation of low-shrinkage and bulk-fill materials was shown to be a promising application for reducing and controlling stress development.

Introduction

Resin-filled dental composites comprise two distinct phases: an organic matrix, mainly composed of resin monomers; and an inorganic phase composed of filler particles. Both phases can be chemically bonded to each other in the presence of a coupling agent (Ferracane, 2011). Dental composite formulations have evolved since their introduction in dentistry, resulting in materials with satisfactory structural characteristics, however, despite all the improvements in physico-mechanical properties (e.g., strength, wear resistance) over the last few years, they still demonstrate a concerning limitation: they undergo shrinkage by volume when they are polymerized (Dauvillier et al., 2000).

Volumetric/Polymerization shrinkage is an intrinsic property of conventional resin-based materials caused by the approximation of monomers during polymerization, i.e., when the distance between monomers is reduced due to the conversion of the weak van der Waals forces into covalent bonds (Kim et al., 2015). However, it is important to note that the most deleterious factor is not the polymerization shrinkage itself, but rather, the stress generated at the tooth-restoration interface while the material is undergoing shrinkage in a confined environment such as tooth cavities or root canals (Braga et al., 2005). Polymerization shrinkage stress has increasingly been considered one of the reasons for the failure of resin-based restorations (Cramer et al., 2011), especially those performed with resin composites or resin cements. Indeed, several negative effects may occur, including but not limited to restoration-post de-bonding, cuspal deflection, microleakage, and post-operative tooth hypersensitivity (Ferracane, 2008, Ferracane and Mitchem, 2003, Gonçalves et al., 2012). Consequently, polymerization shrinkage stress should be reduced and controlled as far as possible in order to prevent the occurrence of these consequences, favoring clinical success of the restorative treatment.

During the last few decades, several strategies have been presented in the literature in an attempt to solve or control the aforementioned problem. Some of the strategies reported were related to modification of the protocol of the technique for placing resin composites, and the light/photo-activated curing method. Nowadays, tooth restoration with conventional resin composites is performed by the ‘incremental technique’, i.e., the restoration is fabricated by adding small amounts of restorative material (increments), layer-by-layer, and by means of individual cycles of photo-activating each increment. Although this has been effective, this technique is time-consuming and may lead to bulk defects (e.g., air bubbles or micro-voids) in the inner portion of the restoration (Petrovic et al., 2010). In the light of the foregoing, other strategies related to modifying the chemical composition of the material have also been suggested. For instance, novel resin-based dental materials may present the SDR™ chemistry (i.e., stress decreasing resin), which has optimized both the organic matrix and filler phase of the material (Ilie and Hickel, 2011, Zorzin et al., 2015). Indeed, the use of alternative or modified resin monomers and photo-initiators, as well as improved filler particles, has increasingly contributing to less shrinkage stress development.

To the best of our knowledge, there is no previous report of a systematic review and meta-analysis regarding this topic. Considering that valuable information can be collected and produced by reviewing studies, the purpose of the present study was to review the literature systematically regarding the potential alternative compositions to reduce and control the polymerization shrinkage stress associated with resin-based restorative materials.