This work aims to review the key factors affecting the polymerization efficiency of light-activated resin-based composites. The different properties and methods used to evaluate polymerization efficiency will also be critically appraised with focus on the developments in dental photopolymer technology and how recent advances have attempted to improve the shortcomings of contemporary resin composites.
Apart from the classical literature on the subject, the review focused in particular on papers published since 2009. The literature research was performed in Scopus with the terms “dental resin OR dimethacrylate”. The list was screened and all papers relevant to the objectives of this work were included.
Though new monomer technologies have been developed and some of them already introduced to the dental market, dimethacrylate-based composites still currently represent the vast majority of commercially available materials for direct restoration. The photopolymerization of resin-based composites has been the subject of numerous publications, which have highlighted the major impact of the setting process on material properties and quality of the final restoration. Many factors affect the polymerization efficiency, be they intrinsic; photoinitiator type and concentration, viscosity (co-monomer composition and ratio, filler content) and optical properties, or extrinsic; light type and spectrum, irradiation parameters (radiant energy, time and irradiance), curing modes, temperature and light guide tip positioning.
: This review further highlights the apparent need for a more informative approach by manufacturers to relay appropriate information in order for dentists to optimize material properties of resin composites used in daily practice.
In modern dentistry, the worldwide use of resin-based composites continues to increase, and in some countries has entirely replaced mercury amalgams. Such trends are due to government legislation on the use of mercury-containing products and obvious material improvements such as esthetic quality, a fast and on-demand setting process, strong physico-mechanical properties and the potential for chemical affinity with tooth tissue . The restoration longevity reported in a long-term retrospective evaluation was 7.8 years for composite and 12.8 years for amalgam . More recent studies showed similar survival rates for amalgam and composite (91.7% at 5 years and 82.2% at 10 years versus respectively 89.6% and 79.2%) or even superior survival for large composite restorations after 12 years . However, in high-caries risk patients, secondary caries was more frequent with large composite restorations than with amalgam ones. Research is therefore required to solve the specific drawbacks of resin composites and improve material performances. A primary disadvantage related to their use is polymerization shrinkage and the associated stress transmitted to the adhesive bond and the remaining tooth structure. Their clinical consequences include crack formation in dentin and enamel, post-operative sensitivity, marginal discoloration and secondary caries . Further drawbacks of resin-based composites include inferior mechanical properties compared to tooth structures , in line with clinical data reporting bulk fracture as the main reason for retreatment , as well as secondary caries and tooth fractures . The biocompatibility of dimethacrylate resin and photoinitiator chemistry extensively described by are further concerns, as it may affect the delicate balance between healing and chronic inflammation in pulp tissues .
Advances in material formulation may improve the shortcomings of resin composites and in this regard, recent reviews have summarized significant developments, including improved filler morphology, progress with existing dimethacrylate chemistry and novel monomer technologies. It appears from those works that, although some non-dimethacrylate monomers materials based on ring-opening epoxy chemistry (Filtek™ Silorane, 3M ESPE Dental Products, Seefeld, Germany) have already been introduced to the dental market, contemporary dimethacrylate-based composites still represent the vast majority of commercially available materials for direct restoration.
As stated by , “the development and implementation of resin composites rely on a comprehensive understanding of each component …”. However, the successful placement of composite restorations depends also on the method of light curing as well as the effect of the components on the efficiency of the photopolymerization process. Therefore, the objective of the present work is to review the different factors affecting the photopolymerization efficiency of dimethacrylate-based dental resins ( Fig. 1 ), be they intrinsic; co-monomer composition and ratio, filler content, photoinitiator type and concentration, or extrinsic; light spectrum, irradiation protocols, temperature and light guide tip positioning, with particular focus on papers published since 2009. It is important for researchers, and also particularly for clinicians, to first comprehend, and second, to optimize the photopolymerization process of resin-based composites in order to improve material properties. To better realize the impact of these different factors, a fundamental description of the photopolymerization reaction will be presented with a critical appraisal of the different properties and methods used to evaluate the polymerization efficiency. Recent advances that improve the photopolymerization process will be reviewed as well.