The Challenge of Shrinking Composite Resins

x500 photo reproduced with permission from the Materials Research Department at the School of Dentistry, Univ. of Copenhagen.

One of he main challenges in the field of materials for dental fillings today is the shrinkage of composite resins.

How this causes problems in the dental clinic is illustrated left, where the lower left corner is the enamel of the tooth and the largest upper right the composite filling. The black band in between is a 20 µm wide crack induced by the polymerisation shrinkage.

A dental composite material is normally composed of 60-70 % filler material and 30 – 40 % polymer resin.  

The polymer resin before curing consists of short monomer units, making the compound material soft and workable for the dentist. Upon curing e.g. by blue light, the monomers combine forming a rigid polymer network – but at the same time contracting and creating micro cracks and tensions in the composite filling.

These microcracks are wide enough to allow inflow of bacteria and colouring agents, thus new caries attacks, maybe even infection of the dental pulp and allmost certainly discolouring over time. The cracks are the main reason why resin composites have a relatively short life span. The average lies at present at 8 years and besides this being unacceptable in itself, it has the unpleasant side effect of hindering the total abolition of the very un-aesthetic and environmentally hazardous silver amalgam.