Polymerization Shrinkage Patterns in Composite Restorations with Different Liners

Objective: To investigate the polymerization shrinkage patterns in composite restorations with or without liners using digital-image-correlation (DIC) technique.

Methods: Sixty intact extracted human molars were mounted and divided into sixty groups to receive a mesio-occluso-distal cavity preparation and a resin composite restoration. The cavity dimension was 2mm(W)×4 mm(D) in Groups 1-3 and 4mm(W)×4 mm(D) in Groups 4-6. Groups 3 & 5 were lined with a flowable composite (Fl), while Groups 4 & 6 were lined with a glass ionomer (GI) on the cavity floor prior to the composite restorations. One proximal side of each restoration was sprayed with TiO₂ powder to create a speckle pattern. The restorations were photographed before light-curing, and every minute until 60min post-curing through a microscope. The displacements on the restoration boundaries generated by the composite shrinkage were analyzed using a DIC program that searched a characterized object on images using correlation operation and interpolation computation. The displacements were measured to a submicron level.

Results: For all groups, the inward displacement on the free surface was significantly greater than the other bonded surfaces. The displacements on both the free surfaces and bonded walls in wide cavities were greater than those in narrow cavities. Use of Fl or GI liners did not reduce the displacement on the free surfaces and axial walls. The Fl lining groups showed greater displacements on the free occlusal surfaces and over cavity floors. The displacements over all the cavity surfaces increased over time.

Conclusion: Neither the Fl or GI liners showed the potential to reduce the cusp deflection or composite shrinkage during the polymerization. The flowable composite lining may increase the inward displacement caused by composite polymerization shrinkage. The cusp deflections increased with cavity width.