Bond strength of restorative materials to hydroxyapatite inserts and dimensional changes of insert-containing restorations during polymerization

• HAP inserts reduced polymerization shrinkage of the insert-containing restorations.
• ‘Total-etch’ approach is recommended for adhesive bonding of composites to HAP.
• Micromechanical interlocking is the major adhesion mechanism.
• A universal composite is recommended to bond HAP inserts to dentin.

ObjectiveTo determine the shear bond strength (SBS) between synthetic controlled porous hydroxyapatite (HAP) inserts and restorative materials and dimensional changes of insert-containing restorations during curing.MethodsCylinder-shaped HAP inserts (4 mm in diameter, 1.6 mm thick) were cemented in dentin discs (5 mm × 1.6 mm), cut mid-coronally from human third molars, using one of the following materials: universal microhybrid composite Filtek Z250, flowable composite Filtek Ultimate or glass-ionomer Vitrebond (all 3M ESPE). SBS of the same materials to HAP inserts was tested in a universal testing machine. Three-dimensional digital image correlation system Aramis (GOM) was used to measure strains and displacements. Data were statistically analyzed using one-way ANOVA with Tukey's post-test (α = 0.05).ResultsSBS of restorative materials to HAP inserts ranged between 12.2 ± 2.1 MPa (Filtek Z250) and 0.7 ± 0.4 MPa (Filtek Z250 without an adhesive). The ‘total-etch’ approach of adhesive application significantly increased SBS of both Filtek Z250 (12.2 ± 2.1 MPa) and Filtek Ultimate flowable (9.5 ± 2.5 MPa) compared to the ‘self-etch’ approach (8.2 ± 1.6 MPa and 4.4 ± 0.9 MPa, respectively) (p < 0.05). HAP inserts reduced polymerization shrinkage to below 0.5% as well as displacements in the central region of the restorations. Peripheral shrinkage of restorative materials was similar with and without HAP inserts as were displacements of Filtek Z250 and Vitrebond.SignificanceReplacing major part of dentin clinically, especially in large cavities, HAP inserts may shorten clinical working time, improve dimensional stability of the restoration by reducing central shrinkage and displacements and provide adhesive bonding to universal composites following a ‘total-etch’ approach.