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The impact of delayed placement and resin matrix composition on adhesion between layers of resin composites
Suvi Vallittu1, Sufyan Garoushi1, Lippo Lassila1
1Department of Biomaterials Science and Turku Clinical Biomaterial Center -TCBC, Institute of Dentistry, University of Turku, Turku, Finland.
Objective:
To investigate the effect of delay time in resin composite placement and resin matrix composition on interlayer adhesion between two successive composite layers.
Materials And Methods:
Three experimental resin composites were prepared by mixing 79 wt.% particulate filler with 21 wt.% dimethacrylate-based resin matrices (urethane dimethacrylate/triethylene glycol dimethacrylate [UDMA/TEGDMA]; ethoxylated bisphenol-A-dimethacrylate/urethane dimethacrylate/triethylene glycol dimethacrylate [Bis-EMA/UDMA/TEGDMA]; bisphenol A-glycidyl methacrylate/triethylene glycol dimethacrylate [Bis-GMA/TEGDMA]). Fifteen groups of specimens were prepared (n = 6/group), consisting of a cured composite substructure onto which a successive surface layer was applied at different time intervals following light-initiated polymerization (0, 10 minutes, 1, 24, and 72 hours). After 2 days of dry storage at 37°C, interlayer adhesion was evaluated by measuring shear bond strength (SBS) using a universal testing machine. Failure modes were visually assessed. In addition, oxygen inhibition layer (OIL) thickness was measured microscopically, and Vickers hardness and viscosity were determined. Data were analyzed using two-way and one-way analysis of variance followed by Tukey Tukey's Honestly Significant Difference (HSD) tests (α = 0.05).
Results:
SBS values ranged from 10.3 ±2.4 to 24.5 ±3.0 MPa. Both placement delay time and resin matrix composition significantly affected SBS (p < 0.05). SBS decreased significantly when the application of the second successive layer was delayed beyond 1 hour, with the lowest values observed after 72 hours, particularly in Bis-GMA-based composites. The highest SBS was observed in the UDMA/TEGDMA group at 1 hour. Cohesive failure within the substructure was seen in all specimens. The UDMA/TEGDMA group exhibited the lowest OIL thickness (29.35 ± 0.69 µm).
Conclusion:
Both incremental application timing and resin matrix composition significantly influenced composite interlayer adhesion. Short delays (10-60 minutes) enhanced interlayer adhesion, whereas prolonged delays (24-72 hours) resulted in a reduction in interlayer adhesion.