Can 4META-based adhesive compensate contraction gap of bulk filled resin?

Objectives: Bulk-fill of resin into deep cavities is contraindicated due to polymerization shrinkage toward the center of the mass and the light source that results in increased contraction gaps and compromised bonding. Recently developed low-shrinkage resins are recommended for bulk-filling without forming gaps with any methacrylate-based adhesives (MABA).  In this study, we evaluated whether a 4META-based adhesive can compensate for the shrinkage when used with conventional resin using a bulk-fill method.

Methods: Class-I cavities(3x3x3mm) were prepared in extracted, gamma-radiated human third molars. 2 MABA systems, EasyBond(E)(3M) or experimental 4META system(4M)(Sun Medical Co., Japan) were used. 4M consisted of TeethPrimer, Bondfill-SB-liquid, Catalyst-V and Radiopaque-L-Powder.  4M was brush-painted to form an adhesive layer (>50 µm thick). 2 resin restoratives, Z100(Z)(conventional,3M) or SureFil SDR flow(S)(low-shrink,Dentsply) were bulk-filled. Of 12 specimens for each combination, 6 were sectioned and polished immediately; another 6 were evaluated after 24 hours storage in DI-water.  Light microscopy and atomic force microscopy(AFM) were used to evaluate the contraction gap width at the center of the pulpal floor and 0.5mm from the pulpal floor along the axial wall. Statistics: 2-way-ANOVA.

Results: E+S group showed significantly less contraction gap than E+Z at both 0 and 24hours(P<0.001). 4M+Z showed less gap compared to E+Z at the sites evaluated(significant,24hour,P<0.05). Only E+Z had significant differences between pulpal and axial sites(p<0.001).

Conclusions: 4M provided improved adhesive/stress-absorbing agent that compensated for polymerization shrinkage even when used with conventional restorative in bulk-fill method.  However, 4M had large variations in the adhesive layer thickness and quality around the cavity wall.  Improvement in technique sensitivity is warranted to achieve clinical acceptability. S is currently used as a base or in non-stress bearing areas due to low mechanical strength, and efforts are needed to improve its mechanical properties for use as a true bulk-fill resin.