Effect of Dynamic Load on Adhesion of Metal-free Onlay Restoration

Objective: The purpose of this study was to examine the effect of dynamic load on the adhesion of a metal-freeonlay restoration.

Method:  Standardized MODB onlaycavitywas prepared in 32 extracted human lower molars using a cavity duplicator.The cavities were coated with an all-in-one adhesive systemand a low-viscosity resin (Clearfil S3 Bond andClearfil Protect Liner, Kuraray).After preparingworking stone-model,onlaysweremade by a hybrid ceramicsmaterial (EsteniaC&B, Kuraray).The cavities were restored by the onlayswitha dual-cureresinluting cement (Clearfilesthethic cement, Kuraray).Restored specimens were dividedinto two groups (n=16);a group with dynamic load stress (S+) and a group without the stress (S-, control). ForS+ group, dynamicloadstress (16.0 kgf×3×10⁵ times) was applied to the restoredspecimensin water at 37 °C. For S- group, no stress was applied.Those specimens wereimmersed in 1% methylene blue solution at 37 °C for 60 min.Two slabs of 1.0mm thickness were obtained from each specimen.Microleakage in lingual(L) and gingival(G) walls of every slab was evaluated by a graded criterion.After the evaluation,each slab was trimmed to a standardized dumbbell specimen, and micro-tensile bond strengths(μ-TBS) topulpal wall (P) and axial wall (A) were measured.The data of microleakage/μ-TBS were examined using Kruskal-wallis test/ANOVA and Student’s t-test.

Result:  Microleakage ofL(S+) was significantly greater than L(S-)at p<0.01and also G(S+) atp<0.05.Mean values(s.d.) ofμ-TBS in MPa were P(S+);7.9(2.2), P(S-);10.5(1.6), A(S+); 9.6(2.2), A(S-); 10.1(2.4).The μ-TBS of P(S+) was significantly smaller than P(S-)at p<0.01and also A(S+) at p<0.05.

Conclusion: Microleakage in the axial wallof MODB metal-freeonlay restorationwasinfluenced by the dynamic load stress, but the gingival microleakage was not affected by the stress. Thedynamic load decreased the μ-TBS of the pulpal wall, although the μ-TBS of the axial wallwas not influenced by the stress.