Fracture and viscoelastic behavior of novel self-adhesive materials for simplified restoration concepts.

OBJECTIVE

The aim of the study was to offer a comparative perspective on the mechanical and viscoelastic behavior of currently developed materials for simplified restoration concepts. These materials have not yet been clearly assigned whether they are complex hybrids of already known material categories or new material classes.

METHODS

A dual-cured, bulk-fill, bioactive resin-based composite (alkasite), a resin-modified glass ionomer cements (RM-GIC) with novel polymerizable acid polymers, and a glass ionomer cement (GIC) with improved adaptation to an acidic environment were compared with regard to their macro-mechanical parameters (3-point bending test, 3-PBT), fracture mechanism, quasi-static and viscoelastic behaviour (instrumented indentation test, IIT), morphology and structural appearance of the filler system (SEM analysis). The influence of surface finishing was quantified on the outcome of the 3-PBT, while the influence of aging and frequency was monitored on the outcome of the IIT. One and multiple-way analysis of variance (ANOVA) with Tukey honestly significant difference (HSD) post-hoc tests (α = 0.05) and Weibull analysis were applied.

RESULTS

Surface finishing strongly influenced the outcome of the 3-PBT for RM-GIC and GIC but not of the alkasite. The highest material reliability (Weibull parameter m) was found with the alkasite, irrespective of the curing mode. Ground specimens showed decrease reliability, except for the alkasite in the light-cured mode. The predominant failure mode originated from sub-surface defects (52.5%), followed by corner (25%), edge (18.1%), and crack arrest (4.4%). The effect of the parameter material on the quasi-static outcome of the IIT was highest on elastic/plastic parameters (p < 0.001; e.g. elastic indentation work, η = 0.875), was moderate on the Martens Hardness (η = 0.420), and was low on the Vickers hardness (η = 0.218). The viscoelastic parameters, in particular the loss factor (tan δ) allow a clear documentation of the ongoing acid-base setting reaction during aging of one month, which was more pronounced in the GIC than in the RM-GIC. The decrease in tan δ with aging for GIC and RM-GIC reflects the maturation process and increased brittleness, while the increase in tan δ with aging reflects the polymer plasticization in the polymer-based alkasite.

CONCLUSIONS

The mechanical and viscoelastic behavior depending on surface refinement, aging and frequency clearly allow to classify the currently developed materials for simplified restoration concepts into known material categories such as RBCs (alkasite), RM-GIC or GIC.