The effect of aging on the wear performance of monolithic zirconia.

OBJECTIVES: Investigate the effect of aging on the wear behavior of glazed vs polished monolithic zirconia and to establish if glazing provides protection against low temperature degradation. METHODS: 40 1-mm-diameter spheres made from four differently treated monolithic zirconia (VITA YZ® HT); polished, polished-aged, glazed and glazed-aged (n = 10), were tested in a wear testing machine (UFW200) against bovine enamel in artificial saliva as per the following settings (ISO20808:2016): ball-on-disc configuration, 5 N vertical load, 0.1 m/s sliding speed, 400 m sliding distance and 37 °C temperature. Vertical substance loss (mm) wear of zirconia and enamel specimens was measured. Data were statistically analyzed with Kruskal-Wallis one-way ANOVA test (α > 0.05). RESULTS: Glazed-aged zirconia specimens resulted in the greatest amount of enamel wear (0.823 mm ± 0.157) followed by glazed (0.729 mm ± 0.289), polished-aged (0.377 mm ± 0.201) then polished (0.247 mm ± 0.125). In the groups with the same surface finish, aging showed no statistical difference in wear (P > 0.008). Glazing resulted in a higher enamel wear compared with polishing that was statistically significant (P < 0.008) except when the polished specimens were aged and the glazed specimens were not aged. SIGNIFICANCE: Aging increases abrasiveness of monolithic zirconia regardless of the type of surface finish. The effect of aging is "latent" and only revealed under mechanical loading during wear simulation which increases surface roughness and wear by adversely affecting zirconia's mechanical properties, making it less capable to maintain its initial surface smoothness. The glaze layer may protect zirconia from LTD, however, it is susceptible to aging which further increases its abrasiveness.

Investigation of the elastic modulus, tensile and flexural strength of five skull simulant materials for impact testing of a forensic skin/skull/brain model.

Conducting in vitro research for forensic, impact and injury simulation modelling generally involves the use of a skull simulant with mechanical properties similar to those found in the human skull. For this study epoxy resin, fibre filled epoxy resin, 3D-printing filaments (PETG, PLA) and self-cure acrylic denture base resin were used to fabricate the specimens (n=20 per material group), according to ISO 527-2 IBB and ISO20795-1. Tensile and flexural testing in a universal testing machine was used to measure their tensile/flexural elastic modulus and strength. The results showed that the epoxy resin and fibre filled epoxy resin had similar tensile elastic moduli (no statistical significant difference) with lower values observed for the other materials. The fibre filled epoxy resin had a considerably higher flexural elastic modulus and strength, possibly attributed to the presence of fibres. Of the simulants tested, epoxy resin had an elastic modulus and flexural strength close to that of mean human skull values reported in the literature, and thus can be considered as a suitable skull simulant for a skin/skull/brain model for lower impact forces that do not exceed the fracture stress. For higher impact forces a 3D printing filament (PLA) may be a more suitable skull simulant material, due to its closer match to fracture stresses found in human skull bone. Influencing factors were also anisotropy, heterogeneity and viscoelasticity of human skull bone and simulant specimens.