Defects-tolerant Co-Cr-Mo dental alloys prepared by selective laser melting.

OBJECTIVES: CrCoMo alloy specimens were successfully fabricated using selective laser melting (SLM). The aim of this study was to carefully investigate microstructure of the SLM specimens in order to understand the influence of their structural features inter-grown on different length scales ranging from nano- to macro-levels on their mechanical properties. METHODS: Two different sets of processing parameters developed for building the inner part (core) and the surface (skin) of dental prostheses were tested. Microstructures were characterized by SEM, EBSD and XRD analysis. The elemental distribution was assessed by EDS line profile analysis under TEM. The mechanical properties of the specimens were measured. RESULTS: The microstructures of both specimens were characterized showing formation of grains comprised of columnar sub-grains with Mo-enrichment at the sub-grain boundaries. Clusters of columnar sub-grains grew coherently along one common crystallographic direction forming much larger single crystal grains which are intercrossing in different directions forming an overall dendrite-like microstructure. Three types of microstructural defects were occasionally observed; small voids (<10 µm), fine cracks at grain boundaries (<10 µm) and cracks at weld line boundaries (>10 µm). Despite the presence of these defects, the yield and the ultimate tensile strength (UTS) were 870 and 430MPa and 1300MPa and 1160MPa, respectively, for the skin and core specimens which are higher than casted dental alloy. SIGNIFICANCE: Although the formation of microstructural defects is hard to be avoided during the SLM process, the SLM CoCrMo alloys can achieve improved mechanical properties than their casted counterparts, implying they are "defect-tolerant".

Finite element analysis of stress distributions in mono- and bi-cortical dental implants.

The finite element analysis (FEA) of the stress distribution in the mono- and bicortically fixed implants subjected to 3-axial loading was performed and verified experimentally on a model mandible to evaluate the benefits of each type of fixation from the viewpoint of the compressive stress reduction in the cortical part of atrophied mandible. The analysis revealed that the highest compressive stresses in the cortical bone are generated at the edge of the cortical bone where the highest torque from the implant is acting. The most effective way to reduce the maximum level of compressive stresses in the cortical bone and in the implant is the recession of the implant thread slightly below the surface of the cortical bone. Shortening of the intraosseal length of the implant and/or thinning of the upper cortical bone result in a substantial increase of the maximum compressive stresses. The comparison of FEA and model experiments suggests that bicortical fixation is the most efficient in the fresh implants and the advantage of bicortical fixation compared to monocortical fixation decreases with time due to osteointegration, possibly as a result of gradual suppression of sliding between the bone and implant during loading.