Comparative Study of the Microstructure and Properties of Cast-Fabricated and 3D-Printed Laser-Sintered Co-Cr Alloys for Removable Partial Denture Frameworks.

Since additive technologies in dentistry are gradually replacing metal casting technology, it is necessary to evaluate new dental constructions intended for the development of removable partial denture frameworks. The aim of this research was to evaluate the microstructure and mechanical properties of 3D-printed, laser-melted and -sintered Co-Cr alloys, and perform a comparative study with Co-Cr castings for the same dental purposes. The experiments were divided into two groups. The first group consisted of samples produced by conventional casting of the Co-Cr alloy. The second group consisted of 3D-printed, laser-melted and -sintered specimens produced from a Co-Cr alloy powder divided into three subgroups, depending on the technological parameters chosen for manufacturing (angle, location and heat treatment). Examination of the microstructure was carried out by classical metallographic sample preparation, using optical microscopy and scanning electron microscopy with energy dispersive X-ray spectroscopy (EDX) analysis. A structural phase analysis was also performed by XRD. The mechanical properties were determined using a standard tensile test. The microstructure observation showed a dendritic character in the case of castings, while in the case of 3D-printed, laser-melted and -sintered Co-Cr alloys, the microstructure was typical for additive technologies. The XRD phase analysis confirmed the presence of Co-Cr phases (ε and γ). The results of the tensile test showed remarkably higher yield and tensile strength values and slightly lower elongation of the 3D-printed, laser-melted and -sintered samples than those produced by conventional casting.

Analysis of load distribution in tooth-implant supported fixed partial dentures by the use of resilient abutment.

INTRODUCTION: Differences between the tooth and implant response to load can lead to many biological and technical implications in the conditions of occlusal forces. OBJECTIVE: The objective of this study was to analyze load distribution in tooth/implant-supported fixed partial dentures with the use of resilient TSA (Titan Shock Absorber, BoneCare GmbH, Augsburg, Germany) abutment and conventional non-resilient abutment using finite element method. METHODS: This study presents two basic 3D models. For one model a standard non-resilient abutment is used, and on the implant of the second model a resilient TSA abutment is applied. The virtual model contains drawn contours of tooth, mucous membranes, implant, cortical bones and spongiosa, abutment and suprastructure. The experiment used 500 N of vertical force, applied in three different cases of axial load. Calculations of von Mises equivalent stresses of the tooth root and periodontium, implants and peri-implant tissue were made. RESULTS: For the model to which a non-resilient abutment is applied, maximum stress values in all three cases are observed in the cortical part of the bone (maximum stress value of 49.7 MPa). Measurements of stress and deformation in the bone tissue in the model with application of the resilientTSA abutment demonstrated similar distribution; however, these values are many times lower than in the model with non-resilient TSA abutment (maximum stress value of 28.9 MPa). CONCLUSION: Application of the resilient TSA abutment results in more equal distribution of stress and deformations in the bone tissue under vertical forces. These values are many times lower than in the model with the non-resilient abutment.

Relationship between microstructure, cytotoxicity and corrosion properties of a Cu-Al-Ni shape memory alloy.

Cu-Al-Ni shape memory alloys (SMAs) have been investigated as materials for medical devices, but their biomedical application is still limited. The aim of this work was to compare the microstructure, corrosion and cytotoxicity in vitro of a Cu-Al-Ni SMA. Rapidly solidified (RS) thin ribbons, manufactured via melt spinning, were used for the tests. The control alloy was a permanent mould casting of the same composition, but without shape memory effect. The results show that RS ribbons are significantly more resistant to corrosion compared with the control alloy, as judged by the lesser release of Cu and Ni into the conditioning medium. These results correlate with the finding that RS ribbons were not cytotoxic to L929 mouse fibroblasts and rat thymocytes. In addition, the RS ribbon conditioning medium inhibited cellular proliferation and IL-2 production by activated rat splenocytes to a much lesser extent. The inhibitory effects were almost completely abolished by conditioning the RS ribbons in culture medium for 4 weeks. Microstructural analysis showed that RS ribbons are martensitic, with boron particles as a minor phase. In contrast, the control Cu-Al-Ni alloy had a complex multiphase microstructure. Examination of the alloy surfaces after conditioning by energy dispersive X-ray and Auger electron spectroscopy showed the formation of Cu and Al oxide layers and confirmed that the metals in RS ribbons are less susceptible to oxidation and corrosion compared with the control alloy. In conclusion, these results suggest that rapid solidification significantly improves the corrosion stability and biocompatibility in vitro of Cu-Al-Ni SMA ribbons.

[The influence of continuous magnetic field on periodontal tissues under overdentures]. / Uticaj stalnog magnetnog polja na parodontalna tkiva ispod mobiinih supradentalnih proteza.

INTRODUCTION: Last remained teeth with reduced alveolar support do not have long-term prognosis, which is the reason for prolonging the life and thus providing a stable support of overdenture. The data from literature point out that static magnetic field has certain possibilities in resolving such problems. OBJECTIVE: Having in mind the pathogenetic factors which cause the reduction of the alveolar ridge and periodontal problems in our population, as well as osteoblastic and antiinflamatory activity, the aim of this investigation was to assess the effect of static magnetic field on periodontal tissue under the overdenture. METHODS: The investigation involved 38 partially edentulous patients, of both sexes and similar oral status who were bearers of a lower complete overdenture and upper classic complete denture as antagonist restoration. In the base of the lower overdenture the micromagnets were installed in the region of the remained teeth, which had static concentrated field of 60-80 mT power. The evaluation was done after 3, 6 and 12 months using the method of light densitometry. Periodontologic analysis was performed by standard and modified periodontologic tests. RESULTS: In patients with overdentures, after exposure to a magnetic field, the density of bone was not significantly changed, but the use of ANOVA disclosed changes in the observed interval. The tendency of increased density of the alveolar part of the observed region was noted. The region of the corresponding tooth of the contralateral side without magnetic influence showed decreased density of this region in the observed intervals. Plaque index and gingival index were improved underthe influence of the magnetic field, while after 6 and 12 months following the magnet insertion statistically significant changes were confirmed. The magnetic devices did not show any influence on the level of the gingival margin and junction epithelium. CONCLUSION: Static magnetic field is to be considered as a noninvasive procedure which is recommended to patients with reduced number of teeth and alveolar support.

[The biomechanics of dental implants and dentures].

INTRODUCTION: Osseointegrated implants are actually replacements for natural teeth, and, like natural teeth, they are exposed to various forces. Rejection and bad osseointegration of implants rarely occur today because oral implants are made from biocompatible materials. Most complications are a consequence of badly planned implant loading. OBJECTIVE: The aim of this work was the optimization of the process of planning and inserting oral implants and dentures based on the analysis of the biomechanical problems in implantology. METHOD: In order to determine the influence of the number of cantilevered superstructures, the number of implants and implant microdesign on tensions within the implant and in the peri-implant tissue, a calculation of tensions and deformations was made in a virtual model (control model) using the finite elements analysis. The obtained values served as reference values in the analysis of the results from three experimental models. RESULTS: In the control model, as well as in the experimental models, the first implant bears the heaviest load with dominant contraction tensions, the second one carries significantly weaker straining tensions, the third one carries weak contraction tensions and the fourth one the weakest straining tensions. The values of tensions and deformations have the same sign (-/+), but the absolute values depend on the number of cantilevered superstructures, implant microdesign and the number of inserted implants. CONCLUSION: Knowing the biomechanics of oral implants and the dentures on them allows for proper indication, a good choice of implants and good superstructure design. The prevention measures for bone resorption caused by biomechanical factors are: insertion of symmetrical screw implants and root-shaped cylindrical implants as long and as wide as possible, insertion of implants with the total supporting area expanded, choosing materials that are rigid enough, the right direction of implants, narrowing of the denture occlusal surfaces and location of the contact point at the centre.