Analysis of Torque Maintenance and Fracture Resistance after Fatigue in Retention Screws Made of Different Metals for Screw-Retained Implant-Borne Prosthesis Joints

Purpose. The aim was to evaluate the effect of different metallic alloys used in the manufacture of retention screws for universal cast to long abutment (UCLA) abutments for external hexagon (HE) and Morse taper (MT) connection implants, as well as of mechanical cycling on torque maintenance and fracture resistance through electromechanical fatigue testing by mastication followed by compression testing. Methods. Sixty implants were used, 30 MT and 30 HE, with their respective titanium UCLA abutments and retention screws of 5 different materials (n = 6): Ti cp grade 2, Ti cp grade 4, Ti cp grade 4 hard, Ti grade 5—Ti6Al4V and surgical steel (DSP® Biomedical). The assemblies were positioned in an electromechanical masticatory fatigue testing machine. The fracture strength test was performed by compression testing in a universal testing machine EMICDL-200. Results. The cycled screws and new screws of each alloy group for each connection type were evaluated, obtaining the maximum force (FM), in order to verify the effect of mechanical cycling. The data were tabulated and submitted to appropriate statistical analysis (α = 0.05). Conclusion. It was concluded that for the MT, the alloy with the best performance was steel, both in the maintenance of torque and in the compression test, and cycling negatively influenced the maintenance of preload for this connection. The alloy material did not influence torque maintenance for HE. The new screws that were subjected to EMIC showed higher strength. The alloy with the lowest strength was Ti grade 2.

Static Compressive Behavior and Material Failure Mechanism of Trabecular Tantalum Scaffolds Fabricated by Laser Powder Bed Fusion-based Additive Manufacturing

Additively manufactured trabecular tantalum (Ta) scaffolds are promising bone repair materials for load-bearing applications due to their good pore interconnectivity. However, a thorough mechanical behavior evaluation is required before conducting animal studies and clinical research using these scaffolds. In this study, we revealed the compressive mechanical behavior and material failure mechanism of trabecular tantalum scaffolds by compression testing, finite element analysis (FEA), and scanning electron microscopy (SEM). Trabecular tantalum scaffolds with porosities of 65%, 75%, and 85% were fabricated by laser powder bed fusion-based additive manufacturing. Porosity has a significant effect on their compressive mechanical properties. As the porosity decreased from 85% to 65%, the compressive yield strength and elastic modulus increased from 11.9 MPa to 35.7 MPa and 1.1 GPa to 3.0 GPa, respectively. Compression testing results indicate that trabecular tantalum scaffolds demonstrate ductile deformation and excellent mechanical reliability. No macroscopic cracks were found when they were subjected to strain up to 50%. SEM observations showed that material failure results from tantalum strut deformation and fracture. Most microcracks occurred at conjunctions, whereas few of them appear on the struts. FEA-generated compressive stress distribution and material deformation were consistent with experimental results. Stress concentrates at strut conjunctions and vertical struts, where fractures occur during compression testing, indicating that the load-bearing capability of trabecular tantalum scaffolds can be enhanced by strengthening strut conjunctions and vertical struts. Therefore, additively manufactured trabecular tantalum scaffolds can be used in bone tissue reconstruction applications.

Mechanical Characterization of Metal Additively Manufactured Contact Aided Cellular Compliant Mechanisms

Additive manufacturing allows for the creation of complex geometries that can be optimized for performance. However, understanding the effect of the manufacturing process on part performance is essential to balance performance with manufacturability. In this work, a cellular contact aided compliant mechanism previously designed by the authors is manufactured using laser powder bed fusion. Failure modes for C3M lattice design are highlighted and recommendations for manufacturability are obtained. The mechanical response of C3Ms is characterized through compression testing of lattice and single cells. Compression testing is used to understand the mode of energy absorption of C3M lattices and compare with mechanical simulations. Lattices with large wall thickness and largest cell size are found to absorb the most strain energy. The increase in stiffness of the C3M lattice is found to depend on the thickness of the cell walls and size of the contact gap. The failure modes based on manufacturing and mechanical test data are synthesized to generate design rules for selecting C3M geometry to minimize manufacturing defects and maximize performance.