Design and Optimization Lattice Endoprosthesis for Long Bones: Manufacturing and Clinical Experiment

Influence of porous titanium-based jaw implant structure on osseointegration mechanisms

The reconstruction of maxillofacial defects caused by anomalies, fractures, or cancer is challenging for dentofacial surgeons. To produce efficient, patient-specific implants with long-term performance and biological suitability, numerous methods of manufacturing are utilized. Because additive manufacturing makes it possible to fabricate complex pore structure samples, it is now recognized as an acceptable option to design customized implants. It is well recognized that a porous structure with proper design promotes accelerated cell proliferation, which enhances bone remodeling. Porosity can also be employed to modify the mechanical characteristics of fabricated implants. Thus, design and choice of rational lattice structure is an important task. The influence of the structure of jaw implants made of highly porous titanium-based materials on their mechanical properties and bone tissue growth was studied. Based on a 3D computer model of Wigner-Seitz lattice structure, the model samples were fabricated from Ti6Al4V powder by selective laser melting to characterize the mechanical properties of the samples depending on their macroporosity. Then two types of jaw bone implants were manufactured to conduct studies of bone tissue ingrowth when implanted in laboratory animals. The research was carried out in several stages: design and production of the implants for replacing incomplete defects of the lower jaw; implantation of SLM-printed implants in laboratory animals into an artificially produced defect of the lower jaw; analysis of the degree of fixation of the "implant - bone" connection (for implantation periods from 2 weeks to 9 months). During the research, Ti-alloy structures with cell diameters of 2-3 mm and macroporosity of 90-97% mimicking the spongy structure of trabecular bone tissue, were characterized by a compressive strength of 12.47-37.5 MPa and an elastic modulus of 0.19-1.23 GPa, corresponding to the mechanical properties of bone tissue. Active processes of tissue growth into implant cells were detected 2 weeks after implantation, the significant differences in the volume and types of filling tissue depending on the size of the cell were described. Recommendations for choosing the cell size depending on the type of bone tissue damage were given. When using SLM-printed implants with lattice structure (cell sizes from 1 to 3 mm), an active osteosynthesis processes occurred, which culminated in the formation of bone tissue inside the implant cells 9 months after implantation, with 68% of the samples characterized by the maximum degree of implant fixation. Implants with 3 mm cells with macropores diameters of 850 μm were recommended for replacing cavities after removal of perihilar cysts. To replace complete and partial defects, it was recommended to use implants with a cell size of 2 and 3 mm.

Extension of the Voronoi Diagram Algorithm to Orthotropic Space for Material Structural Design

Nowadays, the interaction of additive technologies and methods for designing or optimizing porous structures has yielded good results. Construction with complex microarchitectures can be created using this approach. Varying the microarchitecture leads to changes in weight and mechanical properties. However, there are problems with geometry reconstruction when dealing with complex microarchitecture. One approach is to use Voronoi cells for geometry reconstruction. In this article, an extension of the Voronoi diagram algorithm to orthotropic space for material structural design is presented. The inputs for the method include porosity, ellipticity, and ellipticity direction fields. As an example, a beam with fixed end faces and center kinematic loading was used. To estimate robust results for different numbers of clusters, 50, 75, and 100 clusters are presented. The porosity for smoothed structures ranged from 21.5% up to 22.8%. The stress-strain state was determined for the resulting structures. The stiffness for the initial and smoothed structures was the same. However, in the case of 75 and 100 clusters, local stress factors appeared in the smoothed structure. The maximum von Mises stress decreased by 20% for all smoothed structures in the area of kinematic loading and increased by 20% for all smoothed structures in the area of end faces.

Method of computational design for additive manufacturing of hip endoprosthesis based on basic-cell concept

Endoprosthetic hip replacement is the conventional way to treat osteoarthritis or a fracture of a dysfunctional joint. Different manufacturing methods are employed to create reliable patient-specific devices with long-term performance and biocompatibility. Recently, additive manufacturing has become a promising technique for the fabrication of medical devices, because it allows to produce complex samples with various structures of pores. Moreover, the limitations of traditional fabrication methods can be avoided. It is known that a well-designed porous structure provides a better proliferation of cells, leading to improved bone remodeling. Additionally, porosity can be used to adjust the mechanical properties of designed structures. This makes the design and choice of the structure's basic cell a crucial task. This study focuses on a novel computational method, based on the basic-cell concept to design a hip endoprosthesis with an unregularly complex structure. A cube with spheroid pores was utilized as a basic cell, with each cell having its own porosity and mechanical properties. A novelty of the suggested method is in its combination of the topology optimization method and the structural design algorithm. Bending and compression cases were analyzed for a cylinder structure and two hip implants. The ability of basic-cell geometry to influence the structure's stress-strain state was shown. The relative change in the volume of the original structure and the designed cylinder structure was 6.8%. Computational assessments of a stress-strain state using the proposed method and direct modeling were carried out. The volumes of the two types of implants decreased by 9% and 11%, respectively. The maximum von Mises stress was 600 MPa in the initial design. After the algorithm application, it increased to 630 MPa for the first type of implant, while it is not changing in the second type of implant. At the same time, the load-bearing capacity of the hip endoprostheses was retained. The internal structure of the optimized implants was significantly different from the traditional designs, but better structural integrity is likely to be achieved with less material. Additionally, this method leads to time reduction both for the initial design and its variations. Moreover, it enables to produce medical implants with specific functional structures with an additive manufacturing method avoiding the constraints of traditional technologies.

Numerical and Experimental Study of a Lattice Structure for Orthopedic Applications

Prosthetic reconstructions provide anatomical reconstruction to replace bones and joints. However, these operations have a high number of short- and long-term complications. One of the main problems in surgery is that the implant remains in the body after the operation. The solution to this problem is to use biomaterial for the implant, but biomaterial does not have the required strength characteristics. The implant must also have a mesh-like structure so that the bone can grow into the implant. The additive manufacturing process is ideal for the production of such a structure. The study deals with the correlation between different prosthetic structures, namely, the relationship between geometry, mechanical properties and biological additivity. The main challenge is to design an endoprosthesis that will mimic the geometric structure of bone and also meet the conditions of strength, hardness and stiffness. In order to match the above factors, it is necessary to develop appropriate algorithms. The main objective of this study is to augment the algorithm to ensure minimum structural weight without changing the strength characteristics of the lattice endoprosthesis of long bones. The iterative augmentation process of the algorithm was implemented by removing low-loaded ribs. A low-loaded rib is a rib with a maximum stress that is less than the threshold stress. Values within the range (10, 13, 15, 16, 17, 18, 19 and 20 MPa) were taken as the threshold stress. The supplement to the algorithm was applied to the initial structure and the designed structure at threshold stresses σ_f = 10, 13, 15, 16, 17, 18, 19 and 20 MPa. A Pareto diagram for maximum stress and the number of ribs is plotted for all cases of the design: original, engineered and lightened structures. The most optimal was the designed "lightweight" structure under the condition σ_f = 17 MPa. The maximum stress was 147.48 MPa, and the number of ribs was 741. Specimens were manufactured using additive manufacturing and then tested for four-point bending.

Structural Design Method for Constructions: Simulation, Manufacturing and Experiment

The development of additive manufacturing technology leads to new concepts for design implants and prostheses. The necessity of such approaches is fueled by patient-oriented medicine. Such a concept involves a new way of understanding material and includes complex structural geometry, lattice constructions, and metamaterials. This leads to new design concepts. In the article, the structural design method is presented. The general approach is based on the separation of the micro- and macro-mechanical parameters. For this purpose, the investigated region as a complex of the basic cells was considered. Each basic cell can be described by a parameters vector. An initializing vector was introduced to control the changes in the parameters vector. Changing the parameters vector according to the stress-strain state and the initializing vector leads to changes in the basic cells and consequently to changes in the microarchitecture. A medium with a spheroidal pore was considered as a basic cell. Porosity and ellipticity were used for the parameters vector. The initializing vector was initialized and depended on maximum von Mises stress. A sample was designed according to the proposed method. Then, solid and structurally designed samples were produced by additive manufacturing technology. The samples were scanned by computer tomography and then tested by structural loads. The results and analyses were presented.