An acetabular revision is a very challenging intervention, due to moderate to severe bone deficiencies and poor bone quality. Current solutions for this intervention are associated with inconsistent and unreliable clinical outcomes. This leads to substantial complications, including implant migration and loosening. These complications are, among others, caused by the lack of biological fixation, a non-physiological stress distribution and stress shielding. To encounter these problems a novel concept for an acetabular revision has been presented. The aim of this new design is to plastically deform into massive acetabular bone deficiencies. This will stimulate the surrounding bone and therefore diminishes effect of stress shielding (Wolff’s Law). This study explored whether a porous layer made of pure titanium can achieve this space-filling behaviour. The infill of this porous layer is based on meta-biomaterials. The macro-scale properties of this type of materials are determined by their small-scale architecture. The aim of the first part of this study was therefore to systematically study the topology-property relationship of six topological designs, including the cube, truncated cube, truncated cuboctahedron, rhombic dodecahedron, diamond and body centred cubic. These designs were studied by experimentally determining their mechanical properties, including the Poisson’s ratio using the Digital Image Correlation (DIC) technique. Afterwards, three topological designs were selected to be implemented in the novel acetabular component, including the diamond, rhombic dodecahedron and body centred cubic. These unit cells showed the lowest stiffness and the highest positive Poisson’s ratio over the complete range of concerned porosities (80-98%). Besides, they showed bending-dominated deformation without the failure of struts. The results indicated that these unit cells have the highest capacity to plastically deform as well have the potential of space-filling behaviour. The porosity of the porous layer of the implant functionally graded from very porous at the bone-implant interface to very solid at the joint’s articulating surface, which corresponds to bone’s hierarchical structure. These implants were compressed inside a bone-mimicking mould, of which the appearance and mechanics resembled an acetabulum with large bone deficiencies. µCT images revealed that the implant based on the diamond unit cell showed the most promising deformability at the mould-implant interface. Although this deformation was promising, the push-in forces needed to compress the implant into the mould were very high (ranging from 3.33 kN to 14.8 kN). Future work is needed to diminish the need for these high push-in forces by making the porous outer layer even more deformable. This novel implant has the potential to increase the biological fixation, preserve the physiological stress distribution and diminish the effect of stress shielding in the acetabular component of a total hip replacement.

The management of large acetabular defects remains one of the most challenging aspects of revision total hip arthroplasties. Failure of frequently used acetabular reconstruction components are, among others, caused by the lack of biological fixation, a non-physiological stress distribution and stress-shielding. Attempting to diminish these drawbacks of current implants a new acetabular cup has been explored. The double layer of this new acetabular cup better fills the acetabular defect. According to Wolff’s law this should reduce stress shielding and stimulate bone ingrowth. Part one of this study explores the relationship between unit cell size of the titanium and mechanical properties for the body centred cubic unit cell. Three graded lattices using different unit cell sizes were designed and 3D-printed out of commercially pure titanium (Grade 1) using selective laser melting. An unconfined compression test with cylindrical samples as well as a confined compression test with hemispherical samples were performed to obtain mechanical properties. An additional finite element study was used to validate the confined compression test. Acetabular defects were made in five Sawbones hemipelves which served as the basis for the design of the ‘patient specific’ acetabular cups. Three triflange and two unflanged acetabular cups were designed and 3D-printed out of commercially pure titanium (Grade 1) using selective laser melting. Mechanical properties were obtained from the unconfined compression tests. All acetabular cups were inserted by two orthopaedic surgeons at the UMC Utrecht. Cyclic testing was performed up to 1000 cycles to assess femoral head penetration under load after insertion. Additional penetration of the femoral head was found to be between 0.1781 and 0.3793 mm. Future work is needed to prevent strut breakage upon insertion and more research is needed in the fatigue behaviour of these new cups and their biological effect on living bone tissue.

Cavitary bone defects are common in orthopedic surgery and may be present after curettage of benign tumors or as tumor-like lesions. Bone graft substitutes, especially from biocompatible materials, can be used as a solution for lling up those defects. However, these cavitary bone defects differ in size and location. It would, therefore, be convenient to utilize an implant that can change its shape according to its surrounding 3D environment, meaning there is no need to customize the implant for every patient case. In the last decade, additive manufacturing (AM) techniques have been the driving force behind the fabrication of complex three-dimensional objects. The various assets of these procedures enable the production of very complex designs with appropriate material properties. These materials that are engineered to exhibit certain properties are referred to as meta-materials. Functionalizing these materials on a nano-scale, to have certain biological or mass transport properties brings us to a new class of materials better known as meta-biomaterials. In this regard, highly deformable implants with novel properties can be referred to as meta-biomechanisms, consisting of multiple interconnected joints. The current research revolves around the direct fabrication of meta-biomechanisms using Selective Laser Melting (SLM). Subsequently, the general design recommendations and process constraints have been discussed briefly for the fabrication of mechanisms using SLM. In this regard, SLM poses an extra burden on the removal of supports, especially for the vulnerable geometries and within the joints clearance. Implementing a design approach, including both systematic and intuitive methods, was chosen as the main tool towards the accomplishment of the research aim. Consequently, new designs are proposed to minimize these undesirable events and the supports, with the subsequent challenge to obtain multi-joint mechanisms with increased Degrees of Freedom (DOF) and the least amount of supports. The rst joints have been successfully printed, as well as meshes of multiple joints. Surface Morphing Experiments were nally executed to classify them in terms of motion, taking into account the nal application. According to the results, the majority of the proposed meta- biomechanisms approach well the reference acetabular model, with an absolute difference below 1, reassuring the possibility of the proposed meta-biomechanisms as potential implants in cavitary bone defects. Besides the evaluation in terms of mobility, the structures were mechanically tested to derive their behavior under compression loading. The force needed to induce a sharp break was lower than the average peak force reported in hip joints during daily activities. Subsequently, the experiments revealed that their mechanical strength depends on the tting of the structures inside the acetabulum model. The bigger the gap between the structure and the acetabulum, the weaker the structure under compression loading. Overall, their scale and size should be optimized to sustain the average day-to-day forces joints are subjected to.

Bone tissue engineering (BTE) researches the characteristics which are needed to create the ultimate bone scaffold which enhances cell response. Limited research has been done regarding the effect of the Poisson’s ratio on the scaffold-cell interaction. In this graduation project we therefore explore the cell response on scaffolds with a different value of the Poisson’s ratio. Various meta-biomaterials were designed, manufactured, mechanically tested and the response on pre-osteoblasts (MC3T3-E1) was explored. The first experiment was performed at mesoscale. The meta-biomaterials, with cells, were evaluated with SEM imaging, presto blue and ARS staining. The second experiment was performed at micro-scale. The results of this experiment were evaluated with SEM imaging, actin staining and Runx2 staining. It was concluded that the auxetic meta-biomaterial, with negative Poisson’s ratio, high porosity and high stiffness, showed an enhancement of the cell response. However, this could not be confirmed by the 2D SEM images. A potential application for the meta-biomaterial that enhances the cell response is implementing this meta-biomaterial in a design for the surface of an implant to generate fast bone ingrowth.