Background: Patients who suffer from acetabular bone defects are often subjected to a decreased mobility and a loss of independence. To treat these bone defects, standard hemispherical acetabular implants offer in most cases the solution. However, when these defects are large, a custom triflange acetabular implant is needed. Although this type of implant performs well in general, it also has some drawbacks. These include higher costs and a longer design- and development cycle, since every implant is custom-made to the patient. In addition, it is expected that the triflange implant causes more stress-shielding (loss in bone-mineral-density (BMD) due to insufficient loads) than the standard implant. However, the evidence for this last statement is quite weak, since studies on both cups use different parameters,materials and environments. Objective: The objective of this study is to determine if the custom triflange cup causes indeed more stress-shielding than the standard hemispherical cup. Furthermore, the goal is to find a solution for this and the other above-mentioned drawbacks of the custom triflange cup. Therefore, the goal is to study the potential of a deformable acetabular cup, which in theory should solve these issues of the triflange cup. Methods: First a statistical shape model (SSM) of the human pelvis was developed to find patterns in defects of the pelvis, which could eliminate the need for customization. Next, a Finite Element (FE) Model with a bone-remodelling algorithm was developed to determine the difference in stress-shielding between the standard and the triflange cup. Furthermore, a FE model of a deformable implant is developed which is pressed into multiple defected pelvises. Finally, a machine-algoirthm is trained to predict the optimal deformable cup parameters, based on the type of defects. Results: Several modes of deformation were found with the SSM, which were utilised in creating damaged pelvises for the Finite Element Model. Furthermore, it was found that the custom triflange cup decreases the BMD of the pelvis by 28.6% compared to pelvis without cup. With a deformable implant, this decrease can be reduced to 7.1%, which is in the similair range as the standard hemispherical cup. Finally, it was found that the machine-learning algorithm can successfully predict the optimal cup parameters, based on the type and size of defects.

It has been observed that fractured bones which are stabilized with a titanium fracture fixation plate, once healed, often refracture at an edge of the plate. This is believed to be caused by stress concentrations in the bone that take place at the location of the edges of the plate. Various implant parameters are known to affect these concentrations of stress, but we further hypothesized that the material of the plate has the most significant influence in reducing these concentrations, here referred to as ‘peak stresses’. Moreover, it was reasoned, based on relevant literature, that minimizing the peak stresses was not the only criteria that should be considered for the design of an optimal implant, the effect of the Interfragmentary strain on the healing outcome in the early stages after implantation is also crucial for the success of the implant. Otherwise the bone will never heal in the first place.

Thus, by assuming that different regions of the plate have a different influence in the peak stresses, it was suggested that a stiffness graded plate could minimize the peak stresses while still allowing for an acceptable interfragmentary strain at the early stages of healing, through an optimization. In order to prove or disprove all the hypotheses mentioned above, a Finite Element model of a fracture fixation construct was developed. For the selection of many of the modelling assumptions, a literature study was carried out. For the selection of the contact properties to be used, a graphical comparison study was done. For choosing an appropriate mesh, a mesh study was implemented.

Using this Finite Element model, it was possible firstly to show that while the material properties of the plate do in fact appear to be very influential in reducing the peak stresses, the thickness seems to be even more influential. For the latter a parametric study was carried out using the Taguchi method. Secondly, it was shown that different regions of the fracture fixation plate do have a different influence in the peak stresses of the bone. The outer most sections of the plate seem to always be the most influential. Lastly, optimisations were carried out in three different ways and although they all reduced the peak stresses, the method thought to be the simplest, yielded the most useful results. This consisted of dividing the plate only into three sections and assigning one material to the outer sections and another to the inner section.

Knowledge of the influence of loading directions on trabecular bone remodeling in spine is of significant value in understanding the development of spine deformities and vertebral bone quality across different scales. Information on the constitution of a preferred trabecular orientation and mechanical properties of trabecular bone are important indicators in this respect. The current thesis aimed at exploring these aspects across multiple length scales in the spine. The thesis is divided in two parts. The influence of loadings less dominant than compression, i.e. shear, on the constitution of a preferred trabecular orientation in the spine on the macro-tissue level (>10 mm) was investigated in the first part (Part I). This influence was related to mechanical characteristics of trabecular structures on the micro-tissue scale (1-10 mm) in the second part (Part II). In Part I, primary trabecular orientations (PTOsmacro) near the superior and inferior vertebral endplates of L1 and L5 of 6 human spine cadavers were determined on the macro level using micro computed tomography imaging (voxel size = 120 m3), by calculating the dominant fabric principal vector. Their relative deviations to the axial compression vectors in the spines, quantified by the normals to the endplate (NEs), were determined afterwards. The average deviation between the PTOmacro and NEs was 6.24⁰ (±4.34⁰). The PTOsmacro did not show a preference towards the anterior or posterior direction relative to the NE. From the deviations, it was concluded that trabecular bone in the spine predominantly adapts to compression loads. However, secondary loading directions, such as shear, are of additional influence. In Part II, 13 small cubes (6.0x6.0 mm) from the volumes of interest in Part I were analysed on the micro level with regard to elasticity. Components, component ratios and primary elastic orientations (PEOmicro) of elasticity tensors, computed by the simulation of mechanical tests in finite element (FE) models, were calculated. PTOs of the cubes (PTOsmicro) were compared to the PEOsmicro and related to the PTOsmacro and NEs (Part I) qualitatively. Elasticity tensor components were within a reasonable range (approximately 1-250 MPa, excluding outliers) and no material symmetry was found, i.e. the structures were mechanically anisotropic. PTOsmicro deviated 13.90⁰ (±8.04⁰) with respect to the PEOsmicro on average. 10 out of 13 PEOsmicro had similar anterior or posterior tendencies as the PTOsmacro with respect to the NEs. 11 out of 13 PTOsmicro had similar anterior or posterior tendencies as PTOsmacro with respect to the NEs. Elastic properties of typical trabecular structures in the vertebral bodies were successfully determined. Due to a relatively low resolution, PEOsmicro deviated strongly with the PTOsmicro. Such deviations could function as indicators for bone quality in skeletal disease diagnostics using low resolution imaging. PTOsmicro and PEOsmicro agreed relatively well to the PTOsmacro on the macro-tissue level, in terms of anteriorly or posterior tendencies relative to axial loading in the spine. This outcome shows promise for multi-scalar biomechanical analysis of trabecular bone.