Deformable Acetabular Implants

Abstract

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.