Titanium vaulted structures for orthopaedic applications

Medical devices are used to recreate damaged or diseased joints and also to restore pain reduced mobility to patients. Devices such as hip and knee implants are fixed to the host bone by use of acrylic bone cement or by a press-fit interference technique. The surface of the press fit implants may be roughened, but non-porous, which allows new bone to form on the surface, or they may be porous to encourage bone growth into the structure. Traditionally implants are produced by machining, casting or forging, before applying any coatings that are required. More recently Additive Manufacturing (AM) techniques such as Electron Beam Manufacturing (EBM) and Selective Laser Melting (SLM) have attracted interest as they may be able to create the whole device in one operation. This thesis describes the research carried out to produce “Vaulted Structure (VS)” as a method for creating specific porosity with increased strength in porous medical devices. Unit cell structures (Wire frames) are inherently weak because of the struts. It was proposed that VSs, constructed of platelets, would have a higher specific strength and therefore extend the range of available properties while still maintaining a structure that resembles cancellous bone, a structure preferred by surgeons. The creation of a VS begins with a computer model of the object to be created. This is then surrounded by an axis aligned bounding box, a box that has edges parallel to the x, y and z directions and just encloses the object. The bounding box is then voxelised, so it consists of many smaller cubic boxes, these boxes being the size of the required structure. The VS is then constructed by creating holed platelets on some of the surfaces of the smaller boxes, with these platelets being randomly deformed if required. The structure within the bounding box is then trimmed to the boundary of the required structure, this being accomplished using a rapid technique developed during this project. To allow manufacture of the components, the structure is sliced to create a suitable format for the SLM. By using SLM in combination with these new novel structures, a wide range of possible properties and morphologies can be generated, these being controlled by the cell size, hole size, thickness of the platelet and the level of randomisation. ii A range of structures with different morphologies were produced and mechanically tested in compression. It was found that a percentage randomisation of more than 60% and a cell aspect ratio of 1:1:2, produced isotropic mechanical behaviour, while other structures produced very different behaviour. It was also observed that the failure mode in compression could be controlled by the degree of randomisation and this is discussed in the thesis. A property that is important in the medical device industry is the look of the component as this affects up take by surgeons. The preferred structures are ones that look like cancellous bone and the new structures created here match both the look and mechanical properties of this material, but with the added advantage that it is possible, by modifying the control variables, to match the structure to cancellous bone from different aged patients. The research presented in this thesis considers the inter-relationship between the process parameters, architecture, properties of the structures and why these inter relationships occur. The research presented here extends knowledge of how to create these novel material/structure combinations, how to tailor them to a given purpose and by predicting behaviour in software produce the most suitable properties for a given medical device without extensive testing.