The project focuses on the functional and structural design of an innovative hip prosthesis (HP), to be realized through additive manufacturing (AM) technology, with the aim of drastically reducing the typical biomechanical issues of this kind of implants. Taking advantage of the AM capability, a conventional titanium bulky HP is redesigned with a trabecular inner region of the stem, in order to reduce its stiffness and overall weight. Stiffness reduction is beneficial for the stress shielding effect: a conventional implant, more rigid than the bone, takes the great part of the loads during everyday movements, leading the bone to adapt to the partial unload reducing its size and strength. Furthermore, weight reduction is expected to mitigate the impact of the HP on the patient perception. Additionally, by means of AM, the roughness of external regions of the HP can be controlled, thus promoting a better osseointegration.
For the new design to be effective with respect to the above mentioned problems, a large amount of material has to be removed from a bulky HP classical design. Consequently, an assessment of the structural integrity of the new AM-HP design becomes crucial. To achieve this, a parametric Finite Element Model of the implant (prosthesis inserted in the host femur bone) is set up to find the best design parameters which maximize the benefits in terms of stress shielding and weight reduction, and guarantee at the same time that the structural limits are not overcome, identifying the critical points stress state. To improve the prediction of the FE model, tests on dedicated samples will be executed to identify elastic and failure limits of the HP materials. Concurrently, measurements on patients will be performed to acquire the actual loads under everyday and critical activities. As a result, a feasible, safe and efficient HP solution will be presented, along with a thorough comparison showing the gains of the new design with respect to a traditional one.
The major innovative aspects of the research, related to the all-round structural redesign of hip-prostheses, can be summarized as follows:
- The advanced mechanical characterization of the AM Ti6Al4V alloy and damage model calibration that will be performed in the research is per se original: very limited results are at present available in the literature relative to multiaxial testing on an additive manufacturing alloy, for static failure limits identification. Also, the ductile damage model that will be adopted in the FE analysis, which proved to be effective for traditional wrought materials, has not been still studied sufficiently, and should be possibly modified, when used to predict the resistance of an AM alloy. This research will provide a validation for the model and indications on its prediction accuracy on the Ti6A4v.
- The experimental techniques used to identify of the loads acting on the HP and the bone are at the state of the art and will consent to build a sound database of actual critical load combinations the post-implanted human subject will undergo. The experimental data that will be gathered, apart for helping in providing a safe design in the project, will be a valuable information the research community could exploit for future studies on actual AM but also non AM hip prostheses.
- The FE model that will be setup (parametric, with elastic-plastic and failure material models implemented and accurately calibrated, fed with real cases loading conditions. modelling also trabecular regions) is a powerful tool to explore the effects of design choices on the biomechanical response of the implant and in granting the structural integrity of the solution. It is worth reminding that while this last aspect is not so crucial for a bulky Ti6Al4V HP, whose resistance is far beyond what needed, this is particularly important in the proposed design where for the HP to be as light as possible hollow and trabecular regions will be introduced, which significantly reduce the structural resistance of the implant. Basically, the FE model will be determinant in the assessment of feasible HP solutions. Also, FE analysis could play a role in showing potential local or spatially confined unwanted effects, such as stress concentrations or lack of stem-bone contact where expected which could lead to short term or long term issues on the HP.
- The final experimental phase on the HP prototype is a custom test, not usually executed, that will give an actual feedback and confirmation of the robustness and accuracy of the approach.
At the end, it is worth noting that the most of the results of this research will be not limited to the study of a specific HP implant, since material data information and many indications and guidelines of the proposed methodology could be adopted for several other hip replacement surgery cases.