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Optimization of implant design for image-guided failure assessment
Picazo Carpi, Omara
Müller, Ralph
The most common failure in orthopaedic implants occurs at the bone-screw interface with screw pullout being a typical mechanical cause of fault. Even in the case of healthy bones with excellent initial fixation, micromotions experienced by the skeleton potentially lead to a progressive loosening. Although the micromotions might be dissipated ending in a long-term stability, they could also persist causing the vibration of the screw within the much softer surrounding bone tissue. As a result the interface would predictably get destroyed leading to increased liklihood of pullout. One reason for this is the mismatch between stiffnesses. The assessment of the interface becomes even more important in osteoporotic bone. Image-Guided Failure Assessment (IGFA) which integrates the application of a stepwise mechanical loading with the CT-scanning of each evolution, helps to know more about this type of failure. A conflict in CT-scanning of metal screws are the artifacts obtained on the image which disable the definition of an automated segmentation procedure and increase the difficulty to estimate the interface. Those artifacts are a consequence of the metal material and depend on the density and thickness/ volume involved.A detailed methodology was applied to come up with a new design of the current implant. First of all a Fault Tree Analysis (FTA) helped to clarify the weaknesses of the design. Later a morphological box and corresponding benchmarking were carried out to find solutions. After careful reasoning and assessment of the found alternatives, the validity of each proposal was examined. With the resulting configuration a Material Selection Approach was made in order to discriminate among all the available materials and find the ones that better fit the requirements of the project. With Nickel coated PEEK as a final solution, distinct thicknesses were also tried out to optimize the design. A final thickness of 100 μm and two different configurations with and without addition of a protective layer were chosen and applied to the final screws. A realistic test to assess the fatigue resistance was made in a Zwick machine. Ovine vertebral bodies were used to carry out the experiments. It was observed how a lot of the coating came off during the insertion affecting mainly the edges of the screw thread. Improvement of the surface treatment was then defined as an outlook. Other properties like hardness, roughness, friction coefficient and scratching resistance were as well evaluated. By means of XIPL image processing routines implemented by the Institute for Biomechanics within the OpenVMS environment , a script to get an automeated segmentation of the implant and the bone was defined. A validation procedure for the image rocessing routine was also created to assess the validity in comparison with a manually contoured solution, and good agreement was found.
Àrees temàtiques de la UPC::Enginyeria mecànica::Mecànica::Biomecànica
Orthopedic implants
Bone screws (Orthopedics)
Imaging systems in medicine
Implants ortopèdics
Cargols a l'os (Ortopèdia)
Imatges mèdiques
Universitat Politècnica de Catalunya

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