Abstract:
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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. |