Micromechanical aspects of fatigue failure in conventional and carbon nanotube-reinforced acrylic bone cement
Bone cement is required for the majority of implant procedures. The mechanical integrity of cemented implants may be compromised by fatigue failure of the bone cement, mainly due to internal defects or debonding at the implant interfaces; improvements in the mechanical properties of bone cement may therefore be valuable if the implant lifetime of cemented arthroplasties are to be increased and revision rates decreased. The present study investigated the use of synchrotron X-ray microtomography for the observation of internal defects and failure processes that occur during fatigue loading. Initial assessments of fatigue damage processes in in-vitro fatigue test specimens demonstrated the uncertain nature of locating fatigue cracks and other defects, identifying the need for a synthesis of high resolution tomographic imaging with complementary prior damage monitoring methods. This was achieved via a novel amalgamation of acoustic emission, ultrasound and/or microfocus computed tomography scans prior to testing. Location of cracks/defects prior to high resolution tomographic imaging increased the probability of capturing crack initiation, furthering the underlying understanding of crack formation and propagation. Experiments performed at the European Synchrotron Research Facility have shown that the microstructural features of a commercial bone cement are readily imaged using microtomography of short exposure times. Furthermore, interactions (for example crack deflection and ligament formation) have been clearly identified between failure processes and both the cement defect population and internal microstructure. Early stages of crack initiation have also been captured: a new mechanism of crack initiation is proposed where porosity and local BaSO4 distribution are seen to act together to cause resultant crack initiation in the cement matrix rather than directly from pore surfaces. An opportunity for cement enhancement has been identified in the use of carbon nanotubes (CNTs); improved mechanical and physical properties of acrylic bone cement reinforced with CNTs are reported in the literature, although current methods utilised for CNT dispersion in polymers do not immediately lend themselves to surgical deployment. Adding CNTs to bone cement may further provide bio-active and sensing capabilities, beyond the conventional fixation and load-bearing rôle. The present study confirms that CNT-reinforcement (using shear mixing techniques) enhances the fatigue performance of a PMMA matrix and additional acoustic emission parameter based analyses confirm that the presence of CNTs alters the associated failure mechanisms. An insight into the potential capabilities of CNT reinforced cements, using relatively simple preparation techniques suitable for surgical deployment, is provided. These results suggest that enhanced fatigue performance may be achieved by means of CNT reinforcement of the matrix leading to crack shielding mechanisms such as crack bridging. Biologically, the presence of CNTs may reduce local thermal necrosis in the tissue surrounding the cemented construct through a reduction of the peak exothermic polymerisation temperature.