Abstract
The long-term stability of total hip replacements (THRs) critically depends on the lasting integrity of the bond between the implant and the bone. Late failure in the absence of infection is known as ‘aseptic loosening’, a process characterised by the formation and progressive thickening of a continuous layer of fibrous tissue at the interface between the prosthesis and the bone. Aseptic loosening has been identified as the most common cause for long-term instability leading to the failure of ace-tabular cups. There is clearly a need to study the failure mechanisms in the acetabular fixation if the long-term stability of THR is to be significantly improved. The bonding strength in the presence of defects is measured using interfacial fracture toughness, and this information is not available currently.
In this work, interfacial fracture toughness of synthetic and bovine bone-cement interface has been studied using sandwiched Brazilian disk specimens. Experiments were carried out using a common bone cement, CMW, and polyurethane foam under selected loading angles from 0 to 25 degrees to achieve full loading conditions from tensile (mode I) to shear (mode II). Finite element analyses were carried out to obtain the solutions for strain energy release rate at a given phase angle (ratio of shear and tensile stress) associated with the experimental models. The effects of crack length on the measured interfacial fracture toughness were examined. Microscopic studies were also carried out to obtain the morphology of the fractured interfaces at selected loading angles.
The results show that both polyurethane foam and bovine cancellous bone seem to produce a similar type of interfacial failure of bone-cement interface, with cement pedicles being ‘pull-out’ of the pores of the foam/ bone. Damage sustained by the cement pedicles seems to increase progressively as the increase of shear loading component. The measured values of fracture toughness are a function of crack length and phase angle, and are comparable with those published in the literature on cortical bone and cement interface.
The implication of these results on the assessment of fixation in acetabular replacements is discussed, particularly in the light of results from bovine cancellous bone-cement interface.
Correspondence should be addressed to Mr Carlos Wigderowitz, Senior Lecturer, University Department of Orthopaedic and Trauma Surgery, Ninewells Hospital and Medical School, Dundee DD1 9SY.
