In vitro studies of bone-cement interface and related work on cemented acetabular replacement

The lasting integrity of the bond between bone cement and bone defines the long-term stability of cemented acetabular replacements. Although several studies have been carried out on bone-cement interface at continuum level, micromechanics of the interface has been studied only recently for tensile and shear loading cases. Furthermore, the mechanical and microstructural behaviour of this interface is complex due to the variation in morphology and properties that can arise from a range of factors. In this work in vitro studies of the bone-cement interfacial behaviour under selected loading conditions were carried out using a range of experimental techniques. Damage development in cemented acetabular reconstructs was studied under a combined physiological loading block representative of routine activities in a saline environment. A custom-made environmental chamber was developed to allow testing of acetabular reconstructs in a wet condition for the first time and damage was monitored and detected by scanning at selected loading intervals using micro-focus computed tomography (μCT). Preliminary results showed that, as in dry cases, debonding at the bone-cement interface defined the failure of the cement fixation. However, the combination of mechanical loading and saline environment seems to affect the damage initiation site, drastically reducing the survival lives of the reconstructs. Interfacial behaviour of the bone-cement interface was studied under tensile, shear and mixed-mode loading conditions. Bone-cement coupons were first mechanically tested and then μCT imaged. The influence of the loading angle, the extent of the cement penetration and the failure mechanisms with regard to the loading mode on the interfacial behaviour were examined. Both mechanical testing and post failure morphologies seem to suggest an effect of the loading angle on the failure mechanism of the interface. The micromechanical performance of bone-cement interface under compression was also examined. The samples were tested in step-wise compression using a custom-made micromechanical loading stage within the μCT chamber, and the damage evolution with load was monitored. Results showed that load transfer in bone-cement interface occurred mainly in the bone-cement contact region, resulting in progressively developed deformation due to trabeculae bending and buckling. Compressive and fatigue behaviour of bovine cancellous bone and selected open-cell metallic foams were studied also, and their suitability as bone analogous materials for cemented biomechanical testing was investigated. Whilst the morphological parameters of the foams and the bone appear to be closer, the mechanical properties vary significantly between the foams and the bone. However, despite the apparent differences in their respective properties, the general deformation behaviour is similar across the bone and the foams. Multi-step fatigue tests were carried out to study the deformation behaviour under increasing compressive cyclic stresses. Optical and scanning electron microscopy (SEM) were used to characterise the microstructure of foams and bone prior to and post mechanical testing. The results showed that residual strain accumulation is the predominant driving force leading to failure of foams and bones. Although foams and bone fail by the same mechanism of cyclic creep, the deformation behaviour at the transient region of each step was different for both materials. Preliminary results of foam-cement interface performance under mixed-mode loading conditions are also presented.