Colin Lupton

A subject-specific pelvic bone model and its application to cemented acetabular replacements

A subject-specific three-dimensional finite element (FE) pelvic bone model has been developed and applied to the study of bone-cement interfacial response in cemented acetabular replacements. The pelvic bone model was developed from CT scan images of a cadaveric pelvis and validated against the experiment data obtained from the same specimen at a simulated single-legged stance. The model was then implanted with a cemented acetabular cup at selected positions to simulate some typical implant conditions due to the misplacement of the cup as well as a standard cup condition. For comparison purposes, a simplified FE model with homogeneous trabecular bone material properties was also generated and similar implant conditions were examined. The results from the homogeneous model are found to underestimate significantly both the peak von Mises stress and the area of the highly stressed region in the cement near the bone-cement interface, compared with those from the subject-specific model. Non-uniform cement thickness and non-standard cup orientation seem to elevate the highly stressed region as well as the peak stress near the bone-cement interface.

Bone–cement interfacial behaviour under mixed mode loading conditions

Interfacial behaviour of the bone-cement interface has been studied under tensile, shear and mixed mode loading conditions. Bovine cancellous bone was used to bond with acrylic bone cement to form bone-cement interface samples, which were mechanically tested under selected tensile, shear and mixed mode loading conditions. The influence of the loading angle and the extent of the cement penetration on the interfacial behaviour were examined. The failure mechanisms with regard to loading mode were examined using micro-focus computed tomography. The measured tensile and shear responses were utilized in a cohesive zone constitutive model, from which the pre-yield linear and the post-yield exponential strain softening behaviour under mixed mode loading conditions was predicted. The implications of the work on the studies of cemented joint replacements are also discussed.

Spatial resolution and measurement uncertainty of strains in bone and bone–cement interface using digital volume correlation

The measurement uncertainty of strains has been assessed in a bone analogue (sawbone), bovine trabecular bone and bone-cement interface specimens under zero load using the Digital Volume Correlation (DVC) method. The effects of sub-volume size, sample constraint and preload on the measured strain uncertainty have been examined. There is generally a trade-off between the measurement uncertainty and the spatial resolution. Suitable sub-volume sizes have been be selected based on a compromise between the measurement uncertainty and the spatial resolution of the cases considered. A ratio of sub-volume size to a microstructure characteristic (Tb.Sp) was introduced to reflect a suitable spatial resolution, and the measurement uncertainty associated was assessed. Specifically, ratios between 1.6 and 4 appear to give rise to standard deviations in the measured strains between 166 and 620 με in all the cases considered, which would seem to suffice for strain analysis in pre as well as post yield loading regimes. A microscale finite element (μFE) model was built from the CT images of the sawbone, and the results from the μFE model and a continuum FE model were compared with those from the DVC. The strain results were found to differ significantly between the two methods at tissue level, consistent in trend with the results found in human bones, indicating mainly a limitation of the current DVC method in mapping strains at this level.

Near tip strain evolution of a growing fatigue crack

Near tip full-field strains in a growing fatigue crack have been studied in situ using the Digital Image Correlation (DIC) technique in a compact tension specimen of stainless steel 316L under tension-tension cyclic loading. An error analysis of displacements and strains has been carried out, and the results show that the precision of displacements and strains in the wake of the crack is worse than that in front of the crack. A method for the determination of crack tip location is proposed for the DIC analysis. Strain ratchetting is observed ahead of the growing fatigue crack tip and found to be dependent on the distance to the crack tip; whilst normal strains appear to stabilise behind the crack tip.

Mechanical characterisation of a scaffold under monotonic and cyclic loading conditions

Loading from daily activities or from intensive exercise can lead to increased risk of fracture. Implants designed for load bearing purposes, such as repair of articular cartilage and underlying subchondral bone in knees must have the necessary mechanical competence under long term physiological loadings. In this study, the mechanical behaviour of a polymer-based osteochondral scaffold was examined under monotonic and cyclic loading conditions in a phosphate buffered saline solution at 37°C. Monotonic compression tests at selected strain rates were performed in both confined and unconfined conditions to investigate the influence of confinement. The effects of strain rate and sample composition on mechanical properties were also studied. Multi-step cyclic tests were carried out with increasing compressive loads. Changes in secant modulus and residual strain accumulation are monitored. The secant modulus and the number of cycles to failure of the scaffold are obtained and compared with those of human trabecular bone (Topolinski et al., 2011).

Damage evolution in acetabular reconstructs under physiological testing in a saline environment.

Damage development in cemented acetabular reconstructs has been studied under a combined cyclic loading block representative of routine activities in a saline environment. A custom-made environmental chamber was designed and installed on the Portsmouth hip simulator to allow testing of acetabular reconstructs in a wet condition for the first time. Damage was monitored and detected by scanning at selected loading intervals using micro-focus computed tomography (μCT). The preliminary results show that, although, as in dry cases, debonding at the bone-cement interface defined the failure of the cement fixation, the combination of mechanical loading and saline environment significantly affected the damage initiation and development, with drastically reduced survival lives of the reconstructs. Debonding was found to be initiated at the bone-cement interface near the rim of the acetabular cup, or DeLee zone I, in wet condition, as opposed to initiation in DeLee zone II near the dome region in dry cases. The survival time of the reconstruct in wet condition is less than 10% of that in dry condition under a given applied hip contact force.