Imaging and modelling the interference-fit in cementless implantation

Abstract

The survivorship of cementless implants depends upon their initial stability, which is provided by press fitting generated during implantation. Poor initial stability may lead to excessive micromotion between the implant and bone, and potentially loosening leading to pain necessitating revision surgery. Excessive press-fit strains may damage bone, also compromising stability. Previously, measurement of implantation strain had been limited to local discrete or surface measurements. The aim of this work was to characterise strain generated during implantation of cementless orthopaedic devices using full-field, non-contact measurement, primarily Digital Volume Correlation (DVC). Both analogue and real bone were analysed, focusing on characterisation of material, the influence that a metallic implant has on the accuracy of DVC, and implantation strains. The initial phase of work focused on development of non-contact strain measurement methods in analogue bone material. First the experimental measurement error and material’s mechanical behaviour were characterised. Error was assessed from zero strain and virtually, homogenously strained conditions. Young’s modulus and Poisson’s ratio were then determined using DVC analysis with the virtual fields method (VFM) and an optical extensometry technique. The Young’s modulus was found to be in excess of values provided by the manufacturer, due to the present methods’ avoidance of test artefacts, and good agreement was found between the two methods. The material characterisation work was then expanded to include additional analogue bone types, and their anisotropy. Significant mechanical property differences were found between loading directions and foam types. The analogue materials in this study demonstrated anisotropy, but at the lower end of the range seen in real bone, and much higher inter- and intra-sample consistency. A zero interference implantation study was carried out to determine the influence of artefacts caused by the metallic implant upon strain inferred from the CT-based methods. The implant introduced some error to DVC measurements, especially local to the implant surface and its tip. However, error away from this region was relatively small compared to implantation strains. A final analogue study measured strain during an interference-fitted implantation by DVC, and compared the results with a computational model. Large strains and localised deterioration of correlation coefficient were found at the foam-implant interface, obscuring strains in the bulk of the foam. Following correlation coefficient thresholding, the observed strain patterns were similar between the DVC and FE results. The magnitude of FE strains was approximately double the DVC, possibly due to the FEA’s idealised loading and interface failure model.
Finally, real bone studies were conducted. DVC was first used to extract the mechanical properties of two bovine trabecular specimens, pre- and post-failure. Damage was identified by determining regions of permanent strain which would not normally visible by traditional surface measurement. In analysing the effect of metal artefacts, increased strain error was found in regions of cortical bone where insufficient pattern was present to correlate between images. Finally, the implantation strain from press-fitting a metallic stem into bovine bone was analysed. The strain field was qualitatively similar to that in analogue bone, and DVC proved able to capture strain near the interface, despite the majority of strains being transmitted through a growth plate, limiting the interface strain magnitude. In summary, this thesis furthers the understanding of bone material behaviour and full field strain generation at implantation by use of non-contact measurement methods. This knowledge will aid the development of improved computational models and thus more robust design screening prior to mechanical testing, towards enhanced patient outcomes.

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ORCID for Martin Browne: orcid.org/0000-0001-5184-050X

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