In vivo monitoring of bone–implant bond strength by microCT and finite element modelling

Abstract

Immediately after implantation, a dynamic process of bone formation and resorption takes place around an orthopaedic implant, influencing its mechanical fixation. The delay until complete fixation depends on local bone architecture and metabolism. Despite its importance, the temporal pattern of implant fixation is still unknown. The optimal duration of post-operative care is therefore difficult to establish for an individual situation, and a method to evaluate non-invasively the evolution of the mechanical stability would be a significant asset in a clinical environment. The aim of this study was to evaluate the potential of micro-finite element modelling based on in vivo micro-computed tomography to monitor longitudinally the contact between bone and implant and the implant strength in vivo. The model was first validated for screw pull-out in synthetic bone surrogate. Correlation coefficients of R² = 0.94 and 0.85 (p < 0.01) were measured between experimental and numerical results for stiffness and failure loads, respectively. Then, the mechanical integration of screws in the proximal tibia of 12 rats was monitored at seven time points over a period of 1 month. We observed significant increases (p < 0.05) of bone–screw contact (+28%), stiffness (+93%) and failure load (+71%) over the course of the experiment, and more than 75% of these changes occurred during the first 2 weeks. Limitations, such as image artefacts and radiation, still compromise the immediate clinical application of this method, but it has a promising potential in preclinical animal studies, as it provides very valuable data about the dynamic aspect of implant integration with considerably reduced animal resources.

Keywords::

• implant fixation
• micro-computed tomography
• finite element modelling

Acknowledgements

This study was funded by the Swiss National Science Foundation (FNS), the Alberta Heritage Foundation for Medical Research (AHFMR), Canadian Institutes of Health Research (CIHR) and the Skeletal Regeneration Medicine Team (SRMT). We thank Dr Philip Procter (Stryker Osteosynthesis) for generously providing the titanium screws. The study sponsors were not involved in the study. We thank Dr Darrel Florence for his valuable advice on rat surgery, Dr Helen Buie for her assistance and advice on image processing and Mr Eric Nodwell for his assistance with FE modelling.