Modelling bone-implant interaction in fracture fixation devices

Research output: Contribution to conferenceAbstractpeer-review

Abstract / Description of output

Fracture fixation devices, such as half-pin fixators, ring fixators and locking plates, are extensively employed to treat fractures. For any fixation device there are three key clinical requirements and consequent mechanical demands arising from them [1]: (a) it must support fracture healing; (b) it must not fail during the healing period; and (c) it should not loosen or cause patient discomfort. It has been shown that the mechanical demands (a) and (b) are related implant stiffness and its fatigue strength. Consequently stiffness of bone-fixation systems has been extensively investigated over several decades. Implant loosening (mechanical demand (c)), which also entails a risk of infection, occurs due to high strains at the bone-implant interface and due to cyclic loads has received considerable little attention. Computational modelling is an apparent tool for such an analysis as it is not straightforward measure strains at the bone-implant interface via an experimental set-up. We discuss different approaches we have employed to evaluate loosening. For cortical boneimplant interaction we consider two approaches. In the first we assume the bone to be linearly elastic but incorporates anisotropy and inhomogeneity [2]; loosening is estimated by evaluating the volume of bone that crosses a strain threshold [3]. The second approach additionally incorporates strainbased plasticity [4] in the models and estimates loosening by evaluating the pattern and the volume of bone that enters the plastic regime [5]. We consider varied approaches for trabecular bone-implant interaction; all assume isotropy. We examine linear elasticity and Drucker-Prager and crushable foam plasticity [6]. Since implants experience cyclic loading which is known to cause implant loosening we also examine two time-dependent criteria: linear viscoelastic [7] and nonlinear viscoelastic and viscoplastic [8]. Our results indicate that with time-independent models, estimates provided by linear elastic approaches are good, particularly when the aim is merely to compare fixation configurations and options. However, loosening due to cyclic loading can only be provided by time-dependent models and our studies show that it is influenced by bone quality and number of cycles (Fig. 1). So time-dependent
models can be used not only to predict loosening but also to provide post-op rehabilitation advice.
Original languageEnglish
Pages33
Publication statusPublished - 7 Sept 2021
Event17th International Symposium on Computer Methods in Biomechanics and Biomedical Engineering

- Bonn, Germany
Duration: 7 Sept 20219 Sept 2021

Conference

Conference17th International Symposium on Computer Methods in Biomechanics and Biomedical Engineering

Abbreviated titleCMBBE2021
Country/TerritoryGermany
CityBonn
Period7/09/219/09/21

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