An investigation on the daily activity forces and frictional moments and their contribution to the fretting corrosion failure of modular hip joint implants

Author: Hamidreza Farhoudi

Farhoudi, Hamidreza, 2021 An investigation on the daily activity forces and frictional moments and their contribution to the fretting corrosion failure of modular hip joint implants, Flinders University, College of Science and Engineering

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The contemporary design of hip implants for Total Hip Arthroplasty (THA) comes with modular components to provide flexibility for the surgeons during the operation. This modularity increases the number of material interfaces, each of can become a source of failure. Fretting-corrosion at the metallic components of the femoral head- stem neck (head-neck) interface can lead to THA failure due to adverse local tissue reaction to the emitted ions and debris from the interface. Literature review suggests loads on the interface, the geometry of the interface, and corrosivity of the ambient as the main effective parameters on fretting-corrosion of an interface. Loads acting on the head-neck interface of THA comprise body forces and frictional moments caused by sliding of the femoral head inside the pelvic cup. Body forces of daily activities have been well studied and are available in the literature. However, frictional moments of these activities, especially for metal on metal head-cup bearing, is poorly studied and is currently not available. Given the importance of load components on fretting-corrosion, the frictional moment of daily activities needs to be provided as an effective parameter of the study on fretting-corrosion.

An analytical method is developed to provide the currently not available frictional moments for metal-on-metal bearing couples for different patient weights and wide range daily activities. For the nine investigated activities, the maximum magnitude of the frictional moment ranged from 2.6 to 7.1 Nm. The maximum magnitude of the torque acting on the head-neck interface ranged from 2.3 to 5.7 Nm. These induced torques on the head-neck interface were close or even higher than the pure torques previously reported for depassivation of the head-neck interface. Interruption of this oxide layer by fretting initiates corrosion of active metal underneath. Forces and frictional moments of level gait walking found to be in a range inclusive of most of the other daily activity and considering its high daily frequency, it was selected as the loading scenario of this study.

A Finite Element Analysis, which was developed by another PhD student in the research group, was implemented to investigate mechanical responses of the head-neck interface to the frictional moments and forces of walking activity. Loading scenarios were frictional moment only, forces only and simultaneous frictional moments and forces. In the walking forces only loading scenario mechanical response of the head-neck interface had a considerable change as 8.43% initially separated surface nodes came into contact. The stress-field of the press-fitted area also changed temporarily. The frictional moments only did not cause considerable changes in the mechanical response of the interface (only 0.27% of the non-contacting surface nodes temporarily came into contact). The effect of frictional moments was considerable in superposition on forces in simultaneous frictional moments and forces case where the micro-motions and contact stresses changed compared to forces only case. The normal contact stresses and micro-motions obtained from this realistic load-controlled study were typically in the range of 0–275 MPa and 0–38 µm, respectively.

In vitro investigation on fretting-corrosion of the head-neck interface was conducted on two different head-neck material combinations of Ti6Al4V-CoCr and CoCr-CoCr. To keep the stress field, induced micromotion and crevice geometry of the head-neck interface as of its service condition, a full-scale implant was studied under frictional moments and forces profile of walking activity. A range of frictional moment varying from 0Nm to 7.5Nm was combined by a range of forces from 900N to 1800N. It observed that Ti6Al4V-CoCr head-neck combination was more susceptible than CoCr-CoCr material combination. The non-loaded control value of Open Circuit Potential (OCP) of Ti6Al4V-CoCr was -0.311V against -0.116V of CoCr-CoCr material combination, and at the highest drop of OCP, their OCP reached to -0.615V and -0.506V, respectively. Frictional moments effects on fretting-corrosion damage were more considerable in the lower vertical forces (for example, walking force with a maximum magnitude of 900N). In the presence of higher vertical forces equal to 1800N, the variation of moments from zero to 7.5Nm had a smaller effect on the OCP and the fretting-corrosion response of the interface. The general outcome suggests that increase of the forces had a more intensifying effect on fretting-corrosion while frictional moments, is more effective on fretting-corrosion damage when is combined with lower contact forces.

The outcome of this research suggests that frictional moments play a considerable role in both mechanical responses of the head-neck interface and fretting-corrosion of the head-neck interface. Considering the accumulative nature of fretting-corrosion damage in the interface, simplification of loading to forces only scenarios in FEA studies and in vitro studies need to at least come with a justified increase in the force magnitude.

Keywords: fretting corrosion, hip joint implant, taper junction

Subject: Engineering thesis

Thesis type: Doctor of Philosophy
Completed: 2021
School: College of Science and Engineering
Supervisor: Reza Hashemi