Modular junctions are widely used in orthopaedic devices, to provide flexibility in restoring joint alignment during surgery. The head-neck taper junction of total hip replacement is a typical example. However, the cyclic loading, inside the corrosive medium of the human body, results in tribocorrosion (fretting corrosion), at the interface of the head neck junction so called taperosis ( (Gilbert et al. 1993; Oskouei et al. 2016)). The resulting debris and released metal ions has been associated with lytic lesions, pseudotumours, as well as damage the cells body and the production of free radicals that are associated with various pathologies (Borys et al. 2019). Hence, several studies have investigated the mechanism of the material removal at the head-neck junction of hip implants and the parameter that may influence it (Chana et al. 2012; Fricka et al. 2012; Hussenbocus et al. 2015). Due to the complex geometry and multivariable nature of this process, finite element analysis is an effective method for such an investigation. Recently, FE method has been used to predict the material loss at the head-neck interface (Ashkanfar et al. 2017b; Fallahnezhad et al. 2017; Fallahnezhad et al. 2019b).
Previous studies have shown that a number of parameters, such as material combination, taper geometry and the patients’ weight and activities can influence the mechanical behaviour of the head-neck junction and affect the amount of the material loss, due to fretting corrosion (Fallahnezhad et al. 2018a; Fallahnezhad et al. 2017; Farhoudi et al. 2017; Feyzi et al. 2021a; Feyzi et al. 2021b). The force applied by surgeons to assemble the head-neck junction is reported to range from 3000 to 7000 N (Nassutt et al. 2006). This force is proven to play a major role in the mechanics of the taper junction which in turn can influence the tribocorrosion damage (Fallahnezhad et al. 2019b). To date, studies have investigated the influence of the assembly force on the initial mechanical behaviour of the taper junction (Bitter et al. 2017; Pennock et al. 2002; Rehmer et al. 2012). These studies generally suggest that high assembly forces enhance the initial stability and fixation in the head-neck junction and are therefore able to better withstand mechanical loads of daily activities without disconnection.
In comparison, only a few researchers have taken a step forward and investigated the influence of the assembly force on the fretting wear that occur at the taper interface and its associated material removal (Bitter et al. 2017; English et al. 2015b; Fallahnezhad et al. 2017; Fallahnezhad et al. 2019c). Bitter et al. (Bitter et al. 2017) performed an experimental-numerical study to evaluate the influence of assembly force on the fretting wear of a Ti6Al4V/ Ti6Al4V taper junction. Their experimental results showed that fretting wear reduces as the assembly force increases. They also developed a finite element (FE) model, based on the Archard’s law, to estimate the fretting wear damage at the interface. Their model had major simplifications, including ignoring the progressive nature of the fretting wear phenomenon that requires geometry updates to have a precise simulation of the mechanism. They reported poor agreement between their experimental and numerical results. English et al. (English et al. 2015b) used an adaptive numerical approach to investigate the influence of the assembly force on fretting wear in the head-neck junction. They used an energy-based wear law within a FE framework to simulate fretting wear in a CoCr/ Ti6Al4V junction, subjected to several million cycles of gait loading. Their results showed that using higher assembly forces, reduces the fretting wear material loss in the head-neck junction. Fallahnezhad et al. (Fallahnezhad et al. 2019c) investigated the influence of the assembly force on the fretting wear of a CoCr/ CoCr head-neck junction using a 2-dimensional adaptive finite element model. They concluded that high assembly forces reduce the relative micro-motions between the head and neck at the taper junction. However, they can also increase the contact pressures and the contact region at the interface, which may intensify the fretting wear process and, consequently, increase material removal. The results of their study showed that the effect of the last two parameters (contact pressure and contact length) was more dominant in the wear of the CoCr/CoCr junction with a distally engaging taper with a mismatch angle of 0.01◦.
To further improve the estimation of the material loss, during the operation of the hip implant, the tribocorrosion (fretting corrosion) phenomenon at the head-neck junction needs to be investigated. To achieve this, both the mechanical wear and electrochemical corrosion need to be modelled, simultaneously. Modelling tribocorrosion has been a big challenge for researchers, due to its complexity. Several analytical expressions have been proposed by researchers to predict tribocorrosion for simple ball-on-disk configurations (Cao and Mischler 2018b). Mischler and Landolt (Igual Muñoz and Espallargas 2011; Landolt et al. 2001) proposed a mechanistic approach to estimate the chemical wear, by describing the anodic current, as a function of the passivation charge. This model, in parallel with a simplified Archard wear model, was successfully used by Maldonado et al. (Guadalupe Maldonado et al. 2013) to quantify mechanical and chemical volume losses for a CoCrMo alloy in a ball-on-disk tribocorrosion test.
None of proposed analytical models are able to predict the tribocorrosion process for complex geometries and loading configurations of real engineering applications. To address this gap, a few researchers, have tried to use the analytical tribocorrosion models within numerical frameworks (Cao and Mischler 2018a). Dalmau et al. (Dalmau et al. 2018) simulated tribocorrosion using the boundary element method (BEM) to find the contacting elements which were updated using the Archard's wear law. To estimate the contact force profile and contact area, their model needs to define the geometry of two bodies as functions of position (X, Y and Z). This is very hard to achieve when the contacting bodies have irregular/complicated geometries. Moreover, estimation of tribocorrosion damage, using a simple Archard’s model with just one coefficient does not seem to be a generalizable approach to model both mechanical and electrochemical wear components for a wide range of test parameters and geometries.
Recently, the authors have developed a finite element model to simulate the tribocorrosion process to predict material loss due to the both mechanical fretting wear and passivation (Fallahnezhad et al. 2022; Fallahnezhad et al. 2018b). The model used a combination of Landolt’s passive film equation (Landolt et al. 2001) together with Archard’s wear law and was successfully validated by existing experimental tests (Fallahnezhad et al. 2022). This provided a foundation for further analysis on the influential parameters in hip replacements surgeries.
To date, no predictive study has been carried out to investigate the influence of the assembly force on the tribocorrosive behaviour of the head-neck junction of the hip implant, considering both the mechanical (substrate removal) and chemical (oxide removal) material losses. In this work, the developed tribocorrosion algorithm (Fallahnezhad et al. 2022) is employed and used to investigate the influence of assembly force on the tribocorrosiove behaviour of a CoCr/CoCr head/neck junction.