Abstract
Introduction:
The higher resisting torque against dislocation and the large range of motion due to the enlarged effective head diameter substantiate the use of eccentric dual-mobility cups in case of total hip joint instability [1,2]. As a result of force-dependent self-centering mechanism, an increased movement of the intermediate-component can be expected whose effect on wear propagation is unknown so far. Currently available hip joint simulators are only able to vary the load by the absolute value and not by the direction of resulting force. Therefore, the uniaxial force transmission may lead to a unique and stable alignment of the intermediate-component during testing. The purpose of this numerical study was to evaluate relative movements of the intermediate-component during daily life activities with respect to wear propagation.
Method:
The numerical analysis was based on a standard dual-mobility system consisting of a polished metallic cup, a UHMWPE intermediate-component (40 mm outer diameter) with an eccentric offset of 2 mm and a 28 mm ceramic femoral head [Fig. 1]. The relative motion of the intermediate-component was affected by the geometrically generated self-centering torque (TC) and the friction torque for inner (TFi) and outer (TFo) articulation around the centre of rotation Z1[Fig. 2]. In order to consider lubrication conditions the lambda ratio was estimated for different daily life activities [3], including the calculation of composite roughness and minimum film thickness for a ball-on-plate configuration. The friction torque was related to the product of load (FRes), lever arm and the coefficients of friction which were determined using the equation proposed by [4]. Depending on load FRes and load orientation angle βthe equilibrium of moments around Z1 was established.
Results:
Due to surface roughness and calculated minimum film thickness (0.1 to 0.62 μm), boundary lubrication was determined for both inner and outer articulation. The average coefficient of friction was 0.01825 for inner and 0.0751 for outer bearing. Torque patterns were obtained for different load orientation angles [Fig. 3]. In case of an angle β between 15° and 45° the friction torque dominated compared to the self-centering mechanism which resulted in a fixed intermediate-component position. Increasing the orientation angle β above 45° lead to a superiority of the self-centering effect as can be found for varus positions.
Discussion:
In case of daily life activities the load vector FRes varies within the range of 30° in the frontal plane [3]. Therefore, the angle β ranges below 30° which leads to a fixed position of the intermediate-component. Consequently, the self-centering mechanism seems to have no negative influence on the wear propagation of dual-mobility cup systems. The presented numerical analysis permits a variation of implant-specific dimensions, but is limited to polyethylene wear couples.
