Abstract
This study predicts the frictional moments at the head-cup interface and frictional torques and bending moments acting on the head-neck interface of a modular total hip replacement across a range of activities of daily living. The predicted moment and torque profiles are based on the kinematics of four patients and the implant characteristics of a metal-on-metal implant. Depending on the body weight and type of activity, the moments and torques had significant variations in both magnitude and direction over the activity cycles. 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. The bending moment acting on the head-neck interface varied from 7 to 21.6 Nm. One-leg-standing had the widest range of frictional torque on the head-neck interface (11 Nm) while normal walking had the smallest range (6.1 Nm). The widest range, together with the maximum magnitude of torque, bending moment, and frictional moment, occurred during one-leg-standing of the lightest patient. Most of the simulated activities resulted in frictional torques that were near the previously reported oxide layer depassivation threshold torque. The predicted bending moments were also found at a level believed to contribute to the oxide layer depassivation. The calculated magnitudes and directions of the moments, applied directly to the head-neck taper junction, provide realistic mechanical loading data for in vitro and computational studies on the mechanical behaviour and multi-axial fretting at the head-neck interface.
Highlights
The mechanical environment of hip joint implants is complex and not well understood.Mechanical loads can contribute to implant failure via different mechanisms such as fretting-corrosion at the head-neck junction [1,2], loosening of the acetabular cup and femoral stem interface [3], fracture due to fatigue [4], and wear of hard-on-soft or hard-on-hard bearing couples [5,6]
Head-cup contact forces are induced by body weight and muscle contraction forces, bending moments result from the offset of contact forces [9], and frictional moments
There was no z’ component for the bending moment because the contact force passes through the centre point of the head located on the z’ axis of the neck coordinate system
Summary
Mechanical loads can contribute to implant failure via different mechanisms such as fretting-corrosion at the head-neck junction [1,2], loosening of the acetabular cup and femoral stem interface [3], fracture due to fatigue [4], and wear of hard-on-soft or hard-on-hard bearing couples [5,6]. Head-cup contact forces are induced by body weight and muscle contraction forces, bending moments result from the offset of contact forces [9], and frictional moments. Hip implant failure mechanisms such as fretting-corrosion at the head-neck interface [12], especially in larger diameter metal-on-metal bearings, are thought to be influenced by frictional moments. Fretting wear is driven by relative micro-motion at the head-neck interface, which is significantly influenced by the mechanical loading environment [13]
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