An Analytical Calculation of Frictional and Bending Moments at the Head-Neck Interface of Hip Joint Implants during Different Physiological Activities

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.

The Influence of Assembly Force on the Material Loss at the Metallic Head-Neck Junction of Hip Implants Subjected to Cyclic Fretting Wear

“…A 3D model of a 32-mm diameter CoCr head with a 12/14 CoCr neck (as per dimensions reported by Rehmer et al [12]) was first analyzed under normal walking load profiles [24,25] in order to determine the most critical plane of the taper junction in terms of two important fretting wear parameters of normal contact stress (contact pressure) and micro-motion. An elastic-plastic material model was used for CoCr (ISO 5832-12) with a Young's modulus of 210 GPa, Poisson's ratio of 0.30, yield stress of 910 GPa, ultimate tensile stress of 1350 GPa, and tensile elongation of 15%.…”

A finite element study on the mechanical response of the head-neck interface of hip implants under realistic forces and moments of daily activities: Part 2

“…As shown in Figure 1, the forces and moments in this model were applied to the neck while the external surface of the head sphere was fixed in all directions. For each activity, all the three force components [20] and three moment components produced by the head and cup frictional sliding [26] were applied to the head-neck junction (Figure 2). Micromotions and contact pressure are found as the most important mechanical parameters which can control fretting wear and thereby fretting corrosion [14,27].…”

“…Force (N) profiles[20] and moment (Nm) profiles[26] for: (a) Stair up, (b) Stair down, (c) Sit to stand, (d) Stand to sit, (e) One leg standing, and (f) Knee bending activity cycles.…”

A finite element study on the mechanical response of the head-neck interface of hip implants under realistic forces and moments of daily activities: Part 1, level walking

“…The assembly force (4 kN) and the load components of level gait were applied to the bottom face of the neck. The forces were obtained from Hip98 software [12] and the frictional moments were calculated from a previous study for a 32 mm metal-on-metal bearing couple [26], as shown in Figure 1a. The maximum magnitude of the frictional moments in this 32 mm metal-on-metal bearing (with a friction coefficient of 0.20) was found to be 3.68 Nm.…”

“…In addition to the previously available contact forces [12,24,25], head-cup frictional moments have recently become available for walking activity [26,27]. This provides 6 DoF load inputs including three force components and three moment components [26]. However, the contribution of the frictional moments to the stress field, relative displacements between the contacting surfaces and consequently fretting wear behaviour has not been reported yet.…”