Ion release during fretting at taper joint interface of hip joint prosthesis

Abstract

Metallic implants can be subjected to surface damage resulting in wear debris generation and ion release. Large amounts of metal ions released into body fluids may cause various harmful phenomena, from minor allergic reactions to more dangerous tissue necrosis and implant loosening. In hip joint prostheses, ions can be released from the bearing surfaces as a result of sliding at the head/cup interface and from non-bearing surfaces as a result of fretting or corrosion phenomena. In this paper, the focus is on the head–neck taper joint of the modular hip joint prosthesis subjected to high contact pressure fretting corrosion. Tests were performed on a Cr–Co–Mo tribocouple at 37°C in 0·9%NaCl solution using a ball on disc fretting tester. The free corrosion potential measurements were carried out during the friction process to assess how the sliding action at the interface affects the passivity of the material and how the passivity is re-established once the fretting wear ceases. Loading conditions were selected in order to simulate 60 and 80 kg patient body weight. Inductively coupled plasma measurements were carried out for solution samples collected after fretting experiments to detect metallic ions released during tribological contact. The wear tracks were analysed, and wear volumes were assessed using a white light profilometer. For all tests, the fretting loops characterising sliding behaviour of the interface and open circuit potential curves characterising electrochemical reactions were obtained. The average ion concentrations as a function of experiment duration were plotted and compared with volumetric wear. From the fretting loops and potential curves, the correlation between sliding conditions and depassivation/repassivation rate is discussed. The origin of metal loss for mechanical (fretting) and electrochemical (corrosion) processes is discussed. Analysis of the head/neck interface indicates the need for a long term fretting degradation model to consider various patient weights and forms of activity.