Fretting, Corrosion and Connection Strength of Sleeved Ceramic Modular Heads Tested Under in-Vivo Like Conditions

INTRODUCTION

Modular metal-on-metal hip implants show increased revision rates due to fretting and corrosion at the interface. High frictional torque potentially causes such effects at the head-taper interface, especially for large hip bearings. The aim of this study was to investigate fretting and corrosion of sleeved ceramic heads for large ceramic-on-ceramic (CoC) bearings.

METHODS

The investigated system consists of a ceramic head (ISO 6474-2; BIOLOX® Option), a metal sleeve (Ti-6Al-4V, ISO 5832-3) and different metal stem tapers (Ti-6Al-4V, ISO 5832-3; stainless steel, ISO 5832-1; CoCrMo, ISO 5832-12). Three different test methods were used to assess corrosion behaviour and connection strength of head-sleeve-taper interfaces:

1. –

   Fretting corrosion acc. to ASTM F1- Corrosion under in-vivo relevant loads

2. –

   Frictional torque under severe i like conditions

Standardized fretting corrosion tests were carried out. Additionally, a long term test (0.5 mio. cycles) under same conditions was performed.

Corrosion effects under 4.5 kN (stair climbing) and 10 kN (stumbling) were determined for three groups. One group was fatigue tested applying 4.5 mio. cycles at 4.5 kN and 0.5 mio. cycles at 10 kN in a corrosive fluid. In parallel two control groups (heads only assembled at same load levels) were stored in the same fluid for same time period. Pull-off tests were performed to detect the effect of corrosion on the interface strength.

A new designed test was performed to analyse the connection strength and fretting-corrosion effects on the head-sleeve taper interfaces caused by frictional torque of large CoC bearings (48 mm). Two separate loading conditions were investigated in a hip joint simulator. One created bending torque (pure abduction/adduction), the other set-up applied rotational torque (pure flexion). A static axial force of 3 kN and movements with a frequency of 1 Hz up to 5 mio. cycles in the same corrosive fluid as in the second set of tests were applied for both tests. Surface analysis of the taper and sleeve surfaces was peformed. In order to detect loosening caused by frictional torque, torque-out tests were conducted after simulator testing.

RESULTS

The measured currents (static and dynamic) from standard ASTM testing showed low values for all investigated taper materials even for long term testing (0.5 mio. cycles).

The strength of the head-sleeve-taper connection was not affected by storing and fatigue testing in corrosive fluid at 4.5 kN and 10 kN. No critical increase or decrease of pull-off force could be observed.

No loosening of the head-taper-sleeve connection was detected after hip simulator testing applying high frictional torque. For large CoC bearings (48 mm) with titanium alloy sleeves on appropriate stem tapers no critical corrosion effects could be found. Even testing low corrosion resistant stainless steel tapers as a worst case material showed only tribo-chemical layers and plastic deformation of the taper surfaces.

CONCLUSION

All different tests of large ceramic modular heads (48 mm) with titanium adapters on various taper materials exhibited only minor effects on the surfaces of the modular connections. Even worst case material combinations, high loads, corrosive fluid and high frictional torque did not show any critical results using such aggressive test methods.