Abstract
Background
Large diameter chrome cobalt modular heads in hip replacement surgery have caused one of the biggest challenges to face orthopaedic surgeons, manufacturers and regulators in a generation. Increasing numbers of retrieved implants offer the opportunity to look in detail at the modes of failure of these devices.
Methods
We have examined the mechanical behaviour of the head taper/trunnion using a variety of techniques including:
- 1.
mechanical testing in a custom-made jig
- 2.
3D nano-precision form profilometry (RedLux)
- 3.
optical 3D microscopy with focus variation technology
- 4.
electro-chemical testing.
Results
Our findings show that there is micro-movement between the taper and trunnion. This results in damage to the taper surface, causiing corrosion. The susceptibility to corrosion increases in proportion to the torsional load at this interface. We have also observed that taper corrosion occurs in both metal on metal, and metal on polyethylene bearing hip replacements.
In addition, we have found evidence of significant material loss and corrosion at the stem/cement interface in retrieved chrome/cobalt femoral implants. This interface is therefore likely to contribute to the generation of chrome/cobalt particles and corrosion products within the effective joint space.
The clinical implications of this will be discussed.
